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Making Fittings - Part 1
By Tony Bingelis (originally published in EAA Sport Aviation, September 1980)
FITTINGS, ALTHOUGH UNDESIRABLE from a design-structural point of view, are, none the less, essential because to build a one-piece airplane would impose horrendous construction conditions on any builder (or manufacturer).
Fittings are undesirable because they add weight, because they add to the cost of construction, because they add to the time taken to build, and because they add to the complexity of the structure. Additionally, they are difficult to analyze accurately and pose installation and alignment problems which would not exist if the structure were in one piece.
Don't let these introductory remarks dupe you into assuming that fittings are unimportant. They are important, indeed! Poorly made fittings can result in lost wings in flight, the engine falling out, the gear busting off and other even more serious consequences. So, make your own fittings with care.
Some Things Aren't Obvious
Using the proper material for making the fittings is very important. You should abide by the designer's plans call-out. This means the correct steel alloy or aluminum alloy should be used, and in the thickness designated. Sometimes a fitting could be made of a lighter gage and still have the strength necessary for its assigned function. However, there is another consideration . . . that of rigidity. Substituting a lighter gage metal may result in a fitting that lacks the rigidity it needs for a particular function and location. This increased flexibility could contribute to an early failure of the part or, in some locations, result in jammed controls.
You are on safe ground, usually, if your substitution is to a heavier gage (when you have no practical alternative), but then, you will suffer the consequences of added weight. The substitution urge, if encouraged, soon develops into a mental attitude hard to break.
Grain Orientation In Metal
The direction of metal grain is a matter of importance when laying out any part that must be bent. That is, the bend should always be made across the grain or as close to that orientation as practical. If the fitting is to be a flat one (with no bends), the direction of the grain in the part is of no concern and may be ignored. See Figure 1.
About Fittings In General
Some plans provide full size layouts (flat patterns) for all fittings but many plans show only the more important fittings. A few plans show none at all in detail.
A first-time builder will naturally try to do the best he can when drawing his own full-size flat patterns. However, not knowing the intricacies of metal working, things like bend allowance, setback, bend radius and sight line mean nothing to him, and the builder simply doesn't consider any of that at all. As a result, the scenario that often takes place is as follows.
The builder draws his layout using the overall dimensions shown on the plans. Then, he cuts out and traces this pattern onto the metal. Next, he drills the correct size bolt holes . . . most likely without first punch-marking their locations. After that, he saws the fitting out with a hack saw following the outline as closely as possible. And finally, he clamps the part in a vise, and after many less than gentle whacks of the hammer, he has his first bent-up fitting . . . but what kind of a part does he really have? Maybe he has a fitting something like the one illustrated in Figure 2. A fitting that doesn't fit, or worse, one that is unsafe to use.
An informed builder knows that he must not make sharp corner bends in metal, and that, instead, the part should have a generous radius for the bend in order to avoid creating abnormal stresses and a crack potential along the bend line.
A builder not as informed will probably attempt to make a bend using a vise as his bending block. The vise jaw has a rather sharp edge and it will cut into the bend and cause the fitting to crack or (if he's lucky) break. Mr. Builder now has two pieces where he only needed one. But, even if the part doesn't break, it is almost certain to show visible evidence of damage in the form of an embossed crease on the inside of the bend and stress marks on the outside of the bend. That kind of a fitting would be unfit for use in any aircraft! It should be discarded.
But what of the rest of us builders who know about the need for a minimum radius in bends. Wouldn't our fittings be O.K.? Maybe, maybe not. There are other factors. If we do not first smooth the edges of metal parts to remove all saw marks and imperfections before bending them, we too might find cracks in the edges. Additionally, if we had in our eagerness already drilled the bolt holes to the correct size before bending the fittings, we may also find that they are no longer dimensionally accurate, that on installation they do not match an opposite mating fitting. If the part happens to be a control hinge fitting, we may learn that its hinge axis is now slightly higher than required, or that each hinge is a bit different from the other. Yes, a lot of us still have difficulty in making multiple fittings to exact dimensions due to that invisible culprit known as Ben D'Allowance.
Coping With Bend Allowance
If it were possible to make a sharp 90 degree - some call it a square - bend in metal like you can in a sheet of paper, the dimension for each leg of the part would be the same after bending as it was before the bend was made. However, if an identical piece were to be bent around a radius, the material would, in effect, take a "short cut" to its destination and not as much material will have been used in making the bend. This being the case, the end result will be a part that is slightly taller and longer. See Figure 3. Obviously, bend allowance is a factor to be compensated for when laying out a flat pattern for any metal fitting which must be bent.
