Linear Motion Axis Build Update and Updated Part Drawings

Linear Axis Build Update

Pattern Routing

I took advantage of the Hobby Shop’s hours on Tuesday (while there are no classes) to make some more progress on my linear axis builds. I chose┬áto mount the free-end bearing block for the leadscrew to the foot instead of the boxway body in order to make my system as compact as possible at the bottom end of its travel. This meant that I had to make the feet before I can complete my linear motion axes.

I decided to cut the curved outer profile of the feet using a laser-cut template and a flush trim bit on a router table. The mass production option would have been to set up a job on a CNC router and do the hole pattern as well as the exterior profile at one go. However, I think my approach made sense for the short run I was doing since it takes a lot less time to laser cut a template than to fixture and program a CNC router job with the amount of care necessary to get good results.

Fasteners

After discussions about fasteners in class and during peer review meetings, I decided to move to cross dowels and mating bolts (a.k.a. Chicago screws, sex bolts etc.) instead of using axial threaded inserts like I originally intended. This mitigated the risk of fasteners pulling out of the material due to preload or loads encountered in service, which allowed me to design all my bolted joints as slip-critical joints. This is advantageous because the critical shear area is described by the large head diameter of the bolt or nut in slip-critical joints, whereas the critical shear area in shear joints are described by the projection of the much smaller bolt shank. This is especially important in wood as the strength of most joints is limited by the shear strength of the wooden members instead of the tensile of shear strength of the fastener (I am using class 12.9 socket head cap screws, but almost any steel fastener would be much stronger than wood).

Detailed analysis of the bolted joints in my desk can be found here.

Updated Part Drawings

I also spent some time regenerating my part drawings to bring them up to date with recent updates to the CAD model. These changes were mostly additional details that were omitted the first time round, such as counterbores for bolt heads and seating holes for the cross dowels I recently adopted. I also prepared a preloaded slider design in case I run into trouble holding the 0.2 mm of straightness and parallelism I budgeted for the boxways. This is a backup option. To maximize stiffness and minimize actuation force, I intend to stick with plain sliders with radial clearance.

Updated part drawings can be found here.