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The following method describes the proper procedure to measure X-axis backlash. An analogous procedure is used to measure Y- and Z-axis backlash.

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1. Mount a dial indicator to the mill table along the X-axis to the left of the spindle, with the tip pointing at the spindle.

2. . Jog the Y- and Z-axis to position the spindle head so the indictor tip contacts the outer diameter of the spindle cartridge (see Figure 9.1).

3. Carefully jog the X-axis in the positive direction until the indictor contacts the spindle. After initial contact, continue to jog the X-axis in the positive direction so that the dial makes at least one complete revolution; stop when dial reads 0.

4. Zero the X DRO field in PathPilot

5. In MDI field, program a positive X move of .01” at a feed rate of 5 IPM: G01 X.01 F5. The spindle head moves slightly in +X direction. When finished, indictor should read .010”.

6. Program an X move back to 0: G01 X0 F5. The spindle head moves slightly in the -X direction. The X DRO should say 0; however, the dial indicator should read a number very close to 0. This value is the measured lost motion.

Gib Adjustment

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The gib tightening adjustment procedure for each linear axis is detailed in the table below.

1. Loosen the upper gib screw eight rotations and tighten the lower gib screw eight rotations. This ensures that the gib clearance is quite loose.

2. Use a dial indicator to measure lost motion in the Z-axis (refer to How to Measure Lost Axis Motion earlier in this chapter). With a very loose gib, the majority of the measured lost motion is attributable to the backlash in the angular contact bearing pair. On a new mill, this value measured should be less than 0.0015” on the Z-axis and less than .0013” on X- and Y-axis.

3. Tighten the gib by one turn by loosening the lower screw first, then tightening the upper screw. Measure the backlash again.

4. Repeat this procedure until the measured backlash begins to increase. At this point, the gib setting is slightly too tight.

5. Back the adjustment off to the point just before you saw the increased backlash. That is the ideal setting for the axis.

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title 770M/MX, 1100M/MX, PCNC 770, PCNC1100 CLICK TO EXPAND
To properly estimate the torque needed to overcome angular contact bearing friction, the bearings must be isolated from the motor detent torque. The following procedure illustrates the X axes; other axes will follow suit: For adjusting the X-axis, position the table near the right hand end of its travel (i.e., X near to zero). This ensures that the bearing is near to the ball nut to minimize bending of the screw during tests. Remove X-axis motor mount cover plate. Loosen two screws clamping the coupling between the stepper motor shaft and ball screw end. Remove four cap screws holding the axis motor to the motor mount and remove axis motor. Take care not to put any strain on the motor wires. Insert a 1/2” diameter rod (included in AC Bearing Service Tool Kit, PN 35355) or drill blank into the coupling. This will effectively extend the ball screw shaft outside of the motor mount. Clamp a handwheel or vise grip on the end of the rod; this allows sensitive feel for the torque caused by the preload on the bearings. Rotation should be smooth with a small perceptible drag; this corresponds to a medium preload of about 150 pounds. If the rotation feels tight, you have too much preload and will dramatically shorten the life of the bearings. If the rotation is free, you have little or no preload and backlash will be excessive. This test should be done with the lock nut tight. Using kit’s spanner wrenches, loosen the lock nut and back it off about two turns. Then hold the ball screw to prevent it from rotating with a pair of pliers on the coupling and tighten the adjustment nut until there is slightly more backlash than you ultimately want to achieve. Tightening the lock nut will slightly increase the bearing preload. Re-mount box and motor; ensure that coupling is symmetrically fitted to the motor shaft and the screw end and is fully tightened.

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title PCNC 440 CLICK TO EXPAND
The Adjustment Nut is located on the back of each stepper motor (see Figure 9.4). Mark the current position of the adjustment nut on the back of the stepper motor. Measure the lost motion in the axis. Using a pin spanner wrench, turn the adjustment nut clockwise about 10˚ and measure the lost motion in the axis again. If the lost motion value decreases, adjust another 10° and re-measure. Repeat this step until the lost motion value does not decrease from your changes. Info IMPORTANT! Do not overtighten the adjustment nut; this can cause the stepper motors to stall or may damage the bearings and the motor

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The “ADMIN GET_AXIS_BACKLASH” command will display the current amount of backlash for each axis. The “ADMIN RESET_AXIS_BACKLASH” command will reset the backlash values of all axes to 0.

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Note: There is no backlash compensation for the A axis.