I made the mistake of buying expensive baltic birch plywood to build my router with. Other than my local hardware stores only stocking 1/2" MDF in 4'x4' sheets, not quite long enough for what I wanted, I was trying to make my machine as light as possible, like many other people on this site are trying to do.
I say this is a mistake because MDF is actually a great material to use for a few reasons.
First, its cheap, dirt cheap. Enough said.
Second, it has good accoustic damping properties. So although your router may not thump like a car stereo, not only will it help to keep the noise down but will also reduce nasty vibrations better than other materials will when you're cutting. This is especially beneficial if you're doing a surface finish operation.
Third, MDF is heavy stuff. "But aren't most people trying to make their machines lighter?" you may ask. Yes, yes they are. "Why?" Well, to be honest, I think it's because as some point someone started making a bunch of cutouts to remove "unnecessary" material to save weight and the trend caught on. I'll agree that reducing the weight of MOVING parts has a benefits: more rapid acceleration and deceleration; and less wear on some components. But I argue that trying to lighten the base and table is waste of time/effort (design and machining), causes unnecessary tool/machine wear and will ultimately reduce the performance of your machine. For anyone who hasn't seen/used a full size metal working mill, lathe, etc., they're all large, heavy, steel structures. The bases and machine surfaces are basically steel blocks. This makes them heavy and stiff. Stiff is good because the cutting loads are large. Heavy is good for two reasons. It lowers the natural frequency of the machine [natural frequency = sqrt (stiffness / mass)], to something which is hopefully below the cutting speeds. A heavy machine also takes more force/energy to excite into vibration [vibration amplitude = (some constants / mass)], so if a machine vibrates up and down 1/32" inches, another identical machine twice as heavy would only vibrate 1/64".
What I'm trying to say is that cutting a bunch of material out of your table is a bad idea. In fact, some of that "unnecessary" material may actually be doing something.
I posted some images from an Finite Element model of a section of a router table. It's 36.5" long, 8.5" wide, with 4" high ribs equally spaced at 6" centres. The skins and ribs are all 1/2" MDF. A point load of 500 N (112lb) was applied at the centre. The modulus of elasticity of MDF used was 3 GPa (it typically varies between 2.5 GPa and 8 GPa, so 3 is conservative) and yield strength of MDF (modulus of rupture) is between 20 MPa and 80 MPa. (sorry, I didn't bother to convert to PSI).
So what the analysis showed is that the maximum stress is about 0.2 MPa, or a safety factor of 100 (20/0.2) and the maximum deflection is 0.07 mm (.0028 in). It also shows that some of that "unnecessary" material may actually be not-so-unnecessary near the ends in the red region.
I would love discuss these and related issues and hear what the rest of the community members think and have learned from their own experiences.
Shawn