<update 18th Feb - Shane Mans suggestion of using photopaper works very well, my y-axis galvo (x-axis still ok-ish) has become very unstable, and I was able to tune it in under 1/2 hour, instead of 4 days! although of course the first time documented underneath involved a lot of learning. See my 18th Feb update below for details on a much faster way>
You may have seen my other posts complaining about the Form1 dimensional error. My first printer was out in the X-axis by over 3% compression, and its replacement (promptly and professionally handled by Formlabs support - thankyou) - was also inaccurate, by about 1.5%, again in the X-axis but expansion this time. See the first picture below of a simple 80mm “slice” test part, aligned exactly with the X-axis (the peel direction). Out of the box, ObtainableLamb printed my 80mm test part at 81.16mm when aligned with the X-axis.
Formlabs initial response to this (a different 30mm test part out by similar percentage) was; “the measurements you’ve provided are within our expected range for the Form 1’s accuracy (we’re working on improving this and testing to determine official tolerances)”.
This is simply not good enough in my view - especially when the machine was so heavily promoted for engineering and prototyping, with showcase prints of functional nuts and bolts, planetary gearboxes, and the gyrocube being proudly publicised.
Given that it seemed Formlabs would not fix my inaccurate machine - I wondered if I could tune the galvanometers myself. Having disassembled my first Form1 several times in trying to track down other issues, I had noticed a “trimpot” (finely adjustable potentiometer, a variable resistor driven by a worm gear) on each of the Galvo break-out PCBs. It seemed to me that this could only be for adjusting the deflection of the galvos.
I asked Formlabs for confirmation that the trimpots were used to tune the galvos - and they said they could not confirm this, and that I would void my warranty if I tried adjusting them. Well the warranty’s not worth much anyway since it’s so short, mine may have already run out, and the machines utility is considerably lessened to me when it’s out by 1.5% - so I went ahead and started adjusting the X-galvo trimpot, printing my 80mm test part in between each adjustment.
It’s taken about 4 days continuous printing - partly because it turns out ObtainableLamb had another issue out of the box; the build platform clamp was loose - making it wobbly when locked on and I didn’t notice this until yesterday. More on that in another post - anyway - it worked. My test prints are now consistently accurate to a degree that’s useful for my purposes. See second picture of 80mm test part.
The main reason it took so many test prints (several dozen) though, is that the trimpot behaviour is not consistent or linear over it’s range. You turn it clockwise to increase the galvo deflection globally, that is - parts will be printed larger - and anti-clockwise for the reverse. Or; “righty mighty, lefty lessy”. However trimpot sensitivity is not linear, and the inflection point (where parts go from being larger than spec, to smaller than spec and visa-versa) is variable.
During adjustment, up until you reach the inflection point - the trimpot sensitivity is low, a full half turn will not yield much change in deflection/dimensionality. Once you get near and hit the inflection point though, it becomes ultra-sensitive, even a 20 or perhaps 30 degrees adjustment can change the deflection/dimensionality from being +1.5% to being -1.5%.
Moreover, the inflection point is not fixed, that is - it’s not always in the same “dial” position on the trimpot. It seems to shift in the reverse direction about 1/2 turn each time you hit it. Let’s say you hit the inflection point, but overshoot, e.g. a 30 deg counter-clockwise adjustment when you needed 15 degrees. Your part was printing at 101.5% in size, and now it’s printing at 98.5% in size - so you turn the trimpot clockwise to increase galvo deflection, but now the inflection point has shifted, so you have to go back about a full half turn or 180 degrees clockwise.
It was very confusing, and I’m sure I’ve made it more so in trying to explain it. Suffice to say, if you want (or need) to adjust the accuracy of your Form1, you can - but you will void your warranty, and it may take you a lot of test print + adjustment cycles to get it right.
Of course, the behaviour I’ve described is purely from empirical observation - not from any understanding of electronics. That’s empirical observation of a sample with a size of one, so your trimpots may behave differently. I think it’s safe to say though, that they are all “righty mighty, lefty lessy”.
Good luck if you try this!