Anybody used these linear bearings before?

[irving2008]

Those 2 numbers give the two rails with the integrated sliders. The IGUS arrangement is a fixed rail and an adjustable one (thats the LL), not sure how the adjustment works tho.

The main difference between moving bed and moving gantry is one of loading. For a moderate sized machine a router can weigh 5kg or more and the all up weight of the gantry can be 20kg+, thats a lot of mass to move around, if all you're cutting is some small bits of wood. for an A4 sized work area the moving table is a better bet as the table is light and the workpiece cant be very heavy (size limited) so it makes sense to get the better rigidity by moving the table at the expense, as you point out, of a bigger footprint. One advantage of the moving gantry is that the downward cutting force is always midway between the bearings (although this can be acheived on a moving table by using the rails upside down and attaching them to the table, so that the bearings are stationary and the table/rail moves). the disadvantage of the moving table with bearing attached to the table but not at the edges is that cutting at the extremes puts a twisting loading on the bearing as the bearing nearer the cut acts as a fulcrum when the cutting point is outside the bearing footprint.

With a 350mm rail and bearing spaced 75mm the max working throw is 240mm approx (bearings are approx 35mm long spaced 40mm apart), so to get a real 300mm work area you need rails 410mm long for a moving gantry which adds about £4, but for a moving table you'll need them 600mm long (bearings placed at edges of table) although by not placing the bearings so far apart you could reduce the rail length to ~500mm with the disadvantage described above and an additional cost of £8 approx.

[pjl83]

Again, thankyou for the detailed reply irving.

Your point about the stability being better with the bearing "upside-down" is pleasing to hear. This was how I was thinking of doing it. The rails on the base and the bearings on the moving bed. I'd assumed it would be better so as not to get any twist or bow in the bed. I also thought that by doing this, the bearings would be protected from dust as best as possible.

I think I need to stop thinking so much and start cutting some wood. lol I get to the point where I am happy with a design idea in my head, but then I try and over-complicate things and it changes again. I have the remaining wood I need now to finish my benches in my workshop (garage) so if I can get them all finished over the weekend I'll be all set up and ready for some parts with this months pay :)

I'll be going with green MDF and some aluminium angle as my main construction materials. I'll start a blog as soon as I cut something. Nothing spectacular like some I've seen on here but for a first build for me personally it'll be good fun! ;)

Thanks again
Paul