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Pipe thread depth
I need a bit of advice. I just had a job completed by a machine shop
that called for a 1/2" NTP and 1/4" NPT hole. The part was titled "manifold". The parts just arrived with the 1/2" NPT hole tapped to the bottom of a 1 1/8" deep port (will never seal)and a correctly tapped 1/4" NPT hole. The machine shop says it was my fault because I didn't specify the thread length and I will now have to pay for a "rush run" if I need to meet my deadline. Isn't NPT supposed to be tapped within a certain range? Shouldn't they have asked if there was any doubt? I think that they should have asked. Am I wrong? Thanks, RR |
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#2
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#3
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The problem is not that there was not enough depth for the tap. The
hole is threaded so deep that I run out of male threads on a nipple before the nipple seals to the manifold. If I run the nipple into the manifold to just before the end of the male thread, I can actually "rattle" the nipple in the tapped female NPT hole. All I really needed here (quality wise) was "good enough to work". Would I be out of line by asking for a re-run? RR |
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#6
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Eric R Snow wrote:
On 7 Feb 2005 06:50:09 -0800, wrote: I need a bit of advice. I just had a job completed by a machine shop that called for a 1/2" NTP and 1/4" NPT hole. The part was titled "manifold". The parts just arrived with the 1/2" NPT hole tapped to the bottom of a 1 1/8" deep port (will never seal)and a correctly tapped 1/4" NPT hole. The machine shop says it was my fault because I didn't specify the thread length and I will now have to pay for a "rush run" if I need to meet my deadline. Isn't NPT supposed to be tapped within a certain range? Shouldn't they have asked if there was any doubt? I think that they should have asked. Am I wrong? Thanks, RR Greetings RR, You did not need to call out thread depth. When calling out a straight thread you just need to specify the nominal O.D., the pitch, class, and thread depth. For example: 1/4-20 3A .500 deep. You don't need to specify the limits of the O.D. or the pitch diameter. The nominal size and class do that. Same for the pipe tap, except for tapered pipe. Since the thread is tapered the depth is controlled by P.D. (pitch diameter). So, if the P.D. is correct, the depth muist be correct. Pipe taps are ground this way. Only if the thread is going to deviate from standard is the depth called out. The machine shop is lying to you. They scrapped the parts and want you to pay for their mistake. I would be leery of sending them anymore work. I bet that if a tap manufacturer were consulted they would be ablke to send you written proof that the thread depth of a tapered pipe is controlled by the tap cutting to the correct P.D. If it was me, I'd tell the shop that they screwed up and present them with thread data from a tap maker, or from Machinery's Handbook to prove it. Then give them a chance to do the parts over at their cost. If they refuse, and if the dollar amount is low enough, take 'em to small claims court. If the dollar amount is too high then call a lawer and have a letter sent to them from the lawer. Oftentimes this is all it takes to get some relief. You may have be able to save your parts though. Heli-coils may work. Since I don't know the dimensions of your part, or the material, I can't say. But contacting the makers of Heli-coils should get you the info you need to decide whether your parts can be saved. This URL: http://www.newmantools.com/recinch.htm#nptlink takes you to the RECOIL web site for pipe thread repair. If the preceeding URL doesn't work then this one takes you top their homepage http://www.newmantools.com/recoil1.htm .The Helicoil site makes you register so I tried Recoil and they don't require registration. Cheers, Eric R Snow, E T Precision Machine It may be cheaper to just walk away from them -- lies don't come in sets of one. If they're going to play that sort of game once, they'll do it again and again. Unless they're the only shop in town you should probably tell them that you're happily paying the expedite fees -- then tell them which one of their competitors you're paying the fees to. Suing, or threatening to, may be indicated if they've screwed up a bunch of parts -- but if it were me I'd my money and/or rework out of them and then never darken their door. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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#7
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wrote in message oups.com... The problem is not that there was not enough depth for the tap. The hole is threaded so deep that I run out of male threads on a nipple before the nipple seals to the manifold. If I run the nipple into the manifold to just before the end of the male thread, I can actually "rattle" the nipple in the tapped female NPT hole. All I really needed here (quality wise) was "good enough to work". Would I be out of line by asking for a re-run? RR Their mistake for sure... How many parts are you talking about? I'd probably just bore the 1/2" NPT port out and press in a 1/2" pipe coupling (or a machined insert) |
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#8
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Tim Wescott wrote:
