Dan,

how the coordinate system on a XY table actually works

Depends on how you have your machine setup. You can set your 0,0,0 anywhere you want.
Have a read of the Mach Mill manual.

software produces the g-code with coordinates

In general the software will look at the dxf, it contains the location of all the entities which you have
 drawn and extracts that data on import. Best example is to save a file as a ver 12 DXF and look at the
file with Notepad since it's a text file. You can't do that with later dxf versions and they will contain additional
info on what was contained in dxf ie; the third axis locations etc. In general you now must define what you
want to do with each entity that was drawn, no software is a "mindreader".

will the circle at the end actually be 10cm diameter

Well it depends if the program used outputs the file as vectors based on precise data or as vectors
based on screen pixels/units, whatever. You want to use a vector based CAD drawing program and
 will remark that not all programs generate dxf's exactly the same and some are just outright terrible at it.

 [qoute]how do you calculate how many steps per mm the machine is[/quote]
Try the calculator in Members Docs or use the axis calibrator in Mach to check.
http://www.machsupport.com/forum/index.php/topic,16315.msg110286.html#msg110286

Can you post your DXF file of what you have drawn?

RICH

Hi Rich, thanks for the info.

I tried the text feature in Lazycam, and that comes out flawlessly, so I've come to the decision something is going wrong between the DXF and gcode conversion.

Here is the DXF: http://dl.dropbox.com/u/203420/moreshap.dxf

Cheers,
Dan

Just thought of something. The DXF export plugin for Sketchup has multiple export options. They are polyface mesh, polylines, triangular mesh, lines, and stl.

Are any of these better than others? I've been using polylines.

Post a screenshot from Mach3. That will tell you if the machine is missing steps. I'm pretty sure that's what you're seeing.
You can also post the g-code and I'll verify if it's correct.

Technically, there's nothing wrong with the .dxf file. However, it's not perfect, at least for CNC use.

One all the closed shapes are 3D polylines. Not all CAM programs will support those entities. Note that they are not draw in 3D. They're just drawn with a linetype that's capable of being drawn in 3D.

Two, all the shapes are closed, which means the edges between closed shapes (just about all) get cut twice.
I'm not a big fan of using Sketchup as a 2D CAD program, but I know some people do and it works fine for them.

Here's a shot of the g-code I created from your .dxf in Mach3.

Hi, would it be possible for you to post the gcode you created? That way I can try it on my machine and see if it's just the code, or my actual machine. I'm having a hard time thinking it would be my machine, as I can create some text in lazycam and it'll come out perfectly on the machine, jut seems that anything I bring in from a DXF doesn't work properly.

I did this using all the exact same settings:


What makes me thing it's not skipping steps or slipping the belt, is it can draw a misaligned line, but then it can go right to the corner of the square and draw the diagonal line. If it was skipping steps, the diagonal line wouldn't line up with the corner perfectly.

Cheers,
Dan

Dan,
Have a look at the attached pic as it shows what is being exported based on your drawing. It really leaves something to be deisred and is no wonder that LC or some other porgrams will be confused. I strongly suggest you have a read of the LC Manual.
RICH

Dan,
I used your dxf and LC to generate gcode. I converted the gcode and backplotted it back to CAD. Red lines are rapids and blue is the pathing.
Nothing wrong with the gcode as you can see from the pic. 
Should machine just fine.
RICH

Here you go.

You might have a mechanical issue, like a loose coupling somewhere.

Hmm, OK.

This is why I am doubting it's a mechanical problem.

This is the image drawn normally:



And this is the image flipped 90 degrees in lazycam:



How, notice how the outer square is perfect? Parts of that outer square are drawn after the glitches.

You'll also notice that the diagonal lines meet up in the corners of the outer square perfectly. 3 of these 4 diagonal lines are drawn AFTER the glitch in the corner of the box, yet they are still where they should be.

My theory is, if the steppers were skipping steps, or the belt was slipping, wouldn't the outer square also be effected, and the diagonal lines wouldn't match up either?

Does anyone have some different software they know works with their machine, that they could do up a simple DXF like this for me? Just so I can eliminate some things?

Cheers,
Dan

The outer square is NOT perfect. My eyes are going bad, but not that bad.

Reduce your X and Y acceleration by half, and see if it's better.