MCM: Boxway Build 1

I had a couple of days of free time on my hands over spring break and decided to get a head start on my motion axis build. While much of the engineering design for the rest of my structure is yet to be nailed down, I have spent sufficient time tweaking my error budget and doing first-order analysis that I am confident that the preliminary component sizes I specified are at least in the ballpark — sufficiently good estimates for me to put in material orders.

I decided to build the structural members of my desk out of beech for its high stiffness-to-weight and strength-to-weight ratios. There’s also the high Janka hardness which makes it a better bearing material that resists denting under extreme load events. Finally, beech’s neutral grain appearance and favorable machining characteristics also make it a good fit for my purposes. The desktop will be made from a sheet of 18 mm Baltic birch plywood, which would give me a nice void-free edge without resorting to gluing on an edging strip. I probably could have saved ┬ásome money by using cheaper material, but I decided against doing that as the time I am spending on this desk already far outweighs material costs anyway…

To date, I have successfully glued up my boxways and waxed the internal bearing surfaces to minimize moisture absorption and provide a non-porous surface for further lubrication (I am planning to use a light PTFE-infused grease). Now that I am using solid lumber, I can build my boxways to size and plane or sand down my sliders to give the requisite clearance. This allows me to avoid shimming the glue joints with paper like I did for my linear axis demonstrator toy — I am convinced that reduces the strength of the glue joint.

Another lesson I learned from the previous build is to wax the internal surfaces before gluing in the keeper rails, which both gives me better access when buffing out the wax and acts as a glue-release agent to keep the squeeze-out from adhering to my carefully prepared bearing surfaces.

The next step for me is to build my sliders and install my leadscrews on my boxways. My original plan was to laminate three 18-mm beech boards to produce a solid 50 mm-thick slider. The primary manufacturing concern with that was my ability to drill a sufficiently straight hole for the leadscrew nut to register against. This is critical for my design as I am using flangeless leadscrew nuts that are retained with a press fit. I am considering moving to a hollow slider design that would allow me to cut a precise counterbore on the endplate to press in the leadscrew nut using a CNC router since it has a much lower profile than the full slider — which would have been easy to cut with a horizontal boring mill, but which maxes out the vertical work envelope of the small router in Makerworks.

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