sheet metal

I just installed a dust colletor and will be piping to different tools. I would like to make some custom collector hoods at the tools out of sheet metal. Never worked with it before. Any sites on forming and bending it?
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I have had great luck bending sheet metal with a long piece of angle iron and a rubber hammer. Make a pattern from cardboard that you have fitted to the use. Then transfer that to the metal with a sharpie pen and "break" it over the angle with the rubber hammer. I set the angle on a saw horse and lay the metal over that one and another one. A clamp or two will hold it in place until you get the break going. I finally bought a harbor fright metal brake but I still use my rubber hammer most of the time.
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Lots of sites out there. Google "bending sheet metal" or even ask in rec.crafts.metalworking newsgroup.
--


Regards,
Joe Agro, Jr.
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wrote:

Go to a HVAC shop. They make better hoods than you can without rolling gear, and the prices are quite reasonable.
Don't make it, modify it. Most of my workshop ducting is cast-offs from factory ducting. It's a lot easier to take an existing hood and modify the shape than it is to make it from scratch. Even if you just keep the pipe mounting flange and replace all the "hood" itself.
Make protoypes in cardboard. For everything. You may even hook them up to the extractor and test that the shape works for effectiveness and for visibility.
Smoke matches (from a home repair shop, as sold to gas fitters) are useful for visualising airflow. Failing that, _natural_ jute or cotton string smoulders well enough to use.

