As others have done before me, I decided to test the Cartesian system by turning it into an over engineered pen plotter. I used the refill out of a Fisher Space Pen which apparently "is the most advanced writing instrument in the world". If so, when controlled to a precision of 6 μm, it must make HydraRaptor the most advanced writing machine in the world! I can't think what I would use it for apart from forging signatures.
I suspended the pen from the z-axis on a random bit of aluminium using a lump of PolyMorph. This made a perfect mount because I pushed the pen through while it was still molten. There was no play, but if the z-axis were to overshoot, it would just pop the pen out rather than ram it into the table.
I ran a simple program which went from the origin to one edge, around three sides and back to the origin again. It then repeated this, stepping in by 0.2 mm each time. I soon realised the line width was bigger than 0.2mm so I stopped it and ran it again with a step size of 0.4 mm. This left a gap between the lines and produced the pattern below.
Here is the top left corner magnified.
You can see the corners are pretty good considering the table was travelling about 4 cm per second with no acceleration, deceleration or pause as it turned. At this resolution the graininess of the paper is the most significant distortion.
The paper was stuck to the metal top of the XY table with masking tape. I set the height of the pen by stepping it down until it just started to leave a mark on the paper and then one step more. The paper was about 0.1 mm thick so that meant the pen was pressed about half way into it. As you can see from the first picture the pen never left the paper so the table must be pretty flat and its movement true. Here is the machine in action :-
The results look promising, time to have a go at milling next.