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Making Fittings - Part 3
By Tony Bingelis (originally published in EAA Sport Aviation, November 1980)
FITTINGS CAN'T HELP the way they look. I mean, their appearance is determined by how they are fastened to the structure and how they are connected to something else. One end of a fitting, be it a flanged bracket or a simple flat fitting, is generally fastened to a primary structural unit with bolts or rivets. The other end of a fitting, most likely, connects some component part which must be attached to or be supported by the structure. For example, we can easily visualize how typical wing strap fittings, landing gear fittings, engine mount fittings, bracket-like fittings for controls, and other gadgets making up aircraft components and systems are commonly installed and connected. If we were to reduce a fitting to its most elementary function, you would see that its shape is really governed by the location and orientation of its mounting holes, and the hole or holes necessary to connect some component to it. As a result, the ultimate shape of a fitting simply evolves because sufficient metal must be provided around the essential holes to unite the fitting into a single part. See Figure 1.
Actually, the external dimensions of a fitting are not nearly as important as the accurate placement and sizing of the holes. On the other hand, determining the proper specification and gage for the material, from which to make the fitting so that it will be capable of carrying out its function through the life of the aircraft is important keep.
There is usually only a slight difference between a fitting that is not strong enough to do the job and one that is excessively heavy. Determination of the exact material need really represents a greater challenge than most builders realize. As with most of the basic structure of an airplane, builders should not make fitting design changes or substitutions of materials unless they are able to substantiate those changes through design analysis or testing.
Bushings Vs Edge Distance
The most effective material shape, strength-wise, around a drilled hole is, naturally enough, a uniform radius. But, uniform radius or not, it is still possible, of course, to make a fitting with an insufficient amount of metal (edge distance) around its holes and this frequently happens (Figure 2). As a result, such a fitting might eventually fail with the bolt tearing away from the fitting. A similar consequence may result from a bolt hole that is mislocated during drilling, or one weakened by becoming oversized or elongated. Therefore, any fitting hole which will be subjected to wear from a bolt that pivots, should be fitted with a bronze bushing or made so there is a generous amount of material between the hole and the edge of the fitting.
When wear occurs in a bushed fitting, the bolt and/or bushing can be replaced. On the other hand, if a hole does not have a bushing, the only corrective action you would be able to take would be to replace the fitting or to redrill its hole to take the next size larger bolt. It is apparent that the latter cannot be safely done, however, unless a sufficient amount of metal was originally provided around the hole. Both aluminum and steel parts can benefit from the improved ability to handle friction and wear provided by bronze bushings.
Bronze bushings . . . especially oilite bronze bushings . . . effectively reduce frictional wear to a minimum. Bushings are easy enough to install provided you have the correct bit or reamer. The hole must be slightly undersized so the bushing will have to be pressed into the fitting. If you do not have a small press, you can still press the bushing in with precision by squeezing it in your vise. Place a washer of the correct thickness behind the fitting hole and the bushing, and the depth to which the bushing must be pressed in can be duplicated for any number of identical fittings.
Bronze, aluminum and steel being unlike metals can have a tendency to enter into galvanic action. This is an electro-chemical action which sometimes results in galvanic corrosion. It is the same sort of action that takes place in your battery . . . but this, of course, is just what you want. However, when the same kind of action takes place between two metals bolted to your airplane . . . this you do not want.
Some textbooks on metals contain tables which show the relative tendency for different metals to enter galvanic action. It seems that the greater the voltage difference between two metals, the greater the potential for corrosive activity. Be that as it may, you and I would probably be more likely to learn of the presence of galvanic corrosion by the build-up of a snow-like powder around a bolt and/or the fitting hole. Such activity and corrosion if not treated would probably continue to build to the point where it could eventually become destructive.
These scientific tabulations of metals with respect to their susceptibility toward corrosion are ordinarily considered merely as theoretical indicators of their tendency toward galvanic action.
If you want to be practical, you can reasonably assume that galvanic action (corrosion) will not take place unless water (moisture) is present. Dry fittings will not generally corrode. I have salvaged 20 to 30 year old aluminum fittings from military aircraft that had bronze bushings pressed into a variety of aluminum levers and bell cranks with no sign of corrosion in any of them. Obviously, to avoid corrosion inducing conditions, an aluminum fitting and its connecting steel bolt should be "insulated" against each other and moisture, with at least a light shot of zinc chromate primer. A protective coating will do much to exclude destructive moisture from the area of contact between the two metals. Bushings pressed into holes might better be sealed by also using "Locktite" in the installation of bushings.
