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SHEET METAL—A BEGINNER’S EXPERIENCE PART TWO BY RON ALEXANDER from Aircraft Spruce
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SHEET METAL—A BEGINNER’S EXPERIENCE PART TWO BY RON ALEXANDER

You now have in front of you the skeleton for the horizontal stabilizer. It looks like an acupuncture exhibit with clecos sticking out everywhere. We have clecoed it together to ensure proper fit and alignment before we begin drilling holes. The next step is to get out the pneumatic drill and begin drilling. Do you have to use a pneumatic drill? No, but it will make the job much easier. As a matter of fact, I would suggest that you always purchase good quality tools. (An added benefit in attending a builder’s assistance program is experience with tools. Our three builders came with no tools at all. They were able to use the center’s tools which helped them decide what they needed to purchase). Back to drilling, use a variable speed drill. It goes without saying that you will need to determine the proper size drill bit to use to drill through the pilot holes.

During installation, rivet holes are drilled slightly larger than the rivet diameter. When properly installed and driven, the rivet will expand to fill the hole. Drill sizes are based on numbers and letters. You will usually encounter a 3/32 or 1/8 inch diameter rivet. Drill sizes for each type of rivet are as follows:

 
Rivet Diameter
3/32
1/8
5/32
3/16
Drill Bit Size
#40
#30
#21
#11
 

The drill size of each of these is slightly larger than the rivet itself. This allows the rivet to go in place without forcing it. During the process of driving or squeezing the rivet, the shank will swell up and completely fill the hole.

After all of the holes are drilled, you will then disassemble the part. This will take about 10-15 minutes versus the hours spent assembling the pieces. Our builders look a bit dismayed after seeing how quickly their work could be disassembled. Words of caution from our three builders—mark the parts before you start the disassembling process. Well, you say, I thought the parts were identified with a small label? That is true in many cases but you are going to remove those labels for priming. So be sure to not only label what the part is but also where it goes. Mark the top, bottom, etc. The more identifying markings the easier the part will go back together for final riveting. Be sure to use a blue Sharpie marker. Why blue and not black? The blue marker will be more easily readable through the green primer that will be applied. Black will be covered up. Another one of those nice to know tips. By the way, do not use a pencil to mark on aluminum. The lead from the pencil can cause the aluminum to corrode.

Our three professionals are now looking at a number of parts lying in front of them that once were assembled together into a fine looking piece. What’s the deal here? The deal is that you must now “deburr” the drilled holes. No, deburring is not a type of haircut given to a military recruit. Rather it is a sheet metal procedure whereby excess metal is removed from the holes you have drilled. Drilling aluminum causes a burr to form on each side of the piece. These burrs must be removed. Failure to deburr could cause a separation between the two pieces being riveted together or it could cause the rivet to not fit tightly. A special deburring tool is normally used for this task. Another tip—you must be very careful to not remove too much metal when deburring the holes. A couple of circles with the tool in each drilled hole will be sufficient. An oversize drill bit may also be used to deburr. Hold the bit between your fingers and twist it to remove the excess metal.

Because you will be using countersunk rivets in several areas, these particular holes will have to be either “dimpled” or countersunk to allow the rivet to be recessed into the metal so it will be flush on the surface. Machine countersinking actually removes a portion of the metal. This is usually reserved for metals with a thickness greater than .040. Thinner metals may be dimpled. Special dimpling tools are available that basically consist of a set of dies (male shaped to match the rivet head and female corresponding to the degree of countersink) that are squeezed together with the aluminum in between. This will press the metal surrounding a rivet hole into the proper shape to fit a flush rivet. It is imperative that the rivet fit securely to achieve maximum strength. The metal is stretched somewhat during this procedure usually opening the hole to the proper size without additional drilling. Countersinking is done with a special bit attached to a drill.

