Instructions

An introduction to thread cutting
in the real world
After designing and putting the enclosed screw cutting
attachment into production, I sat down and started reading
what other people had written about cutting screw threads
before writing my own instructions. It amazed me that I had
been able to cut threads all these years knowing so little.
How and why I was able to do this is going to be the subject
of my instructions. There are plenty of books available at
any library to go into additional detail on the subject if
required, but my instructions are based on using sharp
pointed 60° tools and cutting threads for your own use.
The reason other books go into such great detail on the
precise methods used commercially is that they are telling
you how to cut threads from specifications for other people.
They must have exact methods and standards to make sure
that a bolt made in California will screw into a nut
manufactured in New York. Fortunately, we have the
tremendous advantage of having both pieces at hand and we
can just “keep cutting ‘till they fit.” It’s as simple as that!
You simply select the proper gears from the chart; put in a
60° threading tool and have at it.
A point to ponder about thread cutting is how a lathe
produces a thread. It doesn’t matter whether it is a 20" or a
3" lathe. The principle is the same. The leadscrew that
drives the saddle is geared directly to the spindle. When the
spindle turns, the saddle moves. If they were geared one to
one, the pitch cut would be the same as the pitch of the lead
screw. On Sherline's 3.5" lathe, this would be 20 Threads
Per Inch (TPI). If we turned the lead screw 180° while we
turned the spindle 360° (by using a 20 tooth to a 40 tooth gear
arrangement) we would cut 40 TPI. Please note that we did
not have to consider the diameter of the stock. The only
requirement is that the major diameter is at least twice the
depth of the thread plus enough material to support these
threads while cutting them. One gets used to hearing a
diameter called out with the threads, such as 1/4-20, 6-32,
10-24, etcetera, but while it’s unusual to think of 40 threads
per inch cut on something 2" in diameter, in some cases it
may be entirely practicable to do so.
It may interest you to know how a metric thread can be cut
on a 3" lathe that has American National screw threads on
Purpose of the thread cutting attachment
One of the big advantages of having a lathe is that it gives
you the ability to cut threads. In reality you do not normally
cut threads on a manual lathe if they could be done with low
cost taps and dies. The reason you cut threads is to produce
non-standard threads for which you don’t have taps and
dies. The second important reason is to produce a thread
that is true or concentric with the centerline of the part. If
we put threads on the Sherline spindles and chucks with taps
and dies it would be impossible to produce 10% of our tools
with acceptable runout. You will also find that when you
start designing and building special parts you’ll often need to
use non-standard threads to work within the constraints of
your design. For example, if you needed a 0.900" diameter,
24 TPI (threads per inch) thread, a tap and die could cost
more than your lathe and take weeks to order.
Sherline offers a unique and versatile thread cutting
attachment for its lathe. This low cost attachment enables
you to machine 46 different unified thread pitches (Pitch
range from 80 to 5 threads per inch) and 28 different metric
thread pitches (Pitch range from .25 to 2.0mm). What's
more, it allows you to cut them as either left hand or right
hand threads. Ten optional gears with odd tooth counts are
available for even more pitches. The lathe tool can, of
course, be ground to cut any of the various thread forms, and
any of these various pitches or thread forms can be cut on
any pitch diameter you desire. This versatility enables you
to machine any standard or non-standard thread you might
desire as long as it is within the size limitations of the lathe.
The attachment consists of 15 hobbed metal gears, mounting
bracket, engagement mechanism and 4.00" (102 mm)
handwheel. Threads are generated on the Sherline lathe by
gearing the spindle to the lead screw. As the spindle is
rotated with the handwheel mounted on the headstock
spindle, the tool will advance an amount equal to the ratio of
the gears. When the tool has completed its movement, you
simply stop cranking, back up the tool and turn the spindle
in the opposite direction until the tool is past its starting point,
reset the tool and you're ready for the next pass. It's a simple
as tapping! By using a 127-tooth gear (supplied), true metric
threads can be cut on an inch lathe (Model 4000, 4400 or
4500) or true inch threads can be cut on a metric lathe
(Models 4100, 4410 or 4530).
3.5" Lathe
Thread Cutting Attachment
P/N 3100
SHERLINE
PRODUCTS
INCORPORATED 1974
5/29/09
SHERLINE PRODUCTS INC. ˆ 3235 Executive Ridge ˆ Vista ˆ California 92081-8527 ˆ FAX: [phone]
Toll Free Order Line: [phone]: ˆ International/Local/Tech. Assistance: [phone] ˆ Internet: www.sherline.com