[Zach_G]

Well, after a couple years of putzing around, I've finally came up with the parts and design for a CNC mill I'd like to build. I've contemplated everything from moving bridge gantries to hexapod layouts, but I think this is a good merger of all my design goals:

• Extensive aluminum milling and light steel work
• Large work envelope, at least 10" Y travel
• Compact and lightweight (less than 350lbs) because I move around alot
• Affordably built, with aid of school machine shop ($2k budget)

I guess I should include some features of my design:

• Mini mill spindle driven by treadmill motor ala JFettig's conversion
• Precision ground ballscrews
• Triple stack nema 34's on X and Z, 23 on Y driven by Centent drives
• All steel construction, 3/4" plate and a 7x4" rect column with 1/2" wall thickness
• Travel: 16"X , 10"Y, 13"Z
• Estimated 290lbs
• Bolted construction so I don't have to deal with stress relieving
• Massive 35mm rails on Z, 15mm on others
• Bellows for way protection

The immediate reaction I expect is why the heck is the Y axis attached to the Z? Well, originally while messing around with C frame designs, I was having a hard time keeping everything compact and light yet maintain ridgidity. That 10" of Y travel really put the spindle way out there, so why not have it move along its own support structure llike a half built gantry? With the components I've purchased, this really works out well I think and enhances each of my design goals.

One of the things I worry about most is maintaining ridgidity while milling metals so I took the opportunity to do some FEA in CosmosWorks. The following pictures show the resulting displacement using a 200lb force (purple arrows, randomly selected a force that seemed big) on the spindle 6" above the table and restrained at the interface to the mill base. The spindle tip displacement comes out to about .05mm.


One of the nice things is that this displacement represents a worse case scenario with the Y fully extended. As you can see most of the deflection is in the twist of the Z column, but as the Y moves in there is a smaller moment arm for the spindle force to act upon which drastically decreases deflection. Plus as the Z goes down, the column length decreases as well. I did a comparison FEA with a more standard C frame design utilizing the same components under the same conditions and settings which resulted in about .08mm deflection.


So there you have it, my rather unorthodox design beats out a similarly built C frame mill in ridgidity while being lighter and more compact. This assumes 200lbs is a significant force in milling. The majority of parts have already been purchased, I'd list the cost but I'm too afraid to total it up! Hopefully I'll start building this thing in within the next 2 weeks. Still need to get a small ballscrew for Y axis. I'd also like to find a real milling table instead of using this 3/4 plate. Seems a bit flimsy especially after milling T slots but at least it's ground flat on both sides already.