Aluminum for frame members?

Has anyone tried 8mm aluminum bar stock for the frame of a RepRap? I was looking at the choices available from McMaster and the 6061 Aluminum bar stock has some major advantages: It's available in six foot lengths and each length is only $4.39!

The six foot length is important because many of the cut parts are 500mm long and many of the less expensive steel bars only come in 36" (914.4mm) lengths. Using 36" bar stock would mean buying 16 bars! I think 5 or 6 bars will be sufficient with 6' stock.

The steel is also about three time the price per foot and heavier so the shipping would likely be more expensive.

LOL! Shipping can sometimes be a problem for something six feet long.

John,

Is the Aluminum cheaper than non-stainless steel?
UPS seems to have no problems with 6 foot pieces; mine have come in cardboard tubes -- with sturdy end caps, so nobody gets speared accidentally (what horror movie had that in it?....)

You're also in the Boston area, yes?
Have you bought anything from (or heard anything about) Admiral Metals Outlet in Woburn?

-- Larry

Forrest Higgs Wrote:
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> LOL! Shipping can sometimes be a problem for
> something six feet long.

Back in 2002 I ordered four 6' lengths of 5/16ths free machining brass rod from McMaster. They arrived at my home just fine so I don't anticipate a problem. I was just wondering if the aluminum would be strong/stiff enough for a Darwin.

Larry_Pfeffer Wrote:
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> Is the Aluminum cheaper than non-stainless steel?

Yes. Less than half the price. The cheapest 8mm steel rod they have seems to be 12L14 (free machining leaded steel) which is over $11 for a 6' length.

The cheapest 8mm stainless seems to be Type 303 at $10.76/meter. That would be over $98 for 9 rods where the aluminum would be about $25 for 5 rods. Shipping weight for the stainless would be higher as well.

> You're also in the Boston area, yes?
> Have you bought anything from (or heard anything
> about) Admiral Metals Outlet in Woburn?

I'm in Maynard, MA.

I went to Admiral Metals once, years ago, and it was a good place to buy chunks of metal. That might be a good option for local builders. I don't know how well stocked they are in metric sizes, in case someone wants to stick with metric.

Greetings all,

I think the connections are probably the limiting factor in both stiffness and strength, rather than what metal is used for the rods. None of the metal is coming near its elastic (let alone ultimate) limit under typical reprap operation -- unless somebody is moving/extruding *way* faster than I'm aware of. (Even Nophead, whose machine's performance is nothing to sneeze at!)

So consider stiffness:
Yes, Aluminum is less stiff (~1/3) than steel, but the plastic (e.g. ABS) at the connections is only ~1% the stiffness of steel, so the plastic is the most compliant link, either way. FYI, Young's modulus data avail here: [www.engineeringtoolbox.com]
(As well as many other places online.)

I'd have no concerns about using Al rods for a Darwin.


-- Larry

PS Aluminum threads are more delicate than steel, and tend to gall (especially Al on Al threads.) I put a little (bicycle) grease on Al threads beforehand. It's not as good as hard anodization, but keeps threads from locking tight pretty well.

Edited 1 time(s). Last edit at 12/04/2008 02:16PM by Larry Pfeffer.

Quote

Aluminum is less stiff (~1/3) than steel, but the plastic (e.g. ABS) at the connections is only ~1% the stiffness of steel, so the plastic is the most compliant

I am not sure that is a valid comparison. The plastic is used for brackets and is very thick. The steel is used for long beams and is a lot thinner. Two horizontal steel rods support the weight of the x-axis, its motor and the extruder. I would worry the aluminium would sag in the middle. Also the x-axis rails slide on these horizontals forming a steel on steel bearing. It is not ideal, but I think AL on AL would wear a lot quicker.

---

[www.hydraraptor.blogspot.com]

Nop,

I wasn't aware that the x-axis rails were involved in sliding, metal/metal contact. (Mea culpa; my repStrap cartBot is distictly non-darwinish.)
You're right then; Al on Al won't work well there. (Al/steel wouldn't be great either.) Steel on brass would probably be best for friction, but brass can be expensive vs. (non-stainless) steel. I think Al could be OK outside of sliding members, though.

You're right that the brackets and corner blocks are thicker (and a simple bending model says that bending stiffness ~cube of thickness.) However with such mis-matched stiffnesses, my gut instinct is that there will be local stress concentrations (where the rods meet the plastic), and thus (under load) local deformations at those contact regions that swamp contributions from a simple bending model. It'd probably be faster for some brave soul to try some Al than to break out a finite element code to analyze the two structures.

-- Larry


PS: John, Since you're in the area, if you want to go with slightly larger diameter rod (for extra stiffness), I can help you turn down the ends to match Std. Darwin brackets/corner blocks.

I woundn't go with Al as the current 8

---

Ian
[www.bitsfrombytes.com]

Ian Adkins Wrote:
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> if the Al is cheaper than steel you are
> buying the wrong steel!

Even switching to 5/16th" rod the cheapest 6' steel rods are $6.60 (about 50% more than the aluminum).

Strangely, the 5/16th" 6061 "mill finish" aluminum rod is more expensive than the 8mm 6061 "mill finish" aluminum rod ($8.92 vs. $4.39).

I'm using aluminum for framing, with PTFE glide plates, seems to work quite well, especially since I got all my aluminium from the scrapyard