I’ve been thinking about building a more accurate CNC router/mill/cutting device for a while. My router is alright, but the fact that it’s about as rigid as a business card doesn’t do it any favors. Neither does the fact that my Dremel spindle has over 0.050″ runout. So I pondered, and pondered, and pondered some more, until I arrived at the conclusion that I needed to make something less sketchy. And proper linear bearings seemed like the right place to start.
I scored these four IKO rails for $50 on eBay. They each have 4.5″ travel. I think that will be plenty for making PCBs and the occasional small plastic part. The day they arrived in the mail I spent an hour just sliding them back and forth. I then grabbed a pair of calipers and got to work sketching things out in rhino. After a couple days of this, I got eager to make things in real life, so it was off to McMaster to pick up parts.
And this is what I ended up with. A 36″ length of precision 1/2 – 10 ACME rod, a length of Delrin, and a couple boxes of screws. The bearings are left over from TinyStruder.
Quickly printing out a drawing of my lead screw, I went down to the basement and cut an 8″ length off of a perfectly good 36″ threaded rod. With a coping saw. Needless to say, it didn’t leave the smoothest edge, so I faced the end on the lathe.
I was now left with the challenge of accurately turning two smaller diameters on the ends. Not having a cylindrical grinder or a surface grinder with a ded-tru, I followed the procedure outlined over at 5Bears.
I started by turning and boring a length of scrap Delrin. I made the bore about 0.001″ undersized. I say “about” because calipers really aren’t very good for tight tolerance measuring. At least now I have a good excuse to get an inside micrometer or some bore gauges (or both). The outside was just cut down a bit to make sure it was concentric with the bore. I made this piece long, so I could part it off into two bushings.
They came out like this. Perfectly smooth and accurate.
I swapped out the 3-jaw for the 4-jaw chuck, and put in the rod. The bushing was a very tight fit, and gave me a smooth surface to indicate the screw in with. Not having a 0.0001″ indicator, did the best I could with my 0.0005″. I got it to where the needle was just barely moving, so about 0.0002″. I couldn’t remove the bushing, so I just turned it off. The great thing about it being plastic is that it only took one pass.
And so I began turning down the screw. It’s heat treated 1018 steel, but a fresh carbide insert and light cuts did a nice job. My goal was to end up just under 0.001″ oversize, but on the last cut I forgot to divide my depth by two and ended up 0.0003″ undersize. Ouch.
0.0003″ really isn’t a lot, but it’s more than enough to ruin a perfectly good press fit. I think this one will end up alright. I’m not using super-precision screws or bearings, so a few tenths slop shouldn’t make too much of a difference. Hopefully can get the other side done right tomorrow.
