MF70 New X Axis Complete
Posted by retromaster on January 6, 2010
Here is a photo showing the new X and Y axes of the MF70 CNC. Also shown in the photo is my new standoff design for the Y axis. Both axes perform noticeably better, with the X axis achieving speeds within 250% of the previous version.
That said, the couplings (which I’ve built) still seem to be problematic. I’ve tried building new couplings out of round aluminum stock (using the dividing attachment for the MF70) but they actually performed worse than the ones I had built before! I’ll have to think of a way to build better couplings.
And finally, for a change, I’ve started working on the new stepper drive electronics. I’ve made a new PCB layout for testing the drive electronics for a single axis. I’ll use that for debugging, and if that works, I’ll go on to build a new integrated board with drivers for the 3 axes interfaced to the parallel port. For each axis, I’ll use a PIC16F676 to monitor drive current through a current sensing resistor using the built-in ADC. The idea is to make the switch the coil drivers on and off to make the current amount follow a sinusoidal profile. I opted for the 16F676 because it is cheap, but it is quite possible that its built-in ADC will not be fast enough to current monitoring. In that case, I might try a design based on the 24F04, which has a much faster ADC (and it is not that much more expensive).
Building this debug PCB for the new drive electronics gave me an opportunity to test the PCB drill performance of the MF70 CNC. Unfortunately, the results were less than satisfactory. Locations for some of the holes were inaccurate. I am not sure what caused this. It may be due to positioning errors in the machine (backlash, missed steps, software errors), or due to drilling errors (the drill bit wandering on the copperclad, or even warpage of the copperclad). Over the next few days I’ll do some tests to figure out the root cause and hopefully solve the issue.