# Threading and minor diameter



## JimM (May 22, 2009)

I'm thinking about having a go at making a collet holder for my lathe that will attach directly to the spindle.

The spindle has a 1.5" 8TPI Unified thread and I'm trying to work out what size I need to bore the hole. From the Machinery's Handbook I worked out that it will be either

Minor Dia External Threads - 1.347"
Minor Dia Internal Threads - 1.365"

But I can't get my head around which of these is correct for this situation - I'm assuming the External Thread diameter but want to be sure.

To be honest given my skills it's probably not going to make a lot of difference but I'd like to be at least aiming at the right size 

Cheers

Jim


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## mklotz (May 22, 2009)

You want the minor diameter of the internal thread. The formula for that is:

Dmin = Dmaj - 5*sqrt(3)/8 * pitch

which, for,

Dmaj = 1.5 in
pitch = 1/8 in

works out to 1.364" on my calculator - essentially perfect agreement with the number you got from MH.


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## JimM (May 22, 2009)

Thanks Marv

Out of interest in what circumstances would you need to refer to the minor diameter of the external thread then ?

Cheers

Jim


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## arnoldb (May 22, 2009)

> Out of interest in what circumstances would you need to refer to the minor diameter of the external thread then ?



At the risk of sounding like a newbie (which I am ;D), you would need that if you were turning a male part (for example, a dividing head nose on which your intended collet holder will also fit).

As far as I can "think", the difference in the minor diameters for internal and external treads might be to cater for differences in tool sharpness for cutting the threads, as well as leaving a little clearance for contaminants on the threads not to cause seizure. 

There is more information on http://en.wikipedia.org/wiki/Unified_Thread_Standard

Regards, Arnold


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## kvom (May 23, 2009)

The minor diameter vs. major tells you how deep the threads are.  The minor diameter of the internal thread needs to be larger than the external to allow the two threads to screw together.


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## mklotz (May 23, 2009)

Whenever you need to calculate the bore size for some large single-pointed internal thread, you can look at the problem as calculating the tap drill size needed if you had the corresponding tap.

Most tables won't list recommended tap drill sizes for something as large as a 1.5-8 thread but my DRILL program will calculate the required size for *any* combination of diameter and pitch.

I tried running it for a 1.5-8 Unified with 100% depth-of-thread and it returned a tap drill size of 1.3647" which corresponds exactly with the formula and the value reported from MH.

This confirms two things...

The program agrees with MH

The recommended size corresponds to 100% thread engagement.

Given the latter fact, you may want to think about loosening things up a bit. For example, for a 75% DOT, the program suggests a tap drill size of 1.3985".

Just something to keep in mind.


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