Sheet-metal basics
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Wood Magazine

Sheet-metal basics

Sheet-metal basics

Sheet-metal basics

As a woodworker, you may feel slightly lost venturing into the foreign territory of sheet-metal fabrication. Because you'll need to wander in there a little way when you build the cyclone dust collector (below), we've put together a few pointers to ease your trek.

You'll need a few things You may have some of the tools required already: a portable drill, twist-drill bits, scratch awl, center punch, and measuring tools. For large-diameter curves, a trammel like the one in the photo above will come in handy.

You'll also need snips for cutting the metal, along with pop rivets and a pop-rivet tool for joining the pieces of metal you cut. You can buy them at home centers or hardware stores.

Snips. For cutting sheet metal, we prefer aviation-style compound snips, shown above. Avoid the old-style tin snips with long handles and short jaws; it's a hard tool to cut curves with.

Aviation snips cut in specific directions -- straight, left curve, or right curve. Handle color signifies the direction.

Pop rivets. We like steel pop rivets 1/8" in diameter for light sheet-metal work. (The diameter indicates the size of the hole the rivet fits through.)

You'll find these rivets available in three lengths -- short, medium, and long, corresponding to grip lengths of 1/8", 1/4", or 1/2". Choose the nearest grip length longer than the thickness of the materials you're joining.

Pop-rivet tool. You can't install pop rivets without a special tool, shown below. The tool is inexpensive, and any manufacturer's tool works with any brand of rivets, as long as they're the same diameter. Most pop-rivet tools feature interchangeable nosepieces, allowing one tool to set rivets of any diameter. A long nose like the one shown reaches into tight spots, making the tool easier to use and more versatile.

Safety gear. Sheet metal has sharp edges. And because it's springy, it sometimes seems to jump at you maliciously. Without protection, that can lead to a cut. Just as bad, tiny metal splinters can get under your skin, and they're harder to get out than wood splinters.

So, wear gloves. The yellow Kevlar-fiber cutproof gloves shown above afford great protection. But any kind of work glove, even cotton gardening gloves, can save you from a painful cut. Wear long sleeves to protect your arms, too, especially when cutting curves in metal with snips. And you should always wear eye protection.

Laying out the lines You can't count on your trusty sharp pencil for drawing layout lines on sheet metal. Pencil marks won't show up well enough or last long enough to be of much help. Felt-tip pens and permanent markers write well on metal, but their wide lines hinder precision. Instead, scribe cutting lines with a scratch awl.

You'll also need a straightedge with a nonskid back. If you can't find one at a hardware store, try an art-supply store. Or, you can make a suitable substitute by sticking strips of double-faced tape to a steel ruler. Press the exposed face against your sleeve a few times to reduce the tape's tack before use.

Straight-leg dividers or a trammel work well for scribing arcs or circles on metal. To prevent the pivot point from skidding off center, stick a piece of masking tape or duct tape at the center. If the inside of the circle will be waste, you can simply centerpunch the centerpoint to prevent skidding.

Cutting and joining tips Before making the parts for your project, lay out some straight and curved practice lines on a piece of sheet metal. Then, cut along them to get the feel of your snips. Some snips, like the yellow-handled one shown above, shear out a narrow strip to make a cut. (Often called duct snips, this one cuts straight or curves, and works well on stovepipe or other cylinders.) With this type, make sure you take the strip from the waste side of the cutting line.

When cutting with snips, closing the jaws all the way leaves a jagged-looking edge on the metal, which looks crude and is difficult to eliminate. Instead, close the jaws most of the way, release pressure on the handles to let the jaws open, and then push the jaws forward to continue cutting.

A lap joint is the simplest way to join sheet metal. And pop rivets provide a quick, effective means of fastening a lap joint, as shown at left.

To make the joint, simply overlap the parts by at least 1/4" (or as specified by project instructions). Then, drill rivet holes through both pieces.

Mating holes must align exactly, so the best approach is to drill through both pieces at once. To do that, tape the pieces together or clamp them, using C-clamps or bar clamps and clamp blocks.

Mark the hole positions, and centerpunch them. For a neat job, align the rivet holes and space them evenly. Drill the holes with a twist-drill bit the same diameter as the rivet. Drill the first hole through both parts, and then insert a rivet (without setting it) to keep the parts from creeping out of alignment while drilling the remaining holes.

(Designer/builder Jan Svec used tape a little differently for the cyclone project. He cleaned the joint area on the top face of one piece with lacquer thinner, and then applied a strip of double-faced tape to it. Next, he cleaned the underside of the piece on the other side of the joint, and brought the pieces together. The tape held both parts in position while he drilled the holes, and it helped seal the riveted joint.)

It's easy to remove a pop rivet, too. Simply drill through the rivet head with the same size bit you used to drill the rivet hole.

Written by Larry Johnston with Jan Svec Photographs: Hetherington Photography

For more information on dust collection solutions, visit the Dust Collectors section in the WOOD Store.



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