Thanks Hood.

I accept what you say however the Y axis is a ball-screw and doesn't seem to have any backlash at all that I can find with the DTI? Also I found this info on the CamBan site which I am don't really understand and I have copied it below.  The circles are so far out that I would have thought they would have shown up on my backlash checks is that a fair comment? Any suggestions you have for testing/eliminating the Y axis problem would be appreciated as I cant see how (it being a ball screw) i can adjust it?

Info from CamBam

Crazy looking arcs (crop circles) or 'Radius to end of arc differes from radius to start' error when loading gcode into Mach3 or other CNC controller.

Gcode arcs specify a center point (I & J parameters), which can be absolute coordinates or relative to the first arc point. A problem will occur if CamBam and the controller are configured to use different arc center methods.

To change the arc center method in CamBam, select the Machining folder. Then change the ArcCenterMode property to Absolute or Incremental to match your controller configutation. Alternatively, change your controller software to match the CamBam setting.

For Mach3, it is sufficient to just select Mach3 in the Machining - PostProcessor property. Mach3 recognises non-standard gcodes G90.1 and G91.1 which are used to specify the arc center mode used in the gcode.

ArcCenterMode and PostProcessor properties are set per drawing. These can be set as the default for all new drawings by specifying a DrawingTemplate property in the system configuration settings.

It is not crop circles, you would see them in the toolpath view and the cut path would be a mile out from what you are expecting.
I am not sure how you tested for backlash but you may not see backlash if you have no cutting forces present when checking, that is especially true for slack couplings, flexing or moving brackets etc.

Hood