I'm new to the CNC world, but I have recently completed a CNC with a 2'x4' table.  I wrote a simple G-Code to carve a circle to see how accurate the different axis' were.  The board was placed at X-23" and Y-8".  When I ran the program, instead of carving a circle of 4" in diameter it looked like it was trying to do a 20" diameter or larger circle.  I thought maybe there was an issue with my drives, but when I looked at the amount of completed path on Mach3 it maybe and traveled 10 degrees around the circle.  I was wondering of someone could look at the G-Code and let me know what I did wrong:
g0 x20.875 y8.0 z-1.4
g1 z-1.5470f2.0
f5.0
g3 x20.875y8.0 i2.125j0 z-1.0695
g0 x20.875y8.0
g1 z-1.6720f2.0
f5.0
g3 x20.875y8.0 i2.125j0 z-1.7345
g0x20.875y8.0
g1 z-1.7970f2.0
f5.0
g3 x20.875y8.0 i2.125j0 z1.20
g0z.1
g0x0y0

Try adding G91.1 to the start.

have you checked your axis Setup by MDI call's

for example

-jog x-axis to the left side
-Zero x axis
-mark the pysical Position somewhere
-MDI Input G1X10F10
-measure the driven distance

Yes,  When a placed the piece to route on the table and anchored it down.  I manually jogged the spindle over to the center of piece of wood.  I then measured from the home position to center of the piece and these were the same dimensions as the DRO's on Mach3 were showing.  These dimensions were also resonable (23" on the X-axis and 10" on the Y-Axis).  The bed of my router is 48"x24" and the board to be routed was approximately in the center.  Prior to this I had calibrated all of the motors, I was going to do the circle so I could fine tune the calibration.

Hi,
Mach has two interpretations of I,J; incremental and absolute. Follow ger21's advice.

Craig

10-4 thanks, what is the difference between the two?

Hi,
look at the description of g2/g3

Craig

I/J Mode: You can set the interpretation to be placed on I & J in arc moves. This is provided for compatibility
with different CAM post-processors and to emulate other machine controllers. In Inc IJ
mode, I and J (the center point) are interpreted as relative to the starting point of a center format arc.
This is compatible with NIST EMC. In Absolute IJ mode, I and J are the coordinates of the center in
the current coordinate system (i.e. after application of work, tool and G92 offsets). If circles always
fail to display or to cut properly (especially obvious by them being too big if they are far from the origin),
then the IJ mode is not compatible with your part program. An error in this setting is the most
frequent cause of questions from users when trying to cut circles.

from Mach3 Manual capter 5-36