Ballscrew Noob Question

[BobWarfield]

I've been researching to learn as much as I can about ballscrews in CNC conversions, and had a question. They have extremely low friction relative to ACME screws, so much so that I have read you really can't run a machine manually with ballscrews because the screw can be pushed off its position by the tool.

In thinking about a mill with a Z-axis ballscrew, what happens when you cut the power to the servo/stepper? I have an Industrial Hobbies mill I'll be converting, and the head there is HEAVY! I have visions of the darned thing crashing down as soon as power is cut.

What's the answer here? Is this why we see so many quills driven for CNC instead of the knee/dovetail axis? I know another issue driving the Z on these machines is that there is so much mass you just can't move it very quickly.

[Geof]

On some machines the Z axis is counterbalanced either by a weight or by a hydraulic cylinder and a gas pressurized hydraulic accumulator. Alternatively the Z axis servo has an electrically released brake which locks down when the servo power is turned off.

[NC Cams]

Try this: try to push the table against the ball screw and get it to move. Can't do it on my BPT.

If you run a preloaded screw w/preloaded bearings and run a reduction drive (IE: 2:1 or so) from motor to spindle, your'e going to have a hard time for simple gravity to overdrive the drive. Again, my BPT only drops the spinde when we disconnect it from the Z drive.

This is a generality as there are some heavy spindles/tables out there to move - these have the counterbalances mentioned in prior post.

In most cases, however, this is a problem that one does not have to worry about....

[Geof]

Originally Posted by NC Cams In most cases, however, this is a problem that one does not have to worry about....

It depends on your definition of most. There are thousands of machines in use that do have the Z axis electrically released brake which is definitely needed with a direct drive servo when a counterweight is not used. In some case it is necessary to ensure that the brake release power is cut fractionally before the servo power otherwise a heavy head can drop an appreciable distance during the few tens of milliseconds it takes for the brake to grab.