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#1
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Bench grinder spindle thread
I see that all the bench grinders and buffers have their spindles
threaded in UNF - either 1/2-20 or 5/8-18. Is there a reason for this? I am asking because I just tried to cut a 1/2-20 thread on an adaptor I made and could not do it - most likely because of the Crappy Tire die. I do have a good 1/2-13 SKF die. OTOH it might be God's way telling me to finally learn how to cut the threads on a lathe. Before I do anything so heroic I thought I would ask... Michael Koblic, Campbell River, BC |
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#2
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Bench grinder spindle thread
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#3
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
Pete's recommendation is good advice. Cutting a thread on a lathe generally
gives much better results (and greater gratification). Eventually one will need a thread which will require using a lathe, so it's better to get some experience before that need arises. My recommendation would be to avoid carbide cutting tools. Using HSS will increase your chances of success with better results and eliminate much of the frustration of using carbide on smaller benchtop machines. Grinding HSS isn't extremely difficult, and some of the best advice you can find has been written by Harold V. Some of that advice can be found in RCM archives, but his concise articles can be found in the Chaski forums. Harold has included details regarding proper grinding wheel selection and use of a dressing stick. Sets of pre-ground HSS cutting tools can be found at Grizzly, LMS and some eBag sellers, which can be very helpful for getting started. These sets will show angles of rake, relief and chip breaker examples while eliminating the need to try to duplicate cutting tools from HSS tool blanks and drawings. -- WB .......... wrote in message ... I see that all the bench grinders and buffers have their spindles threaded in UNF - either 1/2-20 or 5/8-18. Is there a reason for this? I am asking because I just tried to cut a 1/2-20 thread on an adaptor I made and could not do it - most likely because of the Crappy Tire die. I do have a good 1/2-13 SKF die. OTOH it might be God's way telling me to finally learn how to cut the threads on a lathe. Before I do anything so heroic I thought I would ask... Michael Koblic, Campbell River, BC |
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#4
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"Wild_Bill" wrote in message ... Pete's recommendation is good advice. Cutting a thread on a lathe generally gives much better results (and greater gratification). Eventually one will need a thread which will require using a lathe, so it's better to get some experience before that need arises. My recommendation ... WB Good suggestions. You can cut most of the thread depth on the lathe and finish with a die, which will cut more easily if it doesn't have to remove much metal. The die will track better and the threading bit doesn't have to be ground as precisely to form and finish. You also get the correct root shape without having to regrind the bit for each pitch. That matters if the thread will be highly stressed. Usually I grind the tip correct for about 32 TPI and leave it that way, and finish with a die if the thread will be in tension. The die will start straight if you screw a short piece of threaded rod into the back of it and hold the rod in the tailstock chuck. Pressure from the face of the chuck may be enough to start it straight. jsw |
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#5
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Bench grinder spindle thread
On Sat, 17 Dec 2011 12:39:45 -0500, "Jim Wilkins"
wrote: "Wild_Bill" wrote in message ... Pete's recommendation is good advice. Cutting a thread on a lathe generally gives much better results (and greater gratification). Eventually one will need a thread which will require using a lathe, so it's better to get some experience before that need arises. My recommendation ... WB Good suggestions. You can cut most of the thread depth on the lathe and finish with a die, which will cut more easily if it doesn't have to remove much metal. The die will track better and the threading bit doesn't have to be ground as precisely to form and finish. You also get the correct root shape without having to regrind the bit for each pitch. That matters if the thread will be highly stressed. Usually I grind the tip correct for about 32 TPI and leave it that way, and finish with a die if the thread will be in tension. The die will start straight if you screw a short piece of threaded rod into the back of it and hold the rod in the tailstock chuck. Pressure from the face of the chuck may be enough to start it straight. Ah, that is the sort of pragmatic approach that I like! I have die holders for the lathe. I tried them and could not even get the thread started. I tried pushing with the tailstock. I tried brute force by hand. Not a thing. As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment. BTW I have developed a great liking for carbide tools but I see that for this job HSS will be better. Michael Koblic, Campbell River, BC |
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#6
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
wrote in message ... On Sat, 17 Dec 2011 12:39:45 -0500, "Jim Wilkins" ... As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment... Michael Koblic, Mine turns at about 50 RPM in back gear and still demands close attention and quick reactions to stop at the end of the thread. I shut off the motor when it's about one thread away and then lift the belt tension lever to stop the spindle quickly. The spindle is fairly easy to turn by hand if the bit is sharp and the chip 1-3 thousandths thick. It's certainly less effort than cutting the full depth thread all at once with a die. Could you make up a safe balanced hand crank for the spindle and use the power only to run the bit back to the start? jsw |
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#7
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Bench grinder spindle thread
You can always run the spindle backwards and flip the cutter upside
down. Then you don't need to worry about stopping at the right time. DOC n Dec 17, 3:40*pm, "Jim Wilkins" wrote: wrote in message ... On Sat, 17 Dec 2011 12:39:45 -0500, "Jim Wilkins" ... As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment... Michael Koblic, Mine turns at about 50 RPM in back gear and still demands close attention and quick reactions to stop at the end of the thread. I shut off the motor when it's about one thread away and then lift the belt tension lever to stop the spindle quickly. The spindle is fairly easy to turn by hand if the bit is sharp and the chip 1-3 thousandths thick. It's certainly less effort than cutting the full depth thread all at once with a die. Could you make up a safe balanced hand crank for the spindle and use the power only to run the bit back to the start? jsw |
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#8
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Bench grinder spindle thread
"doc" wrote in message ... You can always run the spindle backwards and flip the cutter upside down. Then you don't need to worry about stopping at the right time. DOC Milling attachments showed me that neither of my lathes behaves well if an upward cutting force lifts the saddle off the ways. The hold-down clamps and their machined guides aren't precise enough to take out all the play and still move freely. jsw |
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#9
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Bench grinder spindle thread
On Sat, 17 Dec 2011 18:40:16 -0500, "Jim Wilkins"
