Smithy Midas 1220 XL - Manual (Page 74)

Chapter 15
Cutting Screw Threads
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 thread cutting is so much a part of a machine work, anyone interested in
building things of metal should master it. Threading requires a bit of patience and skill.
Before trying to cut a thread on a workpiece, cut a few practice threads on odds bits of
steel, iron and aluminium.
Built for thread cutting, The Midas 1220 XL 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.
Beginners will use the Unified National Standards most of the time, which is V-form thread
slightly flat on top and at the root. Screw threads are usually referred to by pitch
numbers, such as 18 or 24, meaning 18 or 24 threads per inch (tpi). The Midas 1220 XL
cuts standard threads in pitches from 6 to 30 tpi and metric threads from 0.50 to 3mm.
Because the lathe spindle turning the work connects by gearing to the leadscrew (which
moves the cutting tool along the lathe bed), a ratio exists between spindle speed in
revolutions pre minute and cutting tool movement in inches. When you change the
gearing, you change this ratio. For this reason you can cut threads of various pitches by
changing both the change gears at the head of the lathe and the speed selection lever.
Thread charts inside the gearbox door show both inch and metric measures (Figure 7.2).
The inch chart on the headstock shows the tpi from 6 to 30. The metric show the
distance from thread crest 0.12 to 3 mm.
15-1
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