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Metal Working Tips for First Time Builders – Part 1
By Tony Bingelis (originally published in EAA Sport Aviation, January 1993)
Building a metal kit airplane is, in many ways, simpler than building a tube and fabric airplane, a wood and fabric job, or, for that matter, a composite. This is because fewer and less demanding building skills are required of a metal kit builder.
Here’s the way I see it. How difficult a project is to build is really determined by the number of skills needed to construct the aircraft’s basic structure. Of course, the complexity of the project is also a factor.
Perhaps the most difficult aircraft to construct are the traditional steel tube, wood, and fabric designs. These projects require knowledge and skills (not easily acquired) in at least three basic fields of construction . . . welding, fabric work, and wood work.
Even though only one broad skill area predominates in composite projects, the procedural and application methods are quite demanding and critical. And then, there is the ever present health hazard attendant with the use of epoxies. But, to me, the least appealing aspect of a composite project are the work intensive "fill and sand" chores that can tax even the most dedicated builder’s patience and endurance.
A metal aircraft kit project, on the other hand, requires the development of only one skill area. This basic metal working skill is easily acquired by most any builder because the work consists primarily of drilling holes and driving rivets.
Incidentally, almost all projects, regardless of their type of construction, be it steel tube, fabric, wood or composite, require some metal working skill.
The construction and installation of engine baffles is a good example. Fabrication of fittings, brackets and the like is another. So, why not go metal all the way?
Besides, the cost of kits for high performance metal aircraft are considerably less expensive than kits for composite aircraft of similar performance.
And I do, indeed, recommend that a first time builder build from a kit rather than from scratch (raw materials). A lot of frustration will then be avoided. A kit, ordinarily, contains all the materials, many pre-formed parts, and most of the hardware you will need. More importantly, most of the metal skins, ribs and bulkheads will be pre-formed or at least cut to dimension.
All this, naturally, represents a considerable reduction in the total building time that would otherwise be required - another bonus.
A kit will do much to bolster confidence and minimize construction confusion and difficulties. Furthermore, building from a proven kit generally results in a safer airplane.
So, Let's Begin
When you receive your kit, inventory the contents of the plywood cartons within the time specified by the manufacturer. To get the packing crates out of the way, disassemble them carefully and remove all nails, staples, and steel banding, for safety’s sake.
Save the plywood and wood for future use in making temporary storage shelves, templates, jigs, stands, etc.
Unwrap the contents carefully . . . you don’t want to lose any of the small pieces and parts.
I suggest you put all the wrapping material in one pile at some distance away from those nice new parts you intend to keep.
Check off everything against the packing list or material list (usually enclosed).
Take your time, inventorying the kit contents can be educational and useful because you will have to locate and identify each of the many pieces that will ultimately become your airplane.
Each piece of metal, each part, is usually marked so you can identify it. If it isn’t, search through the plans until you locate and identify it. Then, mark it yourself with a waterproof marking pen (Sharpie, or something similar).
Undoubtedly, you are anxious to get on with the actual assembly work - but don’t get into too big a hurry.
In other words, don’t rush. Proceed by taking care of the first things first. In the end this will save you a lot of time.
Here, for example, listed in logical sequence, are a number of often slighted chores you should take care of before you start your construction and assembly work.
Deburr All Metal Edges
Many builders question the need to deburr or chamfer the sharp metal edges on all the sheared metal skins and formed parts before they are really needed.
However, I find it to be a lot easier and quicker to perform most preparatory, and like operations, at one time. Deburring drilled holes and deburring cut or sheared metal edges are good examples.
Expect your aluminum skins and other formed parts to have sharp and rough edges because most of the parts were stamped out, or sheared, during manufacture. Those razor sharp edges can draw blood.
The kit manufacturer certainly doesn’t feel obligated to smooth the holes and cut edges for you as he would have to raise the cost of his kits. Besides, according to the rules governing amateur built aircraft, you are supposed to do 51% of the work.
You should be very much aware that burrs and sharp edges are stress inducing areas that can develop cracks. These could, in time, progress to the point where complete failure of the part can occur. Figure 1 shows how you can deburr your metal edges.
There is another good reason for smoothing and chamfering the edges of aluminum skins and formed parts. Burrs and sharp edges are risky to handle because they can inflict serious cuts on your hands and fingers.
While working on a piece of metal or a part, it is only natural that you will slide your hands across its surfaces and otherwise handle the metal as you measure, bend, or drill it. Not only that, you will from time to time subconsciously brush off the chips with your hands and, OUCH! . . . you guessed it, a painful cut from a metal burr that wouldn’t let go.
How smooth do the edges of a metal sheet need to be? Well, if after passing your fingers along the edges you draw blood, you didn’t quite finish the job.
Almost every hole drilled in metal will have small metal chips or burrs around its edges on both the top and bottom sides.
Drilling a hole with a sharp drill bit produces a minimum of burrs but you can still expect to find some.
All burrs must be removed prior to riveting or the burrs will keep mating parts from fitting flush. When this happens, a rivet installed in such a hole will not seat properly and will tend to swell up between the sheets when it is being set.
Should that happen, the improperly seated rivets must be drilled out, the burrs or chips removed, and you try to do it right the second time.
You might as well take the time to deburr the edges of all of the metal parts that came with the kit. This would include all of the pre-formed wing ribs, bulkheads, tail ribs, and skins.
Figure 2 shows several deburring tools you can use for deburring holes. All of them work quite well any place access is unrestricted. In other less accessible places, however, you may find that one particular tool works best for you.
