Thursday, November 07, 2024
Alpha Prototype Z axis Mod 2
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.
Wednesday, November 06, 2024
Alpha Prototype Z axis
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.
Sunday, November 03, 2024
Imaging 0.7mm high letters
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:
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.
Saturday, November 02, 2024
Yep, broken 'O' ring
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.
Next hardware failure - NOT Y axis 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.
Monday, October 28, 2024
Correcting Probe/Bed Slope
I levelled the coin 'A' letter scan with a spreadsheet. The upshot is that as the Y movement goes positive, the Z position goes positive by 0.06 times the Y movement. When the X movement goes positive the Z height goes negative by 0.02 times the X movement. These effects are cumulative. Correcting this with the spreadsheet gives the following scan:
This can obviously be corrected preferably in the firmware but for the moment I intend to write something to pre-distort the gcode. But now I know by how much!
The dark purple patch in the top right is a visible contaminant on the coin that required additional force to either move or penetrate and so shows as a deeper contact point. If we correct this, and flip the X axis to correct the fact it is inverted, we get:
There is now a visible defect in the crossbar of the 'A', and indeed if we look closely at a micrograph we can see it:
Sunday, October 27, 2024
Touch Probe Scans
I've connected the touch probe up and written a few scripts to generate a grid of probe points and analyse the resulting data by scraping the results from the CNCjs serial console window. This is what I'm getting:
The probed data (a 30x30 grid with 50μm spacing) looks like this, and suggests there is a constant slope to positioning that I'll hopefully determine and compensate for later:
Bear in mid I'm using a crappy but robust probe 'cos I don't want to destroy any nice ones while bashing things about a lot! I've got a better image of the modified slide holder that shows the setup a bit better:
There's a fairly extensive wiki entry here that details the process and provides the scripts etc. because I bet they'll be useful for a heap of other things than calibrating the probe.
Saturday, October 26, 2024
Contact!
I've got the GRBL touch probe working. On a RAMPS board I just used the T2 thermistor input as the touch probe (default config on the Mega firmware). Ground contact for the probe is supplied by a piece of copper wire that has been curled to give it a bit of spring. I've wedged a header pin onto the probe and soldered another onto the ground contact so I can take stuff on and off easily. Not able to play with chemicals to make more conductive glass yet, so I have put a New Zealand 10c piece on it and I'm testing the probe on that. This is being done with the stock CNCjs app's probe function. I just copy the Z touch data out of its serial console - quick hack or wot?
Mapping the data with LibreOffice wasn't possible - apparently 3D Graph means something with 3D eye candy in officespeak. This has meant learning gnuplot, which takes a bit of wrapping your head around but gets there in the end and produces proper 3D maps in perspective. You'll see some later, but a 15x15 grid with 0.1mm divisions takes about an hour to probe ...