I know I said I would do a cheap mdf machine, but the more I look at it the more I trouble I have. I don't think mdf will give me a decent 3x4 cutting area for doing useful work on and at the same time be able to mill new bits for itself reliably. The amount of mdf needed for a strong structure of that size would also end up as scrap when I came to upgrade. I realised that I don't have a problem with building boxes out of wood or steel I have a problem with all the cnc fetish talk of whipping, backlash, ballscrews and the unhealthy obsession with industrial strength vibration.

So steel it is for the frame and gantry.

I am intending to use 80x40x4mm box for the legs and the top of the longest axis held together with plates and M8 threaded rod. For the rest of the frame I'll use 30x30x3mm box. I'll be sheathing it in mdf so that I can use the underneath as a cabinet.

My longest axis will be 1.5m of rail and the gantry will 1.25m of rail. I am intending to construct the gantry out 30x30x3mm box backed by 3/4 inch mdf and also use 3/4 mdf for the gantry base & z-axis plates etc. Later I plan to machine new plates from 20mm aluminium to replace the mdf.

I'll be using sbr20 supported rail for all the rails and 20mm antibacklash ballscrews/couplers driven by nema 23 3.1nm motors driven by a cw8060 with a 600w 48V 12A power supply.

Each side of the longest axis will have a single sbr20 rail mounted directly to the 80x40 and ballscrew above, the gantry and z axis will have 2 rails and one central screw. I'll be ordering an extra long z axis but not using the full amount until I install the 4th axis.

The steppers etc I plan to get from cnc4you and the shiny metal bits & connectors from Chai.

Now for the questions...

1- Is 80x40x4mm & 30x30xmm mild steel good enough for the frame and gantry and will the threads/plates hold up ok without welding?

2- Will steel & mdf provide enough stiffness without cross bracing? (the frame will have a cabinet floor of 30x30x3mm near it's base)

3- Will the sbr20 mount ok directly to the 80x40x4mm or do I need a cunning plan to attach it?

4- Should I drive the two threads with a belt system or just slave an extra motor?

5- Anything horribly flawed with this masterplan?

I need this build to deal with light use, 3D carving of upto 4 inches of laminated ply/mdf, accurate cutting of inlays on veneered wood, occasional soft metal upto 20mm thick (only need that thickness for cutting new machine parts for it) and cutting/engraving on acrylic/pcbs.

Thanks for all the help!