The purpose of this thread is to discuss the merits and drawbacks of adding a top-beam deflection adjustment into the Maslow firmware and GroundControl. This follows:
I used a laser line pointing at the beam tip, and saw a 2 mm vertical change on one beam tip (near motor position) when the sled moves from left to right. Same thing at all sled heights.The deflection apparently follows a simple load distribution equation. So the sled would be 2mm too low on the left, 1 mm too low on the center, then 2mm too low on the right.That means the sled position varies twice 1mm vertically across the workspace width due to this deflection. (For now, I assume left and right beam tip behave the same. I still have to confirm)
My sled is loaded with a Ridgid router, a standard Z axis motor and two bricks.
I have made my top beam reasonnably stiff: It’s made of a 10 feet steel strutt open forward with hard wood extensions to space gearbox shafts roughly 11.5 feet apart. The horizontal distance from gearbox shaft to the closest beam holding attachement is 19 inches.
What about top beam bow?
From a “no sled load” condition, up to a “sled at the workspace top line”, and using a tape measure, I see no change in the distance between gearbox shafts. And I can place my tape tightly on the sprockets. So for now I concentrate on the vertical deflection.
I might be wrong, but the total combined top beam “flex and twist” creating this deflection is likely to be larger on Maslow cnc frames using plain 2x4 birch wood top beams.
I consider this 1mm vertical error a significant value. (Could it confuse the GC calibration process based on optimisations?)
One solution to include this beam deflection would be to correct the true motors vertical positions according to the sled weight ratio they carry as a result of the sled horizontal position. This would insert an adjustment on the motor vertical position in kinnematics.cpp.
The one required calibration value is to measure the vertical deflection when the sled moves from one workspace side edge to the other. Then extrapolate the value for the sled position right under the motor horizontal position.
How this value would be measured may be simplified, but a cheap laser line and a pen worked quite well for me.
I’m currently trying this new calculation on my github branch of the firmware. See documentation folder for diagrams ( beam_deformation_model.svg )
To establish the benefit of that correction, could volunteers from the community share measurements of their beam tip vertical deflection (mm) when their sled goes from left side to right side of the workspace?