NEW TO CNC - Starting design and so forth

[JAZZCNC]

I am not saying you are wrong JAZZ at all, i am glad you raised the potential issue. But i wouldnt use it if i wasnt sure.

No problem but with respect at the same time I wouldn't say it if I wasn't sure.!
Also If that figure of 0.05 expansion over 5yrs is mm's then I'm afraid your fooling your self a little and it will change more than that within a month of varying tempratures.

But all this aside my main point was not the bed but the frame. The frame is the foundation and any weakness or shape shifting here will show in the work and accuracy. Because you where going to mount the rails directly onto the base then your asking for problems over time.

Again speaking from experience the work and time involved making from steel will be less than wood, even if your well tooled up and used to working wood which you obviously are just the amount of work and time to get any where near the strength required will be double that of cutting and welding steel.
Also in the grand scheme unless your getting the MDF free it won't be that much more money has you'll have sealant and all the other stuff required to stop it absorbing moisture.

You have a Mig and know how to use it so that's half the battle over, for Bench mounted machine then you'll have frame done and welded within short day. To build any MDF machine that's half the strength then you'll need at least 2 full days because of all the sealing.

Honestly it's not worth the trouble. Just build a Steel frame and use MDF for the Cutting base, 2 sheets one sealed for base and other untreated for a spoil board that gets replaced and surfaced between jobs.

Regards the screws or R&P then for 4x4 machine then Screws can be used but I would go for 20mm diam with 10mm pitch. 16mm dia is just on the thin side at this length, they do work at this length and are better in some ways because they have less inertia so allow slightly better acceleration etc but again from experience 20mm give that bit better protection from whip and still allow plenty good acceleration and speed.

If 8x4 then R&P is the better choice unless you want to go into rotating ballnuts but I wouldn't go there yet has it will only confuse the issue.

Belt drive does work well and it's actually more accurate and efficeint than R&P but in all honesty it doen't work out that much cheaper than ballscrews from China when you take all the Gearing and ratios into account.

I know your familiar with wood but trust us on this one using MDF is a mistake for light/Med duty home use and massive mistake for small industrial use.!! (Esp if you can weld and have access to equipment.)