1.2 SHEARING

 

• A shear force is applied that will cut off part of a sheet. The cut off ‘blank’ becomes the workpiece.

 

• To find the shear force for a cut we can go back to the basic mechanics of materials (with one adjustment factor).

 

 

• The basic terms used in shearing are,

Punching - a small section of material is sheared out of a larger piece and discarded.

Blanking - outside/surrounding material is cut off a smaller piece and discarded.

Die Cutting - small features are cut into the sheet, such as series of holes, notches (adjacent material removed), lancing out tabs (no material removed), parting to cut the sheet into smaller pieces.

Fine Blanking - dies are designed that have small clearances and pressure pads that hold the material while it is sheared. The final result are blanks that have extremely close tolerances.

Slitting - moving rollers trace out complex paths during cutting (like a can opener).

Steel Rules - soft materials are cut with a steel strip shaped so that the edge is the pattern to be cut.

Nibbling - a single punch is moved up and down rapidly, each time cutting off a small amount of material. This allows a simple die to cut complex slots.

Nesting - a sheet can be used more effectively (reduce scrap) if part patterns are closely packed in before shearing.

 

• Dies used in shearing typically have small clearances between the punch (moving part) and Die (non-moving backing). If this gap is too great the parts will have rough edges and excess shear force will be required. Clearances that are too small lead to premature wear. Typical design issues for clearances are given below,

- for softer materials the clearances are generally smaller

- thicker sheets require larger clearances

- typical clearance values range from 2-8% of sheet thickness

- extreme clearances range from 1-30% of sheet thickness

 

• Typical dies will come in a number of forms,

- bevel/double bevel/convex shear dies - these have an angle on the punch or die so that the shear starts at one point and then moves, much like cutting with scissors.

- compound dies - a die has multiple punches and dies that operate on the piece at the same time

- progressive dies - a single die contains a number of die slots. A part will stop at each die inside the progressive die before it is complete. This type of dies allows slow working of parts.

- transfer dies - a sequence of dies in one or more presses will operate on a piece - this is basically a scaled up progressive die.