I worked with Onefinity support on this exact issue when I upgraded my machine to the Z-20 slider last year. After a few go-arounds, both Onefinity and I got measurements that match the theoretical shift within a couple thousandths.
Theroretical shifts:
- Spindle centerline offset: Compared to the 65 mm spindle mount, a Z-16 with 80 mm mount moves the spindle centerline forward by ~15 mm, per this forum post.
- Z-slider baseplate thickness: The Z-20 baseplate is 1/4" thicker than the Z-16 baseplate. This shifts the bit forward by an additional 1/4".
So the total theoretical spindle-centerline shift is ~15 mm / 25.4 + 0.25" = 0.841".
Here are Onefinity Support’s final measurements:
- The Z16 slider measures 2.767" from the back of the slider mounting plate to the centerline of the 65 mm spindle mount.
- The Z20 slider measures 3.615” from the back of the slider mounting plate to the centerline of the 80mm mount.
So the total measured spindle centerline offset, per Onefinity Support, is 3.615" - 2.767" = 0.848".
I confirmed those numbers during my own Z-slider upgrade.