... too loose, the rail flops around, tilts, and won't hold gauge, and if you make them too tight they tend to break. I've often thought the solution is to introduce some compliance into the chairs (a bit like the spring-steel keys) but so far that has defeated me completely. Then I finally realized there is a "springy" component- the rail! Here's a test piece I cooked up this evening. The rails are truly vertical and the gauge came out right on the money first time 2983_080100_120000000.jpg Keen observers will have already noticed that half the "chairs" seem to be missing. Here's a drawing of the 3D model. 2983_080100_130000001.jpg The pads are 0.6 mm high with a lip that projects 0.1 mm over the base of the BH rail. They are positioned so that each pad applies a small amount of lateral pressure on the rail. The rail is threaded by "weaving" it between the pads, but the insertion force is so small it feels like the rail is threading in a straight line. So, ...