Gear rack for the Z ?

[douglasco]

I see everything used for x and y but only screws used for z. Why is that? Seems like the Z would be the most cooperative.

[Robin Hewitt]

Possibly because the Z axis takes the weight of the torch asembly. If you used a rack you'd want to drive it with a stepper because an ordinary DC motor would be stalled under load.

Rack and stepper should work well but screw studding is cheaper

[jcc3inc]

Sir,

Attached are pix of both stepper drive and DC motor/gearbox drives for the Z axis.

Regards,
Jack C.

[douglasco]

Originally Posted by jcc3inc Sir, Attached are pix of both stepper drive and DC motor/gearbox drives for the Z axis. Regards, Jack C.

And how does that work out for you? Would you say its just as good as a screw for speed and precision?

Robin, Are you saying i can't use a servo for this? That doesn't seem right. The motors for z are always smaller the X&Y.

[Robin Hewitt]

Originally Posted by douglasco Robin, Are you saying i can't use a servo for this? That doesn't seem right. The motors for z are always smaller the X&Y.

You can use anything you want. If everyone followed the herd what a boring old world that would be

I'm always looking for different ways to do stuff. If you like rack and pinion, go for it, i was just mentioning the permanent load snag.

[towertek]

Once you shut off power to the steppers, the z axis with a rack and pinions setup would freewheel until it hit the table without power to hold it in place..

[jcc3inc]

Sirs,

In the case of the small Pittman motor/gearbox, it is quite hard to backdrive it so freewheeling is not a problem. When using the stepper, I had a router as load with vee bearings on the slide; there was no tendency for it to coast downward. When the system is in operation, the stepper is energizer and does not coast downward.

We used the Pittman motor/gearbox with auto torch height controls which we made on a number of our smaller machines.

Regards,
Jack C.

Edit: One should also consider the machine geometry; sometimes a poor design exhibits excessive springiness, so acceleration or loading forces result in a far greater error than does the gearbox backlash. I have seen many amateur designs that are very poor in that they use long unsupported rails, rail systems such as pipes that were not designed to be straight within small tolerances. Some build carriages that are spindly, or the beams are too far from the bearings that support them.

JCC