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To investigate the plotter stalling problems that were encountered during recent tests, I had ordered Keling stepper motors - one of the official choices of the Reprap project. They are rated for 166 oz*in of holding torque (vs 60 oz*in in Alltronics). They also cost about as much more. When the Keling steppers arrived, I assembled the plotter again. 

For some reason, the maximum feedrate that Keling steppers gave without stalling was 300 IPM, and that was on single-axis movements (like "G1 X10" or "G1 Y6"). Alltronics steppers could do about 600 IPM on single-axis movements in previous tests. I thought this was strange and decided to install Alltronics back to compare.

This weekend I tested the linear drive with ACME leadscrew positioned in the center of sliding element profile. In summary, with good alignment of shaft mounts, it works extremely smooth without any visibly detectable backlash. When shaft mounts are not well aligned, the torque needed for rotation gets uneven unless there is a small axial slack.