Nope, aluminum is too soft. Anodizing won’t help, yea aluminum oxide is harder than aluminum, but it’s only a tenth of a millimeter thick. Even the steel conduit will eventually deform, just not as fast.
If you did end up wanting to go with aluminium, you would need something like 5-6mm thick walls to compensate for the strength differences, but still think going SS would be preferable.
I understand that this has probably been discussed else where but couldn’t find it. I have some 1in od 6061-t6 aircraft aluminum tubes there have a .125 wall thickness and it carries a rockwell B of 60-61 and stainless is only 70 hardness just hate to buy SS if what I already have will work. Thanks in advance
Ryan used to have a machine with stainless rails that had flats pushed into them from the bearings being too tight. He cranked them down too tight to test. Any aluminum will be too soft in the long run. You have it, you can use it. Just be prepared to change them out.
I am looking at tubes here in the UK. I read previously that 0.065” (1.65mm) stainless was recommended and that mild steel is stiffer. I can find 3.2mm thick Aluminium, 1.65 mild steel and 1.5mm stainless. From engineering toolbox the yield strength of carbon steel is about 240MPa (less for stainless steel) and for Aluminium about 95Mpa.
This suggests, to me, that 3.2mm thick Aluminium will be deform under the same load as 1.5mm stainless and but mild still will not. The Aluminium will not be quite as stiff as the mild steel but much lighter.
I am looking to build a machine to work 4’ wide sheets. Does anyone have a view if the Aluminium will be stiff enough for this application? Will the weight saving be worthwhile?
Bending of the rail should obey the calculations, but youngs modulus is what matters, not yield strength.
The real issue is how the bearings ride on the rails. They create a very concentrated point load and this can mash flat spots into the rails. I think Ryan did some tests in the early days and it was a problem.
Quick, back-of-fag-packet, calculations suggest to me that, given a Young’s modulus of about 70Mpa for Al and about 180Mpa for Stainless steel (SS), the Al is only slightly less stiff with a wall thickness of 3.2mm vs 1.5 for SS.
As a follow up, if stiffness is a problem I will fill the pipes with “builder’s foam”. Not only will that increase stiffness but it will also help with dampening vibration. I feel like Al tubing is the way to go for me but I will also buy mild steel pipes as they’re really cheap and I can do some comparison tests. To prevent corrosion I will lightly coat the steel with light machine oil - standard practice in machine shops.
The steel bearings can run flats in thick walled stainless if over tightened. Aluminum will get flats with regular tension.
I waxed my steel tubes, so far no rust, even on the bearing tracks.
It isn’t the loads bending the Al as I thought was being implied (and thus denting the rails) but the abrasion of the steel bearings on the surface causing problems. This abrasion wears a flat onto the rails themselves (Different Z axis tubes). Hence the need to avoid soft materials like Aluminium.
No, bearings are there to keep abrasion from happening in the first place. Unless it’s a bushing, then you’ll get some wear. The roller bearings we use will deform the steel tubes they roll over. For steel it’s just a little amount, you’ll see flats happen after a few months to a year. Aluminum is much softer than steel, it will happen faster. In theory it should only deform until there isn’t enough force, but since these guys are flexing a little when they’re moving, it will deform the tubes to the point the bearings rattle.
I see what you’re saying, Barry. I am not sure it isn’t some kind of abrasion. That would be different than an application like a bearing in a router where the outside of the bearing is always firmly in contact with the bearing. Rolling has funny stuff happen because it can’t possibly touch at only one point, but the intersection of a line and a circle is one line. So there is funny stretching/compression that happens when a bearing rolls on a surface.
I would expect the bigger affect is more like a steel hammer on an aluminum bar stock though. The bearing is applying pressure and moving around, the leading edge is like a little hammer smashing the aluminum out of the way. I don’t know the ME terms for that.
I wouldn’t be surprised if the way to characterize it was as an abrasion or as an impact. But it definitely will deform and CF tubes delaminate.
Now I am really confused. Here’s what I think people have observed:
After a while flats appear on the tubes where the roller bearings touch the tubes.
I can think of only 2 ways that this can happen:
Abrasion - surface treatment will help with this (e.g. anodising)
Plastic deformation - increasing the yield strength of the tubes will help (e.g. thicker tubes, work hardening, using a stronger material of the same thickness).
This should be easy enough to prove. Section the tube and measure the thickness of material at the “flat”. If it’s thinner then it has been abraded, if not then it has yielded.
For plastic deformation I can also see that the contact is initially just a point (tangent of the circle) and that a flat will be bent into the tube until there is enough material to support the, now enlarged, contact surface. If this is the case, it seems to me that some kind of adjustment mechanism might be helpful to cope with the bending (or just increase the tube thickness).
Steel becomes significantly stronger as it work hardens whereas aluminium doesn’t. Hence a tube formed by direct extrusion (ODM) should be stronger than one formed by bending and welding the seam (conduit).
As for carbon fibre tubes I am only concerned about abrasion as the yield point is pretty damn close to the Ultimate Tensile Strength (UTS). Thicker tubes would solve that. Delamination of the composite is normally associated with a chemical process which should not happen in this case.