Aircraft Sheet Metal, II
Working with sheet metal, especially the thin sheets used in aircraft construction, can be a daunting experience for the first time builder. One of the problems is that when you ask advice from those in the industry, the response only makes matters worse, as many assume a certain level of knowledge of airframe construction.
What takes, for example, an air force technician several years to learn, must be picked up to some lesser degree in a matter of months by the home aircraft builder. Which can be a real challenge for some.
Some holes are not the correct size after drilling, they must be enlarged to the correct size by some thousands of an inch, this is done with a special tool. Other holes are drilled in steps.
Believe me, this is scary at first, but with practice you will become very good at properly drilling a hole and reaming it to the correct size. As always, if not sure, go for the scrap piece first before drilling the real thing.
Now there is a word that strikes terror into the inexperienced. If you try to seek advice on reamers, you will be given the impression you need to be into the occult to make them work (a bit like all this nonsense about Proseal). A reamer cuts metal in a different manner to a drill bit (you don't need to remember that) and is more precise, does not snatch the work and produces a cleaner hole (you do need to remember that).
Chuckable (you can put them in a drill chuck!). Spiral, medium or high speed reamers work fine in your drills (in spite of what machinists and engineers will tell you: ignore them, they know more and are just trying to confuse us).
All you need to remember is that you must take the hole to at least 0.007" (7 thou) from final size. Less than that and its too much for the reamer to cut (they are expensive and brittle). If you don't have the confidence to use your hand drill with a reamer, you can use a hand reamer, and some people feel this provides even more accuracy.
I can see the look of horror on your face: 7 thou! How do I work that out to get the correct drill bit? Just go to that table I told you about. Its worth scouring Sun and Fun and Oshkosh for bargains on reamers and drill bits.
To end, I just want to mention step drills. To get a nice round hole you always drill the hole in stages. Step drills do this in one go. For example you can get a 1/8 to #11, the first 1/4" or so of the bit being 1/8", ending in #11. You can also get reamers that do this.
Check the picture on the right, so you will know how they look like. This is a piece of tool that is worth your money, buy one! No. Buy two of different sizes, metric and in inches.
One of the skills you must learn if you have one of our metal kit planes is counter sinking and dimpling. Counter sinking involves removing aluminum from the sides of a hole in a conical shape so a flush rivet can be installed. Dimpling is used on skins too thin for counter sinking, and involves using two shaped disks, one convex, one concave, that come together to produce a concave dimple into which the rivet head sits.
Lets start with countersinking. You will need a micro stop. Few people who live normal lives know what these are, so lets look at one. A micro stop is a special receptacle into which you screw the countersinking bit, having basically two rotating barrels, interlocking with fine teeth.
You pull the two barrels apart under spring tension, and rotate them. This alters the depth of the counter sink. The teeth re-engage, locking the position, and jolly accurate it is too. So "micro" adjustable that you will be driven crazy trying to remember which way to turn it, as its almost impossible to detect with the human eye. Counter sinking bits come in a variety of angles, so use the correct one.
Avex rivets have 120° heads, solid rivets usually 100°. The nipple to guide the bit into the hole comes in the various hole sizes, so you need a selection of bits corresponding with the hole diameters you intend to counter sink.
Try to avoid the type that have the cutting edge running partially into this nipple they can enlarge the hole. I like the sets (usually 3 bits and the micro stop) available from various tool suppliers as the micro stop usually has a nylon edge that butts up to the work, preventing scratches.
Chuck the assembly into your drill, get a piece of scrap, with lots of holes drilled in it, and practice. Once you have the correct depth (check it with the same rivet you will use), you are holding the drill perpendicular and no chattering is taking place, you can attack your kit.
Dimpling is usually performed on homebuilt kit aircraft with the two disks mentioned earlier fitted into a hand pop rivet gun. The nail that runs through the disks is inserted into the gun like a rivet mandrel.
Dimpling squeezers are available for extensive dimpling, with various types of bits available, but usually have limited use on large sheets due to their throat size. Dimpling does deform the hole, slightly enlarging it. For this reason some people drill to final size after dimpling.
Original text by Murphy Aircraft technical department