The protective vinyl coating found on the skins must be removed prior to dimpling. We do not want any of the vinyl trapped within the hole. On the aluminum skin, you can use a soldering iron to actually melt away a strip of vinyl about 1 inch wide on each row of holes. See Figure 1. This will remove the vinyl from around the rivet holes but continue to provide protection on the remainder of the surface. It may sound basic but remember that the pointed end of the dimple die set goes to the inside of the airplane. You can use a hand squeezer or a C frame to dimple. The C frame will speed up the operation.

The next step is to prime the pieces if you so desire. It is certainly advisable to prime if you are going to be near a salt water environment. If not, then consider potential resale value of the airplane if someone near the ocean wants to purchase it. I would recommend using an epoxy primer and priming the skeleton and the inside of the aluminum skins. Remember to wear a respirator when you are priming and follow the directions on the label of the primer concerning mixing ratios, etc.

Our builders came expecting to rivet—is it ever going to happen? Yes, now is the time. We are going to reassemble our skeleton by again clecoing it together. We will then be ready to rivet the parts together. Once you have clecoed the parts together, take a look at where you should begin riveting. It is very easy to rivet yourself into a box. Know where you are going to complete the riveting process before you begin. Have a plan and rivet in the correct order. Of course, with our horizontal stabilizer we will rivet the skeleton structure together and then attach the skin.

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 out the rivet gun before buying it and make sure you can control the strength of its impact with the trigger. You must be able to regulate the impact force. This is usually done through a regulator on the rivet gun itself. Test the amount of pressure by pressing the rivet set onto a piece of wood. You can then squeeze the trigger and adjust the pressure to the correct amount. You will need the proper set for the rivet you are shooting. 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. Don’t squeeze the trigger on a rivet gun unless the set is pressed against a piece of wood or is pressed upon a placed rivet head that you are ready to set.

Guns come with a quick-change spring that works with straight and flush rivet sets, separate springs for each, and a “beehive set” for an offset rivet set. 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.

With a rivet gun you use a bucking bar to form the shop head (the head of the rivet that is formed by driving or squeezing the rivet—the rounded or flat head is termed the machine 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.

Squeezing is another way to set rivets, and hand and pneumatic rivet squeezers are available. Squeezing rivets is the preferred method of riveting if at all possible. The tool itself restricts access to many areas requiring the use of a rivet gun. The shop head is much more uniform and balanced when squeezed. When learning to squeeze rivets it is best to apply minimum pressure until you determine how much pressure is required to properly set the rivet. You can always squeeze the rivet a second time if not enough pressure is initially used. If you use too much pressure, you may have to drill out the rivet.

Driving a rivet using a rivet gun often requires an additional set of hands. This is where you can bring the family together for some “quality riveting time.” Riveting requires some practice to gain proficiency. Practice on scrap pieces before working on the “real thing.” The shop head that results from driving or squeezing the rivet must meet certain criteria. A properly driven rivet will have a shop head of at least 1 ½ times the diameter of the rivet shank in width and about 2/3 of the diameter in height. Special tools are available to quickly check the rivet for proper installation.

Many sheet-metal airplanes, Van’s Aircraft RV series in particular, call for back riveting, where you use the rivet gun—not the bucking bar—to create the shop head. This calls for a back riveting set, which includes a large metal place that acts like a stationary bucking bar that contacts the rivet’s machine head. You must have a good riveting plate usually made out of ¼ inch steel. It should be a minimum of 2 feet x 3 feet.

Our builders have now completed the first phase of their project. They have before them a horizontal stabilizer. We knew they could do it. We have followed them through all of the basic steps involved. It really is not complicated. It is just a matter of reading directions, practicing, and having some guidance. Remember that you will build the airplane one piece at a time. Do not allow yourself to become overwhelmed thinking about the entire airplane. Concentrate on the item you are building. You will finish the horizontal stabilizer, the rudder, and the elevators. You will then be ready to begin on larger pieces. You will find that with the RV series of airplanes, most of the sheet metal work will be done on the empennage pieces. Completing the rest of the airplane is largely a task of assembly. It is time for you to start on your airplane.