Repairing old cast aluminum fan blade

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I collect old fans. I like anything that spins really but for some reason I really got into this very particular era of fan design from the 30's to the 40's - it's sort of a deco/machine age era I suppose. Anyway, my favorite fans in my collection are Roto-Beams which have these exquisite cast 5-bladed rotors. There's just never been anything else like them and I have about 7 of them. The largest one I have my assistant dropped off a table and the rotor broke off two blades. I killed him immediately. I don't think I have a photo of the break and I was going to put off repairing it until my aluminum welding got more proficient but I just won a second one on ebay and now would like to repair the first.The blade is cast, unknown what type of aluminum and it's pre war (WWII). I've attached a photo of the new one I just won. Replacement blades are rarer than hen's teeth - I've looked for over a year - so repair is the only option. I have a Tig machine and I imagine I'd bevel it out, clean it really well and then put in a bead. I'd want to grind it back to hide the repair and eventually polish the whole thing. Obviously it has to be aligned perfectly and it has to be strong too as I'd like to be able to use the fan too. That cage won't contain a blade that decides to fly off!Any suggestions?Gregor Attached Images
Reply:Would be helpful to see a picture of the actual break you're considering repairing.  That is a pretty special design - worth repairing I'd agree.If it's a "clean break", and what I mean by that is one that "fits" back together just right,  I would suggest beveling one side of the break only, and welding a bead in the groove, aiming for about 50%-60% penetration.  The idea is to leave the cleanly broken material to perfectly align the blade.Then, let it all cool down, use a carbide burr to grind out the back side that is not yet welded togther, and TIG weld a pass on the back side to completely fuse it together with full penetration (make sure to melt the puddle out deep enough.)Also - since you said you plan to polish afterwards, that means color match of your filler rod is going to be important (if you want to be able to truly make it "disappear.")  Cast aluminum usually has silicon, so 4043 is a good first guess (but no guarantees it will match.)  I would recommend grinding a small crater, and TIG depositing one droplet, then polishing that to test the color match.  If the match is not good, you can make a recovering by grinding it out with a carbide burr, and trying a different filler rod.  Best to get this figured out before you perform the full welding operation because you will not have an easy opportunity to do that one over.This may not be easy to pull off and if you need practice, I'd do as much as is needed on scrap material of similar thickness and procedure before you take to the actual part.You didn't mention which TIG setup you are going to use, but you should be using AC TIG process, ideally with a footpedal.  No scratch start DC-only.You should probably plan on balancing it (or at minimum, doing some sort of test for acceptable balance) after repairing it as well.
Reply:This is probably not a learning project, especially for someone with limited or no alum tig experience. I'd look for someone with a high degree of skill with tigging alum.Cast alum frequently welds like crap due to all the impurities in the original material. Not much you can do if that's the case and it's really bad.  Also the piece looks very thin on the edges which will add to the challenge.One option might be to take your good one and have a new casting made using it as the master pattern. Size will be slightly smaller due to shrinkage, but should be close enough to work. If you hunt a bit you can probably find a small foundry that will do it for you. You can do basic alum casting at home also if you want. The big trick will probably be doing the mold for the casting. I'd look at lost wax casting myself. You can easily make a silicone mold from the original to cast the wax pattern from. Silicone won't stick to the master and is easy to work with, but not inexpensive. Urethane's are another option that's less expensive, but you need to do a bit of prep on the master so it will not stick. If you go this route, you might be able to recoup your costs by having more than one cast, and offer reproduction blades for sale.No matter which way you go, doing this right probably won't be inexpensive..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:There's a blade on eBay right now, not sure if it's the right size but it looks the same.http://bit.ly/MMaSvySimon.
Reply:Yeah, I did want to post a photo of the break but I'd planned on leaving that one on the shelf for a while until I was either really confident or found a second one. Since I found a second one for less than the first cost (around $400) I thought I'd cast about for some ideas. I do have a AC Tig - Dynasty 200DX with a water cooled torch - so I'm good on equipment. The advice to only bevel one side is fantastic. It didn't occur to me but makes absolute sense and was exactly the sort of advice I was hoping for. I've repaired some cast aluminum parts before (shroud for my cold saw) but they neither had to be aesthetic or very strong (although they turned out fine) and I used 4040 rod. You're right though that I'd like to practice a fair bit more before I try this. What put me over the edge was that a blade showed up on ebay today and I figured if it was the right one (I don't think it is though) I'd really have some leeway to attempt the repair. The part has that sandy, grainy look of old cast aluminum and it does "fit" back perfectly as it broke very cleanly with no accompanying bend or distortion. Much like my stainless exhaust I'll keep practicing until I feel ready but I wanted to get some advice to start the thinking process. The idea of having a casting made is interesting but I think far more work than it's worth. To me it's more interesting to repair the one I have and gain the skills needed to do so and if it's less than perfect I'll be okay and the fan will become a very beautiful "runner" in the shop or home and not one for display. Either way one will run since once I have a second I enjoy the first one working and these make a really big soft breeze and are very quiet. I'll be back at my shop to finish my exhaust in a couple of weeks and I'll take some photos then and post them up. Thanks so much for the great advice!Gregor
Reply:When you have repaired it, you can give it a light sand blast with a hand held blaster to get that sandy/grainy look back.
Reply:Like Jakeru said, it will more than likely need to be balanced, and maybe even trued.  A good prop repair shop can help you there.Lincoln SA 200Esab Caddy 160Thermal Arc 201TSMiller Dialarc HFI don't like making plans for the day because then the word "premeditated" gets thrown around the courtroom....
Reply:Originally Posted by dubl_tLike Jakeru said, it will more than likely need to be balanced, and maybe even trued.  A good prop repair shop can help you there.