Open loop accuracy compensation question...

Hi all,

I'm new to this list, and VERY new to CAM/CNC equipment - my primary
use atm is to mill out pcb's for small test circuits and such.

I currently own a MaxNC 10 open loop, and am running into a bit of a
little problem and am wondering if anyone has seen this type of
problem or might have a suggestion for compensating for this problem.

here's the situation - let's say I have the max engrave a small
circle (seems to only happen on circles, squares and rectangles and
such all appear fine). It starts to cut the circle, and traces
around, and the end doesn't meet up with the beginning.

basically - if I am using the following gcode...

N0002 G00 X0.0000 Y0.0000 Z0.1000
N0003 X0.5300 Y0.4415
N0004 G01 Z-0.0150 F2.00
N0005 X0.5312 Y0.4296 F3.00
N0006 X0.5411 Y0.4019
N0007 X0.5495 Y0.3900
N0008 X0.5690 Y0.3693
N0009 X0.5808 Y0.3623
N0010 X0.5986 Y0.3540
N0011 X0.6611
N0012 X0.6685 Y0.3570
N0013 X0.6798 Y0.3626
N0014 X0.7017 Y0.3790
N0015 X0.7155 Y0.3962
N0016 X0.7254 Y0.4183
N0017 X0.7300 Y0.4356
N0018 Y0.4529
N0019 X0.7246 Y0.4856
N0020 X0.7154 Y0.5028
N0021 X0.7076 Y0.5139
N0022 X0.6973 Y0.5254
N0023 X0.6805 Y0.5365
N0024 X0.6691 Y0.5423
N0025 X0.6566 Y0.5460
N0026 X0.6034
N0027 X0.5908 Y0.5422
N0028 X0.5742 Y0.5334
N0029 X0.5627 Y0.5253
N0030 X0.5523 Y0.5138
N0031 X0.5415 Y0.4975
N0032 X0.5356 Y0.4858
N0033 X0.5300 Y0.4634
N0034 Y0.4415
N0035 G00 Z0.1000

I have to modify N0033 and N0034....

N0033 X0.5240 Y0.4634
N0034 Y0.4380

to make the circle complete itself properly.

I had asked someone about this and they talked about adjusting the
backlash in the machine or mill software.

Unfortunately, I'm really not sure what the guy was talking about
(well, to be honest, I understood where he was coming from, but just
have no idea where to look, or even if that is proper terminology?).

I realize this may not be the proper forum for this question, but in
going over some of the archives, I've seen people talking about
accuracy and repeatability and comparing the MaxNC mills and such,
including open and closed loop operations - I was hoping someone
might have run into this and might be able to offer me a bit of
advice on solving my problem.

I know I could upgrade to a closed loop system, but it is outside my
financial means right now, and I doubt I will be able to afford it
for about 3 months - but I also read in someones post that really,
the steppers should be fairly accurate on their own, which I
interpreted more as - I really shouldn't be having this
much "offness". Then again, I may be expecting too much, but I have
seen other peoples pcb's done by mill and this should work...

I'm sorry for the long letter, I'm just confused =)

-Tony