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Metal Working Tips for First Time Builders - Part 2
By Tony Bingelis (originally published in EAA Sport Aviation, February 1993)
Working With Rivet Holes
It’s true, anyone can drill a hole. However, not everyone can drill a nice round hole - in thin metal - where it needs to be - and do it right every time. After all, with thousands of holes to be drilled in a typical all-metal aircraft project, the "opportunity" exists for some of those holes to be:
• Drilled with the wrong size drill bit.
• Drilled off your center mark.
• Drilled too close to the edge.
• Drilled sloppily and elongated.
• Drilled in the wrong place.
• Drilled O.K., but only after scratching the skin.
However, as if all that isn’t bad enough - after the hole is drilled you can still create other problems by:
• Forgetting to deburr the holes in both skins.
• Forgetting to dimple the top and/or bottom skin for flush riveting.
• Making machine countersunk holes for flush rivets (where needed) too shallow or too deep.
Read on to see how you can avoid or at least limit your encounters with these, all too common, hole preparation problems.
A Bit About Drills and Bits
The type of drill bit you will probably use the most is the High Speed Drill (jobbers length) because of its ready availability and low initial cost. However, the tougher, more expensive, Cobalt bits should be used to drill hardened steel parts like landing gear legs and engine mounts.
In addition, you should acquire a couple of 12" long (#30 and #40) drill bits for drilling in hard to reach places.
Because these long bits can be easily flexed with your fingers they can be made to drill close to corners, and just about anywhere else that a 90 degree angle drill would otherwise have to be used.
I also recommend the acquisition and use of a good lightweight air drill because it is less tiring to use than the average electric drill and has an easily teased trigger for precise speed control.
Your control (either air driven or electric) will very definitely have access limitations.
For example, you will find that you cannot drill closer than, maybe, 3/4" to a corner, or a perpendicular rib surface or bulkhead. At best, an ordinary electric drill will allow you to get within 1" of a corner but certainly no more.
In addition, access to the area where the hole is to be drilled must be accessible enough to allow you to maneuver and position the drill with the bit installed. Even with a short bit installed, this means you will need approximately 10" to 12".
It is in a situation like this that a right angle drill becomes necessary for making those almost inaccessible areas accessible.
You can count on it . . . a desperate need for some sort of an angle drill will arise from time to time.
Even though you are building a metal airplane, you may have to drill holes through fiberglass fairings, cowlings, and other component parts.
Drilling holes through fiberglass dulls drills quickly. Expect this and count on resharpening or replacing the dulled bits before using them again in aluminum.
Before You Drill . . .
When a line of rivet holes is to be drilled, a centerline is normally drawn first and then short cross marks are made to establish each hole’s location. Be alert. Before you mark a rivet location, assure yourself that there is no obstruction (or flute) in the underlying frame where you intend to drill a rivet hole. It is permissible to alter the hole spacing slightly to avoid such problem traps.
Use a center punch and a light hammer tap to pinpoint and slightly emboss the marked location for every important hole.
Better still, use an automatic center punch to pinpoint every rivet location.
An automatic center punch is much easier to use and you may find it to be more accurate for your own use.
Although a punch mark indentation helps ensure that the drill will not stray off the mark, you can help, too.
Hold the drill perpendicular to the surface, exert a little pressure, and turn the drill chuck by hand a little before pulling the trigger. This will almost guarantee that the drill bit will stay accurately seated in the punch mark.
Ignore this technique and you will risk the chance that your drill will take a walk and scoot off the marked hole location.
When this happens, the drill will leave an ugly scarred trail across that nice smooth metal surface . . . what a shame.
Here’s a useful tip passed on from builder to builder. Lay masking tape over the route where your rivet layout and spacing is to be drawn on the aluminum skins. Instead, draw and mark the hole locations on the tape. Drill the holes through the tape - then remove the tape.
Don’t leave the masking tape on more than a day or so because it becomes increasingly difficult to remove with the passage of time.
But why use the tape in the first place? Well, the tape protects the metal from drill scratches and helps prevent the drill from walking.
