I fabricated my linear motion axis (boxway) from scrap plywood and a plywood-oriented strand board laminate scrounged from around campus. As mentioned in my previous post, I wanted to keep the slider cross-sectional dimensions to a minimum of 1″ x 1″ in order to accommodate the flange nut when I incorporate the lead screw. Unfortunately, the only sufficiently thick material I could find was the plywood-oriented strand board laminate. This forced me to use the porous and irregular surface of cut oriented strand board as bearing surfaces instead of a smoother material. To compensate for the surface asperities and higher coefficient of friction associated with this material, I increased the radial clearance from my design value of 0.005″ to 0.01″.
After cutting the component pieces to size with the table saw, I glued up the assembly using copier paper (thickness = 0.0035″) as shims to achieve the necessary clearances. For example, a 3-layer stack of paper brings me within 0.0005″ of my design clearances. A mistake I made during this step was neglecting to account for the thickness of the glue layer, this ended up causing my boxway to have excessive radial clearance, increasing error motions.
I tested the geometric error in my linear motion axis using a laser pointer. I moved the slider between extreme positions on the axis of travel, while applying slight moments to take up the angular “backlash” caused by radial clearance. The position of the projected beam on a surface 4845 mm away was recorded between each adjustment.
The maximum lateral displacement of the laser beam was 109 mm, which corresponds to an angular error of 1.29°. This is the total side-to-side rotation, which we expect to be twice that predicted by our deterministic geometric error analysis utilizing radial clearances. For my boxway with 0.01″ of radial clearance, I predicted a sine error of 48.5 mm when measured 4845 mm away. The actual value is slightly higher than expected, which I attribute to the mistake I made in not accounting for thicknesses of adhesive layers as well as imperfect clamping during the glue up.