an accurate 1000mm steel rule and microscope over the full length of the rule
Just a few thoughts on the matter.
The basis for any measurement is the reference you are using which provides comparison of length.
You should have complete confidence in the reference you use. The optical metal scales we used were
calibrated. A 1000mm calibrated scale would be VERY expensive and any deviation from a start point
to over the entire length and between divisions are known. Can measure to 0.001" ( 0.0254 mm) @ 17 feet
for example.
The scale you have is surely not to the caliber above,but, is usable to a degree, and only if the calibration
reference used is acceptable. I doubt very much that your scale is accurate to 0.05mm (0.002") at all locations
along it's length. Remember any measurement is only as accurate as the reference used! Also note that a mesurment
is only relative to the length being measured. Yes, you can then compare different items measured, but any difference is
realy only relative to the reference used ( may i also add that setup for measuring is very important).
So one can ask a seemingly stupid question "whats your reference to the linear length measured".
Just some things to think about ..........
Practicaly speaking, one must ask "What is being measured"?
It is length, but that lenght is made up of individual components. The gear box should be repetative in one direction
over it's complete reduction, but it probably will not be in the opposite direction and would say the same for the rack & pinion.
Now if you are using an electronic device which is based on steps to relate to length then likewise you should have confidence
that the steps are correct. Now you need a different calibration device. We used a calibratated counter accurate to four parts
in 29x10 6th.
So if one wants to be anal about calibrating their machine, I will just say that you have what you have, each component is contributory
to the system accuracy, and what you measure is only as good as the basis.
BTW, I also believe it is your rack & pinion along with the gear box. Need to isolate each and compare to the other axis to
be meaningfull.
FWIW,
RICH