Eric R Snow wrote: snipped Suing, or threatening to, may be indicated if they've screwed up a bunch of parts -- but if it were me I'd my money and/or rework out of them and then never darken their door. Good thoughts there. I'm gonna celebrate my 69th birthday this Wednesday, and I swear it took until a year or so ago for me to "have an epiphany" over stuff like that and realize that you can't win 'em all. Anything over 95% "wins" is enough to keep me happy now. Neither can you make a profit on every job you do, it's the end of year total that counts. I wish I'd come to that realization 45 years ago. I've stopped agonizing over those sloppy schmucks and wondering why they don't have the class to treat people the way my folks tought me to treat others. Maybe they weren't fortunate enough to have the kind of parents I did. There is an old Jewish saying that goes like... "Treat others as you wish to be treated. This is the whole of the law. The rest is commentary." Just my .02, Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "As long as there are final exams, there will be prayer in public schools" |
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#9
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A few months ago I needed to make a similar manifold and investigated
pipe taps for the project. I discovered that "NPT" can refer to either tapered or parallel treads. A 1/2" tapered will screw into a 1/2" parallell, but not seal. The opposite is not true. The parallel thread requires a sealing mechanism such as an "O" ring. The same threading problem occurs with metric pipe threads, excep that each country with metric standards has it's own standard. That is why there is a 1/8" pipe tap in my set of metric tape/dies. Many Asian countries use the American pipe standard. I wonder if the shop doing your work happened to use the parallel tap instead of a tapered tap? Paul |
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#10
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According to the 26th edition of Machinery's Handbook, pages 1849-1850, and my Starrett tap drill chart, if you called out that you wanted the thread a 1/2-14 NPT on your print the shop should have done as follows:
Drill hole 23/32 in Diameter Chamfer hole Tap 1/2-14 NPT to a depth of .5343 or 17/32 (.53125) In the diagram on page 1849 that depth is the "Handtight Engagement Length (L2=.320)" plus the "Wrench Makeup Length For Internal Thread (L3=.2143)" Any reputable shop should know that if it is specified as an NPT thread it is because a seal is required at that joint. If you would like a copy of those pages from my Handbook if you don't have one, let me know. Also if you happen to be in the Chicago area and need any more of this work done again let me know. As far as boring the hole out and pressing in an insert, I am assuming from what you said that this is for an intake manifold on an engine. If that is the case then you probably won't want to do that. Aside from the fact that there probably isn't enough wall stock to bore the hole larger, if you did that you would have to put an insert in that has the exact same expansion rates as the rest of the manifold, ie: Same material. Probably not the best idea. Go back to the shop and see what they say when you have proof, also review your print and make sure everything is called out right. Hope this helps. |
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#11
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Thanks to all for the replies! Due to the thoughts expressed here, I
was able to successfully argue my case. The job is being re-run at no charge. Obviously my limited machining knowledge will not allow me to pay you folks back by contributing to this group. Take a look at the following URL. If anything there interests you let me know via email (off group). I'll see what I can do to work you a deal. Only those that responded prior to this post please! www.probotics.com Send to rrold1 "at" aol.com Thanks again! |
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#12
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In article .com,
says... A few months ago I needed to make a similar manifold and investigated pipe taps for the project. I discovered that "NPT" can refer to either tapered or parallel treads. A 1/2" tapered will screw into a 1/2" parallell, but not seal. The opposite is not true. The parallel thread requires a sealing mechanism such as an "O" ring. No, NPT is a complete, non-ambiguous specification. There are a few straight pipe threads, and variations on tapered pipe threads, but only one NPT. Here's a chart that looks pretty inclusive... http://www.parker.com/ead/cm2.asp?cmid=3562 Ned Simmons |
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#13
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#14
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"Eric R Snow" wrote in message ... snip---- Since the thread is tapered the depth is controlled by P.D. (pitch diameter). So, if the P.D. is correct, the depth muist be correct. Pipe taps are ground this way. .. I bet that if a tap manufacturer were consulted they would be ablke to send you written proof that the thread depth of a tapered pipe is controlled by the tap cutting to the correct P.D. Yep------and the rule of thumb is to tap until you have 5 threads (on the tap) above the finished surface, unless you happen to have a plug gage to check the proper depth. The rule may not be perfect, but it will always yield a functional thread. Harold |
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#15
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On Mon, 7 Feb 2005 11:53:23 -0800, "Harold and Susan Vordos"
wrote: "Eric R Snow" wrote in message .. . snip---- Since the thread is tapered the depth is controlled by P.D. (pitch diameter). So, if the P.D. is correct, the depth muist be correct. Pipe taps are ground this way. . I bet that if a tap manufacturer were consulted they would be ablke to send you written proof that the thread depth of a tapered pipe is controlled by the tap cutting to the correct P.D. Yep------and the rule of thumb is to tap until you have 5 threads (on the tap) above the finished surface, unless you happen to have a plug gage to check the proper depth. The rule may not be perfect, but it will always yield a functional thread. Harold Harold, I learned 7 threads. And when I checked this against my gauges it worked. Most of the time. ERS |
