If D is the blank diameter in mm, r is the corner radius (in mm), h is the height of the shell as shown in Figure 21.26.
Then for thin shells whose wall thickness is t and bottom thickness is T,
If wall thickness t is not equal to bottom thickness T which is also the blank thickness. Since, volume before and after drawing is equal. Therefore, 
.
To find a blank diameter for a shell of irregular C/S equal the weight before and after when a sample is available.
where W = Weight of finished shells (g), and w = Weight of metal per cubic mm
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