I use 1950's school or apprentice-level metalworking textbooks. Once you understand how to draw a "development" (the flat plan of a complex curved shape) then the rest is just donkeywork.
The basic development technique is to lay out out the major edge as a baseline. Straight for a straight edge or a cylinder, curved for a cone. Then use dividers to lay out measurements along this edge and up from it, according to the shape you need. The way to work out the radius of the curved edge for a cone is to imagine the cone extended all the way to a single point, then measure the radius from this virtual point. Practice with cardboard first!
The tools vary somewhat, depending on what materials you use. The hardest part is cutting it out to shape - I use a few grand worth of plasma cutter, which is just about the most fun you can have in a workshop. A set of "aviation snips" will do much of it, but they're a bit of a chore to use. Roller shears, nibblers, notchers all help, but they're nothing like plasma. A bench guillotine (HF grade) is cheap and useful, but won't do curves. If you ever see a "Beverley shear", then snap it up - you can always sell it to armour makers later.
The best material for rolled or folded ducting is thin steel, hot-dip galvanised. You can't shape this into a compound curve though (which you rarely need for ducting). The easiest material to work with using limited tools is thin aluminium. Caravans / trailers / aircraft are clad with thin aluminium sheet. Use that scrapyard !
To form, bend and roll it, you use a press brake and a rolling mill, maybe even a swage roller. These are expensive, so you're not going to buy them just to make some ducting (why having it made is a good idea - they have tools you don't, so they can just do it more quickly).
For folding, you _need_ a folding machine. These can be bought affordably, or you can make your own. The simplest version is two lengths of heavy gauge L-section angle iron a few feet long, clamped by a bolt at each end and held in a bench vice. The complicated version (bought for <$50 from Taiwanese importers) has a third L section on bearings, which is used to do the actual bending. If you don't have this, use a big mallet and work the whole edge down together in gentle stages. An improved bending machine has the clamp nuts welded to the bars, a wing-nut handle welded to the bolt heads, and light springs under the bolt heads to keep a light friction on the clamp, even when you undo the bolts.
To make tubes you can use rollers (expensive) or you can use the mallet over a former (tricky). Neat cones are hard. Scrap steel tube is your friend here - I mostly use scrap gas cylinders in a range of sizes. You can also make formers by woodturning.
Bare edges should have a "wired" or at least a "beaded" edge. Beaded edges are just hemming for sheetmetal. Fold 3/8" - 1/2" of the edge up neatly to a right angle, then bend it down with the mallet and maybe tighten it up with a cross-peen hammer. Wired edges are the same, but you place a wire (diameter two or three times the sheet thickness) inside the bead as you fold it down. Use a sharp-peened cross-peen hammer to tuck the edge in neatly under the wire. Big square corners should be wired, not beaded, because the wire helps to tie the edges together - you can't work a continuous fold around the corner.
Neat joins into tubing use swaged changes in diameter. This is _very_ hard to do without a swaging roller, so most people cut the overlap into petals and fold those one-by-one, as neatly as you can.
Metal is joined by overlapping and pop riveting. Ducting vibrates, so any bolts must have shakeproof washers or locking compound - or else it _will_ fall apart!
You can't make it airtight. That needs rollers and swages, or else genius. A substitute for genius is copper. This is a ductile material, so you can work it by hand into shapes that would be unattainable in steel. Not cheap, but sometimes a hand-worked copper nozzle or adapter will let you make something you couldn't otherwise do. Aluminium is also fairly ductile, but not quite so easily worked.
If you hammer-work the aluminium or form compound curves in it, you should anneal it before starting, and between workings, or else it will go brittle and crack. You can feel this when it hardens up under the mallet, Wipe aluminum or copper with liquid soap (or cooking oil, jam, maple syrup, or many other kitchen things) and heat it with a gas torch until the soap blackens. Allow to cool naturally in air.
Tools you really ought to have (buy them if you don't) are:
- Leather gloves. Shears leave _wicked_ slivers on edges. Be careful! These things are evil.
- A hand-held 90 countersink, used for deburring drilled holes. One of my most-used hand tools - well worth getting a good one.
- A hand-held tapered reamer or broach, used for slightly enlarging drilled holes. Quicker and more accurate than filing.
- A real sheetmetalworker's deburring tool (knife sort, for edges). Cheap, quicker than filing and saves a lot of fingers.
- Appropriate range of good files. For aluminium you need new files, rubbed with chalk before you start - otherwise they clog.
- Mallet. Ideally you need a rawhide mallet as it doesn't mark the edges like a hammer and it doesn't bounce like a rubber mallet. Cast iron "Thor" hammers with rawhide faces are more compact than simple rolled rawhide. You might use a shot-filled "dead-blow" plastic hammer instead, but _don't_ use a hammer. A malleted edge can sometimes be adjusted if slightly inaccurate. A hammered edge is in there for ever.
- Drill bits for rivet holes. Buy a few of the right size and ideally ground with split-points or proprietary magic grinds for sheetmetal work. These save deburring time, compared to the usual jobber's drills, and they don't skid around when starting a hole.
- Automatic centre punch. You can use this on poorly supported sheetmetal (even onto a clenched fist inside) where a hammer would dent the metalwork. It's also usable single-handed.
- Pop-rivet tool. Simple lever style, but get the sort with a _three_ jawed jaw, not a two-jaw (ten bucks, not two bucks). You don't need it for light work, but you'll be glad when you get to those big rivets. A lazy-tong riveter is heavy and clumsy to use. If you have a low boredom threshold, think about a pneumatic riveter.
- Clecos. These are temporary holding pins that look like a cross between pop-rivets and hairgrips. They're not widely known outside the aircraft industry, but they're an _enormous_ timesaver. Several different styles and makes. It doesn't matter much which you use, but some have a thumb nut, some are applied with magic pliers. You should find a handful of these in a size to match your rivets fairly cheap from eBay.
- The air force manual of how to repair battle damage on fighter aircraft (try eBay). This is the _best_ manual I've got on how to do this sort of sheetmetal work.
- There's also a pop-rivet variant that inserts a nut into sheetmetal and can do it from just one side. You don't _need_ this; you can use nuts and bolts on each side, or you can pop-rivet on an external tab. But they're a great tool for anyone who works sheetmetal or attaches things to cars.
- A small range of shaping hammers and dollies. These are for forming tight internal shapes, so you probably don't need them unless you're making a "nozzle" to fit something specific. Any old lump of heavy iron is probably usable - metalworkers collect their favourite box of old shapes, or make new ones with a grinder.
--
Smert' spamionam

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These are called Riv-nuts. Very handy.
--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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Handy, hell... they're grrrrreat!
--

-Mike-
snipped-for-privacy@alltel.net
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http://www.thesheetmetalshop.com/pn/index.php

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wrote:

I have made several DC shoots, hoods, cyclone, etc from sheet metal. I built my own planer hood because the manufacturer one was $70. The 4" connection is a small coffee can soldered and metal taped to the hood. No plans or sites needed.
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If you get good at creating custom hoods, I'd like one with a mini blast gate to connect my old Boston pencil sharpener to the DC. Everything else that makes shavings in my shop is "online" except that, and it's the one i never remember to empty until it's so packed full and a PITA to empty! ;) --dave

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With a triac or a relay?
Patriarch
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On Sun, 23 Jan 2005 20:59:47 -0600, the inscrutable "Rich"

CAUTION: Buy some aramid gloves before you even START to work with sheetmetal. It's sharp and WILL cut you to the bone if you give it half a chance.
I just returned Audel's "Millwright's & Mechanic's Guide" to my library. It has formulae and info on sheetmetal for hoods. See if your library has a copy. Also look for books by Tim Remus and Ron Fournier while you're there.
Harbor Freight has sheet metal benders and fabrication machines.
--

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