LinuxCNC Wood Router cuts get larger as measurements change

Hello. I am new to this forum and CNC in general. Background is IT and Networks.
We recently had a Windows 98 PC die along with some old software and I built a newer PC with Debian Wheezy and LinuxCNC.
We got the machine working and dialed in with step wizard.

The issue we are seeing is as follows:

4" x 4" is good as it was during testing.
Russ began to cut out new tests and while checking accuracy noticed the following.

12" x 12"
X-axis is long by 1/16"
Y-axis is long by 1/8"

22" x 22"
X-axis is long by 1/16"
Y-axis is long by 1/4"

While cutting a 20 1/2" circle, it comes out slightly oval.
X-axis is fine
Y-axis is long by 1/4"

I apologize for posting this if it was answered. I cannot find the right words to search. I've tried searches like item cuts larger as measurements increase, etc. My newness probably has my vocabulary limited to search for answers...
If I should post this in a specific, let me know and it shall be done.
Being in IT I often look for help or offer help via the forums and generally know proper etiquette.

Thank you to anyone willing to help a newbie.

Keith Budurka

If you were missing steps it would be short so it must be a configuration issue. In your ini file under each axis, there is a value called STEP_SCALE which is the steps per device unit (eg. inches or mm).

I had one axis I could never calibrate perfectly with maths so I had to dial it in by trial and error. From your data, how many steps are you out for a given distance? Divide that by the distance travelled and change the value. Then try again. It won't take long to converge on an an accurate setting

how did you calibrate your machine initially? My guess is that your axis have a large amount of backlash (big issue that needs addressed.) and you calibrated using the 4" moves. (rather than calculating from screw/rack/gear ratios.) Then on long moves the backlash error becomes less significant and shows the error in your calculations. (but this is only a guess.)

I have added my INI and HAL. name is 212.
Thank you for the reply.

I know we are controlling the router. Just the 3 axis.
The PC connects to a "controller"? via Parallel cable.
The box is an Anaheim Automation - Model DPK65004XDTI from 5/1995

The motor on the X-Axis says:
Applied Motion Products, Inc.
Synchronous Stepping Motor.
Possibly a 4034-338 at 200 steps per rev.
I will attach a photo

You asked for:
Also info on the deives, motors, pulley or leadscrew, etc.
I added info on stepper motor. I don't know what pulley or leadscrew is or where to find the info.

Ah, old Digital Tool machine, We have a few of those running Linuxcnc. If you set up the parallel ports for X-mode you can plug the cables directly to the the original plugs on the DPK65004XDTI without any rewiring. I could supply one of our Configs as an example. On our machines though I use a 3rd parallel port for other inputs and outputs such as e-stop, probe, touch-plate, VFD control... If you want to rewire the connection to the DPK65004XDTI you can control all of the step/dir signals and all the limits using a single parallel port set to x-mode, (I've done that on one of our machines.)

Be sure to check the rack and pinion for proper engagement. If the pinion gears are badly worn or aren't properly meshing with the rack, it will result in lots of backlash and or not smooth motion.