I don't intend to use fast feed rates, and never plunging. I have tried to get as much clearance between the spindle and the bed.(150mm) . If I can get 1.5mm of cut on each pass (and I'm sure I will) then that's great. The Y rails are 25mm thick over 500mm ish. I never had a problem with the gantry before I beefed it up. In fact using a plunge, the spindle would push the original bed down so hard it would bend near the ends and the X axis 25mm unsupported rails would bend without the gantry moving at all. The weakest point is the Y axis rails IMO. I will find out soon enough. I don't care if a job that should take 10 mins actually takes 40mins. I just want accuracy. The next step will be to put a 5mm plate behind the rails and change them for supported ones, but that's if I get any problems.