Linear bearings

Have a look at

there is part of the fourm dedicated to plasma cutters, with quite a few good ideas for cheap slides. The cheapest for plasma cutters seems to be cartridge bearings or rollers running on bright steel box section. For plasma cutters you need speed, not accuracy. The plasma cutter itself is not very accurate, and as you said, you don't need the cnc table to be micron perfect.

Also have a look at the torchmate cnc table site, as they have some good pictures showing the set-up for some of their earlier machines.

I have had two commercially made horizontal photographic copy benches in the past - both used just what you describe. Four sets of three ballraces at

120 degrees supported the camera platform, running on a pair of 2" steel tubes. It was a good system, as the mounts for the races were fixed with bolts into slotted holes, so the makers could true everything up, and reduce "slop" to a bare minimum without the carriage binding or having too much friction.

Roller races weren't necessary - ballraces are plenty good enough - the loading on them is minimal. Use two races above the tubes at each support point, and the single ones below.

Dave(uk)

Dave.

I have seen several machines that use square tube and normal bearings. It's quite easy to make a carriage for the bearings from bits of scrap steel bolted together...normally eight small bearings, four at each end of the carrier.

You can often omit the bottom bearings, this can be handy if you can rely on the carriage resting on the bars in use, as the carriage may be able to be lifted off.

For the sort of use you are thinking of 80 x 40 x 3 square tube for rails bolted ( welding will distort it ) on a frame with 6000 2Z ( 25mm OD, 10mm ID ) ( R.S. part 409-0013 £2.66 ) for the top bearings and 626 2Z ( 19 OD,

6mm ID ) ( R.S. 408-9782 £ 2.39 ) for the side bearings might work for you.

Depending on details of your carriage design you might find that two top and four side bearings on one rail and a single top bearing on the other rail are all you need.

Our old PCB factory had a multi-spindle drill with the main table moving on recirculating ball bearing packs which were running on hardened tracks. The bearing units weren't too expensive IIRC and the tracks could be something fairly tough if not hardened.

Peter