The prototype I built has racking problems while radiusing fingerboards. One of the support guides was flexing outward because I took a shortcut drilling the holes in the base. I braced the guide and lengthened the trunnions but it still racks. Since it is most likely tolerances, I will have to find a way to adjust the fit, perhaps with set screws and shims. Also, the long axis is flexing under the weight of the router. I reinforced it but it still flexes slightly.
I went back to the drawing board on the router guide. Since the previous version had flexing and racking problems, I decided to beef up the vertical supports and widen the anti-racking guides. I also added some plastic bushings as bearings on the guide surface. The bearing positions are adjustable along the axis to accommodate various radii. This new design should hold the router weight and move more smoothly to resist racking. Also, it can be used for both concave/convex runners.
I built this version and made the 40'-radius runners--they worked great. Unfortunately, the 2" platform is too thick--standard router bits won't extend down far enough, but the 1"-thick (MDF) platform was too flexible. I could try an extra-long endmill for $60, or a router-bit extension for $30, but I want this to be usable without any special tools or significant costs. Looks like I will have to make a 1" torsion box. Third time's a charm.
Guide to designing torsion boxes, AKA stressed-skin panels.
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