Sport Aviation - 4/87
By Tony Bingelis
Flush rivets are used, primarily, on external metal surfaces where good appearance and the elimination of unnecessary aerodynamic drag are important. The sleek smoothness of flush riveted skins is considered to be an essential factor in obtaining the maximum performance expected of high performance aircraft.
Unfortunately, since flush riveting is an expensive process, most manufacturers of general aviation aircraft have long limited the use of flush rivets to as few components as possible. This is understandable for the extra labor costs could outweigh, by far, the slight performance gain realized from the extensive use of flush riveting.
While labor costs aren't ordinarily a matter of concern in the construction of a homebuilt, the extra work could be. However, extra work for a good purpose doesn't seem to deter most builders as they are more apt to be freer with their time than with their money. Still, it is well for the first time builder to be aware that extra, time consuming, preparatory steps will be required if he elects to go all the way with flush riveting.
As in the installation of conventional protruding head rivets, flush riveting starts with the drilling of the correct size hole for the rivet selected. The additional work generated by the use of flush rivets stems from the requirement to modify the drilled rivet hole to accept the cone shaped head of the flush rivet. This generally means that the drilled rivet hole will have to be either machine countersunk, or compression dimpled to provide the proper nest for the rivet head.
But, before we get into countersinking and dimpling procedures, a few important comments regarding the drilling of rivet holes may be useful.
Notes About Rivet Holes
The Whitney punch is a small hand held lever type punch with interchangeable dies. It can accurately punch the correct size holes, and do it quicker . . . quicker than by drilling them. A built-in adjustable margin stop allows holes to be punched in exact alignment along the edges of a part. Ah, that's the primary limitation to this handy tool. It has a very limited reach.
For the most part, rivet holes are drilled either with a hand held electric or pneumatic drill, or on a drill press. And it goes without saying (but I'll say it anyway) that all rivet holes should be drilled with a new bit, or one that has been accurately sharpened and will drill on-size holes.
Before you start drilling that hole, be sure the drill bit is the correct size! If you err on the small size, no harm is done. But to date, no one has figured out how to shrink a drilled hole in an aluminum skin.
Even though a rivet shank expands when the rivet is set with a rivet gun and bucking bar, this is not to be taken as license to drill sloppy or oversized holes. By drilling the correct size hole, the rivet expansion is such that an added shear strength is developed in the rivet. On the other hand, an oversized hole will deprive you of this added strength and will weaken the joint.
I know that we all, occasionally, drill holes without first fixing their locations with accurately placed center punch marks, but this is a risky practice. Sometimes at the worst possible time, the drill will "walk" in spite of your efforts to control it, and your nice shiny surface is marred. But even worse, an important hole may have been drilled slightly off-center.
Here's a useful tip. Before starting that drill motor, rotate the chuck by hand a turn or two with the drill point pressed against the punch mark. This is equally effective in starting the hole when using the drill press.
Drilling holes in external skins takes extra care because, if you aren't careful, the chuck will hit and grind against the surface when the drill bit breaks through. To prevent this and possible internal damage, support the drill with your free hand using the thumb and, if you prefer, the first two fingers to control the drill's penetration. This will help protect the surface metal from possible drill chuck damage.
A good 90 degree offset pneumatic drill is a very valuable tool for anyone building an all-metal aircraft. It will allow access to many otherwise inaccessible areas. Sometimes though a long 12 inch drill bit can accomplish the same thing because you can flex it with your fingers while it is slowly turning.
Before you attempt to drill any rivet hole, always reassure yourself that the parts are tightly clamped and properly positioned. When drilling holes in thin metal, do not rush or forcibly push on the drill as this strong arm routine can cause the area around the hole to become noticeably dented. Whenever you can, use a small wood back-up block to keep the drill bit from bursting through the opposite side. This will insure a neater, almost burr-free hole.
Coping With Burrs
If this deburring is not scrupulously done for each hole, the rivets will not pull up flush and gaps may develop in the joint.
This means that, after drilling all the holes !n an assembly, you MUST take it apart and deburr every hole. Every hole! The traditional way of doing this is with a sharp, larger diameter (about 3/16"-1/4") drill bit held between your fingers and twirled lightly on the drilled rivet hole. The idea !s to remove the burrs - not to countersink the hole.
After you have painstakingly deburred a hundred holes or so, you will develop a great Interest in saving your fingers from further wear and tear. It would help if you were to slip a small file's wood handle on the end of the drill bit to protect your hand from the sharp flutes of the bit (Figure 1).
For all other deburring jobs, I highly recommend the acquisition of a regular deburring tool. It looks like a screw driver handle with a "S" shaped sharpened narrow blade that swivels freely. You draw this gadget along the raw cut edges of aluminum and, voila!, instant deburring. It works good on wing rib lightening hole cutouts, too (Figure 1).
Before flush rivets can be installed, those nicely deburred rivet holes must be prepared for the countersunk heads by dimpling or countersinking the material.
