Hi folks! Dropping a post to see if I could get some advice on a design concept I'm trying to sanity check. I'm in the process of designing my 2nd woodworking CNC machine, which will be dedicated to 4th axis carving. The wrinkle is that I want to use a common x/y/z to carve multiple rotary A axis at the same time. Which got me thinking about the best solution for spindles motors, and concluding that if I can make it work, designing my own system would be the way to go.

Reason is I want to keep the weight and size of the Z gantry down, since that's the axis that really does all the work and I need it to be as snappy as possible. And since each spindle motor doesnt need much power behind it, as I'm carving relatively small diameter stuff, I figured a rack of DIY spindles would be best - for weight, space, and maybe cost. Add to this that I'd really like two spindles per A axis, with different size end mills on them. So this is my current design, exploded view second image. The plan at the moment is to have two of the below assemblies, with a 1.5 or 2.2kw spindle motor between them, then run gt2 belts from this powered spindle to the first pulley on each side, and then daisy chain the rest together.


Only other things not shown are an end plate on the bottom to hold the double bearings in place (they butt against a lip inside the housing on the other side), the belts, and a crinkle spring washer I'll add between the top belt pulley and the top bearing, just to add a bit of backup preload (hopefully unnecessary, as I'll use a clamp collar to compress the inner rings of the double bearings together).

Machine both housings from alu, run water over the top of the front face of the lower one, possibly compress a copper pipe behind to cool from the other side, and run water over the top face of the upper housing if needed also. I'm thinking deep groove rather than angular contact, shielded not sealed as shown. The cutting forces will not be large, so I dont think angular contact are necessary and deep groove are typically listed as having slightly lower friction coefficient (unless perhaps under high load). These are ID8 OD28 thickness 9mm bearings, although I am considering bigger might be better - I cannot find confirmation on this, but it seems logical that larger balls mean less frictional heating, and of course more rugged. I've found a relatively cheap and decent quality supplier for the ER11 chuck and shaft, tested one out and it looks good, they have options for 8, 10 of 12mm shaft diameters, so I can scale up a bit if needed. Probably have to get someone to machine the housings for me, maybe give it a go myself, but no big deal.

Sooo... thats the plan! Hoping for around 18K rpm. Am I crazy?! I consider myself pretty well clued up on CNC design now, but this is outside my experience, and I cannot see tips from anyone who has tried something like this before, all of which make me nervous . Trying to consider all the things that could go wrong.... first the force from all the DIY spindles on each side will be going through the final belt, connecting to the motor, although only two each side will be cutting at any one time. I've just pencilled the belts in as cheapo 6mm gt2, but there is space for bigger. And I might need to add idlers above the belts to keep them tight. Bearings will need good alignment, and the housing tightly toleranced . Thermal expansion could cause headaches, although if I keep it all well cooled, maybe not. I kinda feel like it cannot be this simple however, so if any of you have suggestions for what I might be missing, I'd really appreciate your thoughts!!! Any other input from "looks good" to "bad idea" would be welcome. Thanks for reading!