Jump-starting my home-made cheap milling machine (problem with stepper motors)

[shawn254]

Hi guys,

I'm new here in the forum and new to the topic of CNC.

I've been cobbling together a very affordable 3-axis CNC milling machine for a few months now.

About the milling machine:

- Aluminum and steel profiles from the hardware store as guides, with 22mm ball bearings
- Working area approx. 80cm x 55cm x 2cm
- 4x NEMA 17 stepper motors
- M8 threaded rods as drive
- Proxxon FBS 12/EF
- actively cooled 2A stepper motor driver boards
- Arduino with GRBL
- universal G-Code transmitter
But now I have the problem that my milling machine doesn't do what it's supposed to. Most of the time everything starts well, and at some point the milling machine deviates from the intended path and mills something. Without warning and for no apparent reason. Since it is usually already quite deep in the wood at this point, everything bends and I have to switch the whole thing off immediately.
My theory now is that my Z axis stepper motors have a problem. Because when they slow down, at one point it looks as if they are getting stuck and then continue to turn again. The problem also occurs when the engines are removed. The driver boards aren't too hot, so that can't be the problem. I believe that there is a shift on the Z axis, but it is not noticeable because the cutter continues to move in the previously milled path (the cutter is not very stable). At some point the deviation becomes too large and the milling cutter then mills away from what was intended. Has anyone understood this theory and is it realistic?
I would be very grateful if someone could help me as I'm kind of running out of ideas.

[peteeng]

Hi Shawn - If you run the code in air does the machine correctly execute the program? If so the electronics are correct. This means the tool is being overloaded and stalling one or more of the motors (ie missing steps) Try reducing the tool load eg less depth of cut. Plus ensure your tools are sharp. A blunt tool will choke and stall a motor easily. Threaded rod is not very accurate for driving a machine, this means the tool is travelling along a slightly different path each cut. As the tool gets deeper the "channel" is wobbly creating more tool load. Plus you need to read up on chip load. If the relationship between the tool rpm and the feed is wrong then the tool is overloaded, rubbing or choking (or all 3) resulting in the motors stalling... The Proxxon is an engraver at 100W it does not have much power to be a router or mill. One of my routers had a 400W spindle and it was under powered and I had to cut gently. A 100W drill needs very light cuts... If the Z axis motor is the one stalling then lubricate the thread with light wax not oil... Photos of your efforts would help.. Peter


https://cimquest-inc.com/what-is-chip-load/

[awerby]

Those M8 threaded rods are probably part of your problem. Screws like this aren't often used for leadscrews because they have a lot of resistance to motion, often in some spots more than others. That's not an issue when they're being used as intended, and you're tightening nuts onto them with a wrench. But if you're trying to turn them with a weak little NEMA 17 stepper and facing resistance from a tool embedded in material it's trying to cut with an underpowered spindle, it's not surprising that you're having difficulties. It's nice that you're trying to make a very affordable machine, but it seems you're learning why people more concerned with results than economy spend a little more money for more capable components.