HINTS & TIPS
Marking Out Use a fine tipped spirit based pen (usually marked at `permanent marker'. Avoid using a scriber, unless the material marked is to be cut away, because the scribed line could be the start of a later fatigue crack. Mark circles using a plastic template. When marking out the centre of a hole to be drilled, make the cross larger than the drill diameter so that you can see whether the drill has wandered during the drilling.
Drilling Use a centre punch to reduce the chance of the drill point skating across the surface of the job. Unless drilling is done in a drill press the drill is likely to wander. The smaller the drill bit used for the initial hole the less the wander will be. For rivets it's normal to drill 2-4 thousandths of an inch oversize to prevent a burr forming on the rivet stem as it is forced into the drilled hole.
Hole Deburring The normal deburring cutter has three cutting edges. That is, one complete revolution of the debur handle would give three cutting passes, thereby countersinking the hole entry. Note how little land in the centre of the sheet will support the rivet. A half turn, followed by a short reversal, will suffice; the reversal serves to break away any burr caused by the debur cutters.
Countersinking or Dimpling For Flush Rivets? A sheet metal thickness of at least 0.032" is needed for a 3/32" rivet (0.050" for an 1/8" rivet). Below this thickness the metal must be dimpled. There are some special NAS 1097 rivets which can be used in thinner sheet, for example to attach a platenut (see page 72 of the catalogue).
Dimpling With two very thin sheets (less than 0.032"), both pieces to be joined can be dimpled together. With thicker metal it will be necessary to dimple each sheet separately. If a dimpled sheet is to be riveted to a thicker metal it may be necessary to machine a larger countersink hole to take the dimples in the sheet.
Use a small sample piece of dimpled sheet to set the countersink cage's depth of cut by trial and error.
Countersinking When countersinking sheet of the minimum thickness it is easy to end up with an elongated pilot hole. To prevent this drill a pilot hole in a sheet of hard material, and place it under the hole to be countersunk. This pilot hole will then keep the countersink cutter's pilot centred. If softwood is used for this purpose the pilot hole will enlarge after just a few uses and will no longer be safe to use. Modern countersink cutter cages, otherwise known as microstops or vanmar cutters, have a nylon foot. Beware! This can pick up swarf and mark the surface you are countersinking.
Rescuing a Poor Rivet Hole If for some reason you have an elongated or enlarged rivet hole, it is possible to buy NAS 1097 rivets which have the same size of head as a standard 3/32" rivet but have a 1/8" diameter stem.
Filing Aluminium swarf is very inclined to stick in a file's teeth. It then leaves scratches on the surface being filed. Clean your file regularly, and for major filing use the Vixen file, which has large curved teeth in which it's harder for the swarf to lodge.
Trepanning or Circle Cutting By design the circle cutter (or trepanning tool) is asymmetric and must, therefore, be run at a low speed. Moreover, its cutter can easily dig in as it first breaks through the job. The work should be clamped whenever possible, and the operator should wear a heavy glove.
Riveting - Safety Never point a rivet gun at someone else when it's connected to the airline. Always disconnect the gun from the airline to change the rivet set in the gun. If the gun trigger is pulled when the rivet set isn't held to a firm structure the set could be fired out of the gun. Try the gun on a piece of wood on the bench or floor before riveting to get its feel and adjust the pressure as necessary.
Care of the Rivet Gun Put 2-3 drops of pneumatic tool oil (e.g. Marvel Mystery Oil) down the gun inlet at the start of each days use. At the same time, open the air tank water drain until no more water comes out with the air.
Using the Gun Hold the gun firmly and press it firmly against the job. Beware, it will try to jump about when fired unless it is held firmly. Agree a system of signalling between the gun operator and the bucking bar holder. Usually the latter says `Ready' when he's set up for riveting to start. Fire a short burst then stop until the bucking bar holder calls `Again'. This gives him time to check that the stem isn't turning over. Modern flush sets have a rubber edge to stop them skidding about on the flush skin as the gun fires, and are much easier to use. There's also a flush set with an articulated stem (part number 1047) to cater for imperfect alignment and so ease another riveting problem.
Controlling the Bucking Bar Hold the bar firmly and to hold it against the stem of the rivet, but it mustn't push the rivet back out of it's hole. The working face of the bar must be square to the rivet in all planes. If a rivet does tip over, as long as it's caught early enough it should be possible to steer it back to its proper place.
Back-Riveting This is a technique developed by those amateurs who wanted to achieve a really good flush finish. It involves the use of a special rivet set, a narrow flush set surrounded by a spring-loaded sleeve. The flush rivets are loaded into their holes and held in place by a special type of 3M `magic' tape which doesn't leave a residue. The heads of the rivet and the flush skin are supported on a polished steel block while the stem of the rivet is formed into the `shop' head using the back-riveting tool. The tape is removed immediately after the rivets have been set, so that any that were missed become evident in time to correct them.
Use of Snips Proper aviation snips have wide jaws and will only cut a curve in one direction. Equally, when snipping angled material only one side can be used close up to the flange of the angle. The material being cut away curls up into a spring-like swarf as the cut is made. Obviously, only a narrow band can be removed with any one cut. If the jaws of the snips are closed completely, a small nick or tear is made at the point where they close. The correct technique, therefore, is to make small nibbling movements, without ever closing the jaws entirely.
Drilling Perspex Perspex cracks very easily, especially when being drilled. The correct drill bits have no rake on their cutting edges and have a much smaller included angle (60 degrees) at the point. Drilling cuts should be made with only a light pressure and with a backing material, e.g. a piece of wood. Edges can be cut with a disc cutting wheel - they should never be sawn. The edges should have their sharp corners removed with a scraper of sandpaper to prevent cracks developing from any small nicks. If a crack does develop drill a stop hole about 1/8" past its end. Don't be tempted to drill right at the apparent end of the crack - it always goes a little further than you eye can see. This is why you so often see a long crack joining a daisy chain of failed stop holes. Unibit step drills have no rake on their cutting edges and can also be used to drill Perspex.