Linear Thinking

I thought I'd get back to the topic of my mortiser. Or, more accurately, the design of the XY table on my mortiser. Below is my initial design that I drew up almost a year ago. My intention was to use 1/2" linear rails and build bronze bushings that pressed into a solid aluminum block. I went as far as to machine the bottom half; the seats and rails (photo #'s 2 and 3).




I got to this point a few months back but got hung up on the prototype for my bushing block. It turns out that while it isn't difficult to get the two rails to be exactly parallel, it's very difficult to bore two 3" deep holes exactly parallel. Even after getting my milling machine trammed in to within 0.0005 in all directions I couldn't drill and ream the two parallel bushings to within the couple thousands that I needed. Time for plan B.

There are several linear bearing designs, depending on the desired application. Very high end linear rails and bearings like the set shown below can withstand huge side loads and still run smooth due to the inherent stiffness of their design.

These are made by TSK and come in various widths and lengths. They are quite expensive new but I got these from ebay for next to nothing. I'm actually saving them for something else though so for my XY table I bought these:


They are actually considered a 'ball bushing' and can't handle the side load that the THK bearings can. They're also not quite as accurate as the bearings and have a tiny bit of slop in them, but it's negligible. In a cnc application, or in a production machine that runs non-stop, these wouldn't cut it. In my application though I think they'll work just fine. I bought these on ebay too, but I bought a second set from a local bearing dealer who had ordered them in for someone who never picked them up. He gave them to me for cost, which was cheaper than the ebay set.

Perhaps one day soon I'll actually put all these pieces together into something usable!