IN MAY WE BEGAN OUR Dis-

blades t h a t will follow the contour of the work, and a double-edge deb u r r i n g tool smoothes the edges of sheet aluminum. Whichever tools you use, make sure you always deburr.

cussion of sheet metal tools with those used to d r i l l and bend metal. Now we'll explore the tools that deburr, countersink, dimple, and rivet sheet metal. But first, a short review of the basic steps in sheet-metal construction. After you cut the pieces of metal to size and bend them if needed, you drill holes for rivets and hold the adjoining pieces together with Clecos to check their fit. Then you disassemble the pieces, deburr the holes, and countersink or dimple them (as appropriate if flush rivets are called for). If required, you prime the parts, reassemble them with Clecos, and rivet them together.

Dimpling & Countersinking

If your project calls for flush rivets, dimpling or countersinking the rivet holes is your next step. AN426 rivets require a 100-degree countersink, and they should fit securely in the recessed hole and be flush with the metal's surface.

Deburring Holes

Drilling a l u m i n u m creates a burr on both sides of the metal. Not removing these burrs can keep the r i v e t s from f i t t i n g tightly, and loose rivets can lead to separation of the riveted parts. There are several tools made just for deburring, and several of them do double duty as a countersink. The 90

first is a speed deburring tool. You simply place it in the hole and twist the handle several times. You can do the same thing by manually turning a drill bit larger than the hole. Drill-powered burring cutters are similar to countersinks, but they don't have a pilot hole. A swivelhead deburring tool has swiveling

Use a countersink drill on metal thicker than 0.040 inch.

As a general rule, use a countersink when the thickness of the material is greater than the thickness of the rivet head, and use the dim-

pling method on thinner material. Generally you'll dimple metal 0.040 inch and thinner and use a drill countersink on metal thicker than 0.040 inch. A dimpling tool consists of a set of male and female dies that correspond to the size rivet you'll use. The male die is shaped to match the rivet head, the female die corresponds to the degree of countersink, and the two are squeezed together with the aluminum in between, pressing the metal around the rivet hole into the proper shape. To achieve maximum strength, the rivet must fit securely. Dimpling stretches the metal somewhat, usually opening the hole to the proper size without additional drilling. The dimpling dies fit in a handy tool, like the one pictured here made by Avery Tools. You simply place the metal in the tool and strike the shaft with a wood mallet. You can replace the dimple dies with a rivet set to drive rivets, and the tool ensures proper alignment for both procedures. Other types of dimpling tools include hand squeezers for hard to reach places and blind rivet dimplers for use with (obviously) blind rivets. When looking for a drill countersink, getting one with a micro-stop cage will enable you to precisely control the depth of its cut. Riveting

Because you'll be spending a lot of time with this pneumatic tool, selecting the right rivet gun is important. Available in different sizes, a 2X or 3X is the most popular for homebuilders. The X denotes the gun's length, and the 2X is adequate for driving rivets up to 1/8 inch. For larger rivets you'll want a 3X gun, which hits the rivet at a slower rate but with more force. Try a rivet gun before buying it and make sure you can control the strength of its impact with the trigger. This variable trigger works in concert with an air regulator, which give you more precise control by letting you adjust the air pressure. (And once you buy a gun, oil it after every use by placing a few drops of non-detergent oil in its air inlet.) Never fire a rivet gun unless the rivet set is hitting

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something, like the rivet you're driving or a leather-covered piece

of wood, because the set can be-

come a deadly projectile. Rivet guns do have retaining springs that keep the rivet set in place, but not against a "free shot," where the set isn't impacting anything. Guns come with a quickchange spring that works with straight and flush rivet sets, separate springs for each, and a "beehive set" lor an offset rivet set.

Squeezing is another way to set

rivets, and hand and pneumatic rivet squeezers are available in different sizes. If possible, m a n y builders prefer to squeeze a rivet rather than drive it, so you'll proba-

bly be using this tool a lot, and you can use it to create dimples. Rivet sets are the tools that transmit the force of the gun or squeezer to the rivet. Sets are not interchangeable between guns and squeezers, so you'll need gun and squeezer sets for flush rivets and for each of the universal rivet sizes you will be using.

Sets used for flush rivets have a smooth, polished face, and some

gun sets have a protective rubber ring that keeps them from walking across the metal while you're riveting. Be careful not to scratch this set because it will transfer the mark to the sheet metal. Sets for universal rivets are cupped to fit different size rivets. To hold these expensive pieces of metal, make a holder for your rivet sets by drilling appropriately sized holes in a block of wood. With a rivet gun you use a bucking bar to form the shop head, and you should get a selection of different sizes and shapes to fit the different parts of your airplane where you'll need to reach the back side of a rivet. A rivet squeezer uses a second set with a smooth face to form the shop head. Many sheet-metal a i r p l a n e s , Van's Aircraft RV series in particular, call for back riveting, where you use 92

the rivet gun—not the bucking bar— to create the shop head. This calls lor a back riveting set, which includes a large metal place that acts like a stationary bucket bar that contacts the rivet's machine head. A number of kit aircraft are built using blind rivets (these are not Pop rivets, a trademarked brand not designed or intended for aviation use). You set blind rivets with a hand or

tection is essential. Drilling, grinding, and other metal tasks create shavings that can be thrown into an eye. When using a die grinder (highspeed grinder), wear a full-face shield because it throws metal shavings from the aluminum and pieces of fiberglass from the c u t t i n g wheel with tremendous speed and force. Wear hearing protection when riveting. And when drilling, make sure

pneumatic puller, which draws the metal shaft through the center of the rivet to form the shop head. Nice-to-have tools include a rivet gauge, which is specific to the different size rivets and checks for the correct shop head diameter and height. A rivet cutter trims rivets to the proper length before you install them. A rivet fence will help you accurately lay out where to drill your rivet holes.

your body parts, and those of any helpers, are out of the bit's path. Before you change drill bits, disconnect the drill from its power source. This goes for changing sets in rivet guns, too. And don't fire the rivet gun unless the set is impacting something. The tools we've discussed are the basics used to build sheet-metal airplanes. The list of additional tools you can purchase is endless because there's a tool for almost every job. Consult your kit manufacturer for the tools you will need to begin your project. CAA