[Aaronem]

I am currently trying to extend my 1200mm x 1200mm (4ft x 4ft) gantry router to 2400mm x 1200mm (8ft x 4ft). I am being told that a 2400mm long screw is going to whip around like mad, and I believe it, and that I would be better off using either a belt or rack and pinion. Belt seems preferable because the stepper motor does not have to move around, but do belts work reasonably well?
Also at the moment one revolution moves the axis 4mm. With a pinion or a toothed wheel the axis will move maybe 75mm to 100mm. Will I need to gear down the motor and is there a cheap way to do this.
My gantry weighs about 75 kg, I have nema 34 steppers with fairly powerful micro stepper drivers
I would really appreciate some thoughts or experience on this.
Cheers
Aaron

[ger21]

Generally, you'll want to use a gear (belt) reduction of at least 3:1, so you end up with 25mm per stepper revolution or less.

The problem with belts, at high speeds, and when moving a lot of mass, is that they are springy when you start and stop. Here's a belt drive that eliminates this, but you'll need to figure out how to build it yourself.
Best belt drive ever! (If I do say so myself) - CNCzone.com-The Largest Machinist Community on the net!

I'd say that rack and pinion would be your best bet. There are some examples of belt reduction units and rack and pinion on the Mechmate forum.

[Pplug]

I am currently trying to extend my 1200mm x 1200mm (4ft x 4ft) gantry router to 2400mm x 1200mm (8ft x 4ft). I am being told that a 2400mm long screw is going to whip around like mad, and I believe it, and that I would be better off using either a belt or rack and pinion. Belt seems preferable because the stepper motor does not have to move around, but do belts work reasonably well?
Also at the moment one revolution moves the axis 4mm. With a pinion or a toothed wheel the axis will move maybe 75mm to 100mm. Will I need to gear down the motor and is there a cheap way to do this.
My gantry weighs about 75 kg, I have nema 34 steppers with fairly powerful micro stepper drivers
I would really appreciate some thoughts or experience on this.
Cheers
Aaron

You could always do a rotating ballscrew nut. The screw would stay still and the nut would rotate, eliminating whip.