Specs for a 4 3/8-8 thread

[cncwhiz]

We have a part that we need to make a 4 3/8" male and female thread. The handbook does not go that big?

[flick]

It has specs for a 1 3/8-8 thread right? Just add 3 to everything.

[asuratman]

Let me know what handbook you are using for that thread. I need also to cut some thread like nu10rd, eue8rd, buttress, VAM, etc. Thanks.

[mc-motorsports]

Originally Posted by cncwhiz We have a part that we need to make a 4 3/8" male and female thread. The handbook does not go that big?

subtract the pitch to get the minor diameter. 1 divided by 8 = .125" 4.375" - .125" = 4.250" minor diameter.

I'm not 100% sure that that works for threads that large. But it works for up to 1"... 3/8"-16 for example. 1/16=.0625. .375"-.0625"=.3125" minor diameter. For metric, just subract the pitch. M8 X 1.25 = 8-1.25=6.75mm

MC

[beege]

Michael Rainey has a program called "Threadpal". I recommend it, especially for all odd-sized threads you may encounter. I get a pitch diameter of 4.2817 to 4.2910 when I input your givens into Threadpal.

http://closetolerancesoftware.com/index.html

[Geof]

Originally Posted by mc-motorsports subtract the pitch to get the minor diameter. 1 divided by 8 = .125" 4.375" - .125" = 4.250" minor diameter. I'm not 100% sure that that works for threads that large. But it works for up to 1"... 3/8"-16 for example. 1/16=.0625. .375"-.0625"=.3125" minor diameter. For metric, just subract the pitch. M8 X 1.25 = 8-1.25=6.75mm MC

That is not exactly correct is it?

For instance your example using 3/8-16 gives 0.3125" which is the drill size to use for 75% full form thread so it is actually large than the minor diameter.

It is a good approximation for tapping but for cutting a thread I think it will leave a wide flat crest.

[g-codeguy]

Originally Posted by cncwhiz We have a part that we need to make a 4 3/8" male and female thread. The handbook does not go that big?

As already stated, find a thread size with the correct pitch you need. Add/subtract the difference. Size of thread doesn't matter. Thread height will always be the same for a given pitch/lead regardless of the size. A 1/4-20, 1/2-20, 1-20, 4-3/8-20, etc. all have the same thread height. This works for major/minor/pitch diameters.

[cncwhiz]

Beege,
That program is great. My boss is thinking about buying it. Anybody have any more sites like that? Good info.

[flick]

Originally Posted by Geof That is not exactly correct is it? For instance your example using 3/8-16 gives 0.3125" which is the drill size to use for 75% full form thread so it is actually large than the minor diameter. It is a good approximation for tapping but for cutting a thread I think it will leave a wide flat crest.

I'm going on memory here, so correct me if I'm wrong: the basic UN spec calls for a 1/6 flat at the crest and 1/24 flat at the root of a female thread. 1-(1/6)-(1/24) = 0.791 = around 79%. TDS is not 75% of a full-form, or normal thread, it's 75% (actually 73.08%) of a full, sharp V, which is really a negligible difference for most practical purposes. 75% is used in tapping rather than 79% (or 73.08%) because:

A) It's easy for sleepy machinists to remember.
B) The thinnest part of the thread contributes the least to it's strength.
C) The thinnest part of the thread contributes the most to cutting forces.
D) It's easy for sleepy machinists to remember.

Usually when I single-point a UN thread I just bore to tap drill size, for all of the reasons stated above.

I would not do this if a drawing specified a 1B thread, rather than the usual 2B, or if I was making a gage.

[Geof]

Originally Posted by flick I'm going on memory here, so correct me if I'm wrong:....

I think you are correct. It is just if you are making a large coarse thread the flat will look quite wide at 75%. I would do this for an in-house job but for a customer I would be more cautious; if the customer questions it you do not in fact have it exactly to spec.

Another reason for using 75% is because practically all the tap drill charts use this; no memory needed.

[flick]

Originally Posted by Geof if the customer questions it you do not in fact have it exactly to spec.

Too true. I have not been caught on this, but YMMV.

[mc-motorsports]

Originally Posted by Geof That is not exactly correct is it? For instance your example using 3/8-16 gives 0.3125" which is the drill size to use for 75% full form thread so it is actually large than the minor diameter. It is a good approximation for tapping but for cutting a thread I think it will leave a wide flat crest.

My drill chart shows a 1-1/2"-6 thead having a 1-11/32" tap drill ending up with 72% thread.

A 75% thread requires 20% less power to cut, and is only 5% weaker than a full thread.

That's only for "open tolerance threads". If you have a tolerance specified, better look it up and gauge it.