Tool coatings versus wear processes

A hard coating deposited on the surface of the tool substrate reinforces hot hardness at the surface of the tool and improves tool life in high-temperature environments. However, a coating generally must be thick to insulate the tool substrate from heat, and a thick coating will not adhere well to a very sharp geometry. Cutting tool makers are working to engineer coatings that are thin but provide a good barrier against heat.

Austenitic stainless steels exhibit high ductility and a tendency to adhere to the cutting tool. Application of a coating can also inhibit adhesion wear, a condition that occurs when the cut material sticks to and builds up on the cutting edge. The adhered workpiece material may then pull away sections of the cutting edge, leading to poor surface finish and tool failure. The coating can provide lubricity that limits adhesion wear; higher cutting speeds also serve to minimise the adhesion wear mechanism.

Some austenitic stainless steel alloys contain abrasive hard inclusions, and increasing a cutting tool’s abrasion resistance with a hard coating can benefit tool life.
Notch wear results from an alloy’s tendencies toward strain or workhardening when machined. Notch wear can be described as very localised extreme friction wear, and it can be mitigated by application of appropriate coatings and other actions such as varying the depth of cut to spread the wear areas across the cutting edge.
Tool development
Toolmakers focus ongoing cutting tool development efforts on finding a balance between tool properties that will provide optimum performance in specific workpiece materials. Carbide grade research seeks a balance between hardness and toughness so a tool is not so hard that it fractures but is hard enough to resist deformation. Similarly, a sharp cutting edge geometry is preferred but is not as mechanically strong as a rounded edge. Consequently, edge geometry development is aimed at creating tools that balance sharpness with as much strength as possible.
As part of the development process, toolmakers are revisiting their tool application guidelines. Current machining parameter recommendations are based, for the most part, on toughness and hardness characteristics of traditional steels, without consideration of the thermal factors that are so important when machining austenitic stainless steels and other high-performance alloys. Recently, toolmakers have begun working with academic institutions to revise tool testing procedures to take into account certain materials’ thermal characteristics.
The new guidelines reflect the creation of new reference materials. Traditionally, machinability standards were set according to one reference material, an alloyed steel, and based on mechanical loads produced during machining. Now there is a separate reference material for austenitic stainless steels for which baseline values for speed, feed and depth of cut have been established. Relative to that reference material, balancing or calibrating factors are applied to determine changes in the base values that will achieve optimum productivity in materials with different machining characteristics.

Specific geometries for specific materials
Many cutting tools provide very acceptable performance in a variety of materials under a wide range of cutting conditions and machining parameters. For one-time jobs with moderate productivity and quality requirements, these tools can be a cost-effective choice. To achieve maximum performance, however, toolmakers continually manipulate and balance a wide variety of tool elements to create cutting tools that provide top productivity and process reliability in specific workpiece materials.
The basic elements of a tool include its substrate, coating and geometry. Each is important, and in the best tools they operate as a system that produces results beyond the sum of the separate parts.
There are distinctions among the roles the tool’s parts play. The substrate and coating have passive roles; they are engineered to provide a balance of hardness and toughness, to withstand high temperatures, and to resist chemical, adhesion, and abrasive wear. The tool geometry, on the other hand, plays an active role because altering the geometry can change amount of metal that can be removed in a certain timeframe, the amount of heat that is generated, how chips form and what surface finish can be achieved.
Basic examples of performance-altering geometry differences include traditional turning geometry inserts from Seco called e.g. M3 and M5 that feature negative (0° clearance angle) cutting edge geometries and T-lands between the cutting edge and the tool rake face. The M3 geometry is a versatile medium-rough geometry that offers good tool life and chipbreaking in a wide range of workpiece materials. M5 geometries are aimed at demanding, high-feed roughing applications, combining high edge strength with comparatively low cutting forces.
Although versatile, the M3 and M5 geometries are strong, but not fully sharp, and generate a good deal of heat via deformation when machining austenitic stainless steel. In comparison, tool designs that can be more effective in stainless steel machining include the Seco MF4 and MF5 geometries that feature sharp, positive geometries with more narrow, positive T-lands that help maintain sharpness while providing support behind the sharp edge. The geometries are engineered to be open and free-cutting to facilitate for medium to finishing operations on steels and stainless steels. The MF5 geometry is especially effective in high-feed applications.
By: Patrick de Vos, Corporate Technical Education Manager, Seco ToolsNike