I'm in the planning stages of an idea I have for a low-cost homemade CNC tubing cutter for making semi-precise curved cuts in basic thin-walled EMT electrical conduit. For my first proof of concept design, I don't want to spend huge money for a plasma cutter, so I thought I'd try using a basic rotary cutting bit in a mill-type cutter setup. I've read that should work on soft metals like aluminum or brass, but I've never seen EMT mentioned. (FWIW, my requirements are pretty low on precision and speed. I'd be happy if my cuts were within .1" and just about any feed rate would do.)
1) So that I can size a motor for the prototype, what ball park should I aim for as far as spindle speed, motor power and what type of bit should I plan to use? High speed cutting would be nice, but isn't paramount. I just want something economical that will cut through the material reasonably well and won't burn up bits too fast. As far as bit size/cutting width, I'd like to use the smallest bit that still would give good bit life, and shouldn't break. I think all cuts would always be through the wall of the EMT, not partially through, so only plunging and side-cutting would be going on. If I guess too small/cheap and burn up a motor, I won't mind upgrading it, but I'd rather avoid spending too much on the motor upfront. (IOW, if a .5hp motor is probably likely to work, I don't want to over spend $300 to get a 2hp motor "just to be sure")
2) Are there good techniques to use if my motor is slightly underpowered to accomplish the job? (E.G. Would it help if I made multiple passes at increasing depths? Slower feed rates? Something else?) I know a rotary cutter isn't ideal for cutting metal, but I'd like to get it to work for this first prototype.
3) If a rotary tool with a milling bit proves unworkable, are their any other "Plan B" cheap ways to cut EMT in a CNC machine?
Since I haven't gotten any replies, I've been hunting around on the web to try to find what I could. Figuring out feed rates for mill bits (ignoring horsepower) was fairly easy to find, but figuring out the horsepower requirements was something I didn't have much luck finding information for. And, since I'm fairly certain I'll be HP-limited,
The only thing I found was one reference on a different forum that tossed out some figures of .8 - 2.18 cubic inches of material removal per minute per horsepower, but I was a bit lost on which exact material that was referring to, and how that might compare to the soft galvanized steel of EMT.
Using those numbers, with a 4 flute 1/16" carbide bit, I think I would need a 15W motor to get a feed rate of roughly 5 ipm if I use the conservative numbers on everything I've seen. That would let me make a piece out of 1/2" EMT (2 end cuts, plus a few random holes) in under 4 minutes, which sounds fantastic to me. Bigger, 2" EMT might take 10-15 minutes, but that would still work for my purposes.
(Of course, I'll try to use a bigger motor than the bare minimum 15W guesstimate, but since I'll probably use a 12v motor and power supply, I don't want to get beyond the point where power supplies are adding $100+ to get the last extra amp or so. IME, 15-30W DC motors + powers supplies are cheap. 30W-60W the prices go up fast. Above that, power supplies get costly fast.)
Can anyone confirm that my numbers are reasonable? I've got a couple months before I buy parts, and I'd really like get things reasonably close to "right" before ordering.