Play in linear bearings

[Ross77]

Dam thats just complicated things. I thought you were talking about the profile rails and carrages. Ignore the comment on adding or changing balls for preload, thats only for the profile rails I think.

Unsupported rails are ok 'ish' for short runs and thats why some use them for the z axis. Not sure for 400mm tho.....is that travel or total length. you mentioned 200x200 at the start.

I dont think that open bearings and supported rail are a problem as long as they are not subject to large side loadings (note there are varying quaility of these bearings and strength of the mounting blocks.)

To make a decision as to the best rails, you need to know what style of machine, what you plan to cut and what tolerance you require.

If its a fixed gantry cutting mdf to 0.5mm tolerance then you would probably get away with unsupported rails. mention Ali and forget it, there have been quite a few builds that have started with unsupported and then changed later at extra cost.

I seem to remember doing a cost comparison vs performance and the profile rails didnt come out much more expensive because you can get away with one one block and a much smaller size rail. and looking at the whole picture the lower mass also helps with smaller ballscrews, motors and controllers

[leadinglights]

The 200x200 is travel, most of the work will be a lot smaller. Materials primarily aluminium, some PCBS, jewelery and the odd difficult part such as a titanium air turbine 22mm in dia (4 axis work). Cutters uniformly tiny and cutter speeds high - though titanium will present problems if the computer occasionaly stops to pray to Redmond. I will spend this evening looking at profile rails.

Mike