Issue with Repeatability and accuracy

[mahmoudtsh]

Hello all,

I am a newbie in the CNC world and recently bought a relatively inexpensive desktop CNC router from ebay.

I am using this machine to cut simple cutout in ABS plastic enclosures. The problem that I have is that after a job is done and when the router returns to its home position it is always slightly off from the initial zero positions (It always undershoots and never overshoots the home positions).

Now can I just simply conclude that since it was an enexpensive machine, I should expect this kind of errors? Or am I doing something wrong commonly done buy newbies that is causing this? Or can it be my computer or the driver for the motors?

My motors are stepper motors and I use a very low feedrate to cut the plastic. The software I use is Mach3.

I disabled a few processes in my computer to see if that helps, I think it helped a little bit but I still have some ~0.02" of offset when going to zero position.

Any help or suggestion is greatly appreciate it. for the last few days I have been banging my head with no results!

Thanks

[Crevice Reamer]

Do measured moves in all Axes. Make sure that if you called for say a 10 inch move, that is exactly what you are getting. If not, you may need to tweak your steps per inch slightly up or down.

CR.

[mahmoudtsh]

Thanks for your reply,
but if the problem was the steps/inch setup, wouldn't that still bring the router back to the home position accurately, since the error would be both ways and cancel each other?

[ger21]

Correct. I don't think it's a step/unit issue. But you'll need to give us more info. Motor size, power supply, screw, drives, speed...

[mahmoudtsh]

here are some of the information for the machine:

• # Kevlar-reinforced urethane MXL timing belts on all 3 axes
  # ball bearings preloaded on on 1 " square 1/8" thick walled steel tubing on X axis and 1.25" square steel tubing on Y axis for ultra smooth motion with zero slop.
  # Z axis features 5/8 inch Thomson brand hardened linear rail with super-oilite self-lubricating bronze bushings. Replaceable.

I don't have any information on the driver as it is a no brand driver.

The interesting thing that I have noticed is that the actual location of the router always skews from X, Y , Z in the positive direction and not in the negative direction. so instead of reaching point (0,0) my router usually ends up at (0.02, 0.02).

At this point I am assuming that there is something wrong with the driver. Maybe EMI issue (since the Parallel port comes in the driver box very close to the motor cables).

I would really appreciate it if you have any suggestions for me on what to look for and what you think may be the problem.

Thanks for the help