RepRap: Blog

The Y axis failed during a probe test (yes, the Z axis has a probe fitted, kinda). After a bit of poking around, I found the Y axis drive screw was jamming. I wonder why? Let's pull it out and have a look next to a new one:

Well thars yer problem.

That is what we in technical circles call "well shagged, mate." I'll replace it and resume probing, but I think the issue here is the screw resting on a plastic bearing hole, which is dragging plastic into the thread, filling it up, and causing excessive wear. I hope the drive nut is ok. 

Edit: The Y axis has now crapped out and only has 1mm of travel. At this point I'm tempted to begin the Alpha Prototype XY stage.

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.

The Mod. 4 axis was a multiple fail. Its improvements were good, there just weren't enough. Actual frame rigidity was not good. The NEMA17 coupling was a press-fit sleeve with the drive screw bolted onto the end. Unfortunately this tended to set an angle to the screw thread and everything wobbled.

After some time I realised that I needed to hold the drive shaft in the collar flexibly. So I put a fixed nut on the drive shaft, and the motor turns the shaft. The nut hole is tapered, allowing it to be firmly grabbed, yet allowing it to move to re-centre rather than eccentricity driving the shaft.

 

The OpenFlexure stages avoid this by having a gear between the motor and the drive shaft, and that absorbs any wiggle.

The nut trap in the drive end of the pivoting arm has a lose cavity for the nut. This allows the nut to rotate partially. Tighten it, and the nut is hard to insert. The solution again is a conical nut trap. The temporary solution: Jam it in with a bit of copper wire to take up the slack.

Finally, lubrication is very important, particularly if you can't find brass nuts.

I may insert a pair of vertical flexures into the Mod.5 lifter bar to see if this will remove any residual sideways wobble and/or try making the other flexures slightly more rigid.

I may need more anti-backlash tensioning but that may need to wait for decent springs.

Assembling Mod. 3 was difficult as the exact length of the motor shaft and placement of the bearing lock nuts is crucial - and they're real swines to access. In the process, I snapped the flexure frame (see pointer).

Mod. 4 has that bit strengthened, and the bearing beam is removable to improve access to the nuts. Printing now.

In white behind the orange coupling you can just see the crossed wire Z endstop, currently held in place with a zip tie and hot glue. It'll do for now but will change that later.

Last century the rapid prototyping tool of choice was a metal child's consruction set of bolt together girders, straps, and gears etc. called Meccano. Many of the original RepRap experimental prototypes - including the first functional extruder and bed - were constructed from it. I've combined the attributes of this with 3D printing to basically use adjustable straps and brackets based on a 10mm pitch, 5mm beam height, and M3 fittings. OpenSCAD parametric library files to follow as I tease them out of the main files. As the original Meccano was in imperial units, I'm calling this Metriccano.

Here you can see the Mod. 3 prototype with a little thumbwheel on top, supported on the OpenFlexure Block Stage with Metriccano. This runs in conjunction with the existing Z stage so I was able to use the height probe to measure performance. The result: I don't trust my Block Stage calibration. However, I am getting smooth movement and an averaged mechanical advantage of approximately 4:1 - the probe moves in an arc, so it's not an easy thing to measure.

Next step is to motorise the Z drive screw and perhaps add a limit switch. The motor will go underneath the mechanism, and the thumbscrew will become the top bearing.

As to overall accuracy, it is theoretically overkill. Range of movement and permanent deformation of the flexures is the limiting factor. For example, the motor has 400 steps/rev. and an M5 threaded rod has a 0.5mm pitch. So that moves at 800 steps/mm by itself. With a 4:1 advantage that gives 3200 steps/mm or 0.3μm per step. And I can reliably quarter step the motor.

I've added a drive screw to test the Z axis movement. Yes, I know, it uses 'O' rings. I have a box full and have yet to purchase extension springs but that will happen later.

The height for the drive screw bracket is wrong, the underside of the crossbeam prints ugly, I don't like the hooks, and many other things. Also, need to do new probe tip mounts and get away fron the old design that was based on hypodermic needles. Busy, busy.

The OpenFlexure microscope is excellent, and I highly recommend it. Its axes print in one piece, all the moving parts are encased in a sleek outer shell, it uses inexpensive steppers, and pans a microscope slide around nicely.

Unfortunately, it is a microscope, not a developmental printer. Those things do not fit well with a prototype system where the parts need to be observed, fiddled with, modified, swapped out, stuck underneath, and poked in from various angles. There is also a problem with the licence in that most common 3D model web platforms don't support it and it is incompatible with the GPL.

So, I have started development of a more modular 3-axis (possibly more...) system that should be more suitable for incremental development. This is the first crack at a Z axis for the Alpha Prototype. This has a 5:1 mechanical advantage, which with a 1/4-stepped 200 step motor directly driving a 0.5mm pitch M3 screw has a theoretical resolution of 0.5/(200*4*5) 100nm - in my dreams. It is designed to have an operational range of +/- 2mm, but will likely only be used for probing in +/-1mm of that to minimise lateral motion due to curvature. To this end it has a 14mm radius on the platform lifting arms, giving an overall sideways drift over the 1mm probing range of approx 36μm or about a 1 in 28 slope.

 

The probe itself will mount on the right-hand face, which I have studded with mounting holes.

Next step is to design a manually operated anti-backlash base to test it out with a thumbscrew. It should mount on the Titch probe platform, so I can compare it directly with the Z motion on the OpenFlexure Block Stage.

With the same resolution as before (50μm) I scanned the smallest details I could find on the NZ 10c coin: the letters 'IRB' found just above the year numbers and so small most people don't know they are there. I misplaced the probe a bit and only got the first part of the 'I' but the results are clear enough. It's reaching the limits of the robust probe tip:

This is what the original looks like, shown on the stage with my horribly clunky probe below it. The letters approximately protrude 40μm and are 0.7mm high, or for those used to colonial measurements, just less than half the width of a piece of printer filament:

With the previous scan of the 'A', the xy slope adjustment was (0.2,-0.06) and for the 'IRB' scan, (-0.018,0.033) so it is fairly clear that the slope changes drastically depending on where on the stage the probe is. Unfortunately I did a demo to my brother-in-law just before making the scan and lost my zero, so I don't know what point the probe started from.

During the demo scanning process, the coiled copper grounding probe lost its spring and would not maintain contact, so I affixed a little PET plastic arm with a nut to ensure downward pressure. I kept nudging the coin around while putting the ground probe on, and I'm afraid some duct tape was deployed for stabilization...

Here's the little perisher. I know a weak point when I see it so I purchased 3 spares. Now 2 spares.

Fitting the new one requires a 3D printed tool and a lot of brute force. They get hung up inside 2 out of 3 times, requiring fishing around with tweezers before trying again. Still, better case than fixing the Y limit switch.

UPDATE: I thought the Y-axis limit switch (actually a couple of sprung, crossed wires) had failed. This is, of course, right in the guts of the thing and I'd have to unscrew every single component of the base and remove all the 'O' rings that are under tension to get at it.

The update, once I'd taken the Y motor off, is that the switch is fine but the 'O' ring in the Block Stage has failed. These are starting to be a royal pain, and when I design my own stage they will be the first thing to go! But, once I unbolt the stage, I should be able to replace a broken 'O' ring without having to dismantle it. Of course, an attachment point could have broken off, in which case I'll have to print a whole new stage and do all that tedious disassembly/assembly.

Also, the probe with the jumper pin sticking out the side won't fit in the nice storage pot I made for it. Damn. Bigger storage pot needed. Fortunately, this is a very tough probe that I use for test-fitting things so it can rattle around until I'm done.