RepRap: Blog

Got it down to 1 μm in 270μm!

As per previous testing the probe wasn't staying at a consistent height, but the change seemed to be linear. So I tried propping up one side of the slide by sticking layers of that well-known precision shim material, duct tape. Yes, that's right. If it's good enough for bodging Apollo 13, it's good enough for me.

The tape was placed on the edge of the underside of the slide in stacked ~6mm strips. Each strip is approximately 0.35mm thick. The slide was covered with burnished ~10μm kitchen foil, burnished flat, adhered with UV resin that was pressed before curing. I used "Probe Zero" - the robust hypodermic tip. The results:

The Z axis values are reversed because I'm lazy and couldn't be bothered typing all the minus signs. The red and blue lines here show what happens with 3 layers of tape. The lines cross because I did the test with the tape on the -X and +X sides. As expected, the slope is fairly similar but reversed.

The yellow line is with one layer of tape on -X, and the green two layers on -X. As can be seen, one is too little (1:127), two is too much (1:128) because the slope changes somewhere between the two which would put the elevation needed at ~0.5mm. The previous round of testing indicated an error in the order of 75:1. I was unable to split a length of duct tape, but I strongly suspect one and a half layers would have been just about perfect. Total measurement error appears to be about 2μm judging from the delta between readings.

That represents roughly 13μm height gain/loss per millimetre of travel, which is probably enough for resin deposition of small objects. This only tests the X axis, but we know there is a deviation on the Y axis as well which is yet to be tested for.

My conclusion here was that designing a Stage with some kind of levelling in would take out a lot of the height variations and should probably be done before further resin tests to avoid probe damage. This will also facilitate Y testing. So another simple bodge: Stick split washers under the Stage mounting screws:

The maroon line here is the end result of the fiddling with the mounting screws after five tweaking sessions. Range of motion was 2400μm (2.4mm) and the final deviation was 8.7μm. This equates to the height being 1μm out in 277μm of movement on the X axis, so good enough for me to make things about 500μm (0.5mm) across. That's the approximate equivalent of the width of a human hair over an inch in old money.

Now the Y axis...