Chapter 4-The Shearing Process
The use of
shears in sheet metal production has diminished through the use of cut-off tooling in CNC
punching and the use of shake-out technology to separate parts from the sheet skeleton.
Shears are used mainly for rough shearing sub-sizes of sheets for CNC presses or strips
for stamping press dies.
In those cases where finished dimensions are sheared, the
thickness of the material and the X-Y dimension of the part dictate the degree of
precision which is feasible economically. Thicker material and greater X-Y dimensions
require greater tolerances.
In the broad range of sheet metal production, material
thicknesses vary from 0.005 in. (0.13 mm) to 0.25 in. (6.35 mm) in ferrous and non ferrous
materials. Shearing equipment varies, accordingly, from 1Ž4 in. (6.0 mm) capacity x
12 ft. (3.5 m) bed length to tiny hand operated shears with a 0.030 in. (0.8 mm) capacity
and a 12 in. (300 mm) blade length.
While in the upper material thickness range a tolerance of
±0.060 in. (1.52 mm) is common, the thinnest material can be sheared in small X-Y
dimensions to as close as a ±0.005 in. (0.13 mm) tolerance. It is advisable to consult
your metalforming supplier for the capacity of available equipment.
For the cutting of sheet metal, several types of operations
are used, depending on the purpose or shape of the cutting action. This chapter focuses on
the shearing process.
Figure 1.
Characteristics of cut edges.
Nature of Cut Edges
Whenever sheet metal is cut, whether by shear, slitters or
punches and dies, the characteristics of the cut edges are similar (Figure 1).
Cutting action takes place in three stages as the cutting
edge moves through the material: initial plastic deformation, penetration, and fracture.
During initial plastic deformation, the "edge radius" or "roll-over"
is formed. During penetration the "cut band" or "burnish" is created.
And during fracture the "break" or "break-off" and the burr are
developed.
Shears and other metal cutting devices are normally
maintained and adjusted to provide acceptable cut quality with nominal burrs and to limit
wear on tooling and equipment. This produces a cut in which penetration occurs to a depth
of approximately 1Ž3 of the material thickness and fracture occurs through the
remaining material. Proper adjustment generates a burr which seldom exceeds 10% of the
material thickness.
Equipment Characteristics
A wide variety of power shearing equipment is in use. Major
machine elements common to most shears include the frame assembly, bed, table, ram,
hold-down devices, gauges, the activating mechanism and the blades (Figure 2). Hold-down
devices, arranged along the bed near the blade, engage the stock and clamp it firmly in
position for shearing.
Figure 2. Machine elements common
to most shears.
Back gauges serve to position the stock under the moving
blade at a predetermined dimension. They may range from simple, positive, mechanical stops
to a series of probes (proximity switches) which sense the stock and activate the machine
when more than one are contacted simultaneously. Depending on type and sophistication,
back gauges may be set manually or programmed.
Front gauges are often used to position the stock,
especially when large workpieces are involved. They may be either mechanical or
programmable.
Side gauges, also known as "squaring arms," are
mounted perpendicular to the blade on either the left or right side of the bed, and assist
in guiding and squaring the stock to the blade.
Operation
Regardless of construction, size or speed, all power shears
operate similarly. A sheet of stock is advanced on the table until the back gauges are
contacted and the line of cut is beneath the blade (Figure 3). When the machine is
activated, the hold-down devices clamp the stock and the angled moving blade cuts
progressively across the sheet in a guillotine-like action.
Figure 3. Shear
operation
Depending on the application, power shears may be fed from
the front or the back. Back feeding can reduce handling of the stock for subsequent cuts,
but requires an additional operator.
Maintaining Quality
Important quality checks are performed during the shearing
operation. Factors of quality control include the initial flatness of the stock, general
surface and edge condition. Surface flaws and skid marks are common on coil and sheet
products and are generally acceptable to the manufacturer unless such marks would cause
cosmetic rejection of the finished product. Delamination, surface inclusions and other
severe defects in the material may also be identified and are cause for rejection.
Design Considerations
For economical production the knowledgeable designer
recognizes several aspects affecting costs and quality during shearing and in subsequent
operations. Following are several such product design considerations.
- Material Utilization. Material suppliers generally make
sheet stock available in standard sizes--widths of 30, 36, 48 and 60 inches. Significant
savings can result from the effective use of these standard sizes by avoiding charges for
extra slitting or mill preparation.
Early consultation with the metalformer may permit modifying the dimensions of unseen
flanges on the product to achieve an overall part layout somewhat smaller than the
standard sheet size. This can avoid extra costs and reduce waste.
- Grain Direction. Grain direction in flat rolled stock
(lengthwise in the coil) is not always a significant consideration. However, in some
operations such as forming and bending, grain orientation can be important.
On very large parts which have formed flanges or features, the designer should consult a
qualified supplier prior to specifying the grain orientation and bend radius to determine
if material size limitations will permit the formed features to be across the grain. This
subject is explored in more detail in the chapters on Press Brake Forming and Stamping
Production.
- Process Characteristics. Burrs, holddown marks and twist
(Figure 4) are characteristics of the shearing process.
Figure 4. Twist characteristics of
the shearing process.
Burrs are present after shearing (as in any metal cutting
operation) and are normally controlled within acceptable limits through proper shearing
practices.
Hold-down marks, appearing as slight indentations along one
side of the sheared edge of the workpiece, sometimes result from the clamping action of
the hold-downs. These marks are seldom a problem. They may often be accommodated as part
of an unseen flange in the final product, or may be eliminated entirely during trimming in
later operations.
In critical applications, coverings on the hold-downs may
be used to protect the stock. Materials with removable protective coatings are sometimes
used to help reduce holddown marks and scratches that are inherent in the shearing
process. These alternatives are usually at considerable additional cost.
Twist, a spiral-like curvature of the material occurs when
shearing narrow strips. It is caused by the scissors action of the shear and is influenced
by the relationship of the width sheared to the thickness and temper of the strip. Twist
is seldom an important consideration except when shearing narrow strips. When a job
requires very narrow strips, roller slit coil material, (if order is of sufficient
quantity) or bar stock can often be substituted.
Go to the Design Guidelines Overview
Go to the Glossary
Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!
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