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Machine Renovations – Part 2

  • Jul 01 / 2011
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Machine Renovations – Part 2

One of the most frustrating problems with our first Darwin was trying to keep the print platform properly aligned and level all the time. Worse, the polycarbonate bed had a subtle warp to it which made the center higher than the edges, causing all sorts of problems. With print layers dropping to as thin as 0.05 mm (and maybe less in the future?), it’s extremely important that the print platform is flat, level, and at a precisely known height. Otherwise, when the nozzle is aligned at one end of the print bed and moves across to the other, it could crash into the floor and damage the machine.

It’s possible to mount a distance probe on the print head like Nophead has done, and we might experiment with this down the road. That makes it unnecessary to level the bed at the start, but it’s still important to have a very flat surface.

Float glass is an ideal material for this purpose. It’s inexpensive and extremely flat, owing to the way it’s made, and RepRap experiments have shown that it has excellent adhesion properties when printing with PLA. Because the bed will be heated to prevent part warping, I originally thought that something with a low coefficient of thermal expansion would be beneficial, like borosilicate glass. But it turns out that thermal expansion actually helps with the printing process: When the glass cools down, differential contraction between the glass and the printed part makes the parts conveniently pop off the print bed.

The print surface is expected to expand about half a millimetre in each direction when it heats up to ~110 C. To keep the print bed exactly constrained and avoid thermal stress, we’ve designed a four-point kinematic coupling for it, inspired by the National Ignition Facility optics arrangement. Another nice feature of a kinematic coupling is that the entire print bed can be taken off and replaced without worrying about needing to re-adjust it. Typical kinematic couplings use a three-sphere-in-groove combination, but a square table held up at three points isn’t stable.

Four point kinematic coupling

Four point kinematic coupling (Layton, 1999)

Our approach flips this around, hanging the print bed from the top instead of lifting it up from the bottom. That way gravity provides the preload on all four points, and if the printed parts add any extra weight, it will assist rather than counter the preload. There’s two basic ways this can be done:

Rapid coupling

Some variations of the NIF coupling

Note that in the above image, the dark spheres represent attachment points connected to the plate, and the white shape represents the mounting structure. I’m not exactly sure which of these two configurations is technically superior, but the one on the left is a little bit easier to build. So that’s what I’ll go with for now, until I figure out whether there’s an advantage to doing it any of the other way(s).

To make it easy to level the print surface, it would be nice to make it screw-adjustable like a micrometer. But to have stiff, screw-adjustable mechanisms can get very expensive. To resolve this problem I’ve put together a sort of hybrid system, where the V-blocks are held by screws in slots which can be loosened to allow position adjustment with a fine screw. Then the slots are tightened again and the block is fixed in its new position. The V-blocks are all to be made on the waterjet from steel plate, and the spheres (or half-spheres) are off-the-shelf locating pins.

V slider block

With a frame made of extruded aluminum, the baseplate assembly looks like this so far:

Print plate mount

Mount for the glass printing surface