Micro Stepping

I finished my new board design and sent it off to Fritzing Fab. The turnaround is slow, for sure. They collect many designs and batch them together to reduce costs. This is a 2-sided board with solder mask and silkscreen. It's 2"x4.5" and costs a total of $62. I think that's pretty good. Here it is:

I'm particularly pleased with the logo. Hopefully the design is right, too. I looked it over a bajillion times.

While I'm waiting, I thought I'd do some more testing. I have noticed in some of my previous test runs of the PCB trace that the spindle wasn't ending exactly where it started. Or so it seemed. There is plenty of room in the software stack for error, but I was sure I had run a 50-minute program some time ago that ended exactly right.

One issue that could result in a real movement error (other than software) is stepper motor resonance. I have already worked out some of these issues. I've been using my DRO as the double-check of position. Some of my runs were resulting in significant errors in ending position, like 20-30 milles. I drilled a hole in the starting position, ran a test then chucked my center finder, dropped it into the hole and it all seemed to be pretty good, certainly not off by 20-30 milles.

Then I thought, hey maybe my mill is so fast the DRO is missing steps. I posted an inquiry to the Sherline CNC group to see if anyone had similar experiences. This guy Dave suggested using the handwheels to double-check the position. Duh. I've been using my DRO for so long I forgot about the handwheels. The handwheels have 1 mille gradations, 50 per turn. It's the way you keep track of the position of the spindle without all the fancy stuff.

So I did a few more tests, particularly with high-speed (rapid) moves. Several runs showed the physical moves absolutely consistent and spot-on in final position. The DRO was missing about 1% of the steps at top speed. Reducing the speed a tad took care of the problem.

My steppers are 200 steps per turn. The driver board can be configured to run in 1/2, 1/4 and smaller fractional steps, or micro stepping. I have been running my mill at 1/2 step or 400 steps per turn. I decided to try 1/4 step which immediately halves the speed. It seems the smaller divisions can help with resonation. 

I tweaked the various settings to get the speed back up, but not as fast as before. It does seem to move more smoothly now. I even ran a GCode file that had a vector that caused resonating. It doesn't do it any more. There will probably always be resonation problems. I was doing some manual work this evening and I found a speed that resonates. This shouldn't be much of a problem. It moves through the speed just fine accelerating. When I get around to coding feed speed control I'll find the speeds that resonate and map them out of the options.