I just wanted to report what I feel is excellent compatibility of the V-Slot with 12mm hardened linear shaft and bearings. I recently changed my lower axis on my CNC Jr. to use linear shaft instead of the wheels on the sides, because I feel that the arrangement with the wheels on the ends of my sliding table and leadscrew down the middle was not very stiff, it was also next to impossible to align sufficiently. I am keeping v-slot and wheels on my gantry and Z axis, I feel given the configuration they are plenty stiff. All the parts I bought are from Amazon, 12mm Linear Shaft (330 mm), 8 pack of linear bearings (only needed 4, but now I have spares) and 4 pack of shaft mounts. $40 total. I clamped the bearings to some V-Slot using some duct strapping. Mounting the bearings to the V-Slot lengthwise works great to keep the bearings aligned perfectly straight. Also, the bearings are 22mm in diameter, so they work out within 1mm of perfect spacing with the rest of the V-slot, note my sliding table is even with the 20x60 along the side of my supporting structure. I think for a sliding table, this configuration is far superior to using the wheels sideways underneath the table, or on the ends. It's extremely rigid.