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PM-1228VF v3 2021-05
Copyright © 2021 Quality Machine Tools, LLC
TAILSTOCK
The tailstock leadscrew has a 10 TPI thread, with 4 inch travel.
A graduated collar on the tailstock handwheel reads 0.001”
per division. To remove tooling from the tailstock taper (MT3)
turn the handwheel counter-clockwise (handle end view) until
resistance is felt, then turn the handle a little more to eject the
tool. Conversely, to install a taper tool make certain that the
quill is out far enough to allow firm seating.
For taper turning the tailstock may be offset by adjusting the
M8 socket head screws on either side of the base, Figure
3-17. To move the tailstock to the back, for instance, loosen
the tailstock clamp lever, back out the offset screw on the
back-facing side, then tighten the opposing screw on the front
side.
Offsetting the tailstock for a specific taper is not a straight
-
forward job; it is a lengthy, iterative process. The same
goes for re-zeroing for normal operations.
In practice, the only way to precisely determine a specific off
-
set is to "cut and try' on the workpiece, or scrap stock, homing
in on the correct degree of offset in small increments.
The same process is used when re-establishing "true zero" of
the tailstock, in other words returning it to the normal axis for
routine operations. A visual indication of zero offset is provid
-
ed by a scale on the back surface, but this is not reliable for
precise work.
One way to avoid cut-and-try is to prepare in advance a bar
of (say) 1" diameter quality ground stock, with
precise center
drillings
at both ends (do this by indicating for zero TIR in a
4-jaw chuck, not in a 3-jaw unless known to be predictably
accurate). The prepared bar can then be installed between
centers and indicated along its length.
Figure 3-17
Tailstock
The gearbox controls provide 15 choices of leadscrew/feed
shaft speed relative to spindle speed, Figure 3-19. Additional-
ly, the overall ratio is determined by the choice of “upper gear”
(30T, 35T, 49T, 50T and 60T). The resulting 75 ratios give a
wide selection of feed rates and thread pitches.
SETTING UP THE POWER FEED
The PM-1228VF gearbox is driven by a train of external gears
taking power from the spindle gear. It has two power-feed out-
puts, Figure 3-18, one driving the leadscrew for thread cutting
(only), the other driving a separate hex section feed shaft for
routine turning and facing
operations.
Figure 3-18
Leadscrew & feed shaft
Figure 3-19
Gearbox controls
TIP
For a better indica-
tion of control knob
position, mark the
knobs with a fiber
tip pen.
Provided the spindle speed is below 600 rpm there is
no need to stop the motor before changing gearbox
settings.
The hex-section feed shaft drives the saddle through an ad-
justable dog clutch, Figure 3-23. The clutch is typically used
to stop the saddle by decoupling the drive at a predetermined
point set by the saddle stop, Figure 3-24. This same decou-
pling action also protects the system if accidentally overload-
ed, e.g., by a tool collision, when turning or facing under power.
The clutch comprises a pair of spring loaded steel balls bear-
ing on a detent disc driven by the gearbox. Spring pressure
is adjusted by a threaded bobbin on the outboard end of the
assembly, Figure 3-23. Setting the spring pressure is a pro-
cess of aiming for the best compromise between too high —
damaging feed pressure — and too low, stopping prematurely.
Setting the clutch to work reliably with the saddle stop is a
good example of such a compromise: start with low spring
force, then work up in small increments until the carriage stops
in the same location (say ± 0.005″, assuming a constant depth
of cut and feed rate).
This is not a precision feature
because
the stopping point can vary with unknowns such as tool sharp-
ness and consistency of workpiece material.
Offset
screw
Quill clamp
lever
Tailstock
clamp lever
Oiler
Oiler
Zero offset
datum
Continued on page 15
