Installing linear guides

[WayneC]

I'm getting very close to upgrading (read rebuilding) my Shopbot with new linear guides. I don't want to redo the rack and pinion stuff, just install new improved guides onto the unistrut stuff.

I use the Shopbot, a 120x60 inch with stepper motor with a Porter Cable router, to cut pine, cabinet plywood, and sign foam. The current stepper motors/controller, etc are adequate, and will probably remain. This is just a rails upgrade. Shopbot claims .015 accuracy, and I'd be happy with that, but would like to get better if I can.

Some questions:

1) Any comments between the HIWIN guides (20 mm?) versus the lo-pro wheel guides from Bishop Wisecarver? They are in a similar price range. Any obvious advantage/disadvantage to either?

2) How do you install these rails accurately. I'm a woodworker, so I can use a tape measure. But how do you set rails both parallel and straight across a 60" + width within a few thousands? Is that sort of accuracy required?

3) My thinking is to install the wheel rails vertically, and place the gantry load on the radial axis of the wheels. If I use the HiWin, I'm thinking I should mount the rail horizontal, but I don't have a clear technical reason.

Thanks for your input- this is a tremendous resource!

Wayne from White Salmon

[Bruggles]

It seems like the only benifit of the v rails would be that the bishop wisecarver rails would be hardened where yours aren't currently. The hiwin style (assuming you are talking about some of the ones on this page http://www.hiwin.com/lg/index.html) seems like it would offer more benifits, such as the rack and pinion wouldn't be holding the gantry down, and the bearings would hold the gantry in all directions without the help of the rack and pinion. There could be less wear on the pinion, but I'm not sure, but you will still need to keep the pinion preloaded so you don't have backlash.

[Bruggles]

One more thing, the rack and pinion is probably the major limiting factor for the accuracy, vee rails if properly pre-loaded are generally quite accurate, which is what the shop bot has. Also wood will shrink/expand/warp more than the .015 in.

[WayneC]

Bruggles, thanks for the quick reply. I agree totally about the expansion of materials comment- no sense chasing accuracy beyond the expansion of most cab materials, or wood. Or beyond the accuracy of the Shopbot control software, for that matter. The current machine, far below factory specs, still does the job for me most of the time as it is. But the rail design is not reliable or accurate, and falls out of alignment very easily.

BWC has a vrail config that can include ball screws. I may go that way, so that if I decide to upgrade later, I can add the ballscrew to my already mounted vrail. If the rail upgrade works, then I'll upgrade to ballscrews, and maybe even a new controller and servos. Who knows. IF IT WORKS I'll be saving a ton of money over buying the equivalent commercial machine.

What remains in my mind is how to get these rails installed cleanly and what sort of accuracy is required. Any comments?


Thanks again for your help!

Wayne from White Salmon