metals, faster output, and more professional results,
BEND ALLOWANCE FORMULA
machines designed for metal-cutting purposes are used.
By experimentation with actual bends in metals,
Machines used to cut sheet metal may be divided
aircraft engineers have found that accurate bending
into two groups--manually operated and power
results could be obtained by using the following
operated. Each cutting machine has a definite cutting
formula for any degree of bend from 1 to 180:
capacity that should never be exceeded. A few of the
(0.0173 x R + 0.0078 x T) x N = BA
more common types that may be available to you have
been described in the previous sections.
Where
R = the desired bend radius,
BENDING SHEET METAL
T = the thickness of the material, and
Straight-line bends and folds in sheet metal are
N = the number of degrees of bend.
ordinarily made on the cornice brake and bar folder;
however, a considerable amount of bending is also
Refer to the NA 01-1A-1 for the appropriate bend
completed by hand-forming methods. Hand forming
allowance tables.
may be accomplished by using stakes, blocks of wood,
angle iron, a vise, or the edge of a bench.
CUTTING SHEET METAL
Bending Over Stakes
Once a project has been laid out on the metal, the
next step is to cut it to shape. The type of cutting
Stakes are used to back up sheet metal to form
equipment to be used depends primarily upon the type
many different curves, angles, and seams. Stakes are
and thickness of the material. Another consideration is
available in a wide variety of shapes, some of which are
the size and number of pieces to be cut. A few relatively
shown in figure 4-33. The stakes are held securely in a
thin pieces of comparatively soft metal may be cut
stake holder or stake plate, which is anchored in a
faster with hand-trimming methods. But for harder
Figure 4-33.--Stakes and stake plates.
4-18
