RepRap: Blog

Last century the rapid prototyping tool of choice was a metal child's consruction set of bolt together girders, straps, and gears etc. called Meccano. Many of the original RepRap experimental prototypes - including the first functional extruder and bed - were constructed from it. I've combined the attributes of this with 3D printing to basically use adjustable straps and brackets based on a 10mm pitch, 5mm beam height, and M3 fittings. OpenSCAD parametric library files to follow as I tease them out of the main files. As the original Meccano was in imperial units, I'm calling this Metriccano.

Here you can see the Mod. 3 prototype with a little thumbwheel on top, supported on the OpenFlexure Block Stage with Metriccano. This runs in conjunction with the existing Z stage so I was able to use the height probe to measure performance. The result: I don't trust my Block Stage calibration. However, I am getting smooth movement and an averaged mechanical advantage of approximately 4:1 - the probe moves in an arc, so it's not an easy thing to measure.

Next step is to motorise the Z drive screw and perhaps add a limit switch. The motor will go underneath the mechanism, and the thumbscrew will become the top bearing.

As to overall accuracy, it is theoretically overkill. Range of movement and permanent deformation of the flexures is the limiting factor. For example, the motor has 400 steps/rev. and an M5 threaded rod has a 0.5mm pitch. So that moves at 800 steps/mm by itself. With a 4:1 advantage that gives 3200 steps/mm or 0.3μm per step. And I can reliably quarter step the motor.

# posted by vik-olliver @ 9:52 PM