Although a rivet squeezer is very easy to use, its utility is reduced by its limited reach. For this reason, it can only be used for dimpling the rivet holes located along the edges.
Whenever possible, both the top skin and the underlying parts should be dimpled together. However, dimpling each part separately is the more likely occurrence.
You can do your dimpling using a rivet as the male die, and a homemade female die (countersink) drilled into a steel bar. The steel bar can be drilled with a single hole, or several in random locations. These are then machine countersunk slightly oversized to provide the necessary depression. To use it, slip a rivet into a rivet hole in the skin and position the steel bar so that the rivet fits into one of the countersunk holes. A hammer or a rivet gun is then used to force the rivet into the depression creating the desired dimple in the skin.
When a thin surface skin is to be flush riveted to a heavier part under it, it is O. K. to countersink the thick member and to dimple the thin skin into the underlying countersunk nest.
From the standpoint of strength, drilled rivet holes are preferable to punched rivet holes for dimpling. When a punched hole is dimpled, tiny little hairline cracks will sometimes appear around the hole. These tiny fractures are almost invisible unless you examine the dimpled hole with a magnifying glass. I don't believe this is, ordinarily, of much concern unless the assembly is one that will be highly loaded in a highly stressed location, and subject to considerable vibration. Punching the rivet holes slightly undersized and reaming or redrilling them to size could obviate the condition. However, that would complicate the procedure, and would nullify the real benefit of using a Whitney punch in the first place.
Countersinking is permitted only when the surface skin is thick enough to accommodate the cutout depression without enlarging the rivet hole. For example, the surface metal skin must be at least .032" thick before it can be countersunk safely for a 3/32" flush rivet. When a 1/8" rivet is to be used, the material must be at least .040" thick to accommodate the larger countersunk hole (see Table 1).
The Countersink Stop Tool
Alas, there is no corrective action for a rivet hole that has been countersunk too deeply . . . unless it is to rework the hole to take a larger rivet, so be careful.
By being careful, I mean check out the stop tool adjustment on a scrap piece of metal first. See to it that the countersink depth is perfect for the rivet size to be used.
For other locations you naturally will have to set the rivets with a rivet gun and a bucking bar. Where you have access to both sides, and the component is not too large, there is no reason why you couldn't handle it single-handedly. Of course, it would be easier to have a reliable rivet bucker to help.
Once in awhile, a rivet is stubborn about entering the hole in the underlying part and the joint at that point tends to separate a bit. Eliminate this risk by using plenty of clamps or clecos in the area being worked. Also, if you have any doubt about the joint's tightness, use a draw set to force the rivet head snugly into place (see Figure 3).
A draw set is nothing more than a bar with a shallow hole drilled in its end, the hole being a bit larger than the rivet. Slip the bar over the protruding rivet shank and give the rivet head a light tap-tap with the rivet gun or a small hammer. This will draw the mating parts into close contact and the rivet can then be driven in the usual manner. A draw set may be purchased, or you can make your own from a one inch steel bar about 6 inches long.
Riveting Sequence Is Important
Another way to avoid this out-of-kilter problem is by not riveting in the logical manner, That is, don't set each rivet in sequence as you work towards the ends of the skin. Instead, it is better to start in the middle, in the usual manner, but to skip rivet holes and set a rivet in every 5th hole as you work towards the ends. Then return, insert and drive the middle rivet in each open hole, etc. This procedure will limit the amount of slippage and just about eliminate bulges between rivets.
Another way to avoid this out-of-kilter problem is by not riveting in the logical manner. That is, don't set each rivet in sequence as you work towards the ends of the skin. Instead, it is better to start in the middle, in the usual manner, but to skip rivet holes and set a rivet in every 5th hole as you work towards the ends. Then, return, insert and drive the middle rivet in each open hole, etc. This procedure will limit the amount of slippage and just about eliminate bulges between rivets.
First, drill and dimple the rivet holes in the usual manner. Next, insert a rivet into each of the drilled holes and temporarily hold them in place with plastic tape. Some types of "Scotch Tape" should not be used because it is almost impossible to remove after riveting. Ditto for some masking tape. Try your favorite plastic tape on a sample riveted piece to see if it can be easily removed (see Figure 4).
The next step is to flip the skin over so that the rows of rivet stems are exposed. Lay the aluminum skin over a steel plate or steel table. A bench saw is fine but be careful to avoid riveting over a groove. Proceed to set the rivets with a rivet gun preferably. A small hammer can also be used, but be careful that you don't miss the rivet and damage the skin.
The safest rivet set to use for back riveting is the one designed for homebuilders which keeps the rivet set from slipping off, thanks to a specially designed plastic collar. This spring loaded collar also imposes pressure on the skin, thereby helping insure a tight joint.
Your completed back riveting job can be fast and flawless . . . and there will be no waviness along the rivet line.
I set all the rivets for a RV-4 rudder skin in just 6 minutes per side using the method. Of course, that was after all the aforementioned preparations were completed. But the process is fast, and highly recommended. Use it wherever you can.
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