wrote: wrote in message .. . On Sat, 17 Dec 2011 12:39:45 -0500, "Jim Wilkins" ... As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment... Michael Koblic, Mine turns at about 50 RPM in back gear and still demands close attention and quick reactions to stop at the end of the thread. I shut off the motor when it's about one thread away and then lift the belt tension lever to stop the spindle quickly. The spindle is fairly easy to turn by hand if the bit is sharp and the chip 1-3 thousandths thick. It's certainly less effort than cutting the full depth thread all at once with a die. Could you make up a safe balanced hand crank for the spindle and use the power only to run the bit back to the start? OK, I had a go on a piece of scrap today. Everything went fine until I cut the first pass at 40 tpi. Re-reading the chart I corrected the gears and tried cutting ostensibly 20 tpi. I did, BTW what you suggested and run the lathe under power both ways just finishing the last three threads manually. The big disappointment was that the thread ended up 21 tpi. Once I was able to get the die on it I was able to correct that with a resulting pattern of periodic double cuts and a variable major diameter (reminds me of beat frequency oscillators). I can get the nut on but it is not pretty! The other thing that I did not expect was that starting with a rod of 0.493" the final major diameter (before the die application) was 0.507". Even after filing it was still 0.502". So the big question: Is there a reason for the 21 tpi *other* than change gears? Michael Koblic, Campbell River, BC BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? |
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#10
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
wrote in message ... ... OK, I had a go on a piece of scrap today. Everything went fine until I cut the first pass at 40 tpi. Re-reading the chart I corrected the gears and tried cutting ostensibly 20 tpi. I did, BTW what you suggested and run the lathe under power both ways just finishing the last three threads manually. The big disappointment was that the thread ended up 21 tpi. Once I was able to get the die on it I was able to correct that with a resulting pattern of periodic double cuts and a variable major diameter (reminds me of beat frequency oscillators). I can get the nut on but it is not pretty! The other thing that I did not expect was that starting with a rod of 0.493" the final major diameter (before the die application) was 0.507". Even after filing it was still 0.502". So the big question: Is there a reason for the 21 tpi *other* than change gears? Michael Koblic, Campbell River, BC BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? You want me to guess this from the other side of the continent? If the bit didn't cut freely it just might have pushed metal forward, increasing the diameter and shortening the rod. Did you sharpen the bit edge to at least the full depth of the thread? jsw |
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#12
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
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#13
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
I dunno the reason for the dies not starting, but there are some really poor
quality die sets on the market and for China/India standards, the products just looky-like dies but are barely sitable as thread chasers for cleaning up a slightly damaged good thread. Almost always, the side of the die that's marked with the size is the starting side and the exit side doesn't have the widened opening for starting onto the workpiece. Also, the end of the workpiece should have a fairly uniform chamfer which aids in starting the cutting action. If no cutting action can be felt in the first 1/4 to 1/2 turn, it's likely that something is wrong.. either the quality of the die is very bad/damaged, or the workpiece is excessively oversize (assuming that the workpiece isn't hardened steel). -- WB .......... wrote in message ... I have die holders for the lathe. I tried them and could not even get the thread started. I tried pushing with the tailstock. I tried brute force by hand. Not a thing. As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment. BTW I have developed a great liking for carbide tools but I see that for this job HSS will be better. Michael Koblic, Campbell River, BC |
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#14
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
As DoN has mentioned, using a DI and turning the spindle with the carriage
feed engaged will show if there are errors in the selected gear set. I generally just rotate the chuck the number of turns equal to the TPI to make sure the travel is 1.00". Before I assembled a variable speed drive, I was using a hand crank fitted into the back/left end of the spindle for short threaded sections such as lens adapters which may only use 3-4 turns of thread. When cutting up to a shoulder, the variable speed drive is a huge benefit. For a fine thread, using a hand crank is fairly easy work, but there are several other required additional steps which make cutting the thread successful. This is a basic outline: Unplug the power cord for hand crank method Setup the compound at about 29 to 29.5 degrees Cutting lubricant applied to work area Setting the cross feed dial to zero for the scratch cut Set the compound dial to zero for the scratch cut Make the first pass scratch cut Backing out the cross feed enough to clear the workpiece Reverse feed to locate the carriage for a second pass (by hand if using a crank) Set the cross feed dial back to zero for the second pass Advance the compound for a reasonable cut (depending upon hand or power cut) Make the second pass with cutting lube Repeat from Backing out the cross feed It helps to have a (quality) nut of the desired thread nearby to use as a gage.. otherwise, there are cutting depth specs on on threading gages for setting up the cutting tool prior to threading (perpendicular to the workpiece axis). A quality nut would be one that doesn't rattle when run onto a quality tap.. which will insure that the resulting thread will fit any other standard nut of that size/pitch. Adhering to the thread specs will generally prevent cutting too deep, although getting familiar with a machine generally includes seeing how well it responds to operator input, so when nearing the final recommended depth of the cut, trying the nut can prevent undercutting and the need to start over. The edges of the thread crests can be sharp and possibly a little rough.. removing the hand crank and running the spindle at a reasonable speed will allow the operator to wire brush the freshly-cut thread (carbon steel brush bristles for a steel workpiece) and/or run some emery cloth over the area (apply a protective cover/shop rag over the ways to keep any shedded abrasive off them). A 3-sided file can also be used to very lightly chase along the thread to break the sharp edges of the thread. -- WB .......... wrote in message ... On Sat, 17 Dec 2011 18:40:16 -0500, "Jim Wilkins" wrote: wrote in message . .. On Sat, 17 Dec 2011 12:39:45 -0500, "Jim Wilkins" ... As I see another one of these in the near future followed closely by a need to cut 3/4-16 I shall have a go. I suspect the whole thing will be done manually as I have no confidence of my ability to stop the lathe at a speed of 150 rpm just at the right moment... Michael Koblic, Mine turns at about 50 RPM in back gear and still demands close attention and quick reactions to stop at the end of the thread. I shut off the motor when it's about one thread away and then lift the belt tension lever to stop the spindle quickly. The spindle is fairly easy to turn by hand if the bit is sharp and the chip 1-3 thousandths thick. It's certainly less effort than cutting the full depth thread all at once with a die. Could you make up a safe balanced hand crank for the spindle and use the power only to run the bit back to the start? OK, I had a go on a piece of scrap today. Everything went fine until I cut the first pass at 40 tpi. Re-reading the chart I corrected the gears and tried cutting ostensibly 20 tpi. I did, BTW what you suggested and run the lathe under power both ways just finishing the last three threads manually. The big disappointment was that the thread ended up 21 tpi. Once I was able to get the die on it I was able to correct that with a resulting pattern of periodic double cuts and a variable major diameter (reminds me of beat frequency oscillators). I can get the nut on but it is not pretty! The other thing that I did not expect was that starting with a rod of 0.493" the final major diameter (before the die application) was 0.507". Even after filing it was still 0.502". So the big question: Is there a reason for the 21 tpi *other* than change gears? Michael Koblic, Campbell River, BC BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? |
