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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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metric equivalent of imperial thread, 36.3 tpi instead of 36
Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The part has to withstand a pressure of 260 psi while a gasram is pressurised. It looks like it is very close, will it be ok? thanks |
#2
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metric equivalent of imperial thread, 36.3 tpi instead of 36
No.
Steve "misterroy" wrote in message ... Hi, the lathe I have access to cuts only metric threads, I think. I want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The part has to withstand a pressure of 260 psi while a gasram is pressurised. It looks like it is very close, will it be ok? thanks |
#3
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metric equivalent of imperial thread, 36.3 tpi instead of 36
misterroy wrote:
Hi, the lathe I have access to cuts only metric threads, I think. I want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The part has to withstand a pressure of 260 psi while a gasram is pressurised. It looks like it is very close, will it be ok? thanks It depends, as usual. How many threads will be engaged? "psi" doesn't describe force on a fastening. Do you mean "lbs"? If not, what is the setup? What is the diameter of the threaded part? Bob |
#4
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metric equivalent of imperial thread, 36.3 tpi instead of 36
On Aug 28, 1:36*pm, Bob Engelhardt wrote:
misterroy wrote: Hi, the lathe I have access to cuts only metric threads, I think. I want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The part has to withstand a pressure of 260 psi while a gasram is pressurised. It looks like it is very close, will it be ok? thanks It depends, as usual. How many threads will be engaged? *"psi" doesn't describe force on a fastening. *Do you mean "lbs"? *If not, what is the setup? *What is the diameter of the threaded part? Bob around 13 threads will be engaged. The part is an adapter from a bicycle shock pump to a gasram in an air rifle. The inlet to the ram is 36 tpi and the air pressure needs to get to around 260 pounds per square inch whilst the ram is being repressurised, then the adapter will be removed. The diameter is 5.2 mm. The thread is here http://www.gewinde-normen.de/en/valve-thread.html its vg 5 thanks |
#5
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metric equivalent of imperial thread, 36.3 tpi instead of 36
On Sat, 28 Aug 2010 06:10:05 -0700, misterroy wrote:
On Aug 28, 1:36Â*pm, Bob Engelhardt wrote: misterroy wrote: Hi, the lathe I have access to cuts only metric threads, I think. I want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The part has to withstand a pressure of 260 psi while a gasram is pressurised. It looks like it is very close, will it be ok? It depends, as usual. How many threads will be engaged? Â*"psi" doesn't describe force on a fastening. Â*Do you mean "lbs"? Â*If not, what is the setup? Â*What is the diameter of the threaded part? around 13 threads will be engaged. The part is an adapter from a bicycle shock pump to a gasram in an air rifle. The inlet to the ram is 36 tpi and the air pressure needs to get to around 260 pounds per square inch whilst the ram is being repressurised, then the adapter will be removed. The diameter is 5.2 mm. The thread is here http://www.gewinde-normen.de/en/valve-thread.html its vg 5 It seems unlikely that a part with .7mm thread will engage even half a dozen turns with a 36 tpi threaded part. Still, that might be enough engagement. A rule of thumb that I've heard (but conceivably might have misunderstood) is that about 3 turns bear the load anyway, even if more are engaged. Are you planning for the thread contact to act as a pressure seal (in which case I would be surprised if you could get even half the pressure you want) or do you have an O-ring or a gasket to take the pressure? Do you plan to single-point cut an external thread? What is the make and model of the lathe? For lathes with change gears rather than a quick change gear box, you may be able to get some unlisted tpi settings by proper gear selection. If you have easy access to the lathe, just cut the .7mm thread on a piece of scrap bar stock and see if it works. -- jiw |
#6
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metric equivalent of imperial thread, 36.3 tpi instead of 36
misterroy wrote:
around 13 threads will be engaged. ... The diameter is 5.2 mm. .... Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13 threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling is that would work. Be a bit stiff, maybe, but doable. And you could could cut your threads small by .003/13 (.0002) for an easy fit. That is the width of the teeth would be .0002 small. The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so that's no problem. These threads don't seem to be tapered, if so leak-stopping has to be done with some sort of seal. Otherwise, even a perfect thread would leak. And your slightly-off pitch is irrelevant. Bob |
