Cutting circles, is it possible to change to quadrants?

Say I’ve got a DXF with a load of circles in it, is it possible via the sheetcam post processor to change the gcode from a circle motion (1 single G2 or G3 movement) to 2 or 4 gcode movements

This may be me here, but uccnc seems to have a problem (quirk) about complete circular motion.

Basically it can and does resynchronize coordinates which means that it appears to skip complete circles.

I say appears because actually what happens is after resynchronizing coordinates the distance between the start and the end point of the circle is so small it does not cut the complete circle.

To explain this in more detail, say you have a machine which has axis settings of 100 steps per unit,bso the smallest step size is 0.01 units.

Say you have the following gcode
G1 X1.015
G4 P5
G3 X1.015 Y0 I5 J-5

Uccnc will resyn the coordinates when it sees certain G or any macro code.
So, because the maximum resolution of my machine is 0.01 units, when I tell the machine G1 X1.015 it will move to the actual position X1.02 (as it will round up or down to the next physical step), it will buffer the 0.005 as that physical position does not exist to the machine)

Then it will resync the coordinates when it sees the G4 (in this case it may be many other non motion G or M codes too) which means when it sees the next line the distance between the CCW move G3 is too small (physical location X0.02, target destination X0.015) because it is not a physical step uccnc will skip the circular motion.

The recommended “fix” or “workaround” is :

  1. split the circles in 2 or 4 in gcode via post processor {apparently from cncdrive this is possible in sheetcam… I’ve written my own post processors and fiddled with rapid, linear and arc motion in the post processor and I can’t see how to do this easily (I can see a complex way via conditional statement if the arc destination coordinates are the same as the current coordinates, but was more wondering am I missing a simple setting)
  2. don’t use g or m codes before arc motion where resync’ing coordinates can occur

My solution I thought was a bit simpler… Change the post processor number mask / digits from default sheetcam 4 decimal places to 2… That way sheetcam will never output coordinates that exceed the machine precision of 2 decimal places… Seems cncdrive (developers over uccnc did not see this as the solution)

The discussion thread is here
http://www.forum.cncdrive.com/viewtopic.php?f=3&t=1530&sid=e46b7fa225d0c6ef7a61013c6350a1c4


…open to suggestions for simplistic fixes… Their solution #2 was not viable in my opinion…

Sorry for the long post

Thanks

Rob

Hi Rob.
In Sheetcam there is the option to set an “Overcut”, which is a short distance you can cut beyond the start point at the end of the cut. This is usually used to account for the slight lag of the plasma flame at the bottom of the cut, but would also allow you to cut past that small gap you are talking about. The option is located in the Jet Cutting dialogue box.
Steve

Thanks for that Steve, ive used overcut before but that wont work for this problem, as it will just cut the overcut bit and not the circle… also its hit and miss when it can happen

I found this on the old SheetCam forum.
https://www.forum.sheetcam.com/viewtopic.php?t=4213#p12980
I tested it on a circle and it divides it into 4 separate quadrant moves.

Thanks very much, just exactly what i needed

Rob

Cool. Glad it worked.