After a fitting installation is completed, polyurethane varnish may be flowed in around the entire assembly. This varnish has a very tenacious waterproof quality and as long as the film is unbroken, it will provide superior protection against corrosive elements.
Completing Your Fittings
Pen or pencil layout markings on steel are difficult to see (try using a silver pencil . . . art supplies) and many builders, therefore, like to scribe their lines on metal parts as an aid to accurate cutting. This is O.K. but only as long as they make no scribe mark on any portion of the metal that will not be cut away.
Never scribe a bend line as it will only create a possible location for a future material failure.
Now that you have obtained the specified material for the fitting . . . laid it out . . . cut it out and bent it as required, you are only half finished with the job.
One chore remaining is to finish your fittings to a uniform external shape making sure that all of their edges are smooth and free of saw and file marks. Surface nicks, scratches and gouges, if present, should be dressed out.
All steel and unclad aluminum parts should be smooth sanded. I use aluminum oxide sandpaper. Smooth finishing a fitting's edges is much easier with the part clamped in a vise. Use a smooth-cut file and follow that up with number 180 (or finer) wet-dry sandpaper on steel parts. Use it dry as wet sanding 4130 encourages rust to form before you can shake up the spray can of zinc chromate primer.
Sometimes we builders will find a piece or two of 4130 steel that has been lying around for years . . . and it shows it in its surfaces which may have varying degrees of rust, scratches and even pit marks. There is no harm in using the material although it should be cleaned up first to determine its acceptability before cutting out parts. This precaution will serve to assure you that you will not have to discard a part after you have spent much time making it. Rust will sand off easy enough provided that corrosion has not eaten into the metal to any degree. All rust must be removed, otherwise the residual will keep working away on the metal even after it has been primed and painted. Sanding the surfaces to a nice shiny appearance may or may not prove successful in removing all of the rust. Although the new shiny surfaces might look perfect, the grain of the metal will still have tiny little imbedded specks of rust invisible to the (if you will pardon the immodesty) naked eye. To be sure, treat steel surfaces with a metal conditioner to neutralize the rust. A product similar to Osphos or any of those put out by DuPont and other paint companies may be used to take care of that problem. (Be sure to read and follow instructions.)
Aluminum oxide sandpaper should be used on aluminum fittings switching from a medium to fine grit in the clean-up process as necessary.
After you have made the surfaces as smooth as a teenager's freshly shaved cheeks, you need to take a close look at what you have. Examine all bend areas closely for cracks or signs of undue stress before you decide that the fitting is good enough to install in your airplane.
Although a lot of builders of metal aircraft like to see all of their internal parts in their natural aluminum shiny state, it is highly recommended that at least the joining surfaces of assembled parts be given a corrosion proofing treatment of some sort.
To most of us this usually means a good cleaning (degreasing) of the part followed by a spray can squirt of zinc chromate primer. Although this means of corrosion control is far from the best, it is not bad at all when you consider that there are builders who do nothing more than install the aluminum parts bare.
Ideally, steel parts should be cadmium plated by a shop that knows what it is doing. It should know of the necessity for, and the process of, a postplating bake treatment to assure relief from hydrogen embrittlement. Hydrogen embrittlement in plated steel parts subjected to vibration and constantly reversing loads, makes them quite susceptible to failure.
Aluminum fittings may be anodized, but this is an electrical process; one not practical for most of us to accomplish in our own limited work area. More likely you will prefer to treat aluminum parts with an aluminum conditioner to improve adhesion qualities for the zinc chromate coating to follow. Once again, the admonishment to make sure the product is suitable for the metal being used and that you follow the manufacturer's directions.
Before You Install Those Fittings . . .
It is a bit annoying to have completed a nice set of fittings only to realize that you cannot immediately install them because the structure hasn't been prepared to accept them.
Aluminum and steel fittings which will be bolted to wood surfaces must be protected from the latent moisture always present in the wood. It is customary, therefore, to coat underlying wood surfaces with two or more coats of varnish, preferably polyurethane or marine varnish, before any fitting is bolted to them permanently. The fittings, too, should be sprayed with at least a light coating of zinc chromate primer, and if you wish, painted. Don't paint them black . . . it's not that bad guys prefer black, but, rather, because black surfaces are simply difficult to inspect for cracks.
