I noticed the blocks are not quite as smooth as I had hoped, but they do the job quite nicely. I used only one of the rails so far. I had to cut it to fit my cheap 3d printer. I replaced the flimsy 8mm tray rails with something a bit more substantial and much more robust. The carriage is much more sturdy now and auto-leveling the machine is much easier since the bed doesnt wiggle any more. It may have been a bit overkill, but thats my style.