I now prefer to use mainly linear bearings such as THK HRS series. The cost might be a bit higher than other methods but you get what you pay for.
I recently completed a 1350 pound, 63" wide motion system capable of positioning accuracies of less than 1 micron (0.00004", 0.001MM) - for this application, medium preload THK bearings were used to eliminate stick-slip.
I have also used Turcite (sheet material, bonded) for high loads with mixed results.
Moglice (specialized, low shrink epoxy) is an absolutely awesome method, but offers a bit of stick-slip which may create difficulty for extremely precise, small moves. It is far superior for dovetail or box ways where you have the potential for metal to metal contact during initial moves due to the displacement of the way lube. If you ever re-furbish a Bridgeport machine, I would definitely urge you to look into the benefits of this type of material for way construction/repair/alignment - and it is not that expensive or difficult to use.
Using rollers on cold rolled material or the round bearings (Thompson) mentioned in a previous post would be adequate for certain applications, but I have had problems with both of these methods. I previously designed a composite processing machine which used the Thompson type (round) bearings to guide a shuttle over 30' at relatively slow speeds and the rails began to exhibit signs of wear after only 8 months of use - with an automatic lubricator and dynamic loads that were less than 1/5 of the maximum recommendations. I was a bit disappointed. The second machine of similar design used THK HSR type bearing blocks (I think they were NSK or another brand) - more initial cost, but provided much longer life - and less noise....and no warranty replacement costs, so they were actually less costly in the long run. Granted this was a production machine not a hobby machine but the lesson is there.
I would venture to say that the selection depends on your goals and your budget. Each method of guiding has its place and its advantages. Each method has disadvantages of course - sometimes it is only cost, sometimes the design dictates a specific method. The selection of guiding components primarily depends on speeds, forces, accuracy requirements and budget constraints.
Guiding is no place to skimp when sitting down to design a machine in my opinion.
Keep this going - let’s hear from everyone!  |