One way to nullify the bend allowance problem is to ignore it. Ignore it, that is, provided you are willing to do a little extra work in preference to working with traditional metal-working formulas. For my part, I don't mess with the calculation of bend allowances in simple fittings. If the designer was kind enough to provide a full size layout for the part with the bend allowance already provided for . . . fine. Otherwise I use the screw driver and hammer technique (picturesque patter). It is so much simpler to cut a fitting to its given dimensions, ignoring bend allowance as a factor. The bend is made first, and then the standing end and base ends are remeasured to see how much has to be trimmed off. I never drill any holes until after the bends are made. There you have it . . . a fitting with the correct height and base dimensions. Not a very professional way to make fittings, but they will be accurate . . . consistently more accurate than many you'd make using the metal working formulas involving bend allowance charts, setback tables and that sort of thing.
How Do You Cut Small Pieces Out Of A Big Sheet?
Before you can even make your first fitting, you have to cut a small piece out of a big sheet of steel or aluminum. This chore alone can become an overwhelming problem for some builders. Here are a number of ways to do this. One or two of them could work satisfactorily for you under certain conditions.
Method One . . . The Hack Saw
Sure, a hack saw will work in a corner of a large metal sheet provided the part you intend to cut out is no more than 3 inches wide. That is about as deep as the hack saw can reach. Sometimes by rotating the blade in its frame 90 degrees a much longer cut is possible.
Of course, you could increase the depth of cut for a hack saw by removing the blade, wrapping it with a rag, and sawing away in that primitive hand-held style . . . but is it ever tiring!
Method Two . . . The Sabre Saw
A sabre saw! Why not use your sabre saw! Wouldn't it be easier than using that hand-held hack saw blade? Couldn't you take a fine tooth (24 to 32 teeth per inch) blade intended for metal cutting and slip it in the sabre saw and be ready to go? Or maybe you could even make your own metal cutting sabre saw blade from a hack saw blade by breaking off a 3 1/2 inch length and grinding it to fit the sabre saw socket. That would work too, wouldn't it?
Sure, but that would be good enough only for cutting aluminum and not for cutting steel. Most sabre saws do not have a slow enough speed for cutting steel, and even a high quality hack saw blade will not last long if you try cutting 4130 steel with a sabre saw having only that one speed . . . too fast!
If your sabre saw is not a variable speed unit, you had better find an alternative way to cut 4130.
CAUTION: Always clamp your work securely and do not overlook the fact that the sabre saw cuts on the up-stroke. Don't take a chance with eye injury . . . wear safety glasses.
Method Three . . . The Cold Chisel
If a sheet of steel is not too large, and if you have a large solid vise, a large cold chisel (about 1" wide) and a baby sledgehammer, you can easily chisel off a small piece. Using this method you should leave about an 1/8" margin for later grinding and filing the part to its exact dimensions. It might sound like a crude method, and it is, but it is also an effective method when properly done. The chisel should be held against the metal at an angle, when struck with the sledge, so that it acts as a shearing tool. This results in a fairly smooth cut along the surface of the vise jaws.
Method Four . . . The Metal Shear
A metal shear completely eliminates the problem of cutting little pieces away from a large sheet unless the metal is too thick for the capacity of the shear. If you don't have access to a metal shear you might, nevertheless, find it worthwhile to have several strips sheared off at some local metal shop. A project having a lot of wing strap fittings, for example, could be expedited by having a number of lengths of a uniform width sheared off the sheet.
Method Five . . . The Aviation (Tin) Snips
Aviation snips are effective only on light gage sheets. They will, however, adequately handle both aluminum and steel stock in thickness up to .050". Some types of snips distort the cut edges considerably and you should therefore not cut closer than 3/32" of the line. These aviation snips are made in three types. That is, in Left Cutting, Right Cutting and Straight Cutting models. There are also offset snips, or band held shears, which can cut easily across a large sheet without distorting the edges.
Method Six . . . The Cutting Torch
Cutting torch? Never! It wastes too much steel and also causes a hardened area along the cut which is very difficult to file or grind away. A large margin must be left in order not to damage the part you want to use. It might be all right to use the cutting torch for very heavy steel plate but even here I wouldn't recommend it because of the possibility of creating localized locked in stresses. This type of cutting introduces a greater risk of crack generation.
Method Seven . . . The Table Saw
With the proper blade, a table saw will cut strips of aluminum to uniform widths but this method is not for steel. I don't think your neighbors will appreciate this method nor will your ears.
Method Eight . . . The Carburundum Disc
These discs will cut through steel easily but are really not suited for making many long cuts. I prefer to use these discs with a drill press and primarily for slotting tubing.
