Steel vs Aluminium

[gavztheouch]

Is it possible for Alu alloys to be as strong as mild steel for cnc machine parts like gantry ends, beams, mounting brackets and z axis plates?

[Clive S]

Is it possible for Alu alloys to be as strong as mild steel for cnc machine parts like gantry ends, beams, mounting brackets and z axis plates?
Probably not as strong as steel but I think most router bit on here are made from 6082 alli , say about 20mm for gantry ends and 15mm motor brackets etc. ..Clive

[Wal]

I was talking to a mate about this recently.

5059 Alustar is pretty strong - nice article here:

All about aluminium (http://www.aluminiumleader.com/en/around/transport/ship)

So, yes - it can be. But it's probably prohibitively expensive for our uses...

Wal.

[routercnc]

Don't get confused between strength and stiffness. Totally different things.

Strength is how much load something can take before it starts to stretch (yield point) and then before it breaks (UTS, ultimate tensile strength).

Stiffness is how much something will deflect for a given load (Young's modulus), below the yield point.

For a CNC machine all we are interested in is the stiffness, as that influences deflection at the tool due to cutting along with vibration. Get the stiffness right through design and gauge etc. and the strength performance is likely to follow.

Strength is simplistically (ignoring fatigue) either good enough or not. It either breaks when you do the first cut, or it does not.

Stiffness on the other hand is a sliding scale.

Take 3 identical machines using different materials:
1. General aluminium
Strength (yield) 400Mpa
Stiffness (Young's) 70GPa

2. 5059 aluminium
Strength (yield) ~450Mpa
Stiffness (Young's) 70GPa

3. Mild steel
Strength (yield) 450Mpa
Stiffness (Young's) 210GPa

Swapping general aluminium for 5059 aluminium does indeed get you a stronger machine (higher yield), but it is not stiffer (similar Young's modulus). It will still bend the same amount for a given load, and will still bend much more than steel. It is this bendiness which is important, not the ultimate strength.

[Wal]

Excellent post routercnc. Thanks for the info.

Wal.