Proper depth of pipe thread

[WJHartson]

How do you calculate the proper depth of NPT when you are tapping? Do you count the number of turns or go for a certain depth? I am tapping 1/8-27 NPT in 304 stainless.

Thanks for the help.

Joe

[Joel]

I just posted on your other thread Joe. Hope it helps some.

[Carl]

Use a male 1/8-27 NPT as a gauge?

[wierdscience]

Joe,the basic rule is 5 complete turns forms the maximum effective seal surface of any NPT thread.Thats what I was taught,I also have a hydraulics formula book and chart that gives thread engagement for various pressures.

Most of the time I have found on standard length NPT taps(not dryseal)four threads from the top is about right.

Hope that helps.

[WJHartson]

Thanks for the responses so far. I have been cutting 10 to 10-1/2 turns and then I have to relieve the top thread. Like you do Joel. This seems to be the right depth but I have broken several taps when I tap by hand getting to that depth. I have tried several different taps from different manufacturers, all high quality taps not foreign made. I am going to build a jig a try using a Tapmatic and see what it does. Have some taps that are long enough to fit the Tapmatic standard taps are to short. Have been using Tapmagic as a tapping fluid and have some Castrol MoliDee, think that is the name,that I will try.

I may be getting the stainless too hot and causing it to harden. Am going to uses a different coolent and see what happens.

Any help or thoughts would be appreciated.

[JCHannum]

Pipe threads are tapered, and for larger sizes, it is a good idea to use a tapered reamer to size the hole before tapping. I don't know if one is available for 1/8".

Stainless may need a size or two larger tap drill to prevent breakage.

[This message has been edited by JCHannum (edited 07-24-2005).]

[WJHartson]

Darin, when you say 4 threads from the top are you talking about the tap?

I am tapping 1/8 NPT and using an R size drill. I have also used and 11/32" drill but it seems to be to big.

Joe

[wierdscience]

<font face="Verdana, Arial" size="2">Originally posted by WJHartson:
Darin, when you say 4 threads from the top are you talking about the tap?

I am tapping 1/8 NPT and using an R size drill. I have also used and 11/32" drill but it seems to be to big.

Joe</font>

Yes,the threads on the tap.304 is very tough on taps,I use either sulpher threading oil (Texaco Sultex-D is what we use at work)or the wax sticks like "Edgelube".Might even try common candle wax.

Like JC said a reamer will help things,also an interupted thread tap may make things easier.Don't know if they make them in NPT,but the Greenfield EM taps work miracles in SS.

I typically use the 11/32 drill for 1/8NPT,it starts out loose,but since the thread is tapered it cuts to full profile anyway.

[WJHartson]

Thanks to all, Darin the last tap I broke was a Greenfield EM. Haven't tried the sulfur threading oil.

I will try the 11/32 drill and see what happens. Who carries the tapered reamers?

Joe

[JCHannum]

Joe, I just looked in Travers, J&L and Production Tool catalogs. They all carry them down to 1/8". Look under pipe reamers.

[WJHartson]

JC, thanks, I will take a look in the catalogs.

Joe

[kap pullen]

Joe,

Interrupted thread taps reduce the thread cutting load.

Any good tool supplier carries these taps.

On these taps, every other tooth is ground away so you have half the teeth cutting, with a heavier chip per tooth.

These taps reduce the chance of work hardening because of the heavier chip load gets under the hardened surface from the previous tooth.

Kap