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#15
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"Wild_Bill" wrote This is a basic outline: ... Adhering to the thread specs will generally prevent cutting too deep, although getting familiar with a machine generally includes seeing how well it responds to operator input, so when nearing the final recommended depth of the cut, trying the nut can prevent undercutting and the need to start over.... WB I had to generate a spreadsheet threading gearbox chart since the original was missing. I added the compound infeed at 29 degrees, assuming the proper bit tip width, and the indicator line-matching rules. As I mentioned before I usually leave the bit more pointed and the calculated infeed cuts a somewhat shallow thread, so I can cut in to the number quickly before measuring with three wires. The bit will cut the proper thread form if I then disconnect the tumbler and move the spindle a tooth backwards, to cut the thread wider but not deeper. jsw |
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#16
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Bench grinder spindle thread
On 19 Dec 2011 05:52:54 GMT, "DoN. Nichols"
wrote: On 2011-12-19, wrote: On Sat, 17 Dec 2011 18:40:16 -0500, "Jim Wilkins" BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? I don't remember even seeing the question. :-) It does not matter what you call it when cutting it on the lathe. It only matters when you are trying to *buy* the tap and/or die. National Fine or National Coarse are just names for what family the threads are in. What matters are the diameter and the TPI (or in metric threads, the pitch (mm per turn). In your case, 1/2-20, which is NF (The 'U' in front comes from "Unified", when thread standards between the US and the UK were somewhat merged. Which brings up the question: Why did they create both families? Why are there two different pitches for each standard diameter of thread? Does coarse grip better/stronger than fine due to its smaller core diameter? I don't recall reading about that in tech school. -- If you're trying to take a roomful of people by surprise, it's a lot easier to hit your targets if you don't yell going through the door. -- Lois McMaster Bujold |
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#17
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"Larry Jaques" wrote in message ... ... Which brings up the question: Why did they create both families? Why are there two different pitches for each standard diameter of thread? Does coarse grip better/stronger than fine due to its smaller core diameter? I don't recall reading about that in tech school. ... The coarse thread standards were developed in the mid 1800s and are well suited to steel bolts in cast iron (or aluminum) tapped holes. The fine series is a better strength match with steel female threads and the larger root diameter makes the bolt stronger. Originally it was called SAE after the Society of Automotive Engineers. http://en.wikipedia.org/wiki/British_Standard_Whitworth http://en.wikipedia.org/wiki/William_Sellers This group introduced the fine thread standard in 1906 after the coarse series proved inadequate for the developing automotive industry.. http://en.wikipedia.org/wiki/Associa..._Manufacturers For completeness: http://www.iceweb.com.au/Tubings/Gla...ad_History.pdf BPT is NOT British Pipe Thread, those are BSP(T). jsw |
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#18
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
Jim Wilkins wrote:
"Larry Jaques" wrote in message ... ... Which brings up the question: Why did they create both families? Why are there two different pitches for each standard diameter of thread? Does coarse grip better/stronger than fine due to its smaller core diameter? I don't recall reading about that in tech school. ... The coarse thread standards were developed in the mid 1800s and are well suited to steel bolts in cast iron (or aluminum) tapped holes. The fine series is a better strength match with steel female threads and the larger root diameter makes the bolt stronger. Originally it was called SAE after the Society of Automotive Engineers. http://en.wikipedia.org/wiki/British_Standard_Whitworth http://en.wikipedia.org/wiki/William_Sellers This group introduced the fine thread standard in 1906 after the coarse series proved inadequate for the developing automotive industry.. http://en.wikipedia.org/wiki/Associa..._Manufacturers For completeness: http://www.iceweb.com.au/Tubings/Gla...ad_History.pdf BPT is NOT British Pipe Thread, those are BSP(T). jsw Properly it should be BSPP and BSPT as British Standard Pipe can be parallel or tapered, the parallel thread largely being used for mechanical fastening as for instance bulkhead fittings, well not BSPP in the US I expect. See http://en.wikipedia.org/wiki/British...rd_pipe_thread for more info and the ISO designations. |
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#19
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"David Billington" wrote in message ... Jim Wilkins wrote: ... BPT is NOT British Pipe Thread, those are BSP(T). jsw Properly it should be BSPP and BSPT as British Standard Pipe can be parallel or tapered, the parallel thread largely being used for mechanical fastening as for instance bulkhead fittings, well not BSPP in the US I expect. See http://en.wikipedia.org/wiki/British...rd_pipe_thread for more info and the ISO designations. I wrote it that way because I've seen both BSP and BSPT. jsw |
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#20
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Bench grinder spindle thread
On Mon, 19 Dec 2011 06:17:00 -0500, "Wild_Bill"
wrote: As DoN has mentioned, using a DI and turning the spindle with the carriage feed engaged will show if there are errors in the selected gear set. I generally just rotate the chuck the number of turns equal to the TPI to make sure the travel is 1.00". Before I assembled a variable speed drive, I was using a hand crank fitted into the back/left end of the spindle for short threaded sections such as lens adapters which may only use 3-4 turns of thread. When cutting up to a shoulder, the variable speed drive is a huge benefit. For a fine thread, using a hand crank is fairly easy work, but there are several other required additional steps which make cutting the thread successful. This is a basic outline: Unplug the power cord for hand crank method Setup the compound at about 29 to 29.5 degrees Cutting lubricant applied to work area Setting the cross feed dial to zero for the scratch cut Set the compound dial to zero for the scratch cut Make the first pass scratch cut Backing out the cross feed enough to clear the workpiece Reverse feed to locate the carriage for a second pass (by hand if using a crank) Set the cross feed dial back to zero for the second pass Advance the compound for a reasonable cut (depending upon hand or power cut) Make the second pass with cutting lube Repeat from Backing out the cross feed It helps to have a (quality) nut of the desired thread nearby to use as a gage.. otherwise, there are cutting depth specs on on threading gages for setting up the cutting tool prior to threading (perpendicular to the workpiece axis). A quality nut would be one that doesn't rattle when run onto a quality tap.. which will insure that the resulting thread will fit any other standard nut of that size/pitch. Adhering to the thread specs will generally prevent cutting too deep, although getting familiar with a machine generally includes seeing how well it responds to operator input, so when nearing the final recommended depth of the cut, trying the nut can prevent undercutting and the need to start over. The edges of the thread crests can be sharp and possibly a little rough.. removing the hand crank and running the spindle at a reasonable speed will allow the operator to wire brush the freshly-cut thread (carbon steel brush bristles for a steel workpiece) and/or run some emery cloth over the area (apply a protective cover/shop rag over the ways to keep any shedded abrasive off them). A 3-sided file can also be used to very lightly chase along the thread to break the sharp edges of the thread. Thanks. Pretty much all that I did except for the indicator thing. I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is? Michael Koblic, Campbell River, BC |
