This week, I built and tested a bearing preload system for my linear motion axis. As described in my post on the design for this system, the preload is provided by a compliant elastomeric layer in an oversized slider that is compressed by the side rails. Since I only need a small piece of material, I was reluctant to incur the cost of ordering some well-documented engineering elastomer. I took a walk in Blick’s instead in search of cheap materials.
I eventually found some relatively flexible carving blocks intended for making printing blocks of rubber stamps that looked like they might work. They had traditional linoleum pads as well as a softer rubber blocks marketed as being easier to carve. The latter is what I went with as my first-order analysis suggested that excessive preload for the required deformation would be a significant challenge, especially considering my relatively anemic motor.
Using the dimensions calculated by my spreadsheet, I made up a new wooden slider core on the table saw and found a scrap piece of 0.25″ aluminum sheet to use as the bearing pad. On initial dry fitting, I found the slider almost impossible to force into the boxway — probably a result of the very approximate modulus value I used for the undocumented rubber compound. To compensate for this, I took the wooden portion of the slider down very slightly using a belt sander, using a guide to keep the sides square. I also gave the aluminum plate a good brush with some grey Scotchbrite to expose a fresh, smooth bearing surface.
I repeated the “along axis” repeatability test I did on the original motion axis a few weeks ago to try to characterize the effect of adding preload on performance. For a description of the test procedure, see my previous post. I found that the preloaded linear motion axis repeated to within 6 mm measured 2.9 m away. This translates to a total (side to side) angular error of 0.12 degrees, which is more than 50% better than the non-preloaded design. I believe the residual error can be attributed to slight movements of the entire system resulting from motor acceleration (The linear motion axis was just placed on a table without clamping), as well as imperfect alignment relative to the wall (essentially an Abbe offset).