I’m working on finishing up the rough design of a smaller CNC mill I’m looking to make in the near future and I’d appreciate some input on some of the decisions I have to make.

I’m shooting for travels of about 14” x 8” x 14” (XYZ), a little less would be fine. I’ve put about a thousand hours on a modified Tormach 1100S3 over the past few years so I have a pretty good handle on how much travel I really need. I plan on putting a pair of 4" vises on the table for most jobs. I don’t want to make the mill larger than it has to be, both for size and rigidity reasons.

Overall, my goal is to build a stout, high-performance prototyping mill for making at most small runs of parts. It will mostly (say 90%) cut aluminum with 1/4” (6mm) and 3/8” (10mm) sized tooling but with some steel and stainless (say 10%). I’m not so concerned with speed when milling the tougher materials, but I want it to be able to cut fairly well with 5/16” (8mm) tooling. To this end, it will be as compact as possible within the constraints of the work envelope. This will allow for both space efficiency and maximization of rigidity.

Spindle:
I’ve been looking at ~10k RPM BT30 spindles with servo drives and pneumatic tool retention on Alibaba as they seem to check most of the boxes at around $2k for the package. At the moment I don’t see much reason to size up to CT40 tooling for a mill of this size. (And I recently got rid of all the CT40 tooling I had so there are no economies to exploit there.)

Rails:
For the rails I was looking at HGH-25 class rails for the Z axis and either HGH-20 or HGH-25 rails for the X and Y axes. Packaging looks to be a little easier with the smaller rails and they save 10mm of stack height per axis too. I’m just not sure if I’m running a risk of leaving performance on the table by going with the smaller rails.

Ballscrews:
I was looking at 5mm pitch, 20mm diameter ground screws as a size that would fit in this package without too much difficulty. I haven’t looked for a source for these yet, but unless I’m missing something, they should be about right for a mill of this size. The longest screw would only be ~600mm long so stepping up to a 25mm screw doesn’t seem necessary, at least from my current perspective. If I’m missing something here, please educate me.

XYZ drives:
I’m really only looking at servos for my axis drives. I want to be able to rapid quickly (~500 in/min or ~200 mm/s). I haven’t looked into what’s available in this regard yet, so I’d appreciate any advice.

Frame:
I’m looking at something along the lines of 8x6” square tube with 1/2” wall, filled with either just sand or an epoxy-aggregate mix for both the base and the column. My quick sketches suggest that this should be sufficient for the smaller form-factor of this mill. I’m planning on very beefy exterior L-shaped brackets to reenforce the column-to-base joint.

This will be my first build of this scale so I’d appreciate any and all advice and feedback you have.