Machining Guidelines

The machining process is very complex, and tool performance is affected by the properties and condition of the workpiece and the cutting condition. For P/M materials, porosity is a major factor that reduces machinability. The cutting tool configuration (in terms of chip breaker profile, stability, and geometry of tool holder, insert style, etc.) also influences the wear processes that determine tool life.

In order to select the right tool and machining parameters, knowledge of the loads on the tool and the properties of the tool material together with an analysis of the wear mechanisms is necessary. The loads associated with the wear process (Ref 3) can be divided into four main groups:

• Mechanical load

• Chemical load

• Abrasive load

To define cutting parameters, the loads on the tool must be controlled based on the active wear mechanisms. Typically, the loads on the edge of a cutting tool are different at different locations. Consequently, different wear mechanisms are activated and proceed at different rates at the various locations. The processes that influence tool life can be plotted schematically on a wear mechanism map (Fig. 2), which delineates the area's wear and "safe zones" for good tool performance.


f ^^^^ Mel! wear

Severe deformalion and Iracture limit

Safe zone

* -»„ ^ P iastrcity-dom in ated wea r Built-up edge

Mild \ \ oxidation/ \ diffusion

\ wear \ \

Severe oxidation/ , diffusion \ wear



Fig. 2 Pressure/feed rate versus cutting speed in a wear mechanism map

The following sections describe machining conditions for common operations (e.g., turning, drilling, tapping, grinding, and milling). In addition, examples are given for machinability evaluations on the turning, drilling, and tapping of various sintered steels. These examples illustrate the influence of chemical composition, tool material, tool geometry, free machining additives, feed rate, cutting velocity, cutting conditions, and surface integrity while forming the basis of guidelines for the machining of sintered steels. Effects from microstructure, carbon content, density, machinability enhancing additives, etc. are examined. Guidelines for optimum machining parameters in turning, drilling, and tapping are stipulated for a wide spectrum of P/M steel conditions based on these examples.

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