In this article we will discuss about the independent input and dependent variables used in Production System.
It is selected to meet the functional design criteria. While some metals like cast iron, aluminium machine easily; others like stainless steel, titanium are difficult to machine.
The size and shape of workpiece is decided by previous casting/forging/forming process. This variable directly influences the machining process selected.
Selection of specific machining process required to convert the raw material into a finished product is based on the geometry of the part, required finish and tolerances, and the quantity of the product to be made.
Type of Machining Process:
Machining processes can be grouped into three broad categories:
(i) Chip formation processes (turning, shaping, milling, broaching, sawing).
(ii) Abrasive machining (chips are formed by very small cutting edges that are integral parts of abrasive particles).
(iii) Non-traditional machining processes. These do not produce chips or a lay pattern in the surface and involve new energy modes. In these processes, heat build up and distortion is less, holding forces are low, and are suited for very hard and delicate parts.
These may include high speed steel, ceramics and carbides, and coated tools.
HSS is used as general purpose tool material. Carbides and ceramics are used for high cutting speeds. Retention of hardness at elevated temperatures as well as long tool life are desirable characteristics of cutting tools.
For every machining operation, it is necessary to select optimum cutting speed, feed, and depth of cut. The selection of cutting parameters also depends on total amount of material to be removed, workpiece and tool materials, machining process, etc.
Tool angles are selected to accomplish specific machining functions. Generally large rake and clearance angles are preferred (on HSS). For carbides and other hard materials, small tool angles are selected to avoid tensile failure and brittle fracture of tool material.
Work Holding Devices:
These may comprise of vises, jigs and fixtures etc. These are the key to precision manufacturing because work-pieces are held in specific position with respect to tools by clamping in work holding devices.
The selection of right cutting fluid for a particular combination of work material and tool material is very important. Cutting fluids cool the workpiece and tools, reduce friction by means of lubrication, carry the chips away from cutting zone, help improve the surface finish, and provide surface protection to workpiece.
These are determined by the process based on the prior selection of the input or independent variables.
Some of these are:
i. Cutting Force and Power:
Every machining operation generates cutting forces and requires power. A change in speed/feed/depth of cut/cutting fluids alters the forces. The forces influence the deflection of tools/workpieces/workholders which may affect part size. Forces also play a role in vibration and chatter. Manufacturing engineer has no control over force, but can predict them and specify the equipment accordingly.
ii. Size and Geometry of Part:
The residual stresses left in parts deserve attention as these can result in wrong sizing, failure from fatigue, and cause corrosion. The input levels have to be selected so that the part product is within prescribed tolerances.
iii. Surface Finish:
It is a function of tool geometry, tool material, workpiece material, machining process, speed, feed, depth of cut, and cutting fluid.
iv. Tool Wear and Tool Failure:
The plastic deformation and friction inherent in machining generate considerable heat, which raises the temperature of tool and lowers wear resistance. To control these, engineer can select slow speeds which produce less heat and lower wear rates. For high metal removal, depth of cut can be increased.
Relations between Input Variables and Process Behaviour:
Machining is a unique plastic deformation process in that it is constrained only by the cutting tool and operates at very large strains and very high strain rates. The tremendous variety of input variables results in an almost infinite number of different machining combinations and thus process of understanding the connection between input variables and process behaviour becomes complex. For understanding of these relations a production engineer is guided by experience, experiments, and theories.
Knowledge is basically gained by trial and error, with successful combinations transferred to other ‘similar’ situations.
Machining experiments are usually expensive, time consuming, and difficult to carry out. However, some tool life equations have been empirically developed by keeping all input variables except speed as constant. There have been many attempts to build mathematical models of the metal cutting process.