Tuesday, November 12, 2024
Mod. 6 Z axis built
Big update. Don't ask about Mod. 5, I got my maths wrong ok? As you can see, Alpha Prototype Mod. 6 Z Axis is much shorter than previous versions. The flexure arm only has a 4:1 advantage (and has a vertical flexure in it), and I've taken the backlash mechanism out because it's a pain in the butt to assemble/disassemble repeatedly. The Z limit switch is not yet installed either. I've substituted a 6mm length of hexagonal brass M3 pillar for the drive nut which is press-fit and it'll do for now.
The screw support beam has an M3 nut in it as a bearing for the top of the screw, drilled out to 3mm. The coupling is again press-fit onto the 5mm NEMA17 shaft, and grips a hex head M3 x 50 by - you guessed - press fit. The coupling itself is a flexure design that gives a bit of play. Behold its labyrinthine internals (GPL'd STL/SCAD on request):
The motor sticks out the bottom a bit, so I've had to prop it up on some Metriccano struts to get it on the stage. I had to move it back a bit to leave room for the probe, so there's a lot of plastic cribbing. TBH still not figured out how to mount the probe to it. Next step.
Things have been moved around on the stand to make room for the Z axis and all the microscopes. I'll probably fit a manual thumbscrew to the OpenFlexure Z axis and run mine off a stepper motor. Comparisons will be entertaining.
So with the short arm, how's the accuracy? Well, promising given I've taken the backlash off. I tested it by pointing a USB microscope at the Z stage, marking the screen on a layer line, and stepping the Z axis up and down. You can certainly step through the 0.2mm layers with a lot more than 10 steps/layer until the lack of a backlash system bites you - the thread wobbles sideways as well as up/down if you don't support it, and that also plays hell with the height. But I have an idea to fix that too, which seemed to work on Mod 5.
On the plus side, dang this Z axis is fast. 20mm/min which sounds like a snail's pace but given the Z axis is only meant to have a maximum range of +/-2mm and an operating range of half that, you can completely clear the work zone in 12 seconds. The coin was probed with a safe working height of 0.15mm, so things are going to speed up a bit. When the probe is fitted I'll test that and compare.
