[ksanalytical]

I've been getting more and interested in DIY CNC projects recently and have actually completed conversion of my HF (Green Monster) mill with great results. I wanted to try a router next and couldn't see any reason not to shoot for a nice 4x8 work area design. However, as I got further into it, I started to think about beefing it up enough to cut tough materials.

I wanted a large work envelope which necessitated more z-axis travel than usual, but it seems inefficient to have a tall, stationary cross bar with a spindle being lowered down to the work surface. It seems to me that this creates a compounded leverage effect which would cause some serious problems. I've tried to address this by using a dynamic cross bar. This creates a situation where leverage on the gantry decreases as the tool approaches the work. I know it's not a new idea, but I don't see much of this on the forum so I thought I'd see what people had to say.

The machine is large enough that I don't think steppers (as on my smaller mill) will be a reasonable choice. I'm looking at servos, but I'll size them after the gantry is built in order to get accurate ideas of the forces involves. Z-drive is shown as a cross bar riding on linear rails and driven by a pair of ball screws which are linked by a belt. This allows for a completely open area under the cross bar.

The Y-drive is a standard dual linear rail with single ball screw.

The X-drive is a little up in the air. The length involved makes the use of ball screws difficult and the fact that I'd like to keep the bottom of the main rails available for mounting legs seals their fate. I'd love to try the RPS (roller rack and pinion) system, but have no idea what this would cost. Especially since the X-drive will be duplicated on each main rail.

All rail and drive mounting surfaces are intended to be precision ground. I'm also exploring precision work surfaces that might be cheaper than a 0.5" steel plate with the necessary grinding, drilling and threading. So far, cast concrete with a good finish looks promising. Granite is a luxury that I doubt I could afford.

I'm new to CNC, but not physics so I'll be interested to hear your comments. Keep in mind that the frame size is a little arbitrary (Pixels are still MUCH cheaper than steel). The sheet in the middle is 4'x8'.

Ken