For producing a shell which is long in proportion to its dia­meter, it is necessary to sub-divide the process into a series of draws; starting with a shallow cup of larger diameter and progressively drawing to a longer and small shell until the desired dimensions are reached. The amount of reduction that may be effected at each operation depends upon various factors like quality of material, die edge radius, bottom radius, material thickness, thickening allowed etc.  This property is defined by the term drawing ratio which is

=  D-d /D × 100%

As a general rule the ratio D/d  for the first draw varies from 45-65%;    21-25%  and 10-15%  in subsequent operations be­cause the wall and bottom thickness do not remain uniform and work hardening is induced every time. Generally when several such operations are necessary, intermediate annealing treatments are necessary to relieve the hardness induced due to several cold work­ing operations. Sometimes ironing operation is also carried out to give uniform thickness to metal.

The general rule for number of reduction for M.S. is

        Ratio    h/d                                                               No.of Reductions

                 ¼ to ¾                                                                             1

                5/8 to 1 1/8                                                                     2

                 Upto 2                                                                              3

                 Upto  3                                                                             4