Yes there is no scaling factor applied in either axis - in fact it's very very simple, there's no transformation or translation at all, beyond the conversion of the coordinate system itself.

I'm not sure what might be afoot here. From your description, I think you're seeing a rectangular plot, where the vertical lines are parallel and vertical, and the top line is horizontal, and in the place you're expecting it, but the bottom line is 50mm lower down than you expect.

It is not unusual to get distortions, but they are always in two dimensions at once - so the top edge might be perfectly sized, but the rest flares out, or narrows down somehow.

There is a button in the controller for "draw outline box". This fires off five commands to draw a rectangle that matches the select area. I would try and draw a handful of boxes all over the surface, and make sure they all come out regular - parallel and perpendicular lines. I'll have a bit of a think.

sn

Thanks for the pics - very helpful.

Two issues that are separate I think. The first one is the geometry, and though it is about right at the top of the drawing, the panel gets more narrow towards the bottom, and that's a sign your calibration is slightly off. This kind of pattern usually indicates that your home point is higher up than the machine thinks it is. You could try moving it down a couple of centimetres and see if it improves.

When you have selected your drawing area, click "draw outline box" to draw a quick outline of that area. A lot faster than drawing the whole picture.

You may find tweaking the home point position solves your geometry problem, or just moves it. I know you've probably done all this, but it's worth reminding:

http://www.instructables.com/id/Polargraph-Drawing-Machine/step17/Back-to-the-drawing-board/

The top edge of the machine is the axis between the centres of the two motor shafts, and the machine width is the distance between the two sprockets at their closest point (on the same axis as the top edge).

The second issue is the pixel density - or lack thereof. Two things: When you hit "render pixel", check to make sure that all the pixel commands you see appear in the command queue have plausible brightness values. Square wave pixel commands are C05, and take the form:

C05,<a position>,<b position>,<pixel size>,<brightness>,END

so the black pixels will have a brightness of 0, the white pixels will have a brightness of 255. That's just as a sanity check to make sure you're actually sending the meaningful commands.

The other is to check and double-check that you are sending the "set pen width" command before each drawing. 25 is quite small as a grid size, but even a 0.8mm pen should be able to render some detail in the darkest pixels.

I've had a few reports of this kind of problem that have resisted the usual troubleshooting, I've even had them before. But I'm not clear at all about what is the cause, if it anything more than absent-mindedness (certainly in my case that's the most likely culprit).

A pen width test may help.

sn