Design Options: Moving bridge vs. Moving Cross Beam

I am working on the design of a bench-top CNC gantry milling machine. Yes, I am calling it "milling machine" because the intent is to build something that will be able to machine aluminum and steel, not just wood like CNC routers do.

I am currently considering various geometries. The most common out there seems to be what I think is known as the "moving gantry" geometry. You find these all over eBay and, of course, in real serious designs:

Moving gantry machine


The other design, less common in the little desktop mill world, is the "moving beam" approach. Here's a big boy that uses this design approach:


Moving beam machine


I am leaning towards going with a moving beam design using either thick welded steel tubular framing, epoxy granite or a hybrid. I have, BTW, read the vast majority of the huge epoxy granite thread.

Here are some of my reasons:

- You can have long work pieces extend out along the x and y directions
- Being that all active components would be on top of the machine, making a machine of a different height is as simple as replacing the frame
- With some creative design I can even think about the idea of having the ability to have huge Z axis space (not travel, just room). Think knee mill table lift married to a moving beam gantry.
- Inherently easy to deal with coolant and chips
- Easy to mount a nice sub-plate
- Sub-plate or machined-piece weight not an issue
- Can be enclosed (for chip and coolant control, maybe noise) very easily
- Less mass to accelerate along the Y axis
- Could be more rigid (highly design dependent)
- Easily adaptable to other configurations. For example, add a second bridge to have dual heads (crazy, but, hey, it's my list)
- If welded steel, easy to fill with epoxy granite for excellent damping
- Can extend frame vertically and have electronics right above machine

I am looking for feedback supporting either design configuration. I can see that in a moving beam mill whatever supports the ends of the beam needs to be design to be more rigid and deflect as little as possible under cutting forces. On a moving bridge machine this function is relegated to the machine's base, which could be quite stiff.

Thanks for your feedback.

Those are both variations on a moving gantry design. The advantage of the "moving beam" is that there's less mass to move, since the walls it runs on are stationary, as opposed to the uprights that must move along with the gantry in the first machine you link to. That makes it inherently more rigid. But either of these options requires moving a very heavy load and being able to reverse its direction constantly, so you'd need very powerful servo motors and solidly mounted screws (or rack and pinion drives) to accomplish that.

The third option to consider would be a fixed bridge/moving table design. In that case you can make the bridge as heavy as you want, since it doesn't have to move. If you're working on extremely heavy stock, it might not make much of a difference, but otherwise it would be easier to move the table than to move the gantry. These designs tend to be the most rigid, since the motion isn't leveraged against you. The disadvantage is that the moving table requires a larger footprint.