Groover/Introduction to Manufacturing Processes
Sheet Metal Stamping Dies and Processes Case Study: Drawing Brass Cups
Three women in rural India have taken out a micro-loan to start up a non-profit
business for making brass cooking and dinner ware. They have plans for
intricately designed pans, plates, bowls, and cups. They have begun to plan the
production of the basic shapes and receive a donated powered mechanical
stamping press that is no longer needed at a local shop.
The women expect one of their most popular items to be a decorated brass cup.
The cup is to be formed from 1.5 mm thick brass sheet stock on the press. It will
be drawn into a cylindrical shape with an inside diameter of 65 mm and a height
of 40 mm. The corner radius on the bottom of the cup (i.e., punch radius) is to be
6 mm.
The women end up modifying the stamping press to add a blankholder to add
pressure to the outside of the sheet stock so that wrinkling does not occur during
the drawing process. They eventually get the blankholder to work and are able
to calibrate just the right amount of blankholding pressure to prevent wrinkles
without tearing the material during the draw.
Parts are finally produced on the press without any tears or wrinkles. One of the
women, however, notices that the top rim of the cups has a wavy, inconsistent
pattern to it. An internet website identifies this type of drawing defect as “earing”
because the irregularities on the rim look like ears. The women hope to resolve
this problem quickly because they cannot earn enough money if they are forced
to manually rework all of the cups to remove the ears.
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1. The ability to successfully complete a sheet metal stamping operation is
called formability. What material property is the most important influence
on formability?
2. Buckling (or wrinkling) usually occurs as sheet stock is drawn into a die
because of the compressive stresses that develop in the stock material.
In a proper drawing operation, how is the buckling prevented?
3. Hemming is a common operation performed on the edges of sheet metal.
What is the function of open or rope hems formed in sheet metal parts?
GO TO THE TEXT: Chapters 3 and 14
4. Use Table 3.3 to justify whether brass is a good material to use with the
cup drawing operation.
5. Is the clearance between the punch walls and the die walls exactly equal
to the stock thickness in drawing? See Section 14.3.
6. Given the starting stock thickness of 1.5 mm, can the women assume that
the walls of the cups they produce will always have at least 1.5 mm
thickness? See Section 14.3.
7. What is the cause of the “earing” defects that the women are seeing? See
Section 14.3.
8. Could the women alternatively use a spinning process to create the cups?
Explain why this might be a good option, especially if production volume is
relatively low. See Section 14.6.
SOLVE
9. Use the method described in Section 14.3 to estimate the starting size of
the blank work part Db needed for the cup drawing operation. For this
calculation, neglect the corner radius at the bottom of the cup (i.e., use 0
radius), and neglect any thinning.
10. Compute the drawing ratio, reduction, and thickness-to-diameter ratio for
the brass cup drawing operation to determine if the process is considered
feasible by these measures. Use Db from Question 9.
11. If the draw die radius is 6mm, compute the drawing and blankholding force
needed for drawing the brass cups. Use Db from Question 9 and the
material properties for copper alloys given in Table 3.2.
Sheet Metal Stamping Dies and Processes Case Study – Solutions
1. Ductility is the most influential material property affecting sheet metal
formability.
2. Buckling is prevented by applying blankholding with the proper holding
force applied.
3. Open and rope hems form a mechanism that allows for joining of sheet
metal parts together, similar to seaming as mentioned in Section 14.2 of
the text.
4. Brass is a good material for drawing because of its high ductility.
According to Table 3.3, the % elongation for brass of 60% makes it among
the most ductile metals.
5. The punch-die clearance is slightly larger than the stock thickness. Eqn
14.9 gives the relationship as clearance = 1.1 x thickness.
6. No, some thinning usually occurs in drawing due to the tensile forces
imparted to the workpiece from the punch. Up to 25% thinning can be
expected in drawing, especially near the cup base.
7. The earing defects are from anisotropic properties of the work material
they are using. The women may need to make a change with their
material supplier or otherwise find sheet stock with more uniform material
properties.
8. Spinning, in which a round, flat blank is gradually formed over a rotating
cup-shaped mandrel by a simple roller tool, can also be used to make
cups. Conventional spinning can be much simpler than using expensive
punches and dies in a drawing operation. The low-cost tooling of spinning
would be a good option for lower production volumes or if little capital
were on hand for investment.
9. The part volume can be estimated from the cup bottom as  x (65 + 2 x
1.5)2/4 x 1.5 = 5,447.5 mm3 plus the cup wall as  x {(65 + 2 x 1.5)2 –
652}/4 x (40 – 1.5) = 12,064.9 mm3, for a total of 17,512,4 mm3. Setting
this amount equal to that of a circular stock of 1.5 mm thickness, results in
a starting radius of r = (17,512.4/1.5/).5 = 61 mm, or 122 mm diameter.
10. Based on the diameter in Question 9, Drawing Ratio is 122/65 = 1.88,
which should be less than 2.0. Reduction = (122 – 65)/122 = .47, which
should be less than .5. Thickness-to-diameter ratio is 1.5/122 = .0123,
which should be greater than .01. The operation appears feasible.
11. Using Db = 122 mm from Question 9 and the material properties in Table
3.2, Drawing Force is from Eqn 14.12. F =  (65 mm)(1.5 mm)(410 MPa)
x (122/65 – 0.7) = 147.8 kN. Holding Force is from Eqn 14.13, Fh = 0.015
x (205 MPa) {1222 – (65 + 2.2x1.5 + 2x6)2} = 81.5 kN.