In time, you will develop other tool preferences and the technique for getting a particular drilling or deburring job done.
Later, when you start drilling holes for the assembly of metal skins and other components, you will learn that it is necessary to take each assembly apart after the drilling is completed in order to properly deburr the holes . . . a pain in the neck but, oh, so important.
About Those Steel Components
Steel parts such as the landing gear, engine mount, control stick assembly, rudder pedal assemblies and the like, will begin to rust almost as soon as they arrive. It is the wise builder who will immediately prime these assemblies to protect them from rusting.
If you delay doing so until you need to install a particular component, you may be faced with the additional chore of laboriously removing a lot of accumulated rust before you can prime and paint the stuff.
You might even have to have all of the components sandblasted . . . another time consuming expense. Bare steel parts must be treated with a good metal treatment system. How you do this will depend on the paint brand you prefer to use. Whichever it is, follow the manufacturer’s instructions to the letter.
For example, if you plan to use DuPont products, you can clean the steel parts with their enamel Reducer Cleaner 3812S or 3832S and spray on their VARIPRIME Self-Etching Primer. This should preserve the parts until you are ready to paint them.
Of course, this is only one of the several systems DuPont has for treating various kinds of metal. Other companies have similar products.
Aluminum Skins Scratch Easily, So...
Handle those large floppy skins carefully, they will pick up scratches in spite of all your efforts. Here’s how it happens and what you can do about it:
1. Cutting an aluminum sheet on the bandsaw invariably results in some light scratches on the under surface as you slide the part through the saw. Be sure the saw table is free of chips and dirt before you start cutting. Putting masking tape strips across the bottom of the sheet along the cut line helps.
2. Sliding aluminum parts around on the workbench also can result in scratches because of the fallen chips and burrs produced by various trimming or drilling operations.
A large piece of cardboard (mattress carton, etc.) laid across the workbench surface helps reduce scuffing. However, you must constantly brush its surface clean of chips or your precautionary efforts will be wasted.
Some builders have had considerable success in reducing aluminum skin damage by laying a rubberized mesh across the workbench surface. This material is similar to the rubber mesh used under throw rugs to keep them from sliding. The stuff looks like hardware cloth but is made of rubber or plastic. It will, I am told, protect an aluminum sheet from scratches because it will be elevated above the stuff that accumulates on the bench. Don’t write, I know of no source . . . try Rubbermaid™ or your local carpet dealer.
3. If you really intend to have a shiny polished aluminum airplane free of scratches, you might be motivated enough to keep brown wrapping paper taped under all your aluminum skins as you work on them.
Otherwise, exercise reasonable care and don’t worry about minor scuff marks. They will not show after the structure has been properly prepared, primed and painted . . . so, don’t panic.
Cutting Out Lightening Holes
Aluminum wing ribs furnished with a kit usually have several large reinforcing rings embossed in the rib metal. The centers of these embossed rings (lightening holes) may be cut out to save several pounds. That is good enough reason to cut them out.
However, if these rings are not cut out, riveting the wing skins could become an impossible task because you wouldn’t be able to reach inside the wing with a bucking bar to do the skin riveting.
To cut out the holes, use a heavy duty adjustable circle cutter or "fly-cutter" (Circle Cutters . . . Sears has them). Clamp the rib securely to your drill press and use a slow speed for cutting. Those fly cutters are dangerous to use so be careful!
I don’t believe an adjustable fly cutter should be used in a hand drill for cutting out large holes as it is hard to keep the circle cutter from jamming in the aluminum during the cutting.
After the centers are all cut out, each hole should be deburred and smoothed. A good finishing tool for this is a small Scotch Brite wheel chucked in a drill press, preferably.
The wing ribs as furnished in an all-metal kit, most likely, will have been stamped out of 2024-0 alclad aluminum in a hydraulic press.
After forming, the ribs are placed in an oven and heat treated to a T-4 condition. This makes them stronger but also harder and more brittle.
That being the case, it is very important that all edges be smoothly chamfered and free of saw or file marks lest they create a starting place for cracks.
Unfortunately, in the heat treatment process the ribs often come out warped and bowed . . . especially in the flanged leading edge areas.
The most important rib preparation you can make is to remove as much of the distortion and bowing as you can.
The bow or curvature in the wing ribs can be completely straightened except, perhaps, near the leading edges.
Straight each rib with the help of fluting pliers.
The commercial fluting pliers form flutes that are quite wide. This may cause a problem when close rivet spacing is called for.
Many builders, therefore make their own fluting pliers by modifying a cheap pair of pliers. They simply braze or weld short pieces of 3/16" rod to the jaws of the pliers. Short pieces cut from an AN3 bolt work quite well.
It is essential that the rivet spacing be precisely established before crimping in the rib straightening flutes.
In order to provide a good alignment reference for straightening the rib flanges, you should first draw a centerline along the flanges using a Sharpie pen or any suitable permanent marker.
Drawing the reference line along the flange may be done free hand using your fingers as a spacing guide. Naturally the reference line will be curved because it follows the curvature of the rib.
To straighten the rib, start the fluting at the rear end and work forward. Don’t make the initial flute crimps too deep . . . only as necessary to straighten the rib. Keep the number of flute crimps to a minimum necessary to straighten the rib.
You can lay a flexible straightedge on the drawn reference line to check its straightness as you flute your way toward the nose of the rib.
Unfortunately, the extreme forward portion of the rib may defy your straightening efforts so don’t get unduly upset when you learn that for yourself. Do the best you can. Later, the rivet holes can be drilled offset a bit to avoid drilling them too close to the edge of the rib flange.