marlin firmware and half height prints

[Roxy]

OK... That isn't all bad. It maybe you just need to tighten down your coupler's so they can't slip. If there is slippage, there is no way the movements will be accurate. You have to get that cleaned up first.

But if that isn't what is going on, your stepper motors might not be able to keep up with the higher step rate. If making your couplers tighter doesn't prevent slippage, some of your other numbers might be close to the limit. If so, Go find these lines. Your actual numbers are going to be different than my numbers (that are posted here).

#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 40} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,7000,100,700} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.

If we are only messing with the Z-Axis, we only want to mess with the 3rd number. Make the 3rd number 1/4 of what it currently is for both of these lines. The reason is we don't want to be right at the edge.

And Doing a quick calculation... We need to change that 3rd number in:

#define DEFAULT_AXIS_STEPS_PER_UNIT

to something like: 5000 * 79mm / 56.7mm = 6966