[footloose]

I originally posted this in the metalworking forum, but got no response, so now I'm trying here.

I would like some feedback on a machine design.


I would like to make precise molds in protoboard, machinable wax, and aluminum.

So, cutting aluminum to .001 is my goal.

I have spent a lot of time reading up on stuff, here is my current design.

Once I get a design nailed down, I'll decide if I want to take the plunge (based primarily on price).

Current plans:
25mm hiwin rails from automation-overstock.com
c7 rolled ballscrews from linearbearings2008
2.2kw spindle from linearbearings2008
8020 80mm and 80mm/160mm framing
shars 18" x 8" cast iron t-slot table
3 axis servo kit from keling

Drawing is a bit rough, I didn't want to take much time to model parts. The ballscrews shown are the wrong size, they're just in there for rough placement.

All the framing is the "correct" size.

Drawing shows just a flat plate rather than the t-slot table.

Drawing is missing:
Mounting brackets for ballscrews.
Mounting bracket for spindle
Holes depicted in plates.
Nuts and bolts
servos

Working area is 450mm x 250mm x 200m, roughly 18" x 10" by 8", except for where the z axis ballscrew might interfere along the right hand side of the x axis. There is enough space to make the table a couple hundred mm deeper, this could be 450mm x 450mm area no problem, I think.


Things I'm proud of in this design:

The axes don't really depend much on each other spatially, so it's relatively easy to adjust the design to a new size. Previous iterations were a real pain in the ass if I decided the spindle anchor plate needed to be bigger, or whatever.

The mill is nice and small. Currently the two sets of 8016 are 750mm long, which comes out to just under 30 inches (without the servos which will stick off the sides). This small size is nice, because then I can build an enclosure and still fit comfortably on a 36" table.

I think it uses its space well. most of the available space is usable working volume.


Things I'm worried about:

Y axis deflection, if the spindle is right in the middle of the wide cross plate and fed in the Y direction. I tried to use deflection calculators with an estimated force of 1000N. 1000N because this paper (referenced elsewhere on these forums http://www.cadanda.com/CAD_4_1-4__35.PDF) suggested 700N for milling aluminum with a 1/2" end mill. I will be doing small work, and probably won't use larger than 1/4" end mill, but I have no idea how to estimate this cutting force. The 8020 deflection calculator was simple and easy to use, but I couldn't get a good design that 'felt' good with a 8020 crossbar, so I just went with 3/4" aluminum plate. Is this good enough?

The ballscrews are off center. Does this matter?
If it really doesn't matter at all, then I'll put the Z axis screw on the 'outside' and eliminate the issue that it takes up some working volume.

Bellows for ballscrews and rails. These seem hard to find, or expensive?

The Z axis is different from most other designs, which would fix a gantry without an axis, and then have Y and Z axes on the gantry crossbar. I put my Z rails directly on the gantry pillars, and I wonder if there's any issue I'm not aware of that makes this a bad idea.

In general:
What things should I be worried about that I'm unaware of?
Are there things in this current design that are problems waiting to happen?