Re: [CAD_CAM_EDM_DRO] lead screw or rack and pinion ?

turbulatordude wrote:

> the question is wheather it is better to use lead screws, driven by
> timing belts, lead screws driven by gears and solid shafting, or rack
> an pinion driven by timing belts or gears and solid shafting.

I guess I would make this decision based on Budget and Needs, but I sure lean toward the rack method for the size machine.

I'd run a Rack down each side of the machine and run a *single drive shaft* that crosses the gantry higher up. I'd drop down each side with toothed belt and pulley to the pinions, happily nestled
into some simple bearing boxes. Do it right, and you can install the pinion shafts that engage the rack right into the vertical gantry uprights, and then install nice little covers over the belts on
the outside of the gantry uprights. A single motor can drive the main shaft also via toothed belt and pulley.

Keep in mind just how convenient it is to use belts and pulleys if you have to bring together a gaggle of different components to make it actually run.

If you try Leadscrews, An 8 foot travel will likely need something like 10 feet of leadscrews down each side. Your accuracy will go up depending on what you use for screws, but, so will the hole in
your wallet. The next big issue is what "lead" to use. To fine, and the shaft will wobble like a drunken bum anytime you try to achieve any acceptable rapid rate. On an 8 foot long machine, even
100ipm is pretty poky slow. If you go with too coarse of a pitch, then you lose all of the accuracy the screws gave you anyhow.

If I was doing it, it'd be rack. Cheaper, and probably plenty accurate enough for the type of work a 4'X8" machine ends up doing. There are a few techniques that can be applied to take any backlash
out of the pinion/rack combination if need be. Some use a spring arangement that binds two pinions towards each other, another actually uses two pinions side by side and a built in spring causes
them to each want to twist apart. if you have some really rigid bearings over the 8 foot travel, you could even "preload" one pinion against the other.

If you get the "racking" issue at all on a flatbed machine, you simply do not have a rigid enough structure for a cross gantry. Done correctly, Even a single lead screw *could* work right down the
center under the table top.

In all reality, the old Digital Tool Router I have to work with (with rack and pinion) exhibits very little backlash without using any method. The pinions fit nice and tight to the rack. I figure
when they get sloppy, it's all cheap enough to replace.

Go for the rack.

Chris L