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Edited by Chuck Larsen, EAA Designee Director
BENDING SHEET METAL
From: "To Set Back Doesn't Always Mean To Relax!" by Bill Warwick, EAA 3775; "Why Bend A Radius" by W. L. Dean, EAA 33953 and a comment by EAA's Washington Representative, David Scott, EAA 1004. Each article was published in the "TIPS" manuals previously distributed but no longer available from EAA.
To Set Back ... Doesn't Always Mean To Relax! By Bill Warwick, EAA 3775
032.
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L HE FOLLOWING IS intended exclusively for those people who do not have, or may not understand the purpose of a setback chart. Let's assume that we're making the part shown in Fig. E. If the part had to fit between two bulkheads or such, which were fairly solid, it would be necessary for an accurate fit. If the dimensions are simply added together (Ex. 4 + 1 + 1 = 6) and bent up, it won't fit. The reason becomes obvious by looking at Diagrams A, B and C. Flange dimensions are given from the outside mold lines. It's apparent that the metal is taking a short-cut home by means of the radius rather than following the plane of the mold line. Therefore, total length of metal required will be somewhat less than six inches. By referring to the setback chart and picking out the correct radius, which is .30, follow this across until it lines up with column under the right thickness, and we read in thousandths the amount of material to be deducted for one bend. The fig-
ure is .295, so we double it for two bends, which is .59, and subtract from
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be exact. Please note that the chart is for 90-degree bends only. There is
This is the one to use for open or
another chart which is a graph type
through your local industrial training shop or high school shop.
18 MARCH 1984
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our six inches and come up with a
and is commonly called a "J" chart.
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correct developed length of .41 inches. Now, all you have to do is bend two one-inch flanges, and the length will
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closed angles, and should be available /.O
Why Bend A Radius By W. L. Dean, EAA 33953
IN A RECENT fine article by Bill Warwick, EAA 3775,
(To Set Back . . . Doesn't Always Mean To Relax — April, 1967) you were given a set of backs to use to make your bends come out to the correct dimensions. Now here are a couple of tables to go along with them that will help make your sheet metal work look more professional and be much safer. One of the most common failure areas in sheet metal structure is in the corner bends of the various components. This is usually because the bend is made without regard for the established bend radius. These bend radii were established as minimums tor the various thicknesses and hardnesses of the metals. As the thickness and hardness increase so do the bend radii. Even the thinnest, softest metal should not be bent in a sharp corner as the strain Minimum Inside Bend Radii for Aluminum Alloy Sheet and Plate 1100-0 3003-0 5052-0 6061-0
Metal Thick.
.006 . . . . . .
.03
.008 . . . . . . .03 .010 . . . . .03
.012 .016 .018 .020 .022 .025 .028 .032 .036 .040
. . . . . . .03 . . . . . . .03 ni . . . . . . . .03 . . . . . . .03 . . . . .03 . . . . . . .03 . . . . . . .03 . . . . . . .06 . . . . . . .06 n** IMS .050 ..... .06 . . . . .06 .056
.063 .06 .071 . . . . . .09 .080 .090 .100 .112 .125
. . . . .09 . . . . . . .09 19 . . . . . . . .12 . . . . .12
Material Condition 1100-H14 2024-T3 3003-HM 2024-T4 2024-0 7075-0 6061-T6 6061 -T4 7178-0 .03
.03
.03
.03
.03 .03 .03 .03 .03 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .12 .16 .19 .22 .25 .25
.03 .03 .03 .03 .03 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .12 .19 .19 .22 .28 .28
.06 .06
.06
.06 .06 .06 .06 .09 .09 .09 .12 .16 .16 .19 .19 .22 .22 .28 .34 .38 .44 .50 .56
7075-T6 7178-T6 .09
.09 .09 .09 .09 .12 .12 .12 .12 .16 .16 .19 .19 .25 .25 .28 .31 .38 .44 .50 .62
.75 .88
BENDING METAL — Fittings play a pretty important part in a homebuilder's life. My primary aim is to point
out a few of the ways one may go wrong when it comes to making a bend in a metal fitting. When we bend a piece of metal as per the section (Figure 1) we take the metal beyond its Yield-Point. The result is permanent deformation of the metal. Just the result we desired. As Figure 1 shows, we have stretching on the outside of the bend and compression on the inside of the bend. Figure 2 shows a zero radius bend in which stretch is the only stress involved. It is not too hard to see
that the metal went through a lot more deformation than it did in Figure 1.
on the metal could produce hidden failure points and cracks which can give way under low stress. In an aircraft plant, the first few parts completed are X-rayed to check for hidden flaws, but inasmuch as the average amateur aircraft builder does not have access to X-ray facilities, we must be right when we bend the metal. The following tables show the minimum inside, or "bend
radius", to which metal can be bent safely with regard to its temper and thickness.
A word should also be said about the speed at which bends are made. When a bend is made too fast, heat is created which, added to the strain of bending, can create cracks which otherwise would not occur. Metal bent slowly and using the bend radii called out should give bends that are as strong as the rest of the metal. Minimum Inside Bend Radii for Ste*) Sheet Material 1020,1025 18-1 Types CRES 4130, 8630 4130, 8630 Annealed '/i Hard 17
.036 .040 .045 .050 .056 .063 .071 .080 .090 .100
.. . ... . . . ... ... . . . ... . . . ... .... .. ... . . .... . . . ... . . .. . . . ... .. .. .. ... . . . . . . ... . . . ... . . . ...
.03 .06 .06 .06 .06 .06 .06 .06 .06 .06
. ..
.12
.06 .06 .06 .09 .09 .09 . . . .12 . . . .12
19S
.140 . . . ... .12 .160 . . . .. .12 ISO . . . .12 .190 ... . . . .12
.03 .03 .03
.03
.06
.03
.03 .03 .06 .06 .06 .06 .06 .06 .09 .09 .09 .09 .09 .09 .12 .12 .16 .19 .19 .19 .19 .19 .25 .25
.03
.06 .06 .06 .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .09 .09 .09 .12 .12 .16 .16 .19 .19 .25 .25 .38 .38
.03
.03 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .12 .12 .16 .16 .19 .25 .25
.25 .25 .25 .38 .38
.06 .06 .06 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .16 .19 .19
.22 .25 .28 .31 .34 .38 .38 .44 .50 .50
strengths of the finished part due to altered dimensions. I will not go into this more than to say that most fittings are designed for the smallest bend radius which will not crack the material being bent. In the case of 4130 steel the radius may be taken as: Radius (R) = 2 to 3 times thickness (t). Having made the bend according to the rules? Shall we now examine it? On the outside of the bend we see faint lines running transverse to the bend. These lines are the result of having stretched the metal. If you really did follow the rules you will have a pretty hard time seeing these lines. In severe cases these lines will develop into
full fledged cracks. A good deal of thought should be given to MAGNAFLUXING or similar test procedures. Cheap
Insurance.
One last item: Avoid nicks, scratches, or any visible
Fig.1
Fig. 2
In making a bend, we aim at the least amount of deformation while at the same time not altering the
marks on the finished fitting. And paint may cover it. It
will not eliminate the problem. ANY MARK ON THE FITTING WILL CREATE WHAT IS CALLED A STRESS CONCENTRATION POINT, i.e. If one scratches a piece of glass, where will it break??? Hope this helps a little.
SPORT AVIATION 19
