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Old Wed 18 July 2007, 00:52
Gerald D
Just call me: Gerald (retired)
Cape Town
South Africa
Aluminum vs. Steel

I used to get quite involved in discussions of aluminum versus steel for the frame construction in CNC routers. (See this example on ShopBot forum). In the last few weeks a couple of guys have mailed me some questions about using Alu and so that I don't have to keep on repeating myself, let's try and pursue it here.

Often repeated points. . . . . .

a. Aluminum is 3 times more flexible than steel. Two beams of identical dimensions, one of steel other of alu, with identical loads : the alu beam sags 3 times more than the steel beam.

b. Alu is 3 times lighter than steel.

c. If you want alu and steel beams to be as stiff as each other, the alu must be thicker than the steel and you need to aim for around the same total weight. (put very simplistically)

d. The grooves extruded into typical alu frames make them inherently weaker than a simple box shape without grooves.

e. The torsional stiffness of a grooved beam is way below that of a non-grooved beam.

f. Aluminium beam producers do not publish data on their torsional stiffness. (shy?)

g. When a load is applied horizontally 300mm [12"] below the center of a beam which is vertically orientated, that beam experiences :
1. a bending load in the wrong (horizontal) direction, but also
2. a torsional load that twists the beam
h. Extruded alu is not necessarily straighter than rolled steel. (most often the alu is straighter or less twisted, but not always)

i. "Bosch" alu is no different than anyone else's in terms of strength and stiffness, although they might have better quality control for straightness. Here is some Bosch data (267kB pdf file)

j. When woodworkers move to metalwork, they find alu very attractive because it can be worked like a very hard wood.

k. Alu has its uses - I won't fly in a steel plane

l. But I also would not make a knife of alu. Nor a lathe, or any other heavy-duty machine tool.
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