Drill Bit Selection
Choosing the correct size bit for the hole to be drilled is extremely important. Small drill sizes are very difficult to judge by eye, so use a metal drill gauge to remove all doubt.
In the typical flush riveted metal project most of the rivets used will be either 3/32" (AN426-3) or 1/8" (AN426-4) in diameter.
You cannot use a fractional size 3/32" drill bit for a 3/32" rivet . . . the hole will be too small and the rivet simply will not go in. The same applies to the 1/8" rivet and a 1/8" hole.
To accommodate these rivet sizes you must use what are known as numbered drill bits. A #40 drill bit is used for 3/32" rivets, and a #30 drill bit is used with 1/8" rivets.
Notice that the bigger numbered (#40) drill bit is for the smaller holes, and the smaller numbered (#30) drill bit is for the bigger 1/8" rivet holes. Who said anything has to be logical?
Drill aluminum, especially the thinner sheets, at a fairly high speed. You can improve your hole drilling accuracy and quality by making it a practice to drill all your holes, initially with either a #41 or #40 drill bit.
You will find that drilling all holes initially with a small bit first is quicker and helps maintain greater accuracy.
Later, if necessary, different holes can be redrilled to whatever larger size is required. This is called "line drilling" and will ensure that the final hole alignment on assembly is exact.
When you can redrill a small hole to a specified larger diameter, you may even be able to correct a slight hole misalignment . . . all the more reason to drill your holes undersized first.
Drilling accurate round holes in thin gauge metal skins is difficult. Unless extra care is taken, you can end up with holes that tend to be irregular and oversized (often taking on a hexagon appearance) instead of neat and round like you want.
If the metal thickness is less than half the diameter of the drill bit, the described difficulty is almost sure to occur.
Improve the quality of the critical holes you have to drill in thin metal by clamping a scrap metal back-up block behind your work before you start drilling. This will allow the body of the bit to be in solid material at all times . . . before, during, and after penetration. The result - a nice round hole.
When drilling holes in thicker aluminum parts, withdraw the drill bit frequently to clear the bit of chips. If you don’t, the aluminum chips inside the hole will build up and bind, preventing you from withdrawing the bit. This is more likely to be a problem when drilling softer 6061-T6 bar stock than it is with the harder 2024-T4 bar aluminum.
When using a hand held electric, or an air drill, to drill deep holes in aluminum, take care when withdrawing the drill that you don’t accidentally tilt it sideways. This would be bad because it will cause an elongation of the hole. If at all possible, do all your important drilling in a drill press.
Never hold small parts in your fingers when drilling holes on a drill press. The drill bit will surely grab just as the bit exits the metal. Before you realize what is happening, the drill will lock (jam) in the metal and the part you are drilling will whip right out of your hands and spin wildly out of control.
No need to go into the gory details of what would happen to your hands and fingers if contacted by that spinning cleaver.
Making Large Holes in Sheet Metal
Sometimes you will have to drill or make holes much larger than those needed for rivet installation.
For example, rudder control cables are usually routed through holes drilled in fuselage bulkheads. These holes must be large enough to allow the passage of the rudder control cables and their formed terminals. Because these holes may have to be 5/8" in diameter, or larger, a regular drill would do a shoddy job in the thin metal.
The solution? Use a step drill (UNIBIT®). This step tapered drill will drill several sizes of holes and will do a beautiful job without chattering or messing up the metal.
Holes in Stainless Steel Firewalls
A stainless steel firewall is mean to work with, and drilling larger holes in one is a challenge. As always, start with a punch mark, use a small bit, and apply steady pressure. Enlarging a hole with a regular drill bit in stainless steel is something less than a precise operation.
A UNIBIT works well if you enlarge the hole progressively from both sides. The stainless is very ductile and tends to bend or deform at the cut line.
To cut a very large hole in the firewall for the installation of a cabin heat valve, or for fresh air ducting, use a "nibbler." Use it to rough out a large opening which can then be evened and trued with a file. You will need an initial 7/16" hole or larger to insert the nibbler.
