troubleshooting non linear motion on axis

[d954r]

arduino grbl controller
x y axis similar to the cheap langmuir type of plasma table.
using 9mm timing belt mounted to both corners/ends of the table the motor is mounted on the x/gantry the belt runs from one side of table then under a pulley on the gantry and up to the motor gear then back down to another pulley then over to the other table corner.
y axis is about 6' long
if I put a ruler on the table in y direction and start my torch tip moving over the ruler from zero toward 10" I can alter my steps per mm to correctly move from 0 to 10".
If I then move from 10 to 20" mark I'm 1/8" short. move to 30" mark I'm 1/4 short.
had found conversations that said:
check belt tension - since I've no idea the best tension I tried various tensions and same results.
check voltage - checked and supply is providing 23.9v the nema 23 stepper requires 24v.
belt can be stretched unevenly - to see if it was an uneven type of problem I switched the belt direction figuring if it was uneven then I would see the opposite problem.. but no.. same problem.
Also note that I can tell it to go to zero from any of those points and it correctly goes to zero every time. So I wouldn't think any steps are being missed.

Maybe someone knows something I'm not testing to get a little further.
Oh I also switched the stepper motor out and no difference.

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[hfjbuis]

It seems that moving from 10 to 20" and from 20 to 30" are equal in distance and that the move from 0 to 10" if off.
I suspect you have a back lash problem (play) Move from 0 to -1" and than back to 0" to set your starting point. If there is any backlash (play) it will be reduced to zero for moves in the + direction.
Now do your test and check the moves from 0 to 10, from 10 to 20" and from 20 to 30".

In general, backlash or play can be checked by using an indicator. Have a look at this video: