View Full Version : linear bearings for copy router
02-12-2005, 05:06 PM
Copy routers look like a miniature (table size is about 30" by 36") CNC gantry routers only they have a lead screw feed for one dimension and hand feed for the other two. ( and obviously no computer) Some of these parts slide back and forth on round drill rod. What bearings would be best for sliding a copy router? The drill rod that they slide on would likely be about .750" dia. rod and there would need to be two bearings on each side about 8 inches apart. The bearings should slide really easily and not bind up. Would bronze bearings or HMW plastic be the best choice for the homeshop machinist? How about some homemade air bearings? Anyone ever try to make their own? Thanks--Mike.
02-12-2005, 05:24 PM
Linear ball bearings would probably run nice & freely - but I'm not sure if the shafts would need to be hardened.
02-12-2005, 06:20 PM
Linear bearings work best on shafting made especially for them. Shafting is available soft (1070 steel), which is turned to a fine surface finish (32 microinch). Tolerance is held to +0.0000"/-0.0030". About $20 per 36" shaft.
Hard shafting is 60 HRC, case-hardened with annealed ends (so you can drill, etc.) Costs about twice as much as soft.
Soft shaft should work fine, as you probably won't slide back & forth a million times. Bearings cost runs from $15 each, and you can pay more if you need great precision. The big bearing distributors always have this, and I see it on eBay from time to time.
02-13-2005, 12:15 AM
I currently have a machine weighing almost more than I can lift sliding on uhmw feet on square steel tubing. I top-koted the steel, and the bearing surfaces total about 6 sqare inches. It slides smoothly, but that's also about 150 lbs bearing flat down on about 6 sq in of uhmw, so there is considerable force required to slide it. It doesn't seem to stick, it just need a certain minimum force before the machine starts to move.
I would have to say one of the biggest problems using hmw or uhmw is getting it to a totally accurate shape to fit the rails. Because it's springy when being machined, you might want to test turn a few, adjusting the tool setting so that it can turn out a close fitting sliding bearing, then make a few extras. Choose the best of the lot for use. You may not be able to remove a thou or two from the blank without mucking up the surface, so plan on having ten or more thou as a finish cut, and adjust either way from there to get to the desired size to fit the rod. Others here will probably know better how much the final cut should be. One thing I like about uhmw is that for a round bushing, you can turn a bit of a v groove in the ends of the bushing, near the inner diameter (your critical dimension). The action of turning that groove creates a lip that will be a tighter fit to the shaft. That becomes your way wipers, one each end of each linear bearing.
I'm not trying to say that this is better than buying proper linear bearings, but I think your loads are light, and this plastic does work well for that. Use the top-kote as well, it's amazing how well it reduces friction, and allows swarf to slide off. I would aim for a loading on these bearings of maybe 10 lbs per sq in or less. I would assume about 1/2 inch wide active bearing area for 3/4 inch rod, so that would mean making bushings about 2 inches long for each of the four you need, if the total load were to be estimated at 40 lbs.
Something else I've found to be appropriate for hmw bearings is good support for the bushing, so it has nowhere to go when squished with the loading put on it. I've used aluminum channel to fit the plastic chunks into, so the plastic stays dimensionally stable. That seems to work well.
You can also heat form hmw and uhmw. Heat a strip, then bend it around the shaft. I won't go any further here with this idea. That's experimenter's stuff, but I will say I've dome some of that, and there are some benefits, chief among them that your bearing surface gets to be very smooth, as long as your shaft is smooth and has a release agent on it. Top-kote here again- usual disclaimer, I have no affiliation with the Bostich company, just some good experiences with using the product, in this case firstly as an anti-friction agent, and secondly as a mold release that can tolerate a certain amount of heat.
02-13-2005, 06:14 AM
<font face="Verdana, Arial" size="2">Originally posted by mikem:
Copy routers look like a miniature (table size is about 30" by 36") CNC gantry routers only they have a lead screw feed for one dimension and hand feed for the other two. ( and obviously no computer) Some of these parts slide back and forth on round drill rod. What bearings would be best for sliding a copy router? The drill rod that they slide on would likely be about .750" dia. rod and there would need to be two bearings on each side about 8 inches apart. The bearings should slide really easily and not bind up. Would bronze bearings or HMW plastic be the best choice for the homeshop machinist? How about some homemade air bearings? Anyone ever try to make their own? Thanks--Mike.</font>
Have a look at this link, you might find what you want here.
02-13-2005, 07:06 AM
The $5 plotter I got at auction has a 3/4 bar, motors, belts all to rob.
I'd look around what I could adapt before I started engineering.
Them are expensive on ebay, linear bearings. The UHMW is the closest thing most us could afford to build at home.
Ever see the copy router made with a box running on inline skate wheels? It looked interesting. Simple to make, had percentages of Increase from model to cutter. I think his plans were only about 10 or 15$ I am on the linux box, link is on the windowz machine.. I can post it later if you like.
[This message has been edited by ibewgypsie (edited 02-13-2005).]
02-13-2005, 10:16 AM
A little OT, but this guy makes his own linear bearings. You might get some ideas from him.
02-13-2005, 01:55 PM
That's the slickest little plunge router I've ever seen! Gonna make one for my laminate trimmer (mini-router).