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Making Fittings - Part 2

By Tony Bingelis (originally published in EAA Sport Aviation, October 1980)

 That is one way, a fool-proof way of making angle fittings that is as practical as it is predictable. Making your bent parts that way, you don't have to know about bend allowance set back, sight line, mold line and neutral axis . . . or how to use that information to your advantage.

Well, you say, if I can bend accurate enough fittings without messing with formulas, tables and charts, why should I bother about getting familiar with some other, more demanding methods, and have to learn new terms besides?

Several reasons. Not all fittings are angles consisting of a single 90 degree bend. A number of fittings have two or more bends, and in some instances, the angle of bend may be greater or smaller than 90 degrees.

When making something like a battery box, for example, the material dimensions (flat pattern) must be established before any bending can be started. Sometimes, even the sequence of making the bends is important.

Take for example a "U" fitting (channel) that must be made to fit snugly between some other parts. How large a piece of metal must you use? How would you determine where to make the bends so that part would fit? Trust to luck and use the trial and error method? You would waste a lot of material that way before you got one just right. Or what if you are building an all-metal airplane and have a lot of aluminum angles to bend. You'd find it very wasteful to pre-cut large pieces for bending and then later have to trim them to size. The trial and error method of bending metal has its place, but under certain conditions could become a very time consuming way of building. Then, too, here's another reason. It is possible that you might become interested in obtaining your FAA Airframe Mechanic license. Quite a few builders have obtained theirs, after receiving credit for the FAA's mandatory practical experience requirement, simply because they did construct an aircraft. They still had to pass a written examination, of course, and possibly answer a few questions thrown in by the inspector. One area of questioning regarding the builder's aircraft knowledge often explored by the inspector has to do with metal working. After all, aren't most present day aircraft all metal? Even those that aren't have a lot of metal parts in them, so any knowledge or experience gained in metal working is important. Yes, it is handy to know how to lay out flat patterns which have been compensated for bend allowance and have the necessary set-back established and sight (or brake) lines clearly drawn. Understand the meaning of each of the following terms, and where and how each is applied and you should be able to lay out, make and bend metal parts with confidence.

Lay Out Procedure . . . Select the Radius of Bend

This is the easiest requirement to understand because most of us are already familiar with the fact that metal has a minimum radius around which it can be bent.. If the bend radius is too small (too sharp) the resultant stresses and strains in the metal will weaken the part and perhaps cause it to crack . . . later, if not immediately.

Incidentally, many builders are not aware that aircraft fittings are normally bent cold nor may they realize that the cold bending strain-hardens the metal slightly and makes it even more vulnerable to cracking unless a reasonable bend radius is used.

Tables giving the minimum bend radii are available from somewhere, I suppose, but not knowing of a source I submit the following abbreviated table of minimums which were derived from reliable USAF and FAA references. The numbers are presented as a function of the thickness of the metal sheet for making 90 degree bends.

(In terms of approximate metal thickness):








Aluminum Alloy























Aircraft Steel 4130







*2024T-3 aluminum sheet 1/8 (.125") should never be bent around a radius less than 4t or 1/2", etc., etc.

NOTE: A good rule is to never bend any kind of metal around a radius less than its own thickness. Notice that the hard tempered aluminum alloys require much larger radii than does 4130 steel.

Annealed steel and aluminum parts can be safely bent around smaller radii than can normalized steel or hardened (tempered) aluminum alloys but they should never be installed in aircraft unless the material has been heat treated after forming.

If a very small radius is bad then a very large radius must be good . . . you know there must be a flaw in that kind of reasoning! A large radius is, indeed, good if its location and use permit it. However, for the most part, a fitting with a very large radius may result in the attachment bolts gouging the metal in the bend area . . . a most unsatisfactory condition.