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#21
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Bench grinder spindle thread
On Mon, 19 Dec 2011 01:28:48 -0500, Gerald Miller
wrote: On Fri, 16 Dec 2011 17:45:43 -0800, wrote: I see that all the bench grinders and buffers have their spindles threaded in UNF - either 1/2-20 or 5/8-18. Is there a reason for this? I am asking because I just tried to cut a 1/2-20 thread on an adaptor I made and could not do it - most likely because of the Crappy Tire die. I do have a good 1/2-13 SKF die. OTOH it might be God's way telling me to finally learn how to cut the threads on a lathe. Before I do anything so heroic I thought I would ask... Michael Koblic, Campbell River, BC You may also find 1/2 24, RH or LH Right. But so far I have not been able to ascertain why a common or garden 1/2-13 thread would not do just as well in this application (L or RH). Michael Koblic, Campbell River, BC |
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#22
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Bench grinder spindle thread
On Mon, 19 Dec 2011 12:47:55 -0500, "Jim Wilkins"
wrote: "Larry Jaques" wrote in message .. . ... Which brings up the question: Why did they create both families? Why are there two different pitches for each standard diameter of thread? Does coarse grip better/stronger than fine due to its smaller core diameter? I don't recall reading about that in tech school. ... The coarse thread standards were developed in the mid 1800s and are well suited to steel bolts in cast iron (or aluminum) tapped holes. The fine series is a better strength match with steel female threads and the larger root diameter makes the bolt stronger. Originally it was called SAE after the Society of Automotive Engineers. http://en.wikipedia.org/wiki/British_Standard_Whitworth http://en.wikipedia.org/wiki/William_Sellers This group introduced the fine thread standard in 1906 after the coarse series proved inadequate for the developing automotive industry.. http://en.wikipedia.org/wiki/Associa..._Manufacturers For completeness: http://www.iceweb.com.au/Tubings/Gla...ad_History.pdf BPT is NOT British Pipe Thread, those are BSP(T). Now I'm thoroughly confused. TMI, but thanks!  -- If you're trying to take a roomful of people by surprise, it's a lot easier to hit your targets if you don't yell going through the door. -- Lois McMaster Bujold |
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#23
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Bench grinder spindle thread
On Sun, 18 Dec 2011 20:24:57 -0500, "Jim Wilkins"
wrote: wrote in message .. . ... OK, I had a go on a piece of scrap today. Everything went fine until I cut the first pass at 40 tpi. Re-reading the chart I corrected the gears and tried cutting ostensibly 20 tpi. I did, BTW what you suggested and run the lathe under power both ways just finishing the last three threads manually. The big disappointment was that the thread ended up 21 tpi. Once I was able to get the die on it I was able to correct that with a resulting pattern of periodic double cuts and a variable major diameter (reminds me of beat frequency oscillators). I can get the nut on but it is not pretty! The other thing that I did not expect was that starting with a rod of 0.493" the final major diameter (before the die application) was 0.507". Even after filing it was still 0.502". So the big question: Is there a reason for the 21 tpi *other* than change gears? Michael Koblic, Campbell River, BC BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? You want me to guess this from the other side of the continent? I have great faith in you. If the bit didn't cut freely it just might have pushed metal forward, increasing the diameter and shortening the rod. Did you sharpen the bit edge to at least the full depth of the thread? I *thought* it cut freely...but I will have another look at the profile. The side clearances may not be all that they can be. As to the TPI issue, another 9x20 owner, no doubt as a result of the same experience, pointed out that I may have missed a step in the change gear set up. In my defence I have to say that the diagram on the machine is not particularly easy to interpret *and* it is different from the machine manual. I will make the necessary adjustments and report the result. Of course the pragmatic Kalashnikov solution would be to cut the first four threads as is and finish the whole thing with a die. But that would not be sporting! Michael Koblic, Campbell River, BC |
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#24
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Bench grinder spindle thread
On 19 Dec 2011 05:52:54 GMT, "DoN. Nichols"
wrote: [ ... ] OK, I had a go on a piece of scrap today. Everything went fine until I cut the first pass at 40 tpi. Re-reading the chart I corrected the gears and tried cutting ostensibly 20 tpi. I did, BTW what you suggested and run the lathe under power both ways just finishing the last three threads manually. And backing the cutting tool out before running in reverse? There is usually some backlash in the leadscrew/half-nuts interface, so it will follow a slightly different track in reverse, and will tend to dull the cutter and do weird things to the thread. Yes. The big disappointment was that the thread ended up 21 tpi. Once I was able to get the die on it I was able to correct that with a resulting pattern of periodic double cuts and a variable major diameter (reminds me of beat frequency oscillators). I can get the nut on but it is not pretty! Strange. Is this with a quick-change gearbox, or a set of loose gears which you have to change on the far side of the headstock? If the latter, I would suggest that you go through the gears and count the teeth on each, and compare them to what the manual claims they should be. This is *every* gear on the way from the spindle to the leadscrew, not just the ones you expect to change. It's a bunch of loose gears. But I think I may have been given the answer. See my reply to JSW. [...] The other thing that I did not expect was that starting with a rod of 0.493" the final major diameter (before the die application) was 0.507". Even after filing it was still 0.502". The tool is not cutting -- it is mashing metal out of the groove, causing it it build up on the crest of the threads. This may be happening in the reverse feed if you did not back the tool out of the groove before reversing. The reverse was done backed out. I shall review the cutter shape, though. [...] BTW what's with the deafening silence on the question as to why these threads are UNF rather than UNC? Could it be that it does not matter? I don't remember even seeing the question. :-) The second sentence in OP. It does not matter what you call it when cutting it on the lathe. Which is what I was trying to avoid. But now I am committed :-) Michael Koblic, Campbell River, BC |
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#25
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
A lot depends upon the overall characteristics of the machine.. rigidity,
