Metal Gurus?

[nickb]

No, not that kind of metal. I'm talking about the element Al.

Not an electronic question but where else can I find such a nice bunch of people to bounce this off?

I have an amp on the bench and the owner gave it a recap. Now the owner seemed to think that only way to get the chassis out was bend the aluminum front panel ends and then bend them back when replaced. So, I have this on my bench now (for new work) and it has these nasty kinks in each end of the approx 16 awg aluminum panel.

My question is, does anyone have any advise on how to de-kink it? BTW, the front of the panel has a stick-on mylar type film for the legend so I have to treat that with some care. Also, it is possible to remove the panel as one piece along with the film.

[J M Fahey]

Sorry but if you mess with aluminum it will crack ... if it isnīt already doing that

Leave it alone, itīs a customerīs problem, donīt make it your own.

[gui_tarzan]

Like JM said, it's already screwed up. It's cast as an ingot, then rolled out to the thickness it needs to be. Since the original form is cast, it has a "grain" running through it that cracks easily. the only way to fix it at that point is to weld it and grind it down. Good luck with that. A person could try to use a hammer and block (behind it) but for an amp you're talking about a ton of work that will be mediocre at best.

[Leo_Gnardo]

I don't think you'll be able to de-kink it, further it would be a good idea to apply some L-brackets to minimize the possibility of having the panel crack off at the bend.

I've seen a couple of Burman amps made late 70's, with chassis metal very thin and tended to crack at the corners just from the weight of the transformers. Otherwise, brilliantly designed and terrific sonic choices. I offered but the owner declined to have brackets installed.

Reminds me a bit of a nice old Gibson hollowbody a customer brought me. His grandpa had no clue about how to get spray cleaner into the pots with a snorkel-tube from the can. So Gramps just took a motor jigsaw and cut a panel out of the back of the guitar. OW!

[J M Fahey]

Ouch !!!!!

[Mick Bailey]

Aluminium cracks when bent because of work hardening. Sheet aluminium has good properties for ductility and malleability in its annealed state. That's how an entire wing of a Morgan car can be rolled, beaten and bent out of a sheet without cracking - it needs to be repeatedly annealed.

Annealed aluminium sheet is soft and has relatively poor strength, so in manufacturing a chassis it begins as a sheet known as 'half-hard'. During the rolling process at the mill it gets rolled, annealed, rolled, annealed etc., and each time the thickness is reduced. The final passes through the finishing rollers harden it up slightly so that the metal isn't too hard (and therefore prone to cracking), but is stronger than the annealed metal.

When the chassis is folded up during manufacture, the metal further work-hardens along the fold, so the metal is stronger along that line. If you try to subsequently bend the chassis at right-angles to the line it will initially bend some distance away, because that metal isn't as hard. As it bends further the new bend work-hardens and approaches the strength of the fold, so eventually the fold will bend.

So this gives a distorted bend.

The trouble is, there are now two areas of increased hardening - the corner of the original folded line and the bit that was bent. If the fold radius was very tight at manufacture it could even crack along the line. This isn't anything to do with grain, its the fact that one side of the neutral axis of the bend has to be compressed and the other stretched, so the overly-hardened metal tears on the outside of the bend.

When you attempt to return the chassis back to its factory-original shape you're fighting those areas of localised hardening. They also become slightly springy. This makes it much harder to re-form the metal due to work-hardening hysteresis; greater force is required to return the metal back to shape than that which bent it. In addition you may have some punched or drilled holes and the metal will want to fold through the centreline of those.

I have a hydraulic press, chassis folder and all kinds of sheet metal tools, and a bent chassis can still be a problem. Try re-shaping a crushed Maz 18. Sometimes hand pressure and shaped wooden blocks, or dollies and panel-beating hammers can achieve a lot, but often the chassis needs stripping and gentle local annealing using a gas torch before attempting to re-shape the metal. Aluminium melts below red heat, so there's the 'soap trick' I use - just mark a line of ordinary white hand soap and only heat until it begins to turn brown.

[J M Fahey]

Same here.
I make my own chassis out of aluminum so I have shears, presses and punchers with all kinds of dies, from 3 mm Leds to different kinds of switches to D shaped fuseholders to 60 or 100mm slider pots, same "knife blade" tools are great to cut rectangular holes for transformers.
I have a supplier who custom makes sheet aluminum for me, I pay the same price a kilo as commercial off the shelf aluminum sheet but the real saving is that I have NO scrap left over because they make custom sized sheets for me.

Truth is, they roll it (annealing between passes as Mick describes) , cut sheet to the exact size I need ... and leftovers go back to the oven .
The Factory looks (and smells) like a 30's film description of Hell, no kidding.

And they supply in 3 hardness levels:
> extra soft (you can bend it by hand) , they call it "recooked" because they heat sheets with a line of gas burners and immediately cool it with a shower of cold water; contrary to what happens with steel it softens aluminum big way and itīs suitable (cut into disks) for turning kitchen pans, pizza molds, etc. or being stamped to different shapes (think teddy bear shaped cake molds or eating trays with stamped divisions)
> semi hard (what I use), meant to be cut, bent *once* and easily punched, great for chassis
> extra hard , meant to punch cut (in a suitable press) flat shapes for different mechanisms (think arms and levers, light duty cams, etc.)

Customers who sometimes come with me always amaze at a little trick: I hit with a coin unlabelled sheet aluminum propped against a wall and know which one is which.
In fact itīs easy: soft is dull, like hitting a sheet of lead, extra hard rings almost like a cymbal and semi hard sounds just "right" ... for aluminum.

Once by mistake I brought an extra hard sheet mixed with the others and it was inconvenient to travel "just for one" so I tried to use it: it was somewhat more difficult to cut, edges were very sharo and clean ... and it cracked miserably when I tried to bend it .

Oh well.
In the end I cut it in small rectangles to hold jacks in cabinets.

[nickb]

Interesting stuff guys. What has happened here it that the front panel was once a flat sheet with ears that stick out past the chassis by a couple of inches. The owner bent the ears slightly, maybe 20 degrees and then attempted to bend them back. And, just like Mick said, it bent along a new line. It's pretty thick, maybe 2.5mm, and there is no sign of fracturing so I suspect it could be beaten back into shape. I'm not going to attempt it, I just said I try to find out for him.

[Mick Bailey]

He'd have to take off the legend - maybe with a hairdryer and patience. Sounds like blocks of hardwood and a hammer would work. So long as the hammer is only used with a block of wood and the metal is well-supported at the back it should be fine, particularly with that thickness of metal. Even squeezing the bent ends between two pieces of hardwood in the vice would probably give some improvement. I use pieces cut from old catering polypropylene chopping boards for this kind of thing.

The problem with those stick-on legends is stretching the material or even tearing it during removal.

[nickb]

He'd have to take off the legend - maybe with a hairdryer and patience. Sounds like blocks of hardwood and a hammer would work. So long as the hammer is only used with a block of wood and the metal is well-supported at the back it should be fine, particularly with that thickness of metal. Even squeezing the bent ends between two pieces of hardwood in the vice would probably give some improvement. I use pieces cut from old catering polypropylene chopping boards for this kind of thing.

The problem with those stick-on legends is stretching the material or even tearing it during removal.

All things considered, I think the best approach is to get a new panel fabricated. Not only are results guaranteed but it'll look and be brand new.

Thanks for all the input!

[J M Fahey]

blocks of hardwood and a hammer would work.

+1