Dayton Die Cushions
6330 Industrial Dr, Eden Prairie Mn 55346, USA
dayton@daytondiecushions.com
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>>Products >>Learn about die cushions >>Topics >>Tonnage and Ring Holding Force Necessary for Drawing Shells

In general, the tonnage necessary to draw a shell of round or rectangular shape is a function of the area of metal in the wall (i.e. = mean perimeter and metal thickness). This is true because at any given time, the force necessary to deform the metal equals the area of metal being deformed multiplied by the yield strength of the material. (In calculating these forces tensile strength is used, since this will either draw the part, push out the bottom or tear the walls). This is generally true for parts having odd shapes, although there are other factors involved which complicate the operation.

 
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ROUND SHELLS
TONNAGE =
Π x D x T x TENSILE STRENGTH/2000
RECTANGULAR SHELLS

TONNAGE =
2 x (L+W) x T x TENSILE STRENGTH/2000

Mechanical punch presses are designed to deliver their maximum tonnage near the bottom of the stroke. Since drawing work actually starts before bottom, the full tonnage will not be available. The above results, therefore, should be multiplied by "G" factor when this type press is used.
H
"G"
H
"G"
H
"G"
H
"G"
1/2
1
3
1.75
10
3
18
4
1
1
4
2
12
3.25
20
4.25
1 1/2
1.5
6
2.5
14
3.5
22
4.5
2
1.5
8
2.75
16
3.75
24
4.75
Ring Holding Pressure or Force
The primary function of the draw ring is to hold the blank while it is being formed to prevent wrinkles. Since the resistance to wrinkling becomes greater as the metal thickness increases, the necessary R.H.F. decreases. Experience indicates that a close approximation to the R.H.P. can be obtained by multiplying the tonnage required to draw the shell by "J" factor (Chart 2). Note: Disregard "G" factor for this calculation.

Press Tonnage
Size of press necessary to draw shell in question is as follows:
MINIMUM PRESS TONNAGE = SHELL TONNAGE PLUS CUSHION TONNAGE (RHP)

Example
Determine the size of mechanical press required to draw the shell pictured.

DRAW TONNAGE = Π x (2.50 - 0.06) x 0.06 x 55,000 / 2000 = 12.7 T x 1.5 ("G") = 19.0 T
 CUSHION TONNAGE = 12.7 x 0.133 ("J") = 1.7 T
MINIMUM MECHANICAL PRESS TONNAGE = 19.0 T + 1.7 T = 20.7 T
MINIMUM HYDRAULIC PRESS TONNAGE = 12.7 T + 1.7 T = 14.4 T