fit, adjustments of gibs and so on. Much of the methodolgy is trial and error, and/or making notes and sticking them on the wall. 25 passes for a fine thread seemed excessive (although slow and easy beats fast and oh-****).. cutting depth will be about half of that mentioned. http://www.engineersedge.com/screw_threads_chart.htm Actual infeed (perpendicular to workpiece axis) is x2 for diameter measurements. When the spindle is hand cranked, the operator gets feedback thru the cranking arm wrt the depth of cut during the pass.. going a little light is better than stopping during the pass, for sure. A very sharp cutting tool and a quality cutting lubricant will definitely enhance performance. When threading under power a 9x20" lathe with the slow speed selected can produce more torque than choosing higher spindle speeds, so cuts can be relatively aggressive.. depending upon the variables mentioned above. Some home shops are equipped with fairly heavy-duty lathes capable of producing lots of blue chips all day long.. yours and mine are jewelers lathes compared to those machines, but very useful none the less. -- WB .......... wrote in message ... Thanks. Pretty much all that I did except for the indicator thing. I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is? Michael Koblic, Campbell River, BC |
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#26
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On 2011-12-20, wrote:
On Mon, 19 Dec 2011 06:17:00 -0500, "Wild_Bill" wrote: [ ... ] Thanks. Pretty much all that I did except for the indicator thing. I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is? Depends on how beefy the lathe is (including the toolpost and the length of threading tool extension, how long the section of workpiece is compared to the diameter (the skinnier, the shallower the pass, but if you can support the other end with a live center, you can get away with a bit more), the material being threaded (I like 12L14, but if you are planning to weld to it, forget that material. :-) If you have 3" or less of the 1/2" stock sticking out, you can likely do it as follows: 1) Light scratch just to verify that the threading gears are set right -- compare the scratches to a thread gauge). 2) I would probably do four passes of 0.010" actual depth 3) And then finish a couple of 0.005" passes. (Make sure you know whether your lathe cross-slide and compound are calibrated in terms of diameter off the workpiece, or radius. (This is why the threading gauges have a "double depth" column as well as the "single depth" one. The threading depths there are with free machining metals, 12L14 steel, 360L brass, 6061T6 brass. If you have some of the gummy "steel" from Home Depot -- experiment until you find values which work for you. And be sure to lubricate the threads each pass. Some of the Rigid high sulfur pipe threading oil for steel, perhaps some Molly-D, nothing for the brass, and just keep spritzing with WD-40 or kerosene for aluminum. I've also got some really high sulfur cutting oil which brushes on with an acid brush and stays put. It is called Sul-Flo, and is only sold in rather expensive quantities, so our metalworking club got together a group purcase, with the one who wanted it the most (from using it before) doing the sub-dividing into gallon plastic bottles. It looks just like flowers of sulfur (the fine yellow powder) in a thick oil. It is really nice when cutting tough steels. I tend to use pre-formed insert tooling specifically for threading most of the time, though I have ground my own Acme threading tools with the proper relief angles for the particular diameter and pitch I was threading. Good Luck, DoN. -- Remove oil spill source from e-mail Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#27
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
Arbor nuts for securing grinding wheels aren't torqued like some fasteners
in other applications, but being able to attain a specific torque value is easy with a fine thread, and mating contact areas are of benefit to holding a position. All bets are off when considering low-grade hardware. A factor someone mentioned is the deeper the thread, the weaker the arbor (or bolt/stud) for a given size. With blotters on each side of a grinding wheel and the appropriate cupped plates, (typical bench grinder) arbor nuts generally only need to be snug, and a fine thread is beneficial for achieving snug.. not even close to a grunt. The blotters become slightly compressed, making good/excellent overall contact between the abrasive and the plates. -- WB .......... wrote in message ... On Mon, 19 Dec 2011 01:28:48 -0500, Gerald Miller wrote: On Fri, 16 Dec 2011 17:45:43 -0800, wrote: I see that all the bench grinders and buffers have their spindles threaded in UNF - either 1/2-20 or 5/8-18. Is there a reason for this? I am asking because I just tried to cut a 1/2-20 thread on an adaptor I made and could not do it - most likely because of the Crappy Tire die. I do have a good 1/2-13 SKF die. OTOH it might be God's way telling me to finally learn how to cut the threads on a lathe. Before I do anything so heroic I thought I would ask... Michael Koblic, Campbell River, BC You may also find 1/2 24, RH or LH Right. But so far I have not been able to ascertain why a common or garden 1/2-13 thread would not do just as well in this application (L or RH). Michael Koblic, Campbell River, BC |
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#28
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
wrote in message ... ... I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is? Michael Koblic, Campbell River, BC I've done only a little threading on lathes bigger than my 10" South Bend. When I experimented with heroic depth of cut the point broke off, so now I don't take over 0.005" per pass on a one-off job on steel or stainless, especially one with previous time invested in it. For multiple parts I make extras and any with mistakes become the set-up and practice dummies. I think I've accidentally taken 0.025" or so when trying to catch the thread after removing the part from the chuck. No matter how beefy (sheepish, ratty) the lathe may be, if the work slips you have to reestablish the bit position in the thread. You can tell if the work and machine have deflected from an overly aggressive cut by taking a second pass at the same setting to see how much of a chip is removed. If the work deflects the thread will become tapered. A thread indicator as shown here speeds up the job considerably. http://www.micro-machine-shop.com/si..._threading.htm Cut a groove with the parting tool at the left end of the thread and pop open the half nuts when the bit reaches it. On my lathe the gear train sound changes when the cutting pressure stops. Retract the bit, crank the carriage the carriage over, advance the bit and feed the compound in, wait for the proper line to match and close the half nuts. I make the retract-one-turn / move-right / advance-to-zero process uninterruptible so I don't lose my place when I stop the spindle to check the progress. jsw |
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#29
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"Larry Jaques" wrote in message ... ... http://en.wikipedia.org/wiki/British_Standard_Whitworth http://en.wikipedia.org/wiki/William_Sellers This group introduced the fine thread standard ... http://www.iceweb.com.au/Tubings/Gla...ad_History.pdf Now I'm thoroughly confused. TMI, but thanks! Now memorize this: http://www.ptable.com/Images/periodic%20table.png jsw |
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#30
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On Tue, 20 Dec 2011 08:54:01 -0500, "Jim Wilkins"
wrote: "Larry Jaques" wrote in message .. . ... http://en.wikipedia.org/wiki/British_Standard_Whitworth http://en.wikipedia.org/wiki/William_Sellers This group introduced the fine thread standard ... http://www.iceweb.com.au/Tubings/Gla...ad_History.pdf Now I'm thoroughly confused. TMI, but thanks! Now memorize this: http://www.ptable.com/Images/periodic%20table.png BTDT (senior HS year in Org Chem) Remember six or seven of 'em, at least in part. Have you seen the Ig-Nobel prize winning table? http://goo.gl/6VApp -- Tomorrow is the most important thing in life. Comes into us at midnight very clean. It's perfect when it arrives and it puts itself in our hands. It hopes we've learned something from yesterday. -- John Wayne |
