Hello everybody, relative newbie here, just getting my plasma table going but I'm having a weird problem cutting circles.

Here are some bullet points that hopefully show my problem, and what works and what doesn't:

1) I have successfully cut a couple of outline irregular shapes.
2) This problem came to light cutting bolt holes in a shaped plate.
3) The hole circle cut starts accurately and then veers off with the circle then being cut offset from the correct position. With multiple holes to cut, the error appears to be cumulative, but the outer panel shape (rectangle with rounded corners) cuts correctly but in the wrong place due to the cumulative circle errors. The final object cut, looks a bit like a cedilla (e.g as in garçon in French)
4)I can cut a rectangle accurately.
5) I can cut a rectangle rotated through 45degrees accurately (ie both x and y axis moving at the same time).
6) I can cut a hexagon accurately.
7)I cannot cut a stand alone circle....doing this displays the same behaviour as described in point 3).
My CAD package correctly draws and displays circles
9) MACH3 displays the correct shape when  I load the GCODE file (for any shape, complex or simple)
10) If I cycle through the GCODE with the scroll bar, MACH3 correctly displays the tool position superimposed on top of the predicted toolpath.
11) If I run the GCODE programme, I get the strange behavior described in point 3).

Clearly the movement instructions in my GCODE are correct because MACH3 displays the shape correctly and follows the shape with the scroll bar

Whilst my x and y steppers have encoders, there is no feedback to MACH3 so the fact that MACH3 displays the 'damaged' cut path when it runs the code, tells me that it has to be MACH3 doing something weird to my GCODE before sending it to the hardware. ie it's not backlash or sticky slides, it must be MACH that is doing this.

Does anyone recognise this behaviour please and know what the cause is?

Many thanks!

Check you have the correct circle mode set incremental or absolute, this can be set in the config or in the g-code using G91.1/G90.1

Check you have the correct circle mode set incremental or absolute, this can be set in the config or in the g-code using G91.1/G90.1

Hi Graham, many thanks for your reply....

After I had posted my question, I realised that I hadn't made it clear that my GCODE is simply a list of incremental X-Y movement instructions, I am not using the R type circle definition. As such then, I don't understand how MACH3 can move my table incrementally along a straight diagonal line but not in a circle.....it's a dumb computer for heaven's sake and it's just being told to put one foot after the other....it surely can't know whether it is a circle or not can it?

So, I have had an extremely frustrating evening trying to implement your suggestion....

I have G91 in my initial set up code (not G91.1) though I tried this and it didn't make any difference.

I've had a thoroughly frustrating evening with it and  I have to say and am now totally confused!...I really  can't get my head around how MACH3 can read the GCODE and display what I intend it to cut, and it can correctly follow the pattern when I scroll line by line through the code, yet when I set the machine off to do it, MACH3 does something different and describes something else! Surely if I tell the machine to move X1.0, Y1.0 it will move 1mm in the X and Y directions.....it is as though it is jumping from Incremental to Absolute as the code is running....but there  aren't any GCODE instructions in my code other than my X-Y movement instructions!

I THINK the final status tonight was that I think I have robustly set distance mode to incremental, yet my problem persists.

I'm away tomorrow but will get back to it asap.

Any further thoughts please?! Could there be some post-precessing occurring? If yes, where do I Look for that?

Arrgghhhh!