Smithy MIDAS MI-1220 LTD, Operator'S Manual, Page: 67 / 133

Chapter 16
Cutting Threads on Your MI-1220 LTD
Threading Terms
Before beginning to cut threads, it's useful to learn the major terms used in thread
cutting:
•
Pitch.
Metric pitch is the distance from the center of a thread to the center of the next
thread. To measure pitch in inches, measure an inch on a bolt and count the threads.
•
Pitch Diameter.
This is the diameter of an imaginary cylinder superimposed on a
straight screw thread, the surface of which would make an equal width of the thread and
the spaces cut by the cylinder.
•
Lead.
The lead is the distance a screw thread advances axially (as through a nut) with
one complete revolution. The lead and pitch of a single thread are identical, but they
differ on multiple threads (the lead of a double thread is twice its pitch; of a triple thread,
three times its pitch).
Because screw-thread cutting is so generally a part of machine work, anyone interested
in building things of metal should master it. Threading requires patience and skill. Before
attempting to cut a thread on a workpiece, cut a few practice threads on odd bits of steel,
iron, and aluminum.
Built for thread cutting, the MI-1220 LTD cuts standard internal and external threads, as
well as special threads. You may cut coarse or fine threads in a great range of threads
per inch, in V or square shapes, in established profiles like Unified National, acme, and
metric. You can cut single threads or multiple threads that run concurrently along the
shaft. You determine the type of thread by how you'll use the screw. Each thread form
requires a different-shaped tool to cut or chase it.
For most work, beginners use the Unified National Standard, which is a V-form thread
slightly flat on top and at the root. Pitch numbers, such as 18 or 24, usually refers to
screw threads meaning 18 or 24 threads per inch (TPI).
Thread charts on the inside of the door of your machine and shows both inch and
metric measurements. The inch chart shows the TPI from 6 to 120. The metric shows the
distance from thread crest to crest from 0.50 to 4 mm.
For right-hand threads, start the threading or chasing tool at the right end of the
workpiece and feed it toward the headstock. For left-hand threads, reverse the
leadscrew's rotation direction and feed the threading tool from left to right.
With practice, you can grind cutters to almost any profile. It is difficult, however, to
sharpen such cutters without altering the cutting form, and almost every resharpening
requires a complete regrinding of profile and clearance angles.
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