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#31
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On Tue, 20 Dec 2011 01:31:45 -0500, "Wild_Bill"
wrote: Arbor nuts for securing grinding wheels aren't torqued like some fasteners in other applications, but being able to attain a specific torque value is easy with a fine thread, and mating contact areas are of benefit to holding a position. All bets are off when considering low-grade hardware. A factor someone mentioned is the deeper the thread, the weaker the arbor (or bolt/stud) for a given size. With blotters on each side of a grinding wheel and the appropriate cupped plates, (typical bench grinder) arbor nuts generally only need to be snug, and a fine thread is beneficial for achieving snug.. not even close to a grunt. The blotters become slightly compressed, making good/excellent overall contact between the abrasive and the plates. OK. That makes sense. SOunds like it comes under the category of "beneficial" but not "essential". Michael Koblic, Campbell River, BC |
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#32
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On Tue, 20 Dec 2011 08:43:30 -0500, "Jim Wilkins"
wrote: wrote in message .. . ... I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is? Michael Koblic, Campbell River, BC I've done only a little threading on lathes bigger than my 10" South Bend. When I experimented with heroic depth of cut the point broke off, so now I don't take over 0.005" per pass on a one-off job on steel or stainless, especially one with previous time invested in it. For multiple parts I make extras and any with mistakes become the set-up and practice dummies. I think I've accidentally taken 0.025" or so when trying to catch the thread after removing the part from the chuck. No matter how beefy (sheepish, ratty) the lathe may be, if the work slips you have to reestablish the bit position in the thread. You can tell if the work and machine have deflected from an overly aggressive cut by taking a second pass at the same setting to see how much of a chip is removed. If the work deflects the thread will become tapered. A thread indicator as shown here speeds up the job considerably. http://www.micro-machine-shop.com/si..._threading.htm Cut a groove with the parting tool at the left end of the thread and pop open the half nuts when the bit reaches it. On my lathe the gear train sound changes when the cutting pressure stops. Retract the bit, crank the carriage the carriage over, advance the bit and feed the compound in, wait for the proper line to match and close the half nuts. I make the retract-one-turn / move-right / advance-to-zero process uninterruptible so I don't lose my place when I stop the spindle to check the progress. At 2 AM last night I was hoping nobody would spot the error and call me on the depth of thread for 1/2-20. It is of course half of the quoted figure, so only 12 passes are necessary. I am pleased to report that with the adjustment of the change gears I was able to cut two perfect 1/2-20 threads today! A nice Christmas present. I re-did the cutter: It is amazing how many different instructions there are to grind one, some from reputable sources which seem plainly wrong (no side clearances). I added those and the cutting improved. One of the joys of the 9x20 is the clutch at the lowest speed which effectively limits the available torque. Changing the cutter geometry stopped the clutch dis-engaging during the last couple of passes which was a problem before. There are those who disabled their clutch by epoxy. I have not addressed that because I tend to use higher speeds most of the time but it may be something I shall have to look at. I use RapidTap, seems to work fine. I do have a threading indicator but a cursory inspection would suggest that it is not working. In any case I felt I wanted to take that variable out of the equation for the moment. If I do more of this I shall re-visit it. All in all I would like to thank everyone for their helpful advice. I feel almost like a proper machinist now. Michael Koblic, Campbell River, BC |
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#33
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On 2011-12-21, wrote:
On Tue, 20 Dec 2011 08:43:30 -0500, "Jim Wilkins" wrote: [ ... ] A thread indicator as shown here speeds up the job considerably. http://www.micro-machine-shop.com/si..._threading.htm [ ... ] At 2 AM last night I was hoping nobody would spot the error and call me on the depth of thread for 1/2-20. It is of course half of the quoted figure, so only 12 passes are necessary. You know better than to hope that nobody spots a mistake. :-) I am pleased to report that with the adjustment of the change gears I was able to cut two perfect 1/2-20 threads today! A nice Christmas present. Congratulations! I re-did the cutter: It is amazing how many different instructions there are to grind one, some from reputable sources which seem plainly wrong (no side clearances). I added those and the cutting improved. Yes -- the clearance makes a big difference. For production work (lots of the same thread) it makes sense to calculate the helix angle and adjust the side clearance angles to give sufficient clearance for that thread in particular, which leaves the maximum meat in the tool and increases its life. The helix angle is calculated by taking the maximum diameter of the thread, calculating the circumference, and the pitch (inverse of the TPI for inch threads), which will give you the sine of the angle, and by looking up or using a scientific calculator to take the arc-sine, you find the angle. As an example, with your 1/2-20 thread, the diameter is 1/2" (0.500"), so the circumference is Pi times that, or 1.5708", and the pitch (from 20 TPI) is 0.050". Divide the pitch by the circumference, and you get 0.0318, and the arc-sine of that is 1.824 degrees. So -- you use that as a starting point, and add 5 degrees to it on the cutting side, and subtract it that from 5 degrees on the other side, giving relief angles of 6.824 degrees on the left and 3.176 degrees on the right (for a right-hand thread). Note, you can ignore this for most thread cutting, except for long production runs where you want to maximize tool life. (The carbide threading inserts have available anvils which adjust the side clearance, usually more necessary with multi-start threads or very coarse pitches.) One of the joys of the 9x20 is the clutch at the lowest speed which effectively limits the available torque. Changing the cutter geometry stopped the clutch dis-engaging during the last couple of passes which was a problem before. There are those who disabled their clutch by epoxy. I have not addressed that because I tend to use higher speeds most of the time but it may be something I shall have to look at. It seems that there is benefit from that -- giving you a clue that things are not as they should be. I use RapidTap, seems to work fine. Sounds good. I do have a threading indicator but a cursory inspection would suggest that it is not working. In any case I felt I wanted to take that variable out of the equation for the moment. If I do more of this I shall re-visit it. What do you see which indicates that it is not working? What you should see, if it is working properly a 1) When you crank the carriage from side to side without the leadscrew turning, the dial should rotate. 2) When the carriage is stationary, but the leadscrew is turning the dial should rotate. 