My flying gantry machine

[irving2008]

Thanks for the replies. What would be the maximum length for the 20mm and 16mm bearings as they seem quite cheap, at least for a small cnc.

Hi, been away for a few days...

how long is a piece of string? It all depends on the rigidity of the mountings, what they are bolted to, the spacing of the trucks and the weight of what is running on the trucks.

As a guide, a pair of 800mm long, 20mm round rail on SK20 supports at each end deflects approx 0.5mm under a 10Kg load. If thats your X rails, your Y/Z gantry and router/spindle is likely to weigh 15 - 18Kg! If you are carving shapes out of MDF then don't expect better than 1mm accuracy vertically or 0.5mm horizontally. For most things you'll want better accuracy than that.

I use 16mm rail on my PCB mill but thats a moving table, fixed gantry design and the load on the table is tiny. On that I get 0.05mm deflection under 4Kg of load which is purely cutting forces as PCB's weigh nothing!