We wanted as little flex as as is possible. Tubing is 4x4,4x6,2x2,8x8 and all is 1/4 thick.
I weigh about 175 and with the machine on and holding position i can only get about .01 flex in any of the axes pushing as hard as i can.
The more steel The less flex, more mass less chatter. At Least that is what we are hoping for. If the machine was just for light foam then yes it is about 85% overbuilt. MDF and dense tooling board isn't light and needs something that is pretty ridged, especially with the x down 36" . If it isn't strong a 100 ipm cut in 3/4 mdf is going to get a lot of flex in the z. The head and the z weigh just under 300 lbs and some test cuts were smooth as butter at ~100 ipm. Not only that but this machine is capable of 550 ipm rapids. If not strong accel and de-cell alone would cause considerable flexing, the gantry weighs around 1300 lbs. Also figure a 4'x8'x3' block of mdf weighs arund 2400 lbs, we have to be able to put that on the table without the base and machine flexing and binding the slides.
Chad