3) When the half-nuts are engaged (for threading, or likely on some lathes, for longitudinal feed as well), the dial is stationary. But -- to minimize wear on the pick-off gear and the leadscrew, is is common to ship lathes with the pick-off gear disengaged. To engage or disengate it, loosen the single screw attaching the dial to the side of the apron, and to engage it. rotate it so the bottom moved into contact with the leadscrew. To disengage it, rotate it so the bottom moves out of contact with the leadscrew. While there are some lathes made with the dial permanently installed in the apron and no way to disengage it, I believe that your lathe likely has it as I have described it above, and certainly the image called "thread indicator" in the web site you pointed to is of that type. FWIW The thread indicator on my 12x24" Clausing had never been *mounted* to the apron, and was in one of the pedestal drawers. This is because the lathe was fitted with a bed turret, and threading was being done with Geometric die heads, not single point (really quick way to thread in production, but tedious to set up properly for a single thread. The choice of what number to close the half nuts on when using the dial varies with the particular thread you are cutting. There should be a table telling you what is what for your machine. But you can get away with always closing them at "1" no matter what, though it can slow you down quite a bit with fine threads, as with many of them you can close on any marking, and with fine threads, the dial turns very slowly. :-) And for most lathes, your 20 TPI would allow you to close it on any marking. For mine, the manual says: ================================================== ==================== When cutting even-numbered threads, engage the half-nut lever at any one of the markings on the threading dial for each cut of the thread. When cutting odd-numbered threads, engage the half nut lever for the first cut and all successive cuts at either the 1 or 2 positions of the dial. When cutting half-numbered threads, engage the half-nut lever at the same mark on the threading dial for each cut of the thread. ================================================== ==================== Note that the illustrated one has only the odd numbers showing with the even ones between, while mine with the same eight index lines, numbers only every other one, so 1 & 2 on mine would be 1 & 3 on this one, so the instructions would be different. So check your manual -- and also compare to your scratch test (make that one on '1', and see whether the outhers track that or not. For 20 TPI, they should on yours. So -- with the threading dial, you engage on your selected number, wait until you get to the runout groove and disengage, crank back the cross-slide, crank the carriage clear of the end of the workpiece, crank the cross-slide back in, advance the depth of cut on the compound, and wait for the right number to come up before engaging the half nuts again. Note that the half-nuts won't engage at the mid-points between marks, so you can start to engage about then and it will fall in at the proper point. (And, if you start far enough clear of the end of the workpiece, you can make sure that it is on the proper number, and disengage if necessary. And -- you don't have to stop the spindle and reverse it to do this. All in all I would like to thank everyone for their helpful advice. I feel almost like a proper machinist now. Being able to single-point cut threads does make you feel like that, doesn't it? Congratulations, DoN. -- Remove oil spill source from e-mail Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#34
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
In article ,
"Wild_Bill" wrote: I believe there would be reasons for using fine threads from an engineering standpoint.. but if I did know, it's been forgotten. In pondering my own experiences with a lot of mechanical assemblies, I'm certain that fine threads are most often chosen for fasteners associated with moving parts in power transmission components. In automotive applications, items bolted to the engine block are often coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts, etc.. have generally always had fine threads IME. In many applications the thread choice is related to the metal/material types which have threaded holes. My understanding is that coarse threads were intended for steel screws in cast iron assemblies, and the fine threads came about for steel into steel. Joe Gwinn |
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#35
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Bench grinder spindle thread
On Wed, 21 Dec 2011 13:12:41 -0500, Joseph Gwinn
wrote: In article , "Wild_Bill" wrote: I believe there would be reasons for using fine threads from an engineering standpoint.. but if I did know, it's been forgotten. In pondering my own experiences with a lot of mechanical assemblies, I'm certain that fine threads are most often chosen for fasteners associated with moving parts in power transmission components. In automotive applications, items bolted to the engine block are often coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts, etc.. have generally always had fine threads IME. In many applications the thread choice is related to the metal/material types which have threaded holes. My understanding is that coarse threads were intended for steel screws in cast iron assemblies, and the fine threads came about for steel into steel. Joe Gwinn Yes, and the reason for the move to finer threads in steel is twofold: Fine threaded *screws and bolts* are stronger in both tension and shear (counterintuitive, perhaps, but check it out) and they are better at self-locking: they're less likely to loosen from vibration. In favor of coarse threads, aside from some installation issues, such as less likelihood of cross-threading, they are less likely to strip out of a soft or fragile material. And it is easier to make a strong threaded hole in brittle material (cast iron) with a coarser pitch. As for total strength, then, it's a matter of the relationship between bolt strength and thread strength in the hole. In hard, strong materials, fine is better. In softer, weak, or brittle materials, coarse is better. In the latter cases, bolt strength is much greater than the hole strength either way. -- Ed Huntress |
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#36
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Bench grinder spindle thread
On Wed, 21 Dec 2011 13:28:50 -0500, Ed Huntress
wrote: On Wed, 21 Dec 2011 13:12:41 -0500, Joseph Gwinn wrote: In article , "Wild_Bill" wrote: I believe there would be reasons for using fine threads from an engineering standpoint.. but if I did know, it's been forgotten. In pondering my own experiences with a lot of mechanical assemblies, I'm certain that fine threads are most often chosen for fasteners associated with moving parts in power transmission components. In automotive applications, items bolted to the engine block are often coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts, etc.. have generally always had fine threads IME. In many applications the thread choice is related to the metal/material types which have threaded holes. My understanding is that coarse threads were intended for steel screws in cast iron assemblies, and the fine threads came about for steel into steel. Joe Gwinn Yes, and the reason for the move to finer threads in steel is twofold: Fine threaded *screws and bolts* are stronger in both tension and shear (counterintuitive, perhaps, but check it out) and they are better at self-locking: they're less likely to loosen from vibration. In favor of coarse threads, aside from some installation issues, such as less likelihood of cross-threading, they are less likely to strip out of a soft or fragile material. And it is easier to make a strong threaded hole in brittle material (cast iron) with a coarser pitch. As for total strength, then, it's a matter of the relationship between bolt strength and thread strength in the hole. In hard, strong materials, fine is better. In softer, weak, or brittle materials, coarse is better. In the latter cases, bolt strength is much greater than the hole strength either way. I can see how a coarse thread may be less likely to strip in a very brittle material, or perhaps in a coarse grained material where the size of the grains is large relative to the threads. But a properly cut (or formed) fine thread should be somewhat more resistant to stripping than a coarse thread in a reasonably ductile material, regardless of the strength of the material. I'm using "stripping" here to mean shearing of the threads as a result of tension on the screw. -- Ned Simmons |
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#37
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On Wed, 21 Dec 2011 19:59:35 -0500, Ned Simmons
