large 3 axis machine

[rutman]

Hi
I'd like some feedback on a design now underway:
its a 3 axis machine based on a rectangular box frame
moving head, workpiece is stationary
x axis - 2meters - rack and pinion
y axis - 75cm - twin ballscrew
z axis - 50cm - single or twin ballscrew

Will nema 34 motors need to be geared down? We dont care about speed
but would like 0.1mm accuracy

Will cutting soft metal be possible on such a machine?


TIA Jeremy

[cyclestart]

y axis - 75cm - twin ballscrew

The most common design is a single y screw placed midway between the slides.
Is your design more like a mechmate/shopbot ?
MechMate CNC Router - Build your own with our detailed plans

z axis - 50cm - single or twin ballscrew

hard to imagine wanting twin ballscrews here

That's a lot of Z height, does your work need that kind of clearance ?

[rutman]

The most common design is a single y screw placed midway between the slides.

Ok we will prob. do that instead

Is your design more like a mechmate/shopbot ?
MechMate CNC Router - Build your own with our detailed plans .

Yes that's the idea just with a much larger Z throw.

hard to imagine wanting twin ballscrews here.

For this I was thinking the large z throw would necessitate sturdier construction - tho' we have a 50cm long ~3cm diameter lead screw so maybe that's enough with a single screw. I suppose the slides are what give most of the suport in any case and the screw is simply for downward thrust thus two are indeed unecessary.

That's a lot of Z height, does your work need that kind of clearance

Ideally we'd like to do things like kayaks, jet cowlings and others so we're going for a lot of Z - my build partner wanted to move the entire y-z setup with rack/pinion and tool in order to change the z distance if wanted, while I favor moving the workpiece by means of an adjustable floor - he beleives that an adjustable y-z is necessary since e.g. the rails will be going out of alignment and need periodic adjustment. I personally was thinking if we fix those in palce insofar as possible by welds, then scallop or otherwise work the rails to reach parallelism, then they will not likely go out of true.
Any thoughts on this?

[cyclestart]

I suppose the slides are what give most of the suport in any case and the screw is simply for downward thrust thus two are indeed unecessary.

Yes. In a perfect world the sliding components would have zero play, the pieces spanning the slides would have infinite stiffness, or we could have very wide spacing between the sliding components along a rail. In my opinion dual drives are a compromise, but when the span gets wide it's a compromise that makes sense. Counting on a centrally placed ball nut to prevent racking is a bad idea, that's the job of the other components. The mechmate design seems to rule out a central y screw ? With decent components racking would be minimal over that narrow span.

I've operated a machine that could raise/lower the bridge but it was a very heavy bridge mill. I'm not sure what is the best approach for a lighter machine.