Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Software is important

Posted by retromaster on February 1, 2010

To the left of the photo above is the latest part I’ve milled using the MF70 CNC. It’s for the third version of the X axis, out of 10mm thick aluminum. It turned out very nice, but I’ve had to solve some problems before getting this result.

First, the X axis started missing steps randomly, then after a short while it stopped moving altogether. It was definitely not a mechanical problem, so after a little bit of debugging the drive circuitry I found out that one of the optocoupler chips on the breakout board had died. Replacing that chip solved that problem.

The next problem was even more critical that it actually ruined a couple of workpieces until I found about it. I was using a very popular piece of CNC software that runs under Windows XP. However, apparently, it was missing steps! I first noticed the problem when I saw that the two circles that form the bearing housing in the middle were not concentric. I did a test, by issuing a “goto to zero” command after milling the first circle. It was off by almost half a millimeter in both X and Y axes. This behaviour was reproducible and I did not think that it was mechanical. So, I installed EMC2 (together with Ubuntu) on the very same machine. Milling the same part with EMC2 gave the perfect results in the photo above. I suppose that the EMC2 installation coming with a Linux kernel patched with realtime extensions may have something to do with that.

I also found locally some good quality flexible couplings for a reasonable price. I’ll use those for replacing the (rather poor) rigid couplings I am using right now. They have a 6.35mm opening on one end with fits the motor shafts perfectly, and the other end is 4mm. So, I’ll need to machine the leadscrew ends to meet that diameter, but that’s not a problem.


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