"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

Yes Eric, this method, as far as I can see, will only work by first
measuring the
major diameter. It may work well if, as you suggest, part to part
variance is
small.

In any case, I wasn't so much thinking of a production set up as I was a
one of a
kind deal where you are single point turning and you want to get a
reading on
how close you are to a specific fit.

I personally have never tried the three wire system (don't have the
wires) and
was just doodling around trying to figure a way to do it with the things
I have in
the shop.

PS I found that both my digital calipers and the mic could be used to
get the
same numbers.

Bill