"Eric R Snow" wrote in message
news
On Wed, 2 Jun 2004 18:53:31 -0400, "Bill Darby"
wrote:

Been wondering if I could get a fairly accurate depth of thread
measurment with only a single wire and a mic.
*****************************

So I took a new 1/2" 13 bolt and miced it. The measurnent of the major
diameter was .489"

I miced it again with a .040" drill bit inserted in the thread. The
measurnent was .496"

In order to balance that reading I subtracted half the major diameter of
the bolt (.496" - 489"/2) = .2515" and doubled the result .2515"*2 =
.503"

So I now had the equivalent to a 3 wire reading. [IE .503"] So the
sharp depth diameter reading would be 3 times the .040" drill diameter
less or .503"- 3 * .040" = .383"

The pitch diameter would be one pitch height greater or .383" +
1/13*.86603" = .4496"

This .4496" pitch diameter falls nicely into the pitch diameter range
of a class 3A thread.
******************************
Bill

May seem complicated to do but you need no special kit, only two hands,
and once you figure out the mechanics of it, it is easy and I would
guess that the accuracy, while not as good as the proper three wires, it
could serve most quite well.

Bill

(Hoping I didn't foul up my numbers)

Of course, your one wire method only works if you first measure the OD
of the part. And if doing several parts you must measure each part on
the OD first. Unless your setup is good enough that the part to part
variance is small enough that you can live with it. I hate thread
wires. They are hard to hold on large diameter parts. And on coarse
threads the wires may be so far apart that you are forced to use only
one wire because the mike spindle is too small to bridge two wires..
And they don't check thread angle or minor diameter. And they are easy
to lose in the chip pan. But I use 'em a lot anyway.
ERS

Yep! They're a real pain on coarse threads, and I assume on large
diameters. Almost my entire experience has revolved around threads from
5/8" and smaller, right down to 80 pitch threads. Wires are the only way
to go as far as I'm concerned.

You hit the problem of a single wire dead on the head. Unless you
carefully measure the diameter of any object from which you would take a
measurement, then calculate the formula for each variation of the major
diameter, it's not worth a damn. Jumping through all those hoops is far
more complicated than using three wires, and certainly it wouldn't be as
accurate. Further, threading tools have a way of leaving a minor burr at
the crest of the thread, so each time you'd measure your thread you'd have
to be careful to deburr the major diameter, but not alter it.

Threads are measured by the three wire method because it is the most
accurate and convenient way to determine pitch diameter. It probably goes
without saying that it is assumes the operator has enough intelligence to
assure that he is using a threading tool with the proper configuration, and
it's set up properly.

Threading blindly is no different from turning blindly. I can't imagine
anyone turning a diameter without measuring somewhere along the way. Why
would threading be any different, especially when threads tend to be tighter
in tolerance than the typical turn? All the wires do is permit the use
of your micrometer so you're not running blindly.

Harold