What do you mount it to?

I see that most of these are screwed into MDF or some other piece of wood. I will be making a smaller one for cutting primarily aluminum, and a wood bed just doesn’t seem right. Do people also mount it to say, a big plate of aluminum? I suppose I’d have to drill and tap a bunch of holes in it for fixturing. Although I suppose the mpcnc itself could do the drilling couldn’t it?

It is a good idea for the spoil board to be of a softer material for though all cuts. Unless you are using coolant I suggest wood.

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Aluminum is pretty pricey. MDF or plywood is stable, and cheap.

Also, and through cuts will end up cutting into the material below. You’d want a “spoil board” which you can easily replace when it becomes trashed, and it will hopefully be softer than the material you are cutting so through cuts will not be higher stress than the cuts you are making.

If you want to make a grid of holes, to tap for screws or to put clamps through, you can use the CNC, but you’ll probably want a few outside where the CNC can reach, since that’s a great place to put clamps. Also, keep in mind that it’s pretty neat to watch the CNC make it’s own spoil board, but it’s several times faster to just have the CNC make some starting holes in the right places, and finish the holes up with a drill and Forstner bit. If you want to make oblong holes for F-clamps, then the CNC would be the right tool.

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Wouldn’t it be better to just leave a small skin of aluminum (and a few solid spots to keep it in place) that you can just file off than to cut the plate underneath?

You’re saying, why not just leave a little of the workpiece, instead of cutting through? Sure, that would work, but it will require more precision, and it will mean more work afterward. You could easily spend an hour carefully filing and cutting that skin off.

w.r.t. better, no, I don’t think it would be better than using an MDF spoil board.

Another thing you can do with MDF, if you are talking a small area, is to mill the entire surface. That will give you a nice, parallel to XY surface to put your work on, which should make a big difference in Z precision. I’m not sure why you think a wooden surface is going to be a poor choice with aluminum milling. AFAIK, that happens a lot.