If it works perfectly at lower speeds but overshoots when you speed up, that's not backlash. Backlash, which is due to a small gap between a screw thread and a nut thread. becomes evident whenever you change directions, but it's not speed-dependent. You'll see evidence of it if you're running at 1 ipm or 300 ipm. What you're noticing might be inertia, as the heavy gantry is flying towards a point, then suddenly changes direction - it's like the "whiplash" effect you'd get if you were going forward in your car and suddenly slammed it into reverse.
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