Tormach PCNC 440 - 9.3.4 Angular Contact Bearing Preload Adjustment; 9.3.4.1 Overview

Chapter 9
165
UM10372_PCNC440_Manual_0221A
MAiNTENANCE
The recommended method for gib adjustment is to measure axis lost moon while incrementally
ghtening the gibs to arrive at the correct seng. The following procedure describes this method
for the Z-axis. A similar procure can be used to adjust the X-axis and Y-axis gibs; however, it should
be noted that the X- and Y-axis gib adjustments cannot be considered in isolaon. Tightening or
loosening a gib on either axis also has an eect on the opposing axis.
1. Loosen upper gib screw eight rotaons and ghten the lower gib screw eight rotaons. This
ensures gib clearance is quite loose.
2. Use a dial indicator to measure lost moon in the Z-axis (for more informaon, refer to How to
Measure Lost Moon secon earlier in this chapter). With a very loose gib, the majority of the
measured lost moon is aributable to the backlash in the angular contact bearing pair. On
a new mill, this value measured should be less than 0.002” on the Z-axis and less than .0018”
on both the X- and Y-axis.
3. Tighten the gib by one turn
by loosening the lower
screw rst, then ghtening
the upper screw. Measure
the backlash again.
4. Repeat this procedure unl
the measured backlash
begins to increase. At this
point, the gib seng is
slightly too ght.
5. Back the adjustment o to
the point just before you
saw the increased backlash.
That is the ideal seng for
the axis (see Figure 9.3).
NOTE: Aer any gib adjustment, ensure
that both adjustment screws are ght or
the gib may move out of adjustment.
9.3.4 Angular Contact Bearing Preload Adjustment
9.3.4.1 Overview
Each axis ulizes a double-nut, pre-loaded ball screw. The preload in the ball nut is set at the factory
by placing a precision ground spacer between the two ball nuts. Lost moon aributable to the ball
screw assembly is 0.00”. Ball nut preload is not operator-adjustable.
The ball screw mount bearings are located in the stepper motor for each ball screw. These are a
pre-loaded angular contact bearing pair and are operator-adjustable. Under typical use, these
bearings should be adjusted so that observable lost moon is between 0.0012” to 0.0018”.
0.004
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Backlash (in)
Backlash Versus Turns of Adjusting Screw
Number of Turns of Adjusting Screw
(0 represents a loose gib, 5 represents a tight gib)
Figure 9.3