ok, the results of tonight's futile experiments!

As mentioned earlier, I am pretty limited to the range of accurate measurement by the 1" capacity of my DTI.

Taking on board what Mr Hood said earlier and adapting it to my limited range I thought would smaller diatances result in smaller errors being recorded.

For anyone who hasn't picked it up, the reason for all the odd figures like 12.7 and 2.54 etc is because i'm operating in mm but have an imperial (inch) DTI.

So starting with the DTI zeroed I drove the table G0 X12.7 (1/2") and the result was 4thou short.

Then driving the table G0 X0 the DTI returned to zero, therefore the error is apparent in both directions.

Next I thought what about "electronic" backlash so I first drove the table from zero to G0 X-2.54 and the same 4 thou error appeared. Driving the table back to G0 X0 returned it to zero on the DTI.

Now the intresting bit. When I now drove G0 X12.7 the needle hit the 1/2" mark right in the middle. Brilliant.

When I drove the table back, i.e. introduced a direction change, to zero the 4thou error reappeared.

Next, because i had just sorted my nice big fat pro MPG I set it to single step mode with a step of 0.1mm (0.0039") and did the same experiment. This gave exactly the same results; take up the electronic backlash and the DTI would read true, change direction and i'd lose some distance, well if I moved 0.1mm (0.0039") whilst changing direction I would only move half that distance.

Now I know that i'm only losing, say, 2thou on the shortest of distances and twice that on long distances but I think we can see the problem.

Question is, is there a cure?

...Sweep