I have finally decided to take the CNC plunge after a couple years of lurking, 3D printing and because I really want one for my woodworking hobby. I’ve decided to go the DIY route and more adventurously design my own machine. Below is an almost finished mechanical plan for the design I have come up with. The 3D rendering is just missing drag chains, proximity limit switches and a few angle braces here and there that would hardly show in this view. I’m not an engineer but I sometimes play one at work since the word engineer is in my official title and I have stayed at a Holiday Inn Express so I’m just barely qualified to design my own machine.
Even though I’ve learned a lot over the last couple of years, there are still some things I’m still fuzzy on, and since I don’t actually have the math and physics background that comes with a mechanical engineering degree, or much real world CNC experience, I have a few questions for the experts here to help me make some informed decisions.
First of all here are some of the specs of this design:
• Ballscrews are 20mm diameter
• Linear Rails are 20mm wide
• Motors are NEMA 34 and what is shown are Chinese 8.0nm Closed Loop
• Table area is 750mm x 1300mm
• X axis will travel a full 750mm so the spindle center point can go edge to edge
• Y axis will travel 1310mm so the spindle center point can go from font edge to 10mm beyond the back edge
• Z axis travel distance is 140mm from 20mm off the flat extrusions to leave room for spoil board strips. (That should be enough for flat work and low relief carving. Much more and the moment arm force could be an issue)
• Spindle is Chinese 3.5KW Air Cooled with ceramic ball bearings
My first question is about what ballscrew lead to use. 5mm vs. 10mm, (SFU2005 or SFU2010) and why choose one over the other?
My second question is around correctly sizing the Motors for the designed dimensions listed above. I am showing this design with 8.0nm motors, but is that totally over-powered or over-torqued for this design? The cost difference between a 4.0nm and an 8.0nm Closed Loop Motor isn’t that much in the grand scheme of the overall cost of this project so if being overly large is not a problem then I’d probably go that way but I’m a bit out of my element on this one. I need some solid technical advice on this decision. (My tendency for Tim Allen grunting about “more power” is not a sufficient justification... AAARRGHHROOO!)
Aside from that I’ll listen to any other advice any of you experts want to give. (I don’t guarantee I’ll follow it, but I will consider what’s offered objectively.) Thanks in advance for your feedback and I hope this isn’t TLDR.
Tom