Frankenplotter - a little rain must fall - but then success!

So not everything was trouble-free yet for the Frankenplotter!

At some point, the old plastic clips that held the scanner's idle pulley broke.  I had to use the Emergency Stop button on the system (i.e., yank out the USB cord) when that happened.

Fortunately, no significant harm was done.  It only took a few screws and washers to tighten that down again.

The bigger problem was one that I hadn't noticed in earlier drawings.  The drawing itself was coming out wrong.  There was drift in the Y axis, but in retrospect that might have drift in both axes, but it was just more noticeable in Y because of the resolution difference.

I lubricated the system and things got a little better, but still it wasn't quite right.  I also scaled up the drawing and came up with more error, but it didn't appear to be linearly scaled error.  The two images below are drawn at different scales, and you can see that the error offsets appear to be Y axis only, and are different depending on scale.

As it turned out, I did have mechanical drift problems -- too much resistance / insufficient torque on the printer carriage -- but also had a rounding error in the code.  It was one of those "I thought I took that into account" moments, but when I converted the current x,y position tracking variables to be floating point instead of integers, it all came out better.

Here's the semi-final plot!

Success at last!

With this done, I feel pretty confident that I have the level of precision I want in X and Y axes -- at least as much as I can get from this build.  And if I move on to using lasers, I won't have all the mass of the pen up/down assembly to carry around.

Next steps:
- Figure out cubic Bezier splines so that I can send c/z commands instead of having Inkscape flatten Beziers
- Go to a laser engraving system
- Configure the system to control motor movement rate so that it doesn't move in X and Y too quickly.  If I'm doing laser engraving, I want the motor movements to go at a steady pace.  Basically, this means slowing down the Y axis movements by increasing its motor delays at the risk of having jerky motions
- Expand the plotting/engraving surface, or do what Mike suggested: cut out the base of the Epson 4180 and mount the whole assembly higher for doing laser work.
- Go to the next precision X-Y-Z stage that I got from Halted the other day (oooooooo!!!)
- Figure out some other plots to draw

A few of the things I'd love to find, if they're available anywhere, are the old vector drawings that we used to use at Nicolet Zeta for burn-in tests.  The ones that were memorable, to me, were the ones of the old WWII airplane diagrams.  I wonder what happened to the Nicolet Zeta / Nicolet Instruments Corp / AM Bruning assets.