This is the machine I'm setting up to (hopefully) grind the v-edges of my linear rails. It's an automatic planer-blade grinder. The carriage, at top front, slides on a well greased v-rail on the top end and a flat one at the bottom on the back side of the carriage directly below the rack. It has rack and pinion drive. When the carriage gets to the end of it's travel, in either direction, it trips a lever, which pushes the drive belt (connected to the back of the twin shaft motor), switching the belt to the next pulley which reverses the travel and repeats the process. Each time the carriage passes a v-shaped deflector thingy, it trips a spring loaded lever which slightly turns the 1/2" shaft in the front. When that happens each end of the shaft has a worm gear which turns another gear ever so slightly with each pass, pushing the top of the carriage (in the "y axis") a tiny fraction closer to the grind wheel. The platen can be set for any angle and the carriage has stops, to prevent any further movement when the job is complete. By just swinging the carriage either way and setting it at an angle to the grind wheel, it will cut hollow ground blades.
The carriage was rocking when I first tried it, but after discovering a bracket, under the back rail, which someone had installed backwards, I found that the machine is now very tight. I have to build some stabilizers for the ends of the carriage,which were missing with this unit so I don't know, yet how uniform it will grind my 10' rails. But I picked it up at a garage sale for only 50 bucks and if it doesn't work to my satisfaction, I'll go a different route