Method Nine . . . The Power Hack Saw
The power hack saw will cut steel and aluminum plate, tubes and rods as easy as pie, but it is not suitable for cutting sheet material. This is more of an off-cut machine for tubing and heavy metal stock.
Method Ten . . . The Bandsaw
Ah! This is the way to go. A metal cutting bandsaw will really turn out the pieces rapidly and you should be able to cut all the fittings for an entire airplane with one blade.
Aluminum may be cut at the regular speed with a handsaw. Try to do the same thing to 4130 steel and the blade will become overheated and quickly ruined.
The handsaw blade speed could be slowed for cutting steel by attaching a speed reducer. Sears has one for their regular 12" handsaw. Or you can obtain the same kind of speed reduction by using large pulleys mounted on jack shafts. When the saw is ready to cut steel, its blade speed will be noticeably slower than it originally was. A blade speed of 125 to 150 feet per minute is good, but in a pinch you might get away with a blade speed somewhat higher.
Method Eleven . . . Hand Operated Nibbler
The manually operated nibbler works fine for lighter gages but cannot be considered as a practical means for cutting many pieces. It is wasteful of metal as it cuts a wide swath and is very tiring to use.
Method Twelve . . . Pneumatic Metal Shears
Most of these are not suitable for heavy gage metal. They do cut easily and smoothly and are enjoyable to use, but do require a source of compressed air.
Any of the foregoing methods used to cut a small piece of metal away from a larger sheet will give adequate results sometimes . . . but not always.
Make Them Smooth
Whatever method you elect to use for the initial separation of a fitting from that large sheet, keep the saw cut well to the outside of your drawn line. Allow yourself plenty of margin for error. The final shaping of the part may be done on a grinder (for steel parts). Follow that up by filing it with a smooth cut file.
You soon find that a file cuts very smoothly when the file is pushed into the work at an angle. Keep the file level . . . you want square edges not sloped or rounded ones. And finally, remove the sharp edges and fuzz by passing the file lightly along the edges of the fitting. All file and saw marks must be removed. Rub the edges over sandpaper or emery cloth laid on a smooth hard surface. For other hard-to-get-to edges you might try a piece of emery cloth wrapped around a dowel or block of wood.
Making Bending Blocks
A single bend up to 90 degrees is simple to make provided the part to be bent is wide enough and you have a solid heavy-duty vise and bending block with the appropriate radius. Making a second bend is more difficult but the blocks illustrated will probably allow more options for bending than other types.
Larger vises have removable jaws with a serrated or knurled surface on one face and a smooth surface on the opposite side. Always use the smooth surface sides for gripping aircraft work. If you wish, you can radius one edge of the vise-jaw-insert and use it as a utility bending block. A separate bending block, however, lends greater versatility and it permits a choice in the bend radius used.
Bending blocks should preferably be made of steel, or of aluminum alloy if only small parts will be bent with them. I find the most practical size for my use is one approximately 8 inches long made of a piece of 1/8 inch aluminum alloy plate. Its width is 3 inches although sometimes it seemed that a 4 inch width would have been better. It really depends on what size a piece of metal plate you can find in the junk yard. A piece of steel plate measuring 3/8 inches thick by about 4 inches square would be adaptable to most small bending jobs.
Prepare your bending block by grinding or filing a radius along one edge. You might consider preparing a different radius for each of the four edges. Unless you require some specific radius I would suggest one edge be radiused to 1/16", another to 1/8", and the remaining two to 3/16" and 1/4". Make templates as shown in Figure 4 to check your radii for accuracy. Punch marks on the ends of the bending block will identify which is which.
That's right, one punch mark for the 1/16" edge, two marks for the 1/8" edge, etc. See Figure 4.
The most effective bending block will be one that has its edge beveled so that it will allow the metal to be bent slightly beyond the 90 degree point. Metal has a spring-back tendency. Beveled bending blocks will naturally be limited to only two different radii per block.
A good bending block is a handy thing to have around the shop and it should serve you well through a couple of airplane and boat projects. If, however, you need a bending block for only one or two bends in light metal, you may do as well with a block made of hardwood. Using a wood bending block for making more than a few bends, however, will result in its radius becoming larger and larger because the wood will flatten slightly with each part bent.
IMPORTANT REMINDER: Never make a bend in a metal part until its edges have been filed and/or sanded to a smooth finish. Saw and file marks certainly increase the risk of cracks particularly in parts subjected to heavy load reversals and vibration.
So much for the basics. More about bend allowance, setback, bend radii, the sight line and difficult bending problems next month.