I’m slated to receive my AltMill in June. The AltMill standard assembly calls for a piece of MDF as the table surface (the surface underneath the wasteboard that screws to the cross-members).
I can get a piece of 1/4" aluminum plate (6061-T651) custom cut to the required 51.75"x34.3" size for about $400. I’m OK with that cost, but I want to make sure there are no other downsides. I will likely also screw T-track to the aluminum which may add to the rigidity. With this setup, do you think the machine would be any less rigid than using MDF?
As I mentioned in another thread, my reasons for wanting aluminum as the table surface are as follows:
The MDF table surface screws in from the bottom of the machine and, for me, would be a major PITA to change. The aluminum would never need changing.
An aluminum base layer will not be harmed by stuff like bit-lubricants for cutting aluminum.
It will not be damaged by repeated screwing in of replacement top layer waste boards.
I feel I would have a secure surface to create some kind of easy attach and remove mechanism for the Vortex 4th axis. (One can unscrew from the aluminum as many times as needed, unlike MDF)
I’ll be using an OPT 45W laser sometimes and I like the idea that there would be no flammable layer underneath the work piece.
The one down side I can see is that you need to be able to consistently locate the mounting holes on your sacrificial board that will be screwed to the aluminum. That is not a problem in itself seeing we are talking about a cnc machine - just run the original program again … BUT … what happens if your x or y home sensors move a bit for whatever reason. You put a new wasteboard on your machine and you want to pre-drill the mounting holes, you load your program, home the mill but now your hole position has moved which means the just drilled holes will not line up with the threaded holes in the aluminum plate below …
I suppose you could over-size the holes in the MDF or somehow register the home position of the mill against the threaded holes …
Oh, you better be good at tapping aluminum (unlike me) because you have a big pile of holes to tap …
I found out some interesting things about aluminum vs. MDF rigidity (stiffness). To cut to the chase, for aluminum to have the same stiffness as 3/4" MDF it would have to be .29" thick (more than 1/4"). But the interesting bit is that the stiffness increases with the cube of the thickness. (cube meaning multiplying a number times itself 3 times, so 3-cubed is 3 x 3 x 3 = 27). So, 5/16" (0.313) aluminum is almost twice as rigid as 1/4" because 5/16 is 1.25 times larger than 1/4, and 1.25 cubed is 1.953, or almost 2. Going back to comparing aluminum with MDF, 5/16" aluminum is 25% stiffer than 3/4" MDF and 3/8" aluminum is over twice as stiff as 3/4" MDF.
To be more precise, stiffness (in bending) is a function of the modulus of elasticity (E) and the moment of inertia (I).
E for MDF is about 3.6 GPa
E for aluminum is about 70 GPa
Since both materials would have a rectangular cross section, we can simplify the moment of inertia and just take the cube of the depth as Dolphran explains.
Therefore the “relative” stiffness for each material would be (ignoring the fact that E is in metric and thickness in inches):
3/4" MDF: 3.6 x 0.75^3 = 1.5
1/4" Alum: 70 x (.25)^3 = 1.09
0.29" Alum: 70 x (.29)^3 = 1.7
5/16" Alum: 70 x (5/16)^3 = 2.13
3/8" Alum: 70 x (3/8)^3=3.69
LOL. Yeah, I knew it was TMI before posting. But I couldn’t help myself. I actually congratulated myself for not providing the complete equations for stiffness.
I was using values of 4 (MDF) and 69 (Alu) for the modulus of elasticity, as found here: Young’s Modulus of Elasticity – Values for Common Materials.
Which gives greater relative stiffness to MDF and yields the thicker aluminum requirement. I prefer to err in that direction.
I’m not convinced that going with aluminum over MDF will provide that much benefit with respect to stiffness. You will only benefit from more stiffness in the Y-axis, in between the cross rails. In the X-axis, the stiffness is mostly provided by the rails themselves.
If stiffness was the only consideration, I’m not sure it would be worth the expense. But you have stated other valid reasons which could make it a good choice.
Personally, I’d rather invest in stiffening the table legs to ensure the table doesn’t rack in the XZ or YZ plane. There are a few posts of plywood used in between the legs that I will emulate.
And I will also have a tight fitting Baltic birch table (instead of MDF) to prevent racking in the XY plane.
Yeah, stiffness was not the primary reason for wanting aluminum (as you see in my original post), but I don’t want to lose any. And more stiffness, all things being equal, is undoubtedly desirable. It turns out that the incremental cost increase to go to 3/8" aluminum is not that great, so that is now my intention. The thicker aluminum will also give me more threads to screw into. I definitely agree on stiffening the legs. I’ll likely be using Baltic birch for that.
