Thing-O-Matic: Un-twisting the Build Platform

Shimmed X-axis rod
Shimmed X-axis rod

The aluminum build platform plates remain both flat and level, but the outline and test extrusions are consistently thin by 0.05 to 0.10 mm in the right rear corner and thick by about the same amount in the right front. That means the rear corner is too high and the front corner is too low, but the whole left side is flat to within my ability to measure it.

The effect is significant, because I’m laying down the first layer at 10 mm/s with a layer thickness of 0.33 mm; the first layer looks exactly like all the other layers in the object. With the middle of the plate at 0.33 mm below the nozzle, the fill can be cramped at 0.23 mm and sparse at 0.43 mm. The long-term Z-min switch repeatability seems to be no better than 0.05 mm, so when the midline goes below 0.30 mm, the higher rear corner really crowds the plastic.

Given that the left side is level front-to-back, the only way a flat plate can appear non-flat is if the X and Y axis rods aren’t quite parallel: the stage rolls or yaws as it moves.

That could indicate a bent rod, but the last time I rolled those rods on a surface plate, they’re perfectly straight. Maybe something horrible has happened, but any stress capable of bending one of those rods will wreck the printer in passing.

Alas, a static platform adjustment can’t fix a dynamic motion, but tweaking the rods to be (more) parallel could reduce the problem. I tried visualizing the possible causes and cures, then decided to stop thinking so much, just change something, then measure the results.

Why my head exploded:

  • The thickness varies from front-to-back, so the Y axis rods are non-parallel, which should affect both the left and right sides. But the left side is perfectly level and the right side is not.
  • The thickness varies from left-to-right, so the X axis rods are non-parallel, which should affect both the front and back sides. But they vary oppositely: the front tilts down to the right and the back tilts up to the right. The midline from left to right, however, is level to within my ability to measure it.

I had shimmed the rear X axis rod quite some time ago, so I decided to try a simple adjustment: move the shim from top to bottom. The picture shows the 0.4 mm shim in its original location at the top of the rod; the edge is barely visible. For lack of anything smarter, I moved the shim to the bottom of the rod to push the end upward.

Shazam! The results of a test extrusion in units of 0.01 mm:

39 35
40 35 40
33 36

[Update: Typo in the rear-left was 49, should be 39. Drat!]

Which says it’s give-or-take 0.05 mm around the middle, with the rear-left corner now a tad low; bear in mind that 0.05 mm is about the limit of my measurement ability. It’s off to a good start, anyway, and we’ll see how it fares over the next few weeks.

Methinks if you’re serious about this 3D printing thing, you need a printer with real axis alignment adjustments and enough stability to make them meaningful. Nophead uses custom code that tweaks the G-Code’s Z-axis coordinates on the fly based on an initial three-point probe, which is a wonderful solution that’s not in the cards for RepG. EMC2 could incorporate that in the kinematics module, but at the moment it does just XY leadscrew mapping. It’s simpler, albeit more expensive on a per-machine basis, to get the mechanical alignment right the first time.