wrote: On Wed, 21 Dec 2011 13:28:50 -0500, Ed Huntress wrote: On Wed, 21 Dec 2011 13:12:41 -0500, Joseph Gwinn wrote: In article , "Wild_Bill" wrote: I believe there would be reasons for using fine threads from an engineering standpoint.. but if I did know, it's been forgotten. In pondering my own experiences with a lot of mechanical assemblies, I'm certain that fine threads are most often chosen for fasteners associated with moving parts in power transmission components. In automotive applications, items bolted to the engine block are often coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts, etc.. have generally always had fine threads IME. In many applications the thread choice is related to the metal/material types which have threaded holes. My understanding is that coarse threads were intended for steel screws in cast iron assemblies, and the fine threads came about for steel into steel. Joe Gwinn Yes, and the reason for the move to finer threads in steel is twofold: Fine threaded *screws and bolts* are stronger in both tension and shear (counterintuitive, perhaps, but check it out) and they are better at self-locking: they're less likely to loosen from vibration. In favor of coarse threads, aside from some installation issues, such as less likelihood of cross-threading, they are less likely to strip out of a soft or fragile material. And it is easier to make a strong threaded hole in brittle material (cast iron) with a coarser pitch. As for total strength, then, it's a matter of the relationship between bolt strength and thread strength in the hole. In hard, strong materials, fine is better. In softer, weak, or brittle materials, coarse is better. In the latter cases, bolt strength is much greater than the hole strength either way. I can see how a coarse thread may be less likely to strip in a very brittle material, or perhaps in a coarse grained material where the size of the grains is large relative to the threads. But a properly cut (or formed) fine thread should be somewhat more resistant to stripping than a coarse thread in a reasonably ductile material, regardless of the strength of the material. I'm using "stripping" here to mean shearing of the threads as a result of tension on the screw. Apparently this is generally true. The older theories that I learned are based on the fact that the stress on the first couple of threads results in failure of those threads before there is significant stress on subsequent threads, and shear strength was dependent on some product of thread contact area and the pyramidal strength of the thread form -- coarser threads producing stronger pyramids. I haven't looked at any of this for 30 years, except in passing. From what I've seen, the current thinking is that shear strength is almost exclusively a product of shear area (divided by a factor for helix angle) and that this says finer threads are always stronger in shear, except, perhaps, in brittle materials. Somewhere there must be some good engineering test results to clear this up, but I haven't looked for them. -- Ed Huntress |
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#38
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
"Ed Huntress" wrote in message news  ... Apparently this is generally true. The older theories that I learned are based on the fact that the stress on the first couple of threads results in failure of those threads before there is significant stress on subsequent threads, and shear strength was dependent on some product of thread contact area and the pyramidal strength of the thread form -- coarser threads producing stronger pyramids. I haven't looked at any of this for 30 years, except in passing. From what I've seen, the current thinking is that shear strength is almost exclusively a product of shear area (divided by a factor for helix angle) and that this says finer threads are always stronger in shear, except, perhaps, in brittle materials. Somewhere there must be some good engineering test results to clear this up, but I haven't looked for them. -- Ed Huntress The explanation I remember from college is that the first few threads are overloaded and fail sequentially because the bolt elongates more than the female threaded element, if it is a solid block. If the female threaded element has the same cross-sectional area and elongation as the male thread, as in a turnbuckle, the bolt is more likely to snap at a thread root as long as at least [3 to 5??] threads engage. When a turnbuckle made personally by the great yacht designer L. F. Herreshoff was tested to destruction every part of it deformed, showing he had balanced the strengths all over without wasting material. http://en.wikipedia.org/wiki/Hyatt_R...lkway_collapse jsw |
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#39
Posted to rec.crafts.metalworking
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Bench grinder spindle thread
On 21 Dec 2011 03:53:33 GMT, "DoN. Nichols"
wrote: [...] One of the joys of the 9x20 is the clutch at the lowest speed which effectively limits the available torque. Changing the cutter geometry stopped the clutch dis-engaging during the last couple of passes which was a problem before. There are those who disabled their clutch by epoxy. I have not addressed that because I tend to use higher speeds most of the time but it may be something I shall have to look at. It seems that there is benefit from that -- giving you a clue that things are not as they should be. Not really. It disengages far too early rendering the low speed useless. I use RapidTap, seems to work fine. Sounds good. I do have a threading indicator but a cursory inspection would suggest that it is not working. In any case I felt I wanted to take that variable out of the equation for the moment. If I do more of this I shall re-visit it. What do you see which indicates that it is not working What you should see, if it is working properly a 1) When you crank the carriage from side to side without the leadscrew turning, the dial should rotate. 2) When the carriage is stationary, but the leadscrew is turning the dial should rotate. 3) When the half-nuts are engaged (for threading, or likely on some lathes, for longitudinal feed as well), the dial is stationary. I do not think it turns at all. Pending detailed assessment. [...] So -- with the threading dial, you engage on your selected number, wait until you get to the runout groove That feature was not an option on the current project. and disengage, crank back the cross-slide, crank the carriage clear of the end of the workpiece, crank the cross-slide back in, advance the depth of cut on the compound, and wait for the right number to come up before engaging the half nuts again. Note that the half-nuts won't engage at the mid-points between marks, so you can start to engage about then and it will fall in at the proper point. (And, if you start far enough clear of the end of the workpiece, you can make sure that it is on the proper number, and disengage if necessary. That I was not aware of. That will make things easier when I do this in future. I wondered how to hit the mark at 150 rpm. And -- you don't have to stop the spindle and reverse it to do this. All in all I would like to thank everyone for their helpful advice. I feel almost like a proper machinist now. Being able to single-point cut threads does make you feel like that, doesn't it? How did one live without a lathe? Michael Koblic, Campbell River, BC |
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Bench grinder spindle thread
wrote in message ... On 21 Dec 2011 03:53:33 GMT, "DoN. Nichols" wrote: .. Being able to single-point cut threads does make you feel like that, doesn't it? How did one live without a lathe? Michael Koblic, Campbell River, BC A quick look at the instructions etc on LittleMachineShop didn't show it, but on my old lathe the threading indicator pivots in and out of engagement with the leadscrew. Quick because then I noticed a puddle around my water heater............... Before modern lathes (~1800) we lived like the Romans. jsw |
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