Hi Ted

I wonder if we both mean the same thing by "major diameter". I mean the
measurement I get if I close a flat faced micrometer over the outside of a
male thread

Yes I meant that they were one in the same IE : "major diameter. I mean the
measurement I get if I close a flat faced micrometer over the outside of a male
thread"

Yes they are one in the same. In regard to Harold's point depending on the
class of thread the actual measurement will not necessarily be the same as the
designated diameter. (as he elaborated upon) IE: A given class of 1/4" thread
might call for a major diameter something less then .25"

I ask because I understand that the only 1/4" thread with a full 0.2500"
o.d. would be one with a sharp V-topped thread.

No this is not a true assertion. The 1/8 th * pitch flat on the crest of the
thread is supposed to be made at the actual major diameter.

Without getting into various classes of threads and their associated major
diameters (see Harold's post) this is why I wanted to stick to a generic 1/4
thread with a 1/4 major diameter because it illustrates very clearly that the
1/8 * pitch flat occurs at the major diameter.

As I see it the thread form designers were quite smart and cost conscious. They
probably wanted to throw away the very weak pip portion of the V-topped thread
because it meant that the diameter of the stock was quit large to accommodate
the sharp V-tops. So they truncated useless top 1/8 th of the thread height and
made the resulting flat occur at the major diameter which is essentially the
same size of the stock used to form the thread.

Also, any thread I have single pointed has always needed to be cut a
little deeper to fit than you indicate, assuming I understand you
correctly. I would like to understand this better.

I also have had this same experience. The answer lies in the various classes of
thread you may be trying to mate your thread with. What I have said (I believe
to be correct) but it is a generic explanation. As you would be aware, all the
various classes of threads call for various dimensions, Not that the various
dimensions change the thread form but they call for various major diameters.

The Major diameter varies with each class of thread but the one thing about the
thread form that is always constant, is the width of the flat. It is always 1/8
th * the pitch. This means that equilateral triangle that forms the thread shape
is always the same, ie 7/8 ths of the pitch length on edge.

All of which simply means that all classes of external threads are cut the same,
except that the start diameter should conform to the stated major diameter of
the class of thread you want to cut.

Sorry I talked so much I am kind of on the fly!

Bill





Ted Edwards wrote:

Bill Darby wrote:

for a 1/4 " shaft the major diameter is supposed to be a 1/4 ".

Bill, you have sadi this many times and now I wonder if we both mean the
same thing by "major diameter". I mean the mesurement I get if I close
a flat faced micrometer over the outside of a male thread having removed
any burrs if present. What do you mean?

I ask because I understand that the only 1/4" thread with a full 0.2500"
o.d. would be one with a sharp V-topped thread. Any comercial 1/4"
thread I have measured has always been rather less than 0.2500 o.d.,
typically by about 1/8 of the nominal thread depth.

Also, any thread I have single pointed has always needed to be cut a
little deeper to fit than you indicate, assuming I understand you
correctly. I would like to understand this better.

Ted