Material Mechanical Properties
I often start a mechanical design with Aluminum (steel is too hard for my China CNC machine to cut (unless I am desperate). Aluminium is easy to cut if you use the right cutter (i.e. single flute). Aluminium angle is light and stiff, but cutting both sides of the angle accurately is an unresolved problem. Aluminium flat bar is a good compromise. It just becomes a design issue.
Aluminium Mechanical Properties
Thickness 3 mm
Young's Modulus 69 GPa
Ultimate Strength 210 MPa
Yield Strength 110 MPa
Working Stress 80 MPa
The stuff looks good and the "good" stuff lasts. I find most of the stuff I get stress cracks after a few months. So although I am keen to use it, I dare not as I don't know if I have the "good" stuff.
Acrylic Mechanical Properties
Thickness 3 mm
Young's Modulus 3.2 GPa
Yield Strength 70 MPa
Working Stress 45 MPa
MDF (that cheap fibrewood board!)
I use an online laser cutting service and the thickest they offer is 6mm.
MDF Mechanical Properties
Thickness 6 mm
Young's Modulus 4.0 GPa
Bending Strength 18 MPa
Working Stress 12 MPa
So MDF is pretty weak but it is thicker. So why use it?
Stress and Deflection
There are lots of failure modes and serviceability requiements that real mechanical design needs to consider, but for toys only "stress" and "deflection" are important.
For a canterlever flat bar (beam) of length (L), width (b), thickness (h) and a point load of (F):
Young's Modulus E
Moment of Inertia I=b*h^3/12 (m^4)
Section Modulus Z=b*h^2/6 (m^3)
Moment M=L*F (Nm)
Stress S=M/Z (Pa)
So what does all this mean?
From a serviceability point of view, deflection is important.
A "floppy" CNC machine is not much use.
We can either use very stiff (high Young's Modulus (E)) materials or use thicker materials.
The deflection equation tells us that if the material is twice as thick (6 mm versus 3 mm) then the deflection will be 1/8. So although Aluminium is 17 times stiffer than MDF, as the MDF is twice as thick the deflection will only be twice that of Aluminium!
One of the advantages of MDF is that you only need to glue two pieces together (for 12 mm thickness) and it will be much stiffer than Aluminium and just stiffer than 3 mm steel (E=210 GPa).
The other reason to use MDF is that so long as it does not get wet it is dimensionally stable.
Although I considered doubling up the thickness of the first arm of the SCARA, I have "another trick up my sleeve", I can epoxy fibreglass on both sides of the MDF to increase both strength and stiffness.