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).

F = 0.7twUTS
where,
F = force needed to shear
t = thickness of sheet
w = width of sheet
UTS = Ultimate Shear Strength of material

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.