X axis varies

I have an 1000mm machine. I have been noticing the X axis moving. The machine does not seem to be losing steps. Also if I run a 20 minute program on the left hand (closest to the power) it will be off by 1/8" or better. If I run the same program to the far right. It is only off 1/32-1/16" needless to say, this is very frustrating. Wheel tension has been checked, as well as belt tension.

Have you calibrated your machine to improve movement accuracy?
Calibrate

http://www.designsbyphil.com/cnc-help.html

Have you test carved a square and verify the machine carve squarly?
Also check your left Y-axis stepper motor / pulley to ensure the stepper have power / dont have a loose pulley etc. Your symptom indicate its lagging behind.

I have calibrated the machine, I cannot understand why there is a variance from the left side to the right side.

Then possibly you have some backlash? Have you checked for that? Not sure there is anything else that comes to mind to explain your findings.

Define backlash for me in this situation, not sure what to check or measure.

Backlash can be from loose belts, or from too tight vwheels that create axis “sticking.”
Correction for the belts is obvious. For the vwheels, they should be tight enough they will not slip/turn with one finger, but will with one finger and thumb.

To measure backlash:
1 - home machine, then jog a short distance from home and set axis zero.
2 - set the zero point of a scale below the bit.
3 - jog a given distance the scale can measure without being moved.
4 - return the axis back to the zero point of the scale in one continuous move.
The distance off of the scale zero is the amount of axis backlash.

This video explains backlash visually:

One possible reason can be the pulley is slipping a little on the stepper shaft on one side, or one of the Y-steppers is not receiving power and being dragged behind.

Aaron is it the X (left to right travel) that varies from one side to the other?
If that is the case I suspect the belt is damaged or won out on the side that is having errors.
Possible cause would be debris build up and the fact that most work is done in the lower left quadrant.