Metal munching moon mice needed. - Help with cutting a light steel plate over concrete.

September 25, 2012 11:47 AM

Efficient cutting of steel plate, backed with concrete-like material? Ideas?

I have a small (800 pound (350 kg) safe with no residual, historic, or functional value that I am attempting to incorporate into a small sculpture. Dates from the mid-late 1800s.

The structure of the safe (as is the case with most of these) is that it ISN'T solid steel, but instead is steel plate encasing a semi-ceramic/concrete-like interior compound that is supposed to release water on being immolated. (This is to protect the safe contents in a fire. Clever Victorians.)

I have partially destroyed it, but am looking for the best way to remove most of the plate. In the end, all I want remaining is the corner elements (in steel) and none of the concrete material. I have tried abrasive cutting wheels on a 4" angle grinder and a saws-all. the grinder is too slow, and the saws all is fast but the short metal blades only last a few minutes as they are dulled by the ceramic.

I don't have recent experience with ox-acetalene cutting and NO experience with plasma cutters, but wonder if either would be better than the saws-all? Will either work, considering that the plate is backed by what is essentially one hell of a thermal sink. Does a plasma cutter care about such things?

Any advice greatly appreciated. It's only art, so not a big deal, but a small amount of order will occur as a result of this.

posted by FauxScot to technology (10 answers total)

I have heard of such things as concrete dissolvers. No experience, though.
posted by Leon at 11:57 AM on September 25, 2012

You need to measure the depth of the steel with a depth gauge (the pin sticking out the end of a slide micrometer will do nicely). Then, adjust the throat depth of a (circular saw/grinder/whathaveyou) to precisely this depth, making sure it's no deeper than this (+0/-0.005", in draftsman's terms) with a short test cut.

Now, remove the metal with confidence.

If your power tool of preference doesn't have a built-in throat guide (I don't own a hand grinder, so I don't know), you can probably jury-rig one by clamping a steel plate to the blade guard with C-clamps. Don't be shy about tightening those clamps; the plate will want to slide under use.
posted by IAmBroom at 12:03 PM on September 25, 2012 [1 favorite]

Plasma cutters apparently can sometimes cut concrete-backed metal.
posted by sebastienbailard at 12:04 PM on September 25, 2012

How about a water jet cutter?
posted by dfriedman at 12:08 PM on September 25, 2012 [1 favorite]

Can you break out the ceramic with a cold chisel before making a cut? Then you'd be able to use the metal blades without them dulling so fast.
posted by echo target at 12:54 PM on September 25, 2012

The drawback here is that the safe was specifically designed to make what you're trying to do as hard as possible.

If it were me (and I'm glad it's not), I'd follow IAmBroom above -- and use a wheel that is correct for hardened steel (contact supplier).

It'll take a while.

Another option would be to take it to a machine shop and ask them what it'd cost to have it done for you (mark out the cut lines ahead of time). They might be able to mount it on a sufficiently large mill (note: very very large mill, something you might find in an "oil country" type shop, not J. Random Precision Machining Co.) and mill out slots using appropriate end mills. To find them, ask around at other machinists ("who's got the largest mill around here?").

There are concrete dissolvers, which work by destroying the cement in the mix and they're mainly intended to help clean equipment (ie., not destroy large monolithic blocks of material). One possible, out of left field option, would be to use a nonexplosive chemical demolition agent.

Basically, these are powders with huge coefficients of expansion-- drill a hole in concrete, mix up the chemical, pour it in the hole, it expands like crazy and destroys the concrete by cracking. For larger expanses you drill a pattern of holes and pour it in all of them. One problem with this approach is that it may also tend to warp the steel (read: it will almost certainly warp the steel unless your geometry is perfect).

Search for "expanding grout" to find these products, but again: they may destroy the safe entirely. Also, you still have to drill holes through the steel into the concrete/ceramic so it's not like they're labor-free.
posted by aramaic at 12:56 PM on September 25, 2012

semi-ceramic/concrete-like interior compound that is supposed to release water on being immolated. (This is to protect the safe contents in a fire. Clever Victorians.)

So, fire will produce some spectacular change in this concrete-like material, right? Have you tried having a go with a blowtorch for, like, science?
posted by Dr Dracator at 2:01 PM on September 25, 2012 [1 favorite]

IAmABroom's idea seems the most plausible for my current tool set, so I used a metal cutting abrasive disk and managed to put in a deep scratch, so it looks like if I get the depth right and the workpiece/tool arrangement right, this might do.

i am also off to borrow a hammer drill to see if I can granulate some of the concrete material, as I have a good bit of it exposed on the back side. if i can relieve it, then the metal shell is saws-all territory. good ideas, though.

since this is an art project, the total budget is $0, and I'm already over that in blades.

Dr Dracator, I think I'd have to burn the house down to get the material to change, and I have no idea what it changes into once it's been heated. I'll torch a piece and see, though. There are about 700 pounds of it in the safe, though, and it's distributed all around the content box. (FYI, the safe is about 3'x3'x2 1/2' and the contents box is about 1 x 1 x 1. The rest is filler material and a door that would kill you if it fell on your foot. ) I drilled in from the back, took off the interior plate, disassembled the lock mechanism and manually operated it to open, then remove the door, after grinding off the hinges. It has been a little pesistent obsession for the last few years. about 20-30 hours invested so far. art is something that can take its time. good thing, too.

thanks for the ideas, gents. they ruled out plasma and acetylene and saved me a bunch of effort.
posted by FauxScot at 2:27 PM on September 25, 2012

Be careful.

I'm not sure when asbestos went into widespread use, but if I were you I wouldn't be betting that there is no asbestos baked into the ceramic. It was very frequently used in things that had to be heat/fire proof.

In which case anything that creates dust would be the last thing you want.
posted by deadwax at 1:55 AM on September 26, 2012 [2 favorites]

Update... if this is indicative of other models (it's a Mosler), the shell is riveted in place. The back is a single plate, held by 3/16th steel rivets at about 4 inch intervals. These are hammered flush and perhaps they are visible on a new unit, but this rusty 100 year old monster concealed them well. The plate is soft steel, and with a suitable lever, can be ripped off. The structure of the safe appears to be 1/4" thick 2" angle iron, hot rolled.

At this point, I think I will dispense with cutting of any form in favor of just ripping off the steel plate. Looks like a few hours of work, if that much. Once I got the rear plate started, it only took 15 minutes to remove the whole plate.

A hammer drill does drill the "concrete" out, but very slowly. However, it fractures using a standard carbide point and 2 pound hammer and can be removed at a rate of about a cubic foot per hour, if you're good with a hammer and chisel. (I have an advantage there as a mable sculptor, but a small jackhammer (electric) would do a decent job, I bet.)

Anyone needing to enter one of these in the absence of a combination, feel free to email me for pix. I think in terms of simple access, peeking inside one of these is a 2 hour job and needs a minimum amount of tooling. The most elegant way is obviously cracking the combination, but this one had a sticky bolt that made it impossible to open from the front, even with the proper numbers.

The safe is a Mosler from 1881, but I have no model number.
posted by FauxScot at 12:27 PM on September 26, 2012