Sheet Metal Working
(Flat Sheet)

Rev. 2000-06-22, 2003-02-26, -05-13, 2005-01-31, 2007-01-02, -07-10

The kind of sheet metal working that a furnace glassworker does is very simple. In fact, most of what needs to be learned is a few suggestions about tools and a couple of tricks. Of course the easiest way to get something done in sheet metal is to take a pencil or CADD sketch to a sheet metal working firm and have them make it. See below on talking to sheet metal shops.

WHAT TO MAKE OF SHEET METAL - Trays for catching glass. Shells for annealers, garages, pans for insulating castable bases.

SHEET METAL - For all practical purposes, what we are talking about is plain and galvanized steel sheet in 24 or 26 gauge. Gauge is a measure of the thickness of the steel although it is now specified as the weight per square foot. Whenever gauge is mentioned, immediately we know that a larger number is a smaller measure, whether shotguns, steel or wire. An easy marker is 16 gauge steel which is just about 1/16" thick, and very hard to work. It is also the thickness of common steel tubing. Sheet metal thinner than 26 gauge is most likely to be found as flashing and be too thin to stand up well. Most sheet metal shops will not have steel thinner than 26 and won't have equipment to work heavier than 16 or 18 gauge. Modern metal gauge is actually based on the weight of the metal per square foot instead of thickness.  These are far enough apart so that weighing a moderate sized piece will yield the gauge, or one can keep samples.

Extracted from table at Sheet Metal Gauge Size Chart - Engineers Edge
(Th=inches, Wt=Lbs/Sq.Ft., Oz=oz/20 sq.in 4x5, 2x10)

DESIGNING FOR SHEET METAL - The overwhelming factor in sheet metal design is simple: bent metal is strong, flat metal is not. When thinking about a design, to function in sheet metal it must have bent flanges and solid corners or it will be weak and wobbly. A corner has to have a bent flange, it will be stronger if both surfaces are bent. Large surfaces are commonly given diagonal bends of a shallow degree to create an X on the surface to reduce drumhead type flexing and noise, but this is very difficult to do cheaply.

TOOLS FOR SHEET METAL - BENDING, CUTTING, FASTENING

BENDING - Bending sheet metal is done by clamping the metal firmly in place and bending along an edge. In commercial shops, this is done with a brake, which weighs a couple of thousand pounds and has a very thin edge to fold against and slots in the edge so a previous fold can be fitted into the machine. (Diagram in A below.) Shops also have a tool that makes matching bent forms to join corners strongly without other connectors.

For home/artist use, there are three steps down from the brake, but I would like to mention that it is perfectly possible to get a sheet metal shop to cut and bend L and U-channels of metal, which goes very quickly, from which you can assemble boxes. For example, if I am building a box 18 by 24, 12" high, I could ask a shop to sheer off a 4' wide sheet, two pieces 13.5" wide (by 48" long) and to bend a 3/4" flange along both long edges. This would give a crisp strong bend on the long sides and I could deal with the short sides as mentioned below.

Step Down #1 - A SIMPLE BRAKE - Part of what a brake does is grip the sheet metal and part applies even pressure all across the bend. If enough bending is done, a couple of angle irons will make the job easier. But there is a problem with the hinges. On a real brake, the hinge center line is exactly in line with the bend (A above), making life easier, but to do this, a heavy structure keeps the edge straight. Carving away at the steel angle could result in a home-built brake with the hinge in line, but that isn't simple any more.

The answer is to make the brake out of regular hinges and angle iron and use added strips to bring the bending surface out (B above) In the diagram a bolt holds the lower angle against a 2x4 with a wood spacer the thickness of the hinge barrel between. Above, the moving part is welded to the hinge which is screwed to the top of the 2x4 and a wood spacer brings that surface out also. The sheet metal is inserted vertically and tightened (and clamped), then the hinged angle is pulled forward to bend the metal. Mounting the lower angle as shown permits bending past 90° to allow for spring back. In this design, the metal is sliding past the upper wood, not just being pushed as in the ideal.

Step Down #2 - CUT TO FIT WOOD - The easiest way to get a crisp corner without a brake is to sacrifice some wood.  That is, cut pieces of 1x2 or 2x4 to the exact inside length of the box and use this block, held in place by C or vise grip clamps (below) to produce good bends, finishing shaping with a rubber hammer.   Depending on the width of the flange, a 2x4 can have grooves cut to fit folds.  Also, with planning, the long bends can be done first, then the wood cut shorter for the short sides.

Step Down #3 - USE SPECIAL WIDE PLIERS - I am not sure this is really a step down, because these pliers are used in sheet metal shops all the time.  They are Vise Grip pliers with flat metal plates with beveled edges, as shown in the upper right of this image.
Slight upgrade - (C)  It is very difficult to juggle angle iron and keep it aligned while clamping to the sheet metal.  So take two pieces of angle iron and drill an place a bolt at one end after carefully aligning the edges.  Sheet metal placed between the irons can be clamped on the other end and then use like a giant version of the wide jaw pliers, clamping with C-clamps or Vise Grip clamps, bending against a flat bench or concrete surface.  2006-07-11 edit

CUTTING - Cutting in a sheet metal shop is first done with a shear, that cuts only straight lines but does is very fast.  In the home shop, it is more likely that it will be done with tin snips or a nibbler for small cuts.

FASTENING SHEET METAL - Depending on the metal, the joints may be fastened by

rivets

usually Pop-Rivets these days.  Rivets are blunt, headed shapes that must have a drilled hole.  They grip by forming a second head on the other side of the metal, a head formed by hammering with old style rivets or by squeezing a special tool with Pop-Rivets, which can be installed from one side.

screws

for sheet metal are two kinds - self-threading require a hole and cut thread in the metal to match their own, while self-drilling have a small drill bit on the tip and can be installed with a power drill without first making a hole.

soldering

the joint is usually folded for most of the strength, then solder to seal it and hold it together.

welding.

less commonly used, due to the thinness of the metal, but useful with unplated steel, as making tanks, etc.

An example of sheet metal working can be seen at my glass garage.