Removing broken steel screws in aluminium (this time it's a siren)

Christopher Tidy · 2006-09-03T23:52:08+00:00

Hi all,On Friday I had some fun. I was passing a old factory building and noticed that it was being demolished. In the past I had noticed a large siren on the building. Whether it was a factory siren or a true air raid siren, I'm not sure, but I had always fancied owning a big siren. I got chatting to the demolition crew and persuaded them to unbolt it and lower it to the ground for me. A little cash changed hands and I had a siren.It turns out that the siren is seized and has suffered from galvanic corrosion, so it's going to be a challenging restoration project, but I'm keen. I think everything can be salvaged apart from the rotor. Making a new rotor will probably be a CNC mill job, so I'll have to look out for someone with a CNC mill and some time to spare! Anyway, to get to the rotor I have to remove quite a few bolts and screws. Some have already come out, some are seized solid, and others are going to break off (one has already). They're BA screws or small Whitworth (approximately 3/16" to 1/4" diameter). Reading through the archives of this group I've found three suggestions for chemicals which will dissolve steel but leave aluminium untouched: nitric acid, sodium hydroxide and aluminium ammonium sulphate (alum). Which is best? I don't care how long it takes, and I can even tolerate a minor amount of etching on the surface of the aluminium.It also made me wonder if there was an electrolytic process for removing corrosion from aluminium. Something similar to electrolytic de-rusting perhaps? And if there is, does it work safely when you have a component made from aluminium and steel bolted together?I also need to undo some bolts which have badly corroded heads. I can't even grind the heads off because of their location. They're inside one cylinder, but outside another, so the only way to get to them is with a socket and extension. It might be possible to get to them with a die grinder, but for the moment I don't have a die grinder. I've seen these Irwin "Bolt...

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Bruce L. Bergman 20 years ago

That one won't bother the neighbors for very far, after all it's just a pennywhistle. What, 1 Horsepower?

Now if you want a *real* siren that'll make some /serious/ noise, Bubba, here's a 180-HP siren that'll wake the dead, gaa-ron-teed...

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;-)

Seriously, if you can get that old one apart far enough to let the Alum or Nitric Acid do it's work getting the waste steel bits (shaft stubs, setscrews, assembly bolts) off the aluminum castings, you might have something there. If anything, sacrifice the motor shaft and save the aluminum impeller & housing castings. The motor windings are most likely toast by now anyway.

You can always find a replacement motor and/or get a new chunk of shafting. But finding a foundry, making a proper pattern, casting a new impeller, and getting it all machined and dynamically balanced (so the siren doesn't shake itself to death) will be a total pain.

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Christopher Tidy 20 years ago

Wish I could afford one. I would LOVE it. I could lend it to Gunner as a burglar alarm!

Got inside the motor tonight. It looks near perfect. The totally enclosed construction has protected it well. Here are some pictures:

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There's no need to sacrifice the shaft. I took off the motor end bell, unbolted the rotor, tapped the shaft with a centre punch and it slid out. But I really am going to have to get a new rotor. This one is flaking apart. I guess I could get new fins welded onto the rotor disc, but this is also a pain as it would need to be done perfectly. The other aluminium castings can be saved though.

Interestingly I found a date on the motor. 20/7/38. This is sounding more like a wartime siren!

Best wishes,

Chris

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Gunner 20 years ago

Of course it is. Few factories used such a siren for lunch etc. Thats an air raid siren.

Gunner

"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."

- Proverbs 22:3

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Christopher Tidy 20 years ago

:-D

I wasn't sure at first. This factory is way out in the country, so I wasn't sure they'd have an air raid siren, but it seems they did. Another thing which suggests wartime use is that the demolition guys who got it down for me didn't have to disconnect any wiring. It had been been disconnected long ago and the siren painted over, despite the fact that the motor looks in good order.

I think I might see if I can look up some old guys who worked at the factory and see if any of them remember the siren.

Best wishes,

Chris

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Larry Jaques 20 years ago

On Wed, 06 Sep 2006 03:49:39 GMT, with neither quill nor qualm, Gunner quickly quoth:

It's quite obvious that you have never eaten homecooked UK food. Steak and kidney pie, their neighbor-island's haggis, etc.

-------------------------------------------- Proud (occasional) maker of Hungarian Paper Towels.

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Christopher Tidy 20 years ago

That's what I was thinking. There's some pretty tasty food in the town :-D.

The pharmacy have got me 1 kg of potassium alum. I'm going to pick it up tomorrow. If it doesn't work, I'll have to look for nitric acid. I don't know if the pharmacy will get that for me, though...

Chris

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Mark Rand 20 years ago

Actually, for the factory that I started off in, the works hooter was the whistle from the SS Mauritania. That was GEC Machines in Rugby... was BTH originally. The whistle did tend to flatten the power house boilers though!

These days It appears that it's not politically correct to wake up the entire town half an hour before the shift starts.

Mark Rand RTFM

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Bart D. Hull 20 years ago

I'll second the aluminum casting idea.

I've used a similar process to make new rubber manifolds for motorcycles as well.

Just siliconed up the bad manifold to spec and added to spots that were too thin in the original. (Manufacturer was a bit too cheap with the rubber!)

I cast the rubber in plaster of paris and have done the aluminum in petrobond (oil) sand. I used an artist's brush to sculpt sand from the mold made from the bad part, to replace the missing areas. Then a classmate and I poured the mold full of aluminum. (In high school metal shop no less, that teacher had big brass balls having teenagers with red hot metal! To be fair only the "good kids" got to do the pours.)

Ah, the good old days. Oh, that was just 1986!

Bart

Bart D. Hull snipped-for-privacy@inficad.com Tempe, Arizona

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