3.2 Shearing

※ Clearance

Because theformability of the blanked part can be influenced by the quality of its shearededges, clearance control is important. The appropriate clearance depends on

√  The type of material and its properties

√   The thickness and size of the blank

√   Its proximity to the edges of other sheared edgesor the edges of the original blank

Clearancesgenerally range between 2 and 8% of the sheet thickness, but they may be assmall as 1% ( as in fine blanking ) or as large as 30%. The smaller theclearance, the better is the quality of the edge. If the sheared edge is roughand not acceptable, it can be subjected to a process called shaving, wherebythe extra material from the edge is trimmed by cutting.

As a generalguideline, (a) clearances for soft materials are less than those for hardergrades; (b) the thicker the sheet, the larger the clearance must be; and (c) asthe ratio of hole diameter to sheet thickness decreases, clearances should belarger. In using larger clearances, attention must be paid to the rigidity andthe alignment of the presses, the dies, and their setups.

※  Punch force

The forcerequired to punch is basically the product of the shear strength of the sheetmetal and the total area being sheared along the periphery. The maximum punchforce, F, can be estimated from the equation

F=0.7TL(UTS)

where T is the sheet thickness, L is the totallength sheared ( such as the perimeter of a hole), and UTS is the ultimatetensile strength of the material. As the clearance increases, the punch forcedecreases, and the wear on dies and punches also is reduced.

Example 3.1,  Calculation of punch force

Estimate theforce required for punching a 25-mm diameter hole through a 3.2-mm thickannealed titanium-alloy Ti-6Al-4V sheet at room temperature.

Solution

The UTS for thisalloy is found from the Table to be 1000MPa. Thus,

F=0.7(3.2)(π)(25)(1000)=0.18MN。