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