The integration of CAD and CAM
has resulted in a paradigm shift in the methodology of NC programming. Today it
is possible not only to generate CNC program required to manufacture a
component directly from the CAD models but also design and model the fixturing
set up, design the blank, process plan, select optimum process parameters, and
simulate the machining operations on the CAD/CAM workstations to ensure that
the program is capable of producing acceptable components
Generating Large Programs
Numerical control was developed
to meet the requirements of the
aerospace industries. In many cases these components are to be milled from
solid parts in order to ensure structural integrity on the components. It is
therefore quite common to start machining a work piece weighing several hundred
kilograms and the final component may have only
a weight of a few kilograms. For example in a typical aircraft
structural component, in order to ensure that the structural member’s
structures are made thin, in order to ensure that the structural member is able
to withstand the heavy stresses and strains during the operation it is
necessary to provide ribs. The geometry is often complex with double curved
surfaces, which necessitate 5-axis machining.
Drawbacks for Manual Programming
Manual programming of aircraft
structures is very difficult because of the sheer size and complexity of these
programs. Mathematical calculations involved in 5-axis contouring is time
consuming and tedious. A programmer is liable to make mistakes and detection
and elimination of mistakes is time-consuming activity. Moreover writing NC
codes of 20 or 30 thousand lines manually is a stupendous task. These
programming operations will involve complex calculations to determine the
co-ordinates of points for the positioning of tools. Many issues like
incorporation of cutter diameter compensation make these calculations very
involved. Further, errors if undetected will result in enormous wastage in
terms o material and time resulting in over runs in project time and cost.
Technology of CAM
The starting point of CAM is the
CAD file. A common approach of the program creation is carried out using
models.
(a) Create a manufacturing model
from the design model and the work piece.
(b) Set up the tool database.
Tools must be defined before an operation is performed. Tool libraries can be
created and retrieved for a manufacturing operation. Alternatively, they also
can be created at the time of defining the manufacturing operation.
(c) Select the set up for the
machining operation. A component may require more than one set up to complete
the machining operation.
(d) Fixtures are necessary at
each set up. Fixtures can be displayed along with the work piece. This is to
ensure that the path of the tools does not interfere with the tools.
(e) Fixtures consist of the
locating an clamping element to hold the work piece on the machine table during
the machining operation. In the case of a turning operation appropriate
chucking devices should be used. Standard fixture elements can be created as a
library of parts and assembled before the manufacturing operation is commenced.
These are also available as standard libraries.
(f) Create a machineability
database. Parameters like spindle speed and feed rate can be selected from the
machineability database.
(g) Create a manufacturing route
sheet at the end of the manufacturing session.
Procedure of CAM
The CAD model represents the
finished product. This is used as a basis for the manufacturing operations.
Features, surfaces and edges are selected on the design model as references for
each manufacturing operation. Referencing the geometry of the design model sets
up parametric relationship between the design model and the work piece. Because
of this relationship, when the design model is changed, all associated
manufacturing operations are updated to reflect the change. The work piece is
the raw material from which the component is obtained after the manufacturing
operations.
The work piece can be bar stock,
casting, solid material, forging etc. It may be easily created by copying the
design model and modifying the dimensions or deleting or suppressing the
features to represent the real work piece.
Selecting Co-ordinate System. Co-ordinate systems are vital components in
manufacturing. They define the orientation of the work piece on the machine and
act as the origin of the computer data generation. The co-ordinate system can
belong to the work piece or the designs model.
(a) Z-Axis orientation. During the manufacturing operations the
coordinating systems should be oriented in such a way, the positive Z-axis away
from the holding fixture.
(b) X-and Y-axis orientations. The orientation of X and Y axes of the
operation co –ordinate system will affect the computer data file. In milling
the main cut feed direction will be parallel to the X-axis.
Manufacturing Operations
Important manufacturing
operations are as follows:
(a) Casting. Most of the machine and huge components are
manufactured with this process. This process is economical when number of
components is manufactured.
(b) Turning. Most of the cylindrical parts are manufactured by turning.
This is the most common method of manufacturing.
(c) Milling. The milling operations normally performed can be classified into a number
of groups.
(d) Grinding. Grinding is used to deal with hardened and unhardened
components.
Non Traditional Methods
Non traditional methods like Electric Discharge Machining,
Abrasive Jet Machining, Electro Chemical Machining, etc. are used to machine
very hard and brittle materials.
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