Hi Ken

for a 1/4 " shaft the major diameter is supposed to be a 1/4 ". at the
same time
the thread form says that each of the thread crests should be flat and
the width
of the flat should be equal to 1/8 th of the thread pitch. So that
leaves 7/8 ths of
the thread pitch to form the top of the equilateral triangle that
defines the shape
of the thread.

So in a 1/4-20 threaded male the pitch is .050" and 7/8 ths of that is
..04375"
So all the sides of the equilateral triangle are .04375"
So the distance from first touch (with a sharp tool) using a 29.5 degree
compound is close enough to .04375" to be just that.

If for some reason you need the double depth then it is cos 30 or .866
times the
length of the sides (.04375") which gives the height of the equilateral
triangle and twice that is the double depth. (.866*.04375*2= .075775")

Nothing to look up!

Bill


"Ken Sterling" wrote in message
s.com...
All,
I also have a question concerning single-point threading.
I recently started practicing making a few *small* parts for
a miniature hit n miss engine I'm going to build (never had to
make stuff this small before). One of the pieces is a
downsized "grease cup" as is seen on some bigger
machinery for greasing the bearings. Simply a threaded
fixture to screw into the bearing cap, and a reservoir with a
grease cap to be tightened to force grease down through
a small hole into the bearing.
As I very seldom had to thread mating parts without having the "other
half" to try for fit, I'm in the process of "bettering" my accuracy so
when I make the female thread, the male thread will fit as it should.
I have a nice threading book (using 29 degrees angle) from Sears/Atlas
showing a lot of info, but I also wanted to try something.
Example:
1/4-20 threaded male piece.
Charts show the major diameter of .2500 with a minor diameter of .1850
which corresponds to the National Form Tool Double Depth of Thread
of .0650. (.2500 minus .0650 equals .1850). The manual I have shows
depth of compound feed for the thread to be .037. So all is well
and good so far.
The chart also shows calculations for VEE form tool being Double Depth
of Thread of .0758. Therefore, .2500 minus .0758 leaves minor
diameter of .1742. The compound feed is shown to be .043 for the VEE
thread. Again, okay so far.
Now... I have Guy Lautard's bedside reader showing a formula for
diameter reduction and also length reduction.
Using the VEE form example above, I need to reduct the diameter
of the shaft by .0650 in the threading process. Dividing this number
by 2 for my lathe, I need a "crossslide" infeed of .0325 for the depth
of thread.
Using Lautard's system for small crossslide infeeds by using the
compound, his formula is:
Amount of crossslide infeed needed divided by sin of compound angle
setting. Therefore .0325 divided by sin(29 degrees) should give me
the correct amount of compound infeed to equal the amount of
crossslide infeed I'm attempting to get. Formula yields .0670.
Wrong answer..... what am I overlooking? The charts in the
manual work well and the fit is good, but I was just trying to
use a method which prevents looking up everything.
Thanks.
Ken.