That old chestnut..

[Ricardoco]

Having trawled the many forums for many hours before starting my new steel 6x4 machine i noticed a very real divide between Aluminium build thought and steel build thought, from what i gleened there are both fors and againsts for both material types. then yesterday whilst browsing i saw this...

http://en.wikipedia.org/wiki/Ultimate_tensile_strength

When i look at things like the human hair in relation to said materials it makes me laugh.

So my questions are these.

1/ What are the pros and cons of using Steel...
2/ What are the pros and cons of using Aluminium...
3/ How do they compare to each other..

I have asked my questions in a very specific way so as not to make it into a direct comparison until question 3..

Rick

[Ricardoco]

I didnt think anyone would want to jump on this LOL..

[JAZZCNC]

I didnt think anyone would want to jump on this LOL..

OK was going to keep out but I get into enough scraps already suppose one more potential brawl won't hurt. . . Go with steel here's why.!!

It's Cheaper, Stronger, Easy to work with, Easy to get, Easy to adapt for ever changing designs.? If you cock up and drill hole in wrong place or change design, weld it up grind flat and start again.. . Not so easy in ALu with out special equipment.!!
It's a blank canvass regards design, by that I mean no having to work round slots or hole positions. Yes it's a bit more work but you have pretty much no restrictions to where you can mount or position stuff.
The few down sides are needs more equipment and little knowledge, but not has much has folks probably think.!! Basicly a cheap stick welder with decent rods and half decent angle grinder, couple of hours practice on some scrap will get you to a point to weld the frame successfully.! A good pillar drill is also required but thats true for Alu and really THE MUST HAVE tool for any CNC build.

NOW don't get me wrong I like Alu profile and it has it's use's, esp if mixed with other materials like plate ALU or steel. . . . But IMO it's too expensive(Look beyond the profile it's self.?), it can be restrictive to design if holes or slots fall in wrong places.
It's rubbish for drilling and tapping, far too soft with not enough support due to it's design making it easy to strip threads, which your then buggerd because you can't easily repair.!

Now some will say "ARGH but profile surface is flat, steel isn’t" . . BUT. . Contrary to what people believe it's not flat or square, infact most profile is designed not to be flat on it's surface.??.
The surface design is angled towards the slot so when bolts are tightened it acts like a spring washer.!! This is fine if you using the whole width of the profile.! But if not like when fastening linear rails etc then your in potential trouble because the rails can be tipped at some unknown angle and if two rails are sharing the same wide piece of profile then one can tip the opposite way to the other causing binding etc.! . . . Now the tipping or discrepancy can be tiny but it's still there with potential to cause head aches, esp when using precision profiled linear rails which don't tolerate much error without affecting performance. IME cheap profile is seldom flat or truly straight and I've used high quality industrial grade ITEM section that’s needed surfacing to have a truly flat surface.!!
So IME you still have some work to do with shimming or surfacing flat just like using steel surface so nothing gained here.!!

It's biggest plus is it's ease of assembly/Disassembly and even this is not all it appears and comes with draw backs and at a premium price to make full use of it.?
The draw backs or hidden aspects are having to drill counter bores for bolt heads because the head is too large to fit thru the slot, this requires Jigs making or careful alignment other wise holes don't align with slots or holes in adjoining profile.! . . . It's also another process increasing assembly time.

To fully make use of fast assembly you need to buy the purpose designed bolts-nuts and fastenings, brackets etc which often work out much more than the profile it's self.? I you don't use these fastenings then the assembly becomes a right pain in the arse.!!

For some some jobs it's great stuff and to some folks it makes a very Prity machine but to me the only prity I find is.!! . . . . Prity awkward, Prity restrictive, Prity expensive, Prity over priced.!!

Go with steel save some money which you can then spend on decent profiled rails.!!

[routercnc]

Wouldn't worry about UTS. If you hit that limit in steel or aluminium then you've not used common sense in the design e.g. simple 2mm thick gantry side plates. In that case the design is not strong enough. It's the point when you have permanantly deformed or broken the material due to the loads you are applying.

What you really need to consider for practical machines is stiffness, which is the slope of that line. Also called Young's modulus, this will tell you how much the material deflects for a given load. If it deflects too much for your requirements it is not stiff enough. Some people confuse this with strength but technically the 2 terms are quite different. Whilst Young's modulus is fixed for a given material, the actual stiffness in your design is related to both the choice of material and the section properties you go for (depth, width, wall thickess) etc.

Steel is 3 times stiffer than aluminium like-for-like. It is also 3 times more dense, so 3 times more heavy. But this is no bad thing for a cnc machine.
If you want to get the aluminium stiffness shortfall back up to steel levels you can increase the wall thickness and/or section size, but except for some unusual cases (all members in pure tension or compression, so not practical on cnc machine), you end up back at the same mass as steel if you want to match the stiffness. But you will have much thicker walls or section sizes than the steel equivalent.

[Ricardoco]

OK was going to keep out but I get into enough scraps already suppose one more potential brawl won't hurt. .!!

No you weren't going to keep out.
. . and i hope you never do, i enjoy reading your scraps, it keeps me interested and i take it in and understand much more clearly the subject matter... there is nothing like a good debate, i love em..

Thats 1 for steel unless anyone else has something to add...

[Ricardoco]

Steel is 3 times stiffer than aluminium like-for-like. It is also 3 times more dense, so 3 times more heavy. But this is no bad thing for a cnc machine.
If you want to get the aluminium stiffness shortfall back up to steel levels you can increase the wall thickness and/or section size, but except for some unusual cases (all members in pure tension or compression, so not practical on cnc machine), you end up back at the same mass as steel if you want to match the stiffness. But you will have much thicker walls or section sizes than the steel equivalent.

2 for steel and no contest so far, hmmmmm im shocked... any Aluminium takers then???

[i2i]

you know the route i would take, and it's none of the above.

[Ricardoco]

you know the route i would take, and it's none of the above.

I do but if you had to build from scratch i wonder which you would use..


Rick

[i2i]

steel plate with machined edges.

[Ricardoco]

steel plate with machined edges.

3 for steel then..

Rick