Varnish, rather than paint, is always the preferred coating for wood surfaces because it is transparent and the underlying wood's condition will always be visible during future inspections. A painted surface, on the other hand, may hide cracks, dry rot and other defects.
Bolts installed in wood also need protection against the natural moisture present. Even though bolts are cadmium plated, they will, eventually, suffer the effects of rust if not given a little extra help by you. Dip the bolts in zinc chromate primer or polyurethane varnish before permanently installing them . . . sure, you can spray the bolts if you haven't the provisions for dipping them. Some builders use a "Q" tip (a dab of cotton on a stick) and also swab the hole with varnish before installing the bolts.
Everything said for the wood-to-metal protection also applies to installations where steel bolts and nuts are in contact with aluminum alloy fittings. These dissimilar metals must be insulated from each other with some corrosion protection. It's a messy but effective practice . . . dipping bolts in zinc chromate and installing them while the primer is still wet.
It is very easy to get a fitting cocked slightly when drilling installation holes. To avoid this horror, clamp the fitting in place checking its alignment. Drill the first hole and insert a bolt. Then recheck the alignment and reclamp the fitting before drilling the second hole. Don't rush! After the second bolt is inserted, any additional holes needed may be drilled with the assurance that the fitting can no longer slip out of alignment.
When two or more 90 degree angle fittings must have matching holes . . . as in control hinges . . . never drill each fitting separately with the anticipation that your skill will prevail.
If you prefer guaranteed accuracy, you might place two opposite fittings on a flat surface, clamp them together back-to-back and drill through both of them at the same time. You can then use one of them to serve as a master jig for drilling the other matching sets. That way you can defer proving your skill and precision for some other more demanding situation.
It is prudent to drill all holes initially undersized. You can always open them to the correct size later by line-drilling through the assembly. Any minor misalignment will usually be corrected in the process. What? You've heard this before? (Must be important.)
. . . They Call Them Relief Holes
There aren't very many instances when you will have to make corner bends in aircraft work. I can think of only a couple; when you make a metal battery box and when you make a flanged fitting similar to one you might find in an aileron bell crank support. In these instances, should you try to bend the metal to form a corner, the material is crowded and it has no place to go. This crowding is particularly aggravated when the bend is attempted without a fairly large bend radius. As a result, cracks will form, radiating outward from the corner. (see Figure 3)
There is a simple way to prevent this forming problem from becoming a problem to you. Drill a small relief hole at each bend intersection. The bends will be easier to make and you will have prevented cracks from starting.
Don't be afraid to drill a good sized hole at each of the bend intersections before making the bends. I would recommend that the relief holes be at least 1/8" in diameter in thin metal and a 1/4" or larger in the heavier gages. Try a few sample holes and bends in scrap pieces to see the results. By the way, relief holes should have smooth edges, or be given a whisper of a chamfer with a piloted countersink or a larger drill bit twirled between your fingers. Just enough to remove the sharp edges and burrs . . . don't overdo it, remember the metal is probably quite thin. A piece of sandpaper rolled into a long taper will work as well.
A Word About Piano Hinges
Yes, I would classify piano hinges as fittings . . . at least as used by many homebuilders. We use them to attach ailerons and flaps, trim tabs, inspection doors, cowlings and small baggage doors. They do make convenient easy-to-take-apart fittings for whatever use we are clever enough to devise.
Two common varieties of piano hinges are currently available to you. One is the relatively inexpensive (MS 20257 series), and the other a much stronger extended type with closed hinge loops (MS 20001 series).
The stronger extruded piano hinge is used for structural applications and the weaker (MS 20257 and old AN 257) continuous hinge is used where the strength requirements are not critical. The hinges are normally obtainable in 6 foot lengths although most of your applications will require but short lengths of it.
The wire holding the hinge together is a stainless steel sort going under the name "piano wire". Hobby shops stock several sizes of piano wire if you need replacements.
Retention of the wire insert, in the hinge, is necessary in some installations to keep it from working out. If you would cut the piano wire so it will be about 1/8" short of each end of the piano hinge, you will be able to crimp its ends to keep the wire from working out. Alternatively, using a slightly shorter wire will permit you to drill a small hole in each end of the piano hinge, with a very small drill bit (say number 60) and through each of these holes insert a small wire to form a safety for the assembly. It is very difficult to bend a short end of the protruding wire, so that third method of ensuring the retention of the wire is not used very often.
Everything considered, making good fittings is time consuming but not difficult provided you use