View Full Version : M Thread Question re Major Diameter

Bruce Griffing
03-07-2009, 02:05 PM
I am probably missing this in Machinery's Handbook, but I am confused about the major diameter for a metric M thread. I am making an ER-11 Collet chuck on the end of a spindle. The ER11 spec calls for and M14x.75 thread. What I have not been able to figure out is this - what major diameter (diameter of starting material in this case) is implied by this spec. For a 7/8 thread depth, I will need to cut 22.4 thousands deep - but I need the starting diameter to proceed.

Peter N
03-07-2009, 02:13 PM
Bruce, the clue is in the description. An M14 thread is 14mm (major) diameter.
However, I don't understand the 7/8 bit you're referring to on this?


Bruce Griffing
03-07-2009, 02:23 PM
I was not sure if the 14 referred to the pitch diameter or the major diameter. The 7/8 is pretty standard - you cut to 7/8 the depth of a full depth thread. This leaves the crests flat and less sharp.

Peter N
03-07-2009, 02:27 PM
Ah! I see. I thought for some reason you were confusing 7/8" with 14mm :D


Bruce Griffing
03-07-2009, 02:38 PM
I just measured a cheap import ER11 chuck. The major diameter on the collet nut threads is 13.88mm. That said - it is a pretty poor looking thread. But I stull wonder if a 14mm major diameter is really the correct answer to this question.

Peter N
03-07-2009, 02:52 PM
But I stull wonder if a 14mm major diameter is really the correct answer to this question.

It is.
The nominal diameter for all metric threads is designated by the number immediately after the 'M'. If you're talking about class of fit that's another matter.


03-07-2009, 03:02 PM
this page has what you need for the threads.


Dr. Rob
03-08-2009, 11:02 AM
You're on the right track, Bruce.

The link above is good info.

Y'see, the M14 designation is true in that the nominal diameter is as others have said, 14 mm. However, an M thread form has 1/8 of the triangle chopped off top and bottom, which means that a proper M14 thread will in fact have a diameter of 13.9-ish mm.

Which entails that if you use a sharpened (pointed, full form 60) vee tool, the thread won't be quite right. Or, it will if you do the full thread, but chop off the 1/8 of the triangle afterwards. That oughta do it.

Which doesn't answer your question entirely, but at least tells you that your question has merit.

Regretful edit later, after a potful of morning coffee:

Nononono, i confused myself. The diameter is indeed 14 mm, but the 1/8 has already been chopped off. So you only need to take into account the 7/8 , and the piece at the root of the thread.