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 There
are three fundamental processes that can occur during sheet metal forming and
they are: 1) Bending, 2) Stretching and 3) Drawing. Any formed sheet metal component is made by a
combination of these three processes. To
help understand what these processes are consider a sheet of unformed metal
onto which you have embossed a pattern of circles as shown in the figure at the
left. Let's call this a state of zero strain, i.e., all of the
circles on the metal surface are in their original shape and size
 BENDING
If we are to form a part solely by bending, say by using a brake press, it is equivalent to
pulling the sheet metal in one direction so that circles we've embossed on the metal
surface elongate in the direction we've pulled but remain unchanged
perpendicular to the direction we've pulled.
In this case we say we have positive
major strain and zero minor strain.
 STRETCHING
If we form a part by stretching,
i.e., by pulling the sheet metal simultaneously in two directions at 90 degrees
to one another, our embossed circles will grow in both directions. In this case we say we have positive major strain and positive minor
strain. Imagine a piece of sheet
metal held rigidly on all four edges while pressing a hemispherical punch into
the center as an example of this forming process.
DRAWING
In the case of drawing,
the sheet metal is pulled on one direction with the perpendicular direction
unrestrained. Thus, as the material
stretches in one direction it actually contracts in the other direction. This is what happens during the formation of
a beer or soda can - a cylindrical punch is pressed into a piece of sheet metal
that is unrestrained or only partly restrained along the edges. In this case we have positive major strain and negative minor strain.
THE REAL WORLD
In the real world, actual sheet metal parts are formed by a
combination of these three fundamental processes, i.e., different regions of
any given part may be formed by bending, stretching or drawing. Also, in the real world, we use a tool called
Circle-Grid Analysis, where
we physically etch a pattern of circles and square grids onto the surface of a
piece of sheet metal prior to forming parts, much like in the illustrations
above. We then form that etched piece of
sheet metal into the finished part, physically measure the major and minor strains
on our now distorted circles, and plot them on something called a Forming Limit Diagram.
 If
the strains that are plotted for a given area on a plot show positive major
strain and positive minor strain, that area of the part is formed primarily by
stretching. If the major strain is
positive, but the minor strain is near zero, you are in the bending region of
the Forming Limit Diagram. Finally, if
the minor strain is negative, you are in the drawing region of the
diagram. If the strains that are plotted
are above the red line in the diagram you are at risk of fracturing the sheet
metal during the forming process. If you
are below the red line, the part can be formed safely without fear of fracture.
Each type and grade of sheet metal has a unique forming
limit diagram. Depending on which
forming process is predominant in making a specific part you may want to
optimize specific material properties.
For example, for parts that are formed primarily by bending, the key
material property is total elongation.
For parts formed by stretching the key property is the work hardening
exponent, usually called the n-value.
Finally, for drawn parts the most important material property is the
resistance to thinning, called the r-value.
Oftentimes we do not have the ability to change materials,
or we have a very limited number of materials to select from; when we do our
circle-grid analysis we find that the strains we measure are very near to the
problem region on our forming limit diagram.
In other cases, our circle-grid analysis tells us we should not be
having any problems but we are fracturing parts in manufacturing. In either of these cases we would look at
other factors aside from material, such as lubrication or tooling condition, or
hold down pressures as ways to get us away from failure.
The bottom line is that there are very useful tools for
analyzing sheet metal forming that can be used to optimize material, part
design, and/or sheet metal processing.
Editor's Note: CR4 would like to thank PJ Sikorsky of GEA Consulting for contributing this blog entry
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