CNC Program Development & Virtual Machining Using CAM

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 gen... thumbnail 1 summary
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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