Instructions

However, the entire chuck, along with the part, could be
removed from the lathe to try it for size. Don’t force
anything when trying the part for size, because you might
move the part slightly in the chuck, and really “screw things
up.”
Why have I made such a point about having the major or
minor diameter wrong and still making the part work? Read
on. You’re probably thinking I must really be a “hacker” if
I can’t cut a diameter within .010". Well, the problem in
many cases, is not how close you can cut to a diameter, but
what the diameter should be.
Example: Your buddy just heard you bought a nice, shiny
new lathe complete with a screw cutting attachment, and like
all good friends he immediately goes to work trying to figure
out how you and your new lathe will be of some use to him.
It doesn’t take him long! He has a camera that he tried to
repair himself last year, but lost an important part. Of course
the missing part has metric threads, but that’s a “snap” for
a Sherline lathe. A quick check with a thread gauge
indicates that it has 0.9 mm Pitch. No problem, yet. It is an
internal thread, so you will have to cut a screw to mate with
it. Here’s the problem: What is the major diameter? You can
measure the diameter of the hole, but you can’t be assured
that the thread form is perfect and that this is really the minor
diameter. You can only assume that it’s close. Now take this
dimension and add to it twice the depth of the thread, which
should give you the major diameter. To get the depth of one
thread, multiply the Pitch x .6. (Note: Pitch x 1.2 + Minor
Diameter = Major Diameter). Total depth of thread using a
sharp pointed 60° tool = P x .65 = .036" x .65 = .023".
The constant .6 is not used to figure depth of an external
thread, it is just one used to get you in the “ball park” in a
situation such as this.
At least we have a fairly reliable place to start now and can
probably get one cut that will work on the first try. Always
keep track of the total depth of cut in case it comes out
undersized. At least you’ll know how deep not to cut it on the
second one!
The example I gave you was one of the more difficult
situations you may run into, not only because you had to do
the job for free to keep a friend, but also because you had
very limited information from which to work.
Usually, you will be cutting both the screw and nut. This is
a case where two wrongs can almost equal one right. You
can rectify any error you may have made in cutting the first
one by compensating for it in the mating part.
Left-hand threads can be cut as easily as right-hand threads
on a lathe; the only difference being the addition of an idler
gear that reverses tool movement so that it travels left to
right instead of right to left.
It’s hard to appreciate just how much money an inexpensive
lathe like this with a screw cutting attachment can save you
until you have had to have a special part made that doesn’t
have a standard thread size. Even though there may be taps
or dies available, a single left-hand 1-32 set would probably
cost half as much as your entire thread cutting attachment.
What I have tried to do in these opening remarks is to show
that screw cutting is really easy, and to give you the self-
P/N 3100, Pg. 2 of 8
its leadscrew. The 127-tooth conversion gear does this by
driving the leadscrew at a ratio that converts 20 TPI to 1
mm. Consider 100T on the spindle driving a 127T. The ratio
is .7874 to 1. The leadscrew has 20 TPI: .050" P x .7874 =
.03937" = 1 mm.
Figure 1—Component parts of a thread cut with a
sharp pointed 60° Vee tool.
MAJOR DIAMETER— Largest diameter of the thread of
either the screw or the nut.
MINOR DIAMETER— Smallest diameter of the thread of
either the screw or the nut.
PITCH DIAMETER— The theoretical diameter that falls
on a point where the thread
width and the groove width are
the same.
PITCH (P)— The distance from point to point
measured parallel to the axis.
Metric threads are always
expressed in Pitch
LEAD— The distance a screw thread
advances axially in one turn. On
a double lead screw, the lead is
twice the pitch.
NOTE: The same methods can be used in figuring dimensions
for American (inch) or Metric screw threads.
1 mm = .03937"
Pitch (Metric) x .03937" x .758 = depth of screw
thread in inches
Take the time to familiarize yourself with component parts
of the screw thread from Figure 1. The pitch diameter is the
important one to consider. Before going on, let’s take the
time to really understand why. The pitch diameter determines
how a screw or thread will fit, not the major diameter.
Suppose you were cutting 20 TPI and the major diameter
was .010" undersize and the pitch diameter was correct.
About the only thing wrong would be that the flat on the point
of the thread would be a little wide, but it would still have
approximately 75% of its strength and work well.
Now let us suppose we cut the pitch diameter undersize by
.010". We would end up with a nut that fits so loose and a
thread that was so weak that we would have to scrap it. This
is where “cutting to fit” comes in. You can compensate for
some pretty bad errors on the major and minor diameters by
having the pitch diameter correct. To get it correct, all you
have to do is to keep trying it for size as you cut. Don’t ever
take the part out of the chuck to try it because it would be
next to impossible to re-chuck it in exactly the same place.