NEW TO CNC - Starting design and so forth

[PSG]

That's good advice so far. What should be my first step then? I think I have the basis for a design, I was set on a disposable top anyway so that's a consolation. What do I use for a bed then that will cost the same money as MDF

[JAZZCNC]

That's good advice so far. What should be my first step then? I think I have the basis for a design, I was set on a disposable top anyway so that's a consolation. What do I use for a bed then that will cost the same money as MDF

Ok well thats the problem isn't it nothing will be better and cost same.!!

The problem you have is stabilty of the surface that important components like linear rails fasten on. Because you plan to use a Torsion box design the strength and flex will be reduced but you won't be immune from material expansion and MDF expands if you so much has show it a cup of coffee. Now your problem will lie with the fact your mounting the rails directly onto the torsion box so if this material deflects or expands the accuracy of the machine changes as the rails are no longer on the same plane.
For this reason your better with a metal frame work for the machine and mounting important stuff like rails, ballscrews etc as it doesn't expand with moisture and less with heat. It's also much stronger will take much more abuse over a much longer time period.

Now the Don't get me wrong I'm not saying you can't get a accurate machine when building with wood, esp when built for cutting wood which requires low tolerences, but unless lots of effort and special techniques are employed in the build you'll spend a lot of time chasing accuracy and keeping accuracy.
The effort required and time required are IME not worth it and in the long run you'll spend just as much money if not more than if you built it from Steel box section. The differance will be very much worth the effort and you'll have a machine that will last for years and if carefully built with correct components give hassle free usage.
Often you can return your investment or very large portion of it when ready for bigger or better machine which you certainly won't if built from wood.

The starting point is where your at now.? The Design.!!
Spend some time looking at designs and sketching designs that you feel are within your skills and abilty's. Don't be afraid of stretching your skills, you'll be amazed what your capabale of and the rewards are very nice but do be realistic.
Don't let welding put you off has where not designing nuclear reactors or needing sky scraper strength welding and just cheap MMA(ARC) welders are more than enough along with short welds. Pigeon Shit welds are often good enough and with quick grind and lick of paint they disappear.!

You have most the basic information you require, like Size required and Materails your mostly going to cut, so now you just need to identify machine design style and select the best components for the machine at this size.
Your biggest and most important choice at this size is whether to use Ballscrews or Rack & pinion for linear motion.? This will also to some degree dictate the design slightly to accomodate which your using.
Ballscrews are more efficient and give best accuracy but at this length your near the limit and in danger of whip if not correctly sized. Rack & pinion is better for longer machines but requires more power has it's less efficient and for this reason it uses gearing ratios so theres slightly more work involved.

I suggest you stay away from flat Plate and Skate bearings type of linear rails and unsupported round type rail. Only use supported rails.

Don't get obsessed with rapid speeds or building light weight gantry's. Fast rapid speeds are only of any use on very large machines or where lots of point to point work like drilling 1000's of holes are needed, most of the time the machine will be cutting much slower and it's this cutting speed that matters the most.
The strength and ridgidty of the gantry is important and often with these comes weight but to a point that can be a plus and allow deeper cutting and better finish. Obviously Gantry weight does have to be considered but upto about 70-90KG then your fine with most stepper systems.

Think that's enough for now but I suggest you look around the forum at similair sized machines, you'll also motice there are not too many built from wood.? and for good reason.

Hope this helps.