The M2 is a club member's (Weelic's) commercially-bought printer that's living in the lab. It is currently the fastest printer we have, at least in part because it is the only one that has been methodically pushed to its limits.
Z-axis Leadscrew Edit
The print bed is 8" on the X axis by 10" on the Y, and there is about 9" of Z travel. The X and Y axes are belt-driven, while the Z axis is driven by a leadscrew. Unlike the other printers (except the Rostock) which use standard threaded rod with a lead (amount of linear advancement per revolution) of about 1mm, this screw has a a lead of about 10mm. This has two main consequences: 1) the Z can move much faster, and 2) backlash is amplified. This means when the Z reverses directions, things may not be positioned accurately. It is very important for print quality on this printer that lifting the Z on retraction is turned off (in Slic3r).
Another consequence of the high lead of the screw is that it can be backdriven (and in fact, the weight of the bed is sufficient to do this when power is not supplied to the stepper). After a couple minutes of inactivity, the steppers will be disabled, and the bed will sink. Sometimes it takes the bed longer than that to heat, so for this printer, homing is done after the bed heats. The time until stepper shutoff can be changed in the firmware.
Z-axis positioning accuracy may also be a limiting factor (along with extrusion consistency) for doing prints with very small layer heights. It does well at 0.08mm; at 0.04mm the layers become a bit wavy/bumpy, indicating either extrusion consistency or Z-positioning accuracy problems (note that 0.04mm is only 16 steps (1.8 degrees of leadscrew rotation!), and these test prints were spiral vase, which might make this worse).
Bed Leveling Edit
Bed level can be adjusted with three screws on the underside of the bed. Since they are upside down and are set up a bit differently from the other printers, figuring out which way to turn them is sometimes confusing. However, since they are spring-loaded and there are three of them, leveling the bed is usually fairly painless.
If the bed is uniformly too high or too low, the Z-axis limit switch (found at the top of the back smooth rod) can be slid up and down on the rod. It is usually easier to do fine adjustments by turning the three leveling screws the same amount, but if those are at the end of their travel or a large adjustment is needed, slide the switch to do a coarse adjustment.
The stick that pushes on the switch used to be a bolt that could screw up and down to achieve this adjustment. This was implemented about as poorly as it could be done: the hole was not tapped; instead there were nuts on either side of the plate. The screw had a flat hex head, which made things more annoying as well. The worst part was it was physically impossible to get a wrench in a position that could tighten the nuts, and hand tightening them was usually insufficient. The new system is much superior.
Speed Testing Edit
The M2 has been pushed to its failure point in terms of print speed. It can successfully print perimeters and solid (rectilinear) infill at about 165 mm/sec. Much faster than this and small bumps tend to get pulled up or made worse on subsequent layers rather than getting smoothed down like they do at lower speeds. I'm not sure why this happens.
Hexagonal infill speeds are limited by acceleration and jerk settings rather than speed, since it is composed of many short segments with direction changes between them. Acceleration has also been greatly increased on this printer, so it prints hexagons quite quickly, but the acceleration (or perhaps the jerk) is the limiting factor.
Looking to increase infill speeds, I tried (non-solid) rectilinear infill. At first the prints came out very weak, because I was using a low extrusion width, and since it prints successive layers at 90 degree angles to each other, the only contact points are at the intersections (and to a limited extent the infill 2 layers below). These parts were easy to destroy. Increasing extrusion width (to about 1.0 to 1.2mm) helped with strength drastically. This was able to print for simple shapes at about 150mm/sec; however with more complicated shapes, where the infill was done in multiple sections with retractions between, extrusion problems occurred (not extruding for a while at the start of a new section).
Another limiting factor is the force required to push the molten filament through the nozzle - the high extrusion width and high speed of this type of infill requires more plastic per second to be pushed through than any other operation (perimeters, hexagons, etc.). The failure mode here is the drive gear slipping on the filament and grinding a notch in it. Once this has happened, extrusion will stop. Increasing the temperature (I went up to 220C) makes the plastic more liquid and easier to push through, and slightly higher speeds could be achieved with the higher temps.
Some people claim to be able to print faster in spiral vase mode (single wall, no infill, continuously lifting Z instead of discrete layers). On this printer, the spiral vase speed limit seems to be about the same as the solid infill limit of 165mm/sec. Beyond this point what tends to happen is the