# understanding thread diameters



## kd0afk (Apr 13, 2013)

any kind of information you get on thread diameters gives you terms like flank and crest and such. But when I use my machinist calc it uses terms like minimum minor, maximum minor and minimum major. What do these terms mean?


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## Swifty (Apr 13, 2013)

All threads have tolerances, making them an easy fit or a precision fit. What you are seeing in the minimum and maximum sizes are the tolerance allowances. Naturally if a shaft were made to maximum size and the corresponding nut were made to the minimum size, it would be a tighter fit. The opposite applies if the shaft were made to the minimum sizes and the nut is made to the maximum size, it would be a much looser fit.

On my 5C collet block set, the ring nuts supplied are really sloppy on the collets, probably to make them easier to get on. In an application where a lot of torque pressure is required when tightening, then a more precise tolerance would be required.

Paul.


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## cncjunior (Apr 13, 2013)

kd0afk, I am not sure by your question if you want those specific terms defined or just a general description of most thread terms.

For your three terms above the are the minimum, maximum that the root diameter can be.  For the major diameter, this is simply the shaft diam. before cutting the thread with a max/min.


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## ruzzie (Apr 14, 2013)

A picture is worth a thousand words


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## Tin Falcon (Apr 14, 2013)

I agree the question was not real clear. and the diagram posted should clear things up. 
 I recommend anyone in this hobby get a copy of the machinery handbook. you can get old out of copyright editions on pdf or older hard copy for $ 20 or so. 

what is really important is to know how to sharpen the tool so the root is the correct shape. 
major diameter is the size stock you need for a male thread ie a bolt. and the minor diameter is the size hole you need to drill for a female thread. 

now keep in mind these may be theoretical numbers as normal thread engagement is only 65%- 75%  so the actual major diameter of a threaded shaft will be about  0.010 in small and tap drills are slightly larger than the minor diameter. 

Hope this helps.
Tin


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## dman (Apr 14, 2013)

kd0afk said:


> any kind of information you get on thread diameters gives you terms like flank and crest and such. But when I use my machinist calc it uses terms like minimum minor, maximum minor and minimum major. What do these terms mean?



minor is the smaller diameter, on an outside thread it is the root, minimum is the smallest that diameter is supposed to be in the blueprint or spec for the specified class of fit in the machinist handbook. so minimum minor is the smallest the root of an outside thread (or crest of an internal thread) is allowed be. i'm sure you can extrapolate the rest from that.


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