Limit Switches, Limited Progress, Reduced Backlash

Progress has been slow. After school got out we took a vacation, then we had a holiday weekend. Weather has been great, lots of barbecues. I spent most of the day yesterday doing some nature photography. Just before vacation I realized I needed to order some parts and I didn't want to do that until we got back.

I have the Sherline DRO that has special collars for mounting the encoder wheels. The X and Y collars mount on the back shaft of the steppers nicely. The Z axis collar is very different. I needed to order another one like the X/Y collar design. Sherline makes a device for the Z axis that allows tightening the saddle nut lock bar to reduce the backlash. I decided to get that, too. The kit includes a new saddle nut and lock bar which I really don't need. I'm not going to use it, just stick it in a drawer until my current Z axis wears out. I also thought it would be good to use Dremel bits with the mill for engraving so I got a 1/8" collet.

After getting the DRO handwheels remounted I was able to check the backlash. The DRO has electronic backlash compensation, basically not counting a certain number of clicks when the direction is changed. Prior to mounting the steppers, the backlash on my mill was 3-5 mils. After mounting the steppers and the Z-axis saddle nut kit, my backlash is now 1-2 mils. That's quite an improvement. The stepper mounts are so solid, there's no play at that end, so that just leaves the backlash nuts to tighten up.

I've been thinking about how I'm going to deal with backlash. I may write about that some other time. For now, I'm going to ignore it altogether since it's so low. I want to move on to getting productive with the system.

I'm supposed to be working on the limit switches now. It's not obvious how to mount the switches, certainly not in a way that looks good and is out of the way.

I did do a functional test, though. I measured the movement of the switch. The switch would move some amount I can't recall before getting to the "click" point, then another distance before bottoming out. I made a quick estimation of how many steps the motor would take to move from the click point to the bottom out point. The limit switch signal is polled in the main loop of the firmware so there would be a delay from the change of the switch to the time the steppers would be told to stop. I don't remember the numbers but it seemed likely the steppers would stop before bottoming out on the switch.

I used my favorite prototyping tool, the hot glue gun, to mount a switch and a stop block to the table. Then I ran the table into it at full speed. The steppers came to a stop even before the switch got to the click point. Sweet. The hot glue held well. I messed up the controls a few times and ran it further into the switch and the piece of plastic I used as a stop block would pop off.

My first automated project will be 2 circuit boards for the stepper box and control panel. I happened to be at the hardware store that sold Dremel bits. They didn't have a certain engraving point I wanted but I did pick up a cutting bit that looks to have like a 45 degree point on it. I mounted it in the mill and mounted a scrap PCB on the table. I brought the spindle down and nearly or lightly ran it into the hard stop. This was my first operation with the "power feed" or manual control. So, uh, yeah, I need limit switches on this thing.

I made a very light cut, maybe 6-8 mils deep. I cut a trace in two with a pretty thin line. It looks like it will do the job of cutting my PCBs just fine.