Determine Set-Back

An angle fitting cannot be laid out to exact dimensions unless you first know where the bend begins. This point (line) is called the set-back. Set-back is actually the distance as measured from the point where the bend begins (or ends, depending on how you look at it) to the fittings base line or mold point. This length actually consists of the bend radius and the thickness of the metal. (Figure 1 - SB = R + T) Therefore, to find the dimensions for the flat portions of the fitting you must subtract the radius plus the thickness of the metal from each leg of the fitting to find the point where the bend tangent line will be. That's all there is to set-back for any part that must be bent to form a 90 degree angle. A part that must be bent to a larger or smaller degree must have its set-back figured in a slightly different manner. The set-back for bends greater or smaller than 90 degrees is normally obtained from standard set-back tables (not included in this article).

After you have your dimensions for the flat (unbent) portions of the fitting, you are ready to accommodate the bend allowance.

Making Fittings

Figure Bend Allowance

Bend allowance and set-back are often looked on as a baffling mystery to many builders who are used to practicing the cut, bend and fit method of metal working, but it shouldn't be at all.

Both bend allowance and set-back are measured from a common point . . . the bend tangent line. Remember, that is the point where the bend begins. The difference between the two is that the set-back is measured along a straight line while the bend allowance is measured along the radius of the neutral axis in the bend. Bend allowance determination is based, primarily, on the thickness of the metal, the radius of the bend used and the number of degrees through which the bend will be made. Better take another look at Figure 1 to make sure your mental and visual images are the same.

There are several ways of determining the amount of bend allowance for laying out a fitting prior to bending it. Here are a couple of them.

The Formula Method of Determining Bend Allowance

A scholarly means of determining bend allowance is through the use of a simplified empirical formula which assumes the neutral axis in a bend to be in the middle of the metal and not at a point - .445 x t from the inside of the bend radius where it is really located. This mathematical transgression apparently doesn't affect the accuracy of the formula method:

    BEND ALLOWANCE (BA) = (0.017438 + 0.0078t)N

    R stands for the preselected inside radius of bend (use decimals of an inch)

    t is the metal thickness . . . also in decimals of an inch

    N is the number of degrees of bend to be made in the metal

Here's how you put it all together:

Let's assume you need to find the bend allowance needed for a .040" part which must be bent to a 60 degree angle around a 1/4" (.250 inch) bend radius. Let's proceed.

    R is .250 inch

    t is .040 inch

    N is 60 degrees

Using the BA empirical formula (.017438 + .0078t>N, the arithmetic works as follows:

    BA = (.01743 x .250 + .0078 x .040) x 60

    BA = (.00435 + .00031) x 60

    Answer: BA = .2796 or .280 inch (reduced to a fraction, about 9/32 inch)

NOTE: The Bend Allowance Table shows that the bend allowance for a .040" metal part bent 60 degrees, as in the above example, is .00468 per degree of bend, or .00468 x 60 which equals .2808'. That is quite close to the answer obtained using the empirical formula above . . . and a lot easier to determine.

If your memory is no better than mine, you will probably forget the formula by the time you need to use it.

Anyway, nobody in this enlightened age figures bend allowance any more . . . they use Bend Allowance Tables. These tables automatically provide the correct bend allowance requirement based on a particular metal thickness, the selected bend radius and will adjust for the number of degrees of bend intended.

Bend Allowance Table
























































































































































































Now that you have the BA what do you do with it? Simply add that figure to the dimensions retained for the two flat portions of the fittings (after you subtracted the set-back). Total the figures in the same manner as shown in Figure 5, Step 6, and you can lay out a perfect template for bending. Of course, your layout could be made directly on the metal. (Use a silver pencil on steel and a soft lead pencil on aluminum.) The only other information you need have is how to align the part for bending. The alignment for bending is solved when you draw a sight line or brake line on the part. The sight line is located by measuring out one radius from the bend tangent line. Align your bending block with this reference line and bend away. The dimensional accuracy of the completed part will surprise you.

Making Fittings

By the way, don't overlook Figure 2, Figure 3, and Figure 4 . . . they should help prove the adage that, "It's easy when you know how!"

Making Fittings

Making Fittings

Making Fittings

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