#7
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metric equivalent of imperial thread, 36.3 tpi instead of 36
On Aug 29, 1:42*am, Bob Engelhardt wrote:
misterroy wrote: around 13 threads will be engaged. *... *The diameter is 5.2 mm. ... Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. *13 threads at .7mm is 9.1mm, giving .08 mm error (.003"). *My gut feeling is that would work. *Be a bit stiff, maybe, but doable. *And you could could cut your threads small by .003/13 (.0002) for an easy fit. *That is the width of the teeth would be .0002 small. The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so that's no problem. These threads don't seem to be tapered, if so leak-stopping has to be done with some sort of seal. *Otherwise, even a perfect thread would leak. *And your slightly-off pitch is irrelevant. Bob Thanks Bob, I hadn't thought to taper it to get a seal. |
#8
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metric equivalent of imperial thread, 36.3 tpi instead of 36
"misterroy" wrote in message
... On Aug 29, 1:42 am, Bob Engelhardt wrote: misterroy wrote: around 13 threads will be engaged. ... The diameter is 5.2 mm. ... Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13 threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling is that would work. Be a bit stiff, maybe, but doable. And you could could cut your threads small by .003/13 (.0002) for an easy fit. That is the width of the teeth would be .0002 small. The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so that's no problem. These threads don't seem to be tapered, if so leak-stopping has to be done with some sort of seal. Otherwise, even a perfect thread would leak. And your slightly-off pitch is irrelevant. Bob Thanks Bob, I hadn't thought to taper it to get a seal. Sounds like you are making an adaptor for a PCP gun. |
#9
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metric equivalent of imperial thread, 36.3 tpi instead of 36
On Aug 29, 3:34*pm, "Bob La Londe" wrote:
"misterroy" wrote in message ... On Aug 29, 1:42 am, Bob Engelhardt wrote: misterroy wrote: around 13 threads will be engaged. ... The diameter is 5.2 mm. ... Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13 threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling is that would work. Be a bit stiff, maybe, but doable. And you could could cut your threads small by .003/13 (.0002) for an easy fit. That is the width of the teeth would be .0002 small. The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so that's no problem. These threads don't seem to be tapered, if so leak-stopping has to be done with some sort of seal. Otherwise, even a perfect thread would leak. And your slightly-off pitch is irrelevant. Bob Thanks Bob, I hadn't thought to taper it to get a seal. Sounds like you are making an adaptor for a PCP gun. close, its an hw90 http://www.sportsmanguncentre.co.uk/...EIHRAUCH+HW90/ To service the seals you have to depressurise it. The adapter puts the pressure back in. |
#10
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metric equivalent of imperial thread, 36.3 tpi instead of 36
misterroy wrote:
Thanks Bob, I hadn't thought to taper it to get a seal. Tapered fittings usually have the male & female tapered to match. E.g., the US NPT. Tapering just one would mean that only a couple of threads, at the most, would be fulled seated. You might get a good-enough seal, especially since you only need it long enough to get the cylinder to pressure. Or maybe not. Made-in-brass might work better - it would deform more easily than steel, to match the existing threads. Bob |
#11
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metric equivalent of imperial thread, 36.3 tpi instead of 36
"misterroy" wrote in message
... On Aug 29, 3:34 pm, "Bob La Londe" wrote: "misterroy" wrote in message ... On Aug 29, 1:42 am, Bob Engelhardt wrote: misterroy wrote: around 13 threads will be engaged. ... The diameter is 5.2 mm. ... Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13 threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling is that would work. Be a bit stiff, maybe, but doable. And you could could cut your threads small by .003/13 (.0002) for an easy fit. That is the width of the teeth would be .0002 small. The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so that's no problem. These threads don't seem to be tapered, if so leak-stopping has to be done with some sort of seal. Otherwise, even a perfect thread would leak. And your slightly-off pitch is irrelevant. Bob Thanks Bob, I hadn't thought to taper it to get a seal. Sounds like you are making an adaptor for a PCP gun. close, its an hw90 http://www.sportsmanguncentre.co.uk/...EIHRAUCH+HW90/ To service the seals you have to depressurise it. The adapter puts the pressure back in. Gas spring? Like the Browning Nitro Spring? |
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