Rivet Spacing and Edge Distances
Drilling holes too close to the edges of the aluminum skins, or fittings for that matter, is not acceptable. How close is too close?
The basic rule is that you do not drill rivet holes closer to the edge than two diameters of the rivet size used. For 1/8" rivets this would mean that the centerline of any hole would be at least 1/4" away from the edge of the skin (material). This is quite a critical factor as in- sufficient edge distance reduces the strength of the riveted joint.
You should not reduce the edge distance from that shown by the designer on the plans.
Actually, too great an edge distance for a line of rivets can be equally undesirable as it may permit the edge of the sheet to curl up slightly leaving a gap that will look lousy after the aircraft is finished and painted.
You might be interested to know that the minimum recommended pitch (spacing) between rivets is 3 x the diameter of the rivet and the normal pitch is accepted to be 8 to 10 x rivet diameter.
As for rivet spacing for your project, use the nominal spacing called out in the plans. However, you should realize the hole spacing may have to be altered here and there to avoid placing a rivet where it cannot be installed, or be properly driven, due to some structural interference behind.
Skin Drilling Techniques
There are at least three basic procedures for marking and drilling aluminum skins (fuselage, wing, tail). You should be familiar with each so you can select the one (or a variation thereof) that would be the easiest and best suited for a particular job.
Clamp and align the skin to the underlying structure. From inside, trace around each rib or frame. Remove the skin and turn it over. Draw a centerline on the skin within each of the traced frame outlines. Mark off the specified rivet spacing and drill all the rivet holes marked on the skin. Reinstall the skin and clamp it in place again. From the outside, sight through each pre-drilled skin hole to verify that the centerline (marked on the rib or frame) is visible before you drill. Insert a Cleco clamp in each (or every other) hole as you proceed.
With the skin clamped to the structure, work from behind with a long 12" drill or an angle drill and back drill through both the frame and skin at one time. Have a helper on the outside to press down on the skin with a wood block as the holes are drilled. Your helper should install Cleco clamps as the drilling proceeds. This method is awkward, and when working from beneath you risk getting drilling debris into your eyes. However, this method does make it easy to avoid flutes and other structural interference. Wear protective glasses.
Mark the centerlines and the rivet hole spacing on each frame (or rib). Drill all the holes in the frames (or ribs). As you drill, it will be easy to avoid flutes and places where the underlying structure would make a rivet installation impossible. Position and clamp the skin in place. From behind, or under the structure as the case may be, drill the skin holes using the pre-drilled rivet holes in the frames as a guide. Here again, you must protect your eyes from falling debris.
Whatever procedure you use, proceed slowly and reassure youself that the drilling will be centered on the frames or ribs; that the holes will avoid flutes; and that each hole will be where a rivet can be inserted and set. For the best results always try to drill from the center of a section toward each end.
Dimpling and Countersinking
Aluminum skins that are to be flush riveted have to be prepared by first machine countersinking, or dimpling each hole before riveting. Aluminum skins that are less than .032" thick must be dimpled . . . not machine countersunk. Dimpling is more time consuming an operation than is machine countersinking. The reason being that both the surface skin and the underlying structure (frames, ribs, etc.) have to be dimpled to match the surface skin’s bulged dimples.
You do, however, have the option of machine countersinking the under- lying frame instead of dimpling it - provided the frame material is at least .032" when 3/32" rivets are used (.050" thick or more for 1/8" rivets). Ma- chine countersinking is simpler than dimpling when the top skin is .032" or thicker. That is because machine countersinking a rivet hole does not cause a bulge under the skin as is the case with dimpling. Therefore, no further preparation of the underlying rivet holes in the frames is necessary. A real time saver.
A Few Parting Words of Wisdom
Finally, before drilling or cutting any metal . . . always stop and recheck! Make sure you didn’t measure from the wrong point!
Although you check and recheck your measured dimension and are convinced that it is perfect, it may still be wrong if you measured from the hole instead of from the edge . . . or vice versa.
In other words, don’t allow yourself to fall into the same mental trap that continues to puzzle my favorite "adviser" and critic. He says he can’t understand what happened. He cut it twice and it is still too short.