270 



HANDBOOK OF MECHANICAL DESIGN 



THE DESIGN OF DROP FORGINGS 



Most metal forgings are made from bar stock wherein the lines of the fibers run 

 parallel to the axis. Best forging results are obtained when the forging pressure is 

 applied along the axis of the bar, which will compel the metal to flow wdth least distor- 

 tion of the fiber hues. When metal is compelled to flow in a direction perpendicular 

 to the lines of the fiber by means of pressure appUed perpendicular to the flow Unes, 

 as in raising a boss on a flat plate, the metal will not be as strong, especiaUy in its 

 resistance to impact loads. An even worse condition is created when the metal is 

 compeUed to flow at an angle to the direction across the grain. 



The accompanying flgures illustrate flow conditions in forging bars or plates. 



Forging pressure 



Forging pressure 



Forging 

 pressure 



''7/777777777777777777777777777777^77/ 



Best forging condition. 



Raising a boss in this manner 

 weakens the metal. 



Worst condition is 

 when a boss is thrown up 

 on a plane making an 

 angle with the flow lines. 



^ 



v 



-^ 



E 

 o 



> 



/ Vo/ume curve 



l^cor^ec^■ 

 design 



mML^J^^yt^ 



( Volume curve 



Correct 

 design 



t. forging 



To assure best forging conditions, a curve of volumes, such as above, should be 

 plotted. This enables the designer to visualize quickly and accurately the flow con- 

 ditions that \\dU exist in the forging operations. Thus to the left is the volume curve 

 obtained from a poor design as indicated by the abrupt changes in volume. To the 

 right is shown the same design corrected so that the volume curve changes smoothly. 

 Smooth changes in volume also indicate a design that is most economical to forge. 

 Poor flow conditions aaoII cause an excessive amount of flash, thereby necessitating an 

 excessive number of forging blows, which favors the formation of cold shuts, the metal 

 not fiUing the die cavity. Cracks and other defects are also hkely to result if the dis- 

 tribution of the metal around the neutral axis is 

 unsymmetrical . 



In making upset forgings, the bar stock is rough upset 

 and is usuaUy so proportioned that the upset ratio on the 

 diameter will be 13^ to 2, the length upset ratio usually 2 to 

 2I2, with 3 as a maximima. If it is greater than 3 diameters, 

 the bariwill usually buckle. However, length upset ratio may 

 exceed 3 diameters, but the die and operation costs will be 

 ■ Gripping die ■' greatly increased. 



