Category: Science

Knowing the number of motor steps required to move an axis by 1 mm, the next step is to figure out how fast each axis can possibly move, given the restrictions of the Marlin firmware driving the motors.

Dan Newman pointed out that Marlin runs with a maximum 10 kHz interrupt rate, with up to four steps issued per interrupt. The constant controlling (or at least defining) that is in Configuration_adv.h (with a comment that seems irrelevant to the M2’s setup):

#define MAX_STEP_FREQUENCY 40000       // Max step frequency for Ultimaker (5000 pps / half step)

Below the 10 kHz rate, the step interrupt occurs whenever the next step must happen, so it does not have a constant frequency. Above 10 kHz, the steps (seem to) emerge in bursts, so there’s likely a good bit of jitter that I should measure. In any event, there’s an obvious loss of resolution at high speeds, which is a problem common to all variable-frequency pulse generators that’s worse for relatively low-frequency software generators used in high-speed applications.

In any event, these numbers show the absolute maximum possible speed for each axis:

• X and Y axes: 450 mm/s = (40 k step/s) / (88.89 step/mm)
• Z axis: 100 mm/s = (40 k step/s) / (400 step/mm)
• Extruder: 94.3 mm/s = (40 k step/s) / (424.4 step/mm)

Due to the low torque available from the Z axis motor, the actual maximum speed seems to be around 30 mm/s = 1800 mm/min. After I replace the motor, I’ll measure the actual performance and see what’s reasonable.

One could quibble about the extruder, as the extrusion multiplier affects the final speed. The extruded thread squirts out at a pretty good clip if the motor turns at full speed:

2350 mm/s = (1.75 mm)2 / (0.35 mm)2 * 94 mm/s

It’s not clear the hot end can melt plastic fast enough to keep up with that pace more than momentarily, but I haven’t measured that yet.

However, if the X and Y axes both move at 450 mm/s, then the nozzle moves at 640 mm/s = √2 * 450 mm/s relative to the platform, so the maximum extruder speed while printing will be roughly:

26 mm/s = (640 mm/s) * (0.35 mm)2 / (1.75 mm)2

That assumes the printed thread has the same cross-section area as the nozzle, which is roughly true for my choice of output:

• Thread: 0.1 mm² = 0.4 mm wide * 0.25 mm thick
• Nozzle: 0.096 mm² = pi * (0.35 mm)² / 4

If you bake the extrusion multiplier into the step/mm value, then compute the maximum speed without applying the same multiplier in slic3r, the plastic should come out of the nozzle at the same speed.

So the speed setup looks like this:

#define DEFAULT_MAX_FEEDRATE          {450, 450, 30, 94}    // (mm/sec)

After that exchange, I dabbed some Oatey PVC Purple Primer/Cleaner on two PLA slabs:

The active ingredient involved in PLA bonding is tetrahydrofuran, which makes up anywhere from 10 to 40% of the primer (the MSDS gives a broad range). The primer immediately marred the PLA surface, which is exactly what you want in a solvent adhesive.

After an overnight clamping, I couldn’t pull or peel that joint apart: the two slabs had become one. That’s unlike the paint stripper test that didn’t bond well at all. Good enough for me.

Obviously, you’d prefer Clear Primer for natural PLA, but Purple Primer is what I had on hand.

Given that this stuff has no solid content, I think it’s more suitable as a PLA adhesive that the thicker PVC Cement. However, clear cement would be less likely to run along the thread seams and ruin the surface finish outside the joint than water-thin primer.

Tradeoffs, tradeoffs… but now I can build things from PLA subassemblies!

The only commonly available PLA adhesive seems to be methylene chloride, which is common only because it’s part of really nasty paint stripper that actually works; I suspect you can’t buy the pure stuff anywhere.

Anyhow, I picked a pair of flat line width test plates from the PLA scrap pile, dabbed paint stripper on each, and clamped them together overnight:

Unlike acetone on ABS, paint stripper doesn’t actually combine the parts into a single fused unit; I could peel the two plates apart with some effort:

That picture shows the results of two glue-and-peel tests, with much the same result along the top and bottom edges. Some solvent damage appears as a thin white line around the edge of the glued joint, but with some care that wouldn’t be too bad.

I think paint stripper makes an acceptable adhesive for PLA, at least for joints that aren’t subject to peeling loads. You must design an interlocking mechanical joint, perhaps filled with epoxy, to withstand peeling loads, which isn’t nearly as good as the ABS option of just fusing the parts together.

While changing to black filament, I measured the force required to pull the (natural PLA) filament through the translucent guide tube arching over the M2’s chassis from the spool to the extruder:

A strike-anywhere kitchen match (bet you can’t buy those any more!) provided more than enough heat to bend the end of the filament into a loop suitable for the pull scale:

The results:

• Tube reasonably straight: 0.5 lb = 2.2 N
• Tube arched to middle of X axis: 1 lb = 4.5 N
• Tube sharply bent to X axis nearest spool: 1.5 lb = 6.7 N

The force increases slightly while tugging filament off the spool, as the spool does not rotate freely on the printed arm jutting out from the frame, but those numbers are in the right ballpark.

The effective diameter of the extruder drive gear is about 11.5 mm, so overcoming the tube friction requires somewhere between 10 and 40 mN·m of torque. That’s applied at the one point in the whole system most likely to show the result of uneven loading, because it directly affects the pressure of the molten plastic behind the nozzle.

That’s considerable motivation to get rid of the filament guide tube…