Category: Machine Shop

Companion Cubes^* being nice handouts, at least in certain circles, I printed three arrays of nine. This being an opportunity for Science, they’re printed at 25, 40, and 50 mm/s, all with 100 mm/s moves.

They’re scaled to about 16 mm on a side, making them about as small as anything you’d want to print. With a 0.66 mm extrusion width (0.33 mm thick and 2.0 w/t, they’re only 24 threads wide, which doesn’t allow for much detail. Here’s the top of one printed at 25 mm/s:

And at 50 mm/s:

Obviously, the faster cube turned out uglier. The 40 mm/s cubes look a whole lot like the 50 mm/s ones, which means the best printing quality occurs well below that.

The front at 25 mm/s:

And at 50 mm/s:

The odd shadows on either side of the central bump become more obvious at an oblique angle. At 25 mm/s:

And at 50 mm/s:

This confirms a suspicion I’ve had for a while that the XY stages don’t have the positioning accuracy required for this level of detail when printing at any reasonable speed: they tend to oscillate slightly after changing position. I think acceleration limiting would help this, but I have no way to prove that. I know a more rigid mechanical assembly would help a lot, but there’s no way to get there from here.

Obviously, slowing down will improve the overall quality. I’m printing at high speeds to see what goes wrong; I have no objection to reducing the speed to a crawl in order to wring the best quality out of the hardware.

Moving at 100 mm/s, however, definitely reduces the opportunity for ooze. Even with Reversal, sometimes the nozzle dribbles a bit and very fast moves tend to produce a hair-fine thread that’s easily removed from the finished object. In this case, all three sets of nine cubes printed with no threads at all.

These were printed with Reversal set to 25 rev/min, 110 ms in and out, and no early motion. That eliminates suck-out as the extruder reverses while the nozzle approaches the end of the thread, but still leaves a zit at the endpoint where the nozzle moves to the start of the next thread:

The zits appear identical with faster print speeds, so they don’t depend on anything other than Reversal.

The other two sides of each cube look about the same.

For completeness, here’s the bottom at 50 mm/s:

Comparing that with the top suggests that Skeinforge used bridging fill, rather than surface fill.

(*) They’re actually Weighted Storage Cubes, as they lack the hearts found on Companion Cubes. My Shop Assistant asserts that if you cuddle them enough, they’ll grow hearts…

So I’d installed an activated charcoal air filter to cut down the hot-ABS stink and it’s been doing fine. Of late, however, I’ve noticed more smell, so I figured it was time to replace the filter…

Guess I gotta vacuum the floor again this decade…

The three Skirt extrusions around those polyhole test plates had thicknesses that looked like this (in the usual units of 0.01 mm):

	28	33
24			38
	29	34

Average: 0.31 mm

	34	35
30			41
	34	37

Average: 0.35 mm

	36	37
32			40
	35	38

Average: 0.36 mm

In round numbers the extrusions all fall within ±0.05 mm of their average (for a loose definition of average, of course) and the test pieces built just fine. The layer thickness is slightly over the desired 0.33 mm, but sheesh the Z-min switch gets it close enough.

[Update: following tweaked to match reality. The motor steps are 0.005 mm, but the conclusion remains.]

However, the Z axis motor drives a four-start leadscrew that moves the Z stage and the thingomatic.xml file specifies 200 step/mm = 0.005 mm/step. As a result, the Z axis positions occur in discrete steps (no surprise there) and the first extrusion happened ten-ish motor steps higher than the other two: 0.31 mm vs 0.35-ish.

Other folks use layer thicknesses below 0.30 mm and I think they’ll start to see the effect of Z axis resolution and consistency on build quality. Putting the height adjustment on the build platform means you can set the initial nozzle height above the platform very precisely (no steps on the screw-and-spring adjusters), but the height will continue to vary due to all the usual effects. The Z-min switch can adjust the Z stage height only in integral motor steps that may not be precise enough.

That means the actual extrusion thickness will vary by ±0.005 mm depending on where the Z-min switch trips with respect to the motor steps. Because the nozzle-to-plate distance varies smoothly with temperature / time / sag / whatever, you’ll see steps of that size no matter how much you twiddle the initial platform height.

For example, if you adjust the build plate height (using the platform screws) to set the nozzle at 0.00 mm for a commanded Z=0.000, the next possible Z stage positions are 0.005 mm above and below that level: the Z-min switch cannot adjust the platform with better resolution. The possible ±0.005 mm variation represents ±2% of a 0.25 mm layer thickness (it’s 1.5% of the 0.33 mm I use).

Using a 1/16 microstepping driver (and a better Z-axis motor!) would cut the nominal resolution to 0.0025 mm, but I think there’s no way to improve the actual resolution and precision given the present mechanical design. In short: you can make the steps smaller, but that doesn’t mean the Z stage will actually pay attention.

There’s also backlash: the Z-min switch sets the height as the stage moves down and the nozzle moves down to the first layer, but the second layer thickness depends on the stage moving up by a precise distance. I now think some of the surface finish problem with the three-layer polyholes test pieces came from the second layer being a bit thinner than it should be: the Z stage didn’t really move up by the full 0.33 mm due to backlash and there’s more plastic than room to put it.

A backlash compensation setting lies buried somewhere in RepG/SF, but I don’t have any measurements to justify anything other than zero. I think the entire Z stage assembly lacks the rigidity for motion on this scale, anyway, and the XY stages flop around more than that, too.

Building a machine tool with positioning accuracy / repeatability / stability below 0.1 mm is an exceedingly non-trivial project; getting that much resolution is no big deal. The Thing-O-Matic works just fine, but smaller nozzles and thinner layers require more attention to the mechanical design. Achieving tighter tolerances with plywood and acrylic probably isn’t feasible, but anything else will drive the cost up far too much.

It’s an interesting set of tradeoffs…

FWIW, I picked 0.33 mm to get a (nearly) integral number of layers per millimeter of object height; I tend to build things using hard-metric sizes and that seemed reasonable. It’s now obvious that the actual heights will be in multiples of 0.005 mm, which doesn’t really matter at all in absolute terms. I should probably use 0.30 mm to make the answers come out easy.