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#16
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"Eric R Snow" wrote in message ... snip---- Harold, I learned 7 threads. And when I checked this against my gauges it worked. Most of the time. ERS Chuckle! Most of the time? That's the beauty of the shop. I can't help but wonder if we're at the mercy of poor work turned out by others in many cases. I've had to tap until I had 4 or fewer threads on more than one occasion, and have kicked myself for tapping too deep at five, so what you say certainly makes sense. If nothing else, it's playing it safe. You can always go deeper. I have never had the luxury of gages (rarely ever tapped pipe threads for product, only my own use) so I tend to tap to about 5½ threads and call it good. Harold Harold |
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#17
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Ned Simmons wrote:
In article .com, says... A few months ago I needed to make a similar manifold and investigated pipe taps for the project. I discovered that "NPT" can refer to either tapered or parallel treads. A 1/2" tapered will screw into a 1/2" parallell, but not seal. The opposite is not true. The parallel thread requires a sealing mechanism such as an "O" ring. No, NPT is a complete, non-ambiguous specification. There are a few straight pipe threads, and variations on tapered pipe threads, but only one NPT. Here's a chart that looks pretty inclusive... http://www.parker.com/ead/cm2.asp?cmid=3562 Ned Simmons Very good Ned. I was about to post a note on another thread that confused what the " T " in NPT stands for. It's not thread but taper. Some people just don't know. ...lew... |
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#18
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In commercial production, NPT thread depth is measured with a gage that has
a ground notch. Unless otherwise specified on the blueprint, and with the gage screwed in as far as it will readily go, the thread depth is correct if the notch is within plus or minus one thread from the surface of the part. There is no need to specify depth for a standard NPT thread. The correct depth is plus or minus one thread from that notch, "coolcamaro79" wrote in message ... According to the 26th edition of Machinery's Handbook, pages 1849-1850, and my Starrett tap drill chart, if you called out that you wanted the thread a 1/2-14 NPT on your print the shop should have done as follows: Drill hole 23/32 in Diameter Chamfer hole Tap 1/2-14 NPT to a depth of .5343 or 17/32 (.53125) In the diagram on page 1849 that depth is the "Handtight Engagement Length (L2=.320)" plus the "Wrench Makeup Length For Internal Thread (L3=.2143)" Any reputable shop should know that if it is specified as an NPT thread it is because a seal is required at that joint. If you would like a copy of those pages from my Handbook if you don't have one, let me know. Also if you happen to be in the Chicago area and need any more of this work done again let me know. As far as boring the hole out and pressing in an insert, I am assuming from what you said that this is for an intake manifold on an engine. If that is the case then you probably won't want to do that. Aside from the fact that there probably isn't enough wall stock to bore the hole larger, if you did that you would have to put an insert in that has the exact same expansion rates as the rest of the manifold, ie: Same material. Probably not the best idea. Go back to the shop and see what they say when you have proof, also review your print and make sure everything is called out right. Hope this helps. -- coolcamaro79 |
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#19
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Sounds like they had either the "new guy" or the apprentice do it, and he ran the NPT tap right through the plate or maifold or what-ever it is, as he would with a standard tap. Maybe they can take it out to 3/4" and use a 3/4 to 1/2 breducing bushing (standfdard stuff anywhere). Take care. Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXXX On 7 Feb 2005 06:50:09 -0800, wrote: I need a bit of advice. I just had a job completed by a machine shop that called for a 1/2" NTP and 1/4" NPT hole. The part was titled "manifold". The parts just arrived with the 1/2" NPT hole tapped to the bottom of a 1 1/8" deep port (will never seal)and a correctly tapped 1/4" NPT hole. The machine shop says it was my fault because I didn't specify the thread length and I will now have to pay for a "rush run" if I need to meet my deadline. Isn't NPT supposed to be tapped within a certain range? Shouldn't they have asked if there was any doubt? I think that they should have asked. Am I wrong? Thanks, RR |
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#20
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The straight threads you refer to are called NPS, they are the same dia. and
pitch as NPT but use an o-ring at the neck to seal. Valve to bottle connections are where I usually see them such as scuba tanks, fire extinguisher CO2 paintball or beverage tanks. wrote in message oups.com... A few months ago I needed to make a similar manifold and investigated pipe taps for the project. I discovered that "NPT" can refer to either tapered or parallel treads. A 1/2" tapered will screw into a 1/2" parallell, but not seal. The opposite is not true. The parallel thread requires a sealing mechanism such as an "O" ring. The same threading problem occurs with metric pipe threads, excep that each country with metric standards has it's own standard. That is why there is a 1/8" pipe tap in my set of metric tape/dies. Many Asian countries use the American pipe standard. I wonder if the shop doing your work happened to use the parallel tap instead of a tapered tap? Paul |
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#21
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