Thursday, March 1, 2012

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.

SHEET METAL PROPERTIES


• The properties of sheet metal determine how well it can be stretched or bent.
• The various properties include,
- Formability - a larger strain rate exponent ‘n’ relates to longer deformation
- Uniform Necking - the higher the strain rate sensitivity ‘m’, the less localized the necking
- Uniform Elongation - when the yield point has upper and lower points the material may
deform in bands - giving long depressions in work surface called Leuder’s bands.
These may occur in low carbon steels and aluminum/magnesium alloys.
- Anisotropy - if the material properties have no directionality deformation will be even.
- Small Grains - finer grains are preferred for better metal properties and surface finish.

Fixture Design Procedure


In the design of a fixture, a definite sequence of design stages is involved. They can
be grouped into three broad stages of design development.
Stage One deals with information gathering and analysis. These include
product analysis such as the study of design specifications, process planning,
examining the processing equipment and considering operator safety and ease of
use. In this stage, all the critical dimensions and feasible datum areas are examined
in detail.
Stage Two involves the consideration of clamping and locating schemes. A
clamping scheme is devised in such a way that it will not interfere with the tools or
cutters and are fully compatible with proposed locating surfaces or areas. The
locating scheme, using standard elements such as pins, pads, etc. is designed to be
consistent with clamping and tool-guiding arrangements.
Stage Three is the design of the structure of the fixture body frame. This is
usually built around the workpiece as a single element which links all the other
elements used for locating, clamping tool-guiding, etc. into an integral frame work.
The above procedures are quite general and can be modified depending on the
relative importance of the various elements in providing for the required accuracy
of the workpiece to be located and secured into the fixturing device. With the
popular adaptation of modular fixturing elements, the fixture body frame is usually
a standard block with fixed arrays of locating and fixing holes or slots. It becomes a
matter of selecting the most suitable body frame to accommodate the various
elements, provide good support of the workpiece and access to cutters and tools.

Fixture Design Criteria


The following design criteria must be observed during the procedure of fixture design:
Design specifications
Factory standards
Ease of use and safety
Economy

Requirements of a Fixture

In order to maintain the workpiece stability during a machining process, an
operational fixture has to satisfy several requirements to fully perform its functions
as a workholding device. The following constraints must be observed while
designing a viable fixture:
Deterministic location
A workpiece is said to be kinematically restrained when it cannot move without
losing contact with at least one locator. The workpiece is constrained by a set of
appropriately placed locators so that it is presentable for the machining operation.
Locating errors due to locators and locating surfaces of the workpiece should be
minimised so as to accurately and uniquely position the workpiece within the
machine coordinate frame.
Total constraint
A workpiece should be fully constrained at all times to prevent any movement.
Clamps should provide locking forces to hold the workpiece in place -once it is
located. A totally restrained part should be able to remain in static equilibrium to
withstand all possible processing forces or disturbance. A necessary and sufficient
condition to warrant workpiece stability is to satisfy the condition of force closure.
Contained deflection
Workpiece deformation is unavoidable due to its elastic/plastic nature, and the
external forces impacted by the clamping actuation and machining operations.
Deformation has to be limited to an acceptable magnitude in order to achieve the
tolerance specifications.
Geometric constraint
Geometric constraint guarantees that all fixturing elements have an access to
the datum surface. They also assure that the fixture components do not interfere
with cutting tools during a machining operation.
In addition to these requirements, a fixture design should have desirable
characteristics such as quick loading and unloading, minimum number of
components, accessibility, design for multiple cutting operations, portability, low
cost, etc.

Elements of Fixtures

Generally, all fixtures consist of the following elements:

Locators
A locator is usually a fixed component of a fixture. It is used to establish and
maintain the position of a part in the fixture by constraining the movement of the
part. For workpieces of greater variability in shapes and surface conditions, a
locator can also be adjustable.

Clamps
A clamp is a force-actuating mechanism of a fixture. The forces exerted by the
clamps hold a part securely in the fixture against all other external forces.

Supports
A support is a fixed or adjustable element of a fixture. When severe part
displacement/deflection is expected under the action of imposed clamping and
processing forces, supports are added and placed below the workpiece so as to
prevent or constrain deformation. Supports in excess of what is required for the
determination of the location of the part should be compatible with the locators and
clamps.

Fixture Body
Fixture body, or tool body, is the major structural element of a fixture. It
maintains the spatial relationship between the fixturing elements mentioned above,
viz., locators, clamps, supports, and the machine tool on which the part is to be
processed.

Fixture Design

A fixture is a device for locating, holding and supporting a workpiece during a
manufacturing operation. Fixtures are essential elements of production processes as
they are required in most of the automated manufacturing, inspection, and assembly
operations.
Fixtures must correctly locate a workpiece in a given orientation with respect to
a cutting tool or measuring device, or with respect to another component, as for
instance in assembly or welding. Such location must be invariant in the sense that
the devices must clamp and secure the workpiece in that location for the particular
processing operation.
There are many standard workholding devices such as jaw chucks, machine
vises, drill chucks, collets, etc. which are widely used in workshops and are usually
kept in stock for general applications.
Fixtures are normally designed for a definite operation to process a specific
workpiece and are designed and manufactured individually. Jigs are similar to
fixtures, but they not only locate and hold the part but also guide the cutting tools in
drilling and boring operations. These workholding devices are collectively known
as jigs and fixtures.