I have a lot of parts and pieces that I have accumulated over the last couple of months and a whole lot of ideas, but I need to go ahead and start building something or it will end up becoming a mental exercise that is eventually forgotten.
I have several pieces from the CNCRouterParts 8020 design, but I do not have any of the 8020 pieces. The steel rails I have are for the 2x3 form factor so I want to go ahead and build a machine that size as a learning experience, but I eventually want to build a table that is 2x8 or 4x8 to use in building cabinets and built-ins for a new house we are planning to build. That being the case, I don't want to spend $400 on 8020 pieces that I won't be able to use on the larger machine. Would gluing 3/4" MDF together to make 1.5" MDF and then cutting it to the appropriate width and length to match the 8020 parts on the CNCRouterParts design result in a usable machine? I know the sizes of some of the bolts, etc. will change a bit since they will be going through the full 1/2" instead of just the t-slot area and I can accomodate that.
Why not build it in 8020 to begin with. There is nothing that says that you can't build a 3' travel axis using longer lengths of profile. I recently saw a thread where the guy did just that. He hadn't yet found the right length thk rails he wanted but had some shorter ones at hand so he used those to get the machine going.
So you could design your 4x8 machine, get the 8020 cut to length for the 4' axis of the machine you want and use them for the 3' axis of the machine you can build now.
The profiles you use to build the 2' axis might not be reusable on your new table but will surely come in handy elsewhere.
This also means that all of the bolts and brackets that you buy to put the test table together will be re-usable. If you look at some of the build logs where people have posted cost listings you will see that the hardware costs are not insignificant.
8020 that you use now could easily be used in a machine in the future. you can also use it for hold downs, jigs, and bracing. you'll get your moneys worth out of it.
Glued MDF won't work.
In general, don't use wood and plastics for machine frame. These materials are not rigid enough to withstand cutting forces. You'll get a lot of flex and chatter which equals bad surface finish and lack of precision.
My DIY CNC machine plans at www.8020CNC.com
I just completed a wooden router based on Ahren's parts and now wish I had used the 80-20. Besides being more rigid the 80-20 construction is amazingly quick. I built my z-axis using the extrusion and it took only 1 hour!
I unfortunately have to agree with folks on the wood vs. 8020. Wood has less than 1/3 the stiffness of aluminum, and MDF is probably more like 1/5. While you can overcome this with geometry (for example, everywhere the aluminum is 1.5" thick, make the wood 3" thick -- resistance to deflection is a function of material stiffness * thickness^3), it is difficult, and will result in a heavy gantry and z. You could potentially get away with making your longest axis out of MDF, however. This is where the majority of the aluminum goes into machines.
Another way to save money with extrusion is to use carriage bolts turned upside down in the t-slots rather than 8020's expensive fasteners. It's just as strong (if not stronger), and 5/16-18 carriage bolts cost maybe 1/8 of the 8020 t-nut/button head solution.
I say you can do it and it will work but the time and effort put in will definitely not be worth it. main reason is you will want a second machine from day one. and a smaller one will still have a ton of uses. build it with the 8020. even if you do not want it when done and want only the bigger one I promise you can sell off the smaller machine as a kit for almost everything you spent on it to some one who wants a smaller machine, if not more.
also seriously what is going to be the difference in price really. I mean if you have the stuff laying around free maybe it would be worth putting the effort into it, but with out a machine to cut it the accuracy will be out the window so you will have to use a different design all together. but if you are buying it then this is a no brainer due to the time the aluminum extrusion saves vs. the extra spent.