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Copyright © 2021 Quality Machine Tools, LLC
PM-1340GT v8 2021-04
Figure 4-11
Carriage gib assembly
LEADSCREW
The leadscrew shear pin, Figure 4-12, is designed to mini-
mize damage if the carriage encounters an obstruction when
traversing under leadscrew power. [If that should occur when
the carriage is powered instead by the feed shaft, the drive is
automatically disengaged by the clutch, bottom of Figure 4-12,
also Figure 3-12.] If the shear pin breaks, use a 1/8" drift to
clear remaining fragments, and replace the pin with a 5/32"
brass rod. When hammering on the drift
avoid damage
to the
gearbox output shaft by placing a solid support block under the
leadscrew collar.
Figure 4-12
Leadscrew shear pin
SPLIT NUT ADJUSTMENT
If the split nut becomes excessiv
ely loose, with appreciable
side to side movement, this may be corrected by adjusting
the gib at the back of the apron, item 27 on the Apron Body
schematic, Section 5. Loosen the screws holding the gib to the
apron casting, then tighten the gib by screws 40. Re-tighten
the securing screws and lock nuts.
CROSS-SLIDE & COMPOUND BACKLASH
When alternating between clockwise and counter clockwise
rotation, the cross-slide handwheel may move freely a few de-
grees but the cross-slide table stays put. There may also be
similar lost motion in the compound. The acceptable amount
depends on the user, but 0.005” is generally a good compro-
mise. Smaller numbers are possible, but overdoing it can lead
to premature wear of leadscrew and nut.
Lost motion is due to two factors: 1. End-float (in/out move
-
ment of the handwheel) caused by insufficiently tight coupling
of the leadscrew and thrust bearings. 2. Wear in the leadscrew
nut.
Factor #1 is correctable in both the cross-slide and compound.
Leadscrew handwheels on the PM-1340GT — cross-slide,
compound and tailstock — are attached in a similar way, Fig
-
ure 4-13. The handwheel is locked to the leadscrew shaft by
a key (not shown). It is held in place by a flange screw in a
threaded well at the outer end of the leadscrew shaft. An in-
ternal locking set screw, bottomed in the well, prevents loos
-
ening of the flange screw. To correct backlash due to loose
coupling between leadscrew and thrust bearings, back out the
set screw a turn or two, then tighten the flange screw using
a pin vise or needle-nose pliers. Do not tighten to the point
where the thrust bearings are over-compressed, resulting in
uneven motion. Back off for smooth rotation, with no appre
-
ciable end-float. Re-tighten the set screw. This tends to back
out the handwheel a small amount, so check end-float again.
Factor #2 is correctable in the cross-slide by compressing
the leadscrew nut, Figure 4-14. Remove the compound from
the cross-slide, then remove the socket head screw secur
-
ing the cross-slide to the leadscrew nut. Turn the cross-slide
handwheel clockwise to drive the nut backward until it can be
worked on at the back. Unless previously installed insert an
M6 x 1 socket head screw, approximately 15 mm long, then
tighten the screw as necessary. Don't overdo this — a 45 de-
gree turn of the screw represents a backlash take-up of about
0.005".
The compound leadscrew nut is not adjustable.
Figure 4-13A
Handwheel attachment schematic
