Troubleshooting Axis positioning problems–Vertical Machining Center
1) First check for “Lost Motion”; best for X & Y axes ballscrews. Z-axis, fights
gravity unless counterbalanced. To check the backlash of the X & Y ballscrew
thrust bearings. Use a .0001”(.002mm) per division “lever type” “test indicator” and
a ball bearing, glass or plastic bead 9/16” (15mm) diameter fitted with heavy
grease into the ground “center” hole at the end of the ballscrew away from the
motor. Move the axis with the handwheel and oscillate back and forth quickly.
There should be no more than .0005” (.013mm) lost motion. More means
mechanical looseness of some type; most likely in the thrust bearings or the
locknut for the inner races or loose bolts of the retaining flange for the outer races.
2) Ballnut Checks: With the indicator fixed on the side of the table and a good rigid
setup; set the indicator at a 45 degree position down in the ground ball groove
track; move the axis back and forth; there should be no more than .0002”(.006mm)
lost motion in the ballnut. Note: A better ballnut check for X&Y axis.
Caution: Z-axis would need to be rigidly blocked up first! Remove the lubeline from
the “X”, or “Y” ballnut and the four ballnut mounting screws; carefully push the
table away from the nut and feel the ballscrew by hand: Rough? Smooth? Loose?
It should not spin and free wheel. On a 30mm (1-1/8”) Diameter screw the preload
pull is at least 1.5-2 lbs. of preload. With a string wrapped around the ballnut it
would be measured pulling it with a fish scale. An FTV X-Axis 85mm
diameter screw has 14 lbs. of preload. If the ballnut spins easily by hand; then the preload has been lost due to
wear; it needs to be rebuilt or replaced. If good, hook-up the lubeline; but leave the 4 ballnut mounting screws ¼
turn loose. Move the ballnut axis close to the thrust bearing housing to allow it to self-align; then snug the ballnut
bolts; move away if needed and torque to spec per prints.
3) If the X or Y Ballscrew has a solid coupling; such as a “crush-coupling”: With the machine shut off, loosen the 4
bolts for the motor. The “solid coupling” for the ball screw should not allow the motor to pull/slide out; it must be
tight. Caution: Z-axis would need to be rigidly blocked up first, as the Z-Brake is in the motor!
Note: QC20-W Renishaw Ballbar is able to quickly diagnose mechanical and servo problems on X, Z axes!
4) After the mechanical issues are resolved: Adjust the Gain and Balance of the servo system if applicable.
Possible causes for mechanical backlash problems
1) Thrust Bearings or Thrust Bear Housing bolts and Tapered pins or Ballnut Bracket bolts and pins.
2) Worn-damaged Ballscrew-Ballnut. Rebuilders will not rebuild if the ballscrew assembly shows brinelling.
3) Improper or lack of stretch on a stretched ballscrew design.
4) Motor to ballscrew loose mechanical coupling.
5) Misalignment of the ballscrew with the linear ways of the axis.
6) Scale; mechanical reader head brackets, or misalignment, loose springs in scale reader head.
7) Z-axis Brake dragging or not properly releasing.
8) Counterbalance problems.
9) Damaged Linear Ways causing Slide-Rock
10) Improperly Leveled beds or poor foundations
9/16” (14mm) Diameter Sphere, Ball bearing, Plastic Pearl, Glass Bead
Left: Checking Thrust Bearing Backlash
Right: Checking the Ballscrew Nut Backlash