how to calculate beginning point when programming pipe threads on a lathe

# Thread: how to calculate beginning point when programming pipe threads on a lathe

1. ## how to calculate beginning point when programming pipe threads on a lathe

I tried this question in the general forum and got no where. Can any of you guys tell me how to mathematically calculate the bginning diameter when programming a pipe thread. My current method is to go over to the tap drawer and pull out whatever size pipe thread I am cutting and measure the tap. Then I program those numbers in and tweak the "x" offset until I get it to gage depth. Not very scientific I admit - but it works. But there must be a formula to calculate this. I have read the Machinery Handbook on this and it is very good about giving the pitch diameter and how to arrive at minimum and maximum thread truncation for NPT's but try as I might I have never been able to knead and massage that formula sufficiently to come up with a number that is any where near what the tap actually measures. So I end up going back and measuring the tap and cutting the threads as such. Your help would be appreciated. Please don't give me the names of books - I am looking for the working formula. If you have worked it and are inclined to pass it on then I would be most appreciative. If you find a formula and have never worked it and don't know how to work it then please refrain from responding - I have already received plenty of those.

Joe

2. see how this works for an internal thread.

L1 and K0 come from any good NPT dimension chart

thread height = (pitch value * .8)

minor diameter at the face = (L1 * .0625) + K0

major diameter at the face = minor + (2 * thread height)

Software For Metalworking
http://mrainey.freeservers.com

3. Tag for when I need to use this!

Thanks M.

mrainey,

Wow! Very nice program. I don't have my paypal info with me right now but when I get back to my desk I will definitely purchase this info. Very nice. What a time saver! By the way, I went back to the Machinery Handbook and figured out the calculations and I now have a working formula for NPT's whether it's a full thread or some part of a full thread (I don't know about you guys but it seems like I am always being asked to make shortened pipe threads for unique applications). Nevertheless, I love the amount of information that your program puts at one's fingertips. Thanks again.

Joe

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Originally Posted by mrainey
see how this works for an internal thread.

thread height = (pitch value * .8)

minor diameter at the face = (L1 * .0625) + K0

major diameter at the face = minor + (2 * thread height)
This is great information.

I have another question about lengths. To make standard internal NPT thread (eg. 3/8 NPT), what is the formula to calculate how long the thread should be?

I never know how far to machine the ID and how far to machine the thread.
I usually machine a length equal to L1+L3. Is that remotely correct?

Thanks.

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