12
PM-1440E-LB 11-13-17 V1
Copyright © October 2017 Quality Machine Tools, LLC
Figure 3-7
Feed shaft clutch
SADDLE STOP
The stop assembly, Figure 3-8, has a micrometer-style collar
graduated in 0.001 in. divisions. It can be clamped at any point
along the lathe bed (two M6 socket head screws on the under
-
side secure the clamp plate to the block). A set screw at the
front locks the barrel.
THREAD CUTTING
KEY FACTS TO REMEMBER ...
TPI threads
See Figures 3-5, 3-6. For inch thread cutting, the 127T larger
gear is an
idler
, transferring the drive from the upper change
gear to the lower change gear.
Metric threads
The upper change gear engages the 127T gear. The smaller
of the transposing gears, 120T, drives the lower change gear,
Figure 3-9.
For metric threads the split-nut on the apron must be left
engaged throughout the entire process.
COMPOUND SETUP FOR THREAD CUTTING
Thread cutting on the lathe is unlike most other turning oper-
ations, for two reasons: 1. The cutting tool must be precisely
ground with an included angle of 60 degrees for most Ameri
-
can and metric threads, and; 2. It is mostly preferable to feed
the tool into the workpiece at an angle so it cuts mostly on the
left flank of the thread, Figure 3-10. The correct angle relative
to the cross-slide (zero degrees) is debatable — should it be
29, 29-1/2 or 30 degrees? Many machinists prefer 29 degrees
because it holds the cutting tool marginally clear of the right
flank of the thread, close enough for cleanup of the flank while
at the same time avoiding appreciable rubbing.
The clutch comprises a pair of spring loaded steel balls bear-
ing on a detent disc driven by the saddle feed gearbox. Spring
pressure is adjusted by two set screws on either side of the
feed shaft, arrowed in Figure 3-7. Setting the spring pressure
is a process of aiming for the best compromise between too
high — damaging feed pressure — and too low, stopping pre-
maturely.
Setting the clutch to work reliably with the micrometer saddle
stop is a good example of such a compromise: start with low
spring force, then work up in small increments until the saddle
stops in the same location (say ± 0.002”, assuming a constant
depth of cut and feed rate).
Figure 3-8
Saddle stop
Figure 3-9
Metric thread cutting setup
The upper gear meshes with the 127T gear,
but the lower gear is flipped — hub inward
— so that it engages the 120T gear.
