Category: Machine Shop

What with printing this and that and those and some other things, we ran through a pound of red filament in short order. That’s fine with me, because I could never figure out a good combination of settings to photograph the objects; the cameras I have on hand seem blind to saturated red and overexpose the daylights out of it, even with the flash turned down a stop or two.

We picked yellow as our least favorite of the remaining colors, yanked out the last few feet of red filament as a show-n-tell for presentations, rammed in the yellow, and extruded a red-to-orange-to-yellow plastic tangle over the platform.

The two filaments had somewhat different diameters:

• Red: 2.91 mm dia → 6.65 mm² area
• Yellow: 3.00 mm dia → 7.07 mm² area

The area ratio is 1.06, so the yellow filament puts 6% more plastic through the nozzle at a given extruder speed. I’d been running the red filament at 1.40 rev/min for 20 mm/s feed, so the yellow should produce the same results at 1.40/1.06 = 1.32 rev/min.

My Shop Assistant designed some beads for a cloak she’s repairing and we tried one at 1.25 rev/min (better to err on the low side, I figured) that came out thoroughly overstuffed. She deemed it usable and whisked it off for painting before I could get a picture.

I ran another one at 1.15 rev/min that was still a bit overstuffed. A pair (using Multiply) looked better at 1.10 rev/min. This is a group photo:

So I ran off two polyhole test sheets at 1.15 and 1.10 rev/min, the latter of which looked much better. At 1.10 rev/min, the holes run a consistent 0.3 mm smaller than the programmed diameter: 3% on the 9 mm hole. Close enough!

Then it’s time for some things from coasterman’s calibration set.

A thin-wall box at 1.10 rev/min and 20 mm/s came out with exactly 0.66 mm wall width, so that extrusion speed produces a physical thread that matches the Skeinforge thread parameters used to make the extrusion: 0.33 mm thickness and 2.0 w/t.

And then a perimeter test box came out perfect. After scraping off some Reversal zits, the small block fits into the recess in all possible orientations. The base is slightly larger than the top, but that just makes for a somewhat more snug fit. The block is upside-down in the recess so you can see both sides of the printed objects:

So.

The initial extrusion speed ratio based on the filament area gets you close, but it still takes a few calibration objects to achieve perfect results. What’s really nice: after that calibration, it’s spot on!

A miniature version of the Swig Cup came out reasonably well. It’s supposed to be a platform-filling monster; at only 25 mm tall, it’s cute:

There’s a bit of trouble with the overhangs at the bottom and top of the handle and the spouts at the top look a bit lumpy. What you can’t tell from that crappy picture is that the panels don’t quite seal to the cylinders: it’s not watertight.

You’ll also have trouble seeing the fine hairs connecting all the spouts. The Reversal plugin in combination with 100 mm/s moves makes the ooze hairs easily removable, although I tend to leave them in place for show-n-tells.

This worked out better than I expected: printable chainmail!

The back view may be easier on the eyes:

As the writing says, printed at 20 & 100 mm/s, 0.33 mm thickness, 0.66 mm width, and bridge speed at 1.0 to 1.3 times the usual.

I tried a few variations and got decent results with the bars set to 3 threads wide (the pix show 4 × bars). Making it fairly tall (11 × thread thickness, IIRC) helps get enough clearance below the sagging bridges between the vertical pegs. I’m amazed it works as well as it does.

Dropping to a width of 2 threads doesn’t work: the vertical pegs simply disappear from the G-Code! Turning the pegs into cylinders might help.

A pair of flush-cutting wire nippers applied to the top of the pegs along one edge allows you to lace a pair of sheets together. Apply a micro-drop of plastic cement to each cut, put a roll of duct tape on the joint overnight, and it’s all good.

My Shop Assistant has some interesting ideas for this, although I was mostly interested in its build-ability. It’s wonderful to see the printer lay down a sheet of tiny vertical pegs, five layers tall, and clear the top of every one, every time, on its way back and forth.

I love it when a plan comes together…

OK, so I printed a Stanford Bunny to haul along to the 3rd Ward Make-A-Thon:

It’s one of the few objects I’ve printed with enough interior to show the honeycomb fill pattern:

Obviously, this was before I sawed three segments off the LED ring light.

It had a bit of trouble with overhang under the ears, but I figure rabbits are soft and fluffy there anyway, so I’ll define this as a feature rather than a bug:

What’s nice: all I did was slice the STL and build the rabbit. No muss, no fuss: It. Just. Works.

Some parameters:

• 0.33 mm thickness, w/t=2.0 -> 0.66 mm width
• 50 mm/s print, 75 mm/s move
• Reversal 25 rpm, 75 ms, early action

An organic object like this eliminates any problems with waviness due to axis instability.

If you look very closely, you can see early Reversal suckouts just to the left of the zit marking the start of the thread. This was the object that prompted me to turn off early Reversal action, but I still haven’t figured out how to get rid of the zits:

(Is it just me or does that not look like part of a rabbit?)

All in all, though, this bunny marks the end of the Intense Thing-O-Matic Hackage era. The printer now works dependably, prints parts accurately, and doesn’t require a lot of babysitting. I’ll present some test pieces over the next few days that explore some variations.

While I didn’t quite jump & clap my hands, life is good…

This one didn’t work out at all and, after a few attempts, I gave up:

It turns out that the myriad gear teeth curl up slightly as they cool. At some point, one of them will snag the nozzle and, even with good steppers in full effect, yank the XY stage to a dead stop. The missed steps cause that ledge a few millimeters up from the plate and, of course, the gears don’t mesh at all.

I watched it happen and stopped the print as soon as I could, but I didn’t catch exactly which tooth did the damage. Then I extracted just the bottom few layers by importing the STL into OpenSCAD, subtracting a block from the top, exporting what’s left as another STL, then built just that chunk.

Of course, that worked perfectly:

Printing each object separately should eliminate the problem: the nozzle would remain within the outline at all times and, with a smaller part, the plastic would stay bendy.

Maybe next year…

A 3×3 array of dodecahedra printed at 50 mm/s with 100 mm/s moves:

You can clearly see the axis oscillation near the left edges.

What’s nice: the total lack of threads between the parts: snap ’em off the platform and they’re done!

After they built halfway up the top facets, I dropped a ball bearing in each one. They rattled around something fierce, but didn’t quite hop out.

Building a single dodecahedron at 20 mm/s showed that the oscillation problem really is due to the speed. More accurately, the problem is the abrupt change in velocity as the axes change direction without any deceleration / acceleration in the middle.

Here, the single line near the edge matches up with the internal fill, so it’s not an oscillation:

The small ripples come from a mechanical resonance in the geared stepper mount pumped by its full-step drive at 1.28 rev/min. I’m using a failed MBI stepper driver board that can only do full stepping, so trying 1/2 stepping won’t happen until I build a 4-axis space transformer for those tiny Pololu stepper boards.

As you’ve probably noticed, I’ve gone back to Kapton tape on the build platform, rather than the ABS I’d been using. AFAICT, the Kapton didn’t work well on my earlier attempts because I didn’t have good control over the first-layer thickness and was probably printing too fast for conditions.

The Z-min switch solves the layer thickness problem and printing at 10 to 15 mm/s for the first layer glues the thread in place. So far, so good!