When processing metals using cutting tools, which is usually accompanied by chipping (sometimes no chipping is involved), tools wear and get damaged. Worn tools are usually re-sharpened for re-use or replaced when unrecoverably damaged.
The causes for this phenomenon are various and result from the nature of the different machining processes involved (metal cutting, alloy cutting, cutting other types of material) and also from all other subjective factors involved and influencing the process.
Processing, i.e. cutting conditions usually involve significant energy consumption, occurrence of substantial forces, vibrations, shocks and emission of heat. In this sense, cutting conditions are heavy processing conditions and therefore lead to faster tool wear or damage especially when hard, tough and high-strength materials are to be processed or when high-speed processing or fast-feed processing aimed at increasing production efficiency is involved.
Causes and significant factors
Generally, causes for cutting tool wear or damage are cutting edge wear or the occurrence of obvious breaking out on cutting edges or internal cracking and stress. These are determined by the extent of applied pressure and slipping of metal chips as well as the nature of surface being machined. Included in the cumulative load is also the tool temperature in the area where the load is applied. Tool temperature usually rises due to the heat Q emitted during the processing (cutting) operation.
where Q is the emitted heat; Pz is the shear force [dN] and V is the cutting speed [m/min].
Although cutting speed is an independent variable, the forces and temperatures generated are dependent variables and are functions of numerous parameters. Similarly, wear depends on tool and workpiece materials (their physical, mechanical and chemical properties, tool geometry, cutting fluid properties and various other operating parameters). The types of wear on a tool depends on the relative roles of these variables. Due to the complicated relations and numerous factors influencing tool wear, various experimental methods and data is usually used to define the type of wear.
Let us consider, for example, tool wear on a conventional lathe knife – figure 1.
- Front face
- Rake face
- Flank face
- Cutting edge
- Tool tip
- Auxiliary cutting edge
Considering the geometry and characteristic elements of a lathe knife, tool wear usually occurs in indicated significant cutting edges and faces along with unrecoverable breaking (damage) with significant breaking out of the cutting tool and internally observed and hidden cracking. In other words, tool wear results in the tool being incapable to continue the process carried out between the machine, tool and workpiece (due to different tool size, surface integrity, internal structure, etc.). The term “tool life” is used to identify the time period until the tool is made incapable to perform its functions.
The most significant types of wear are crater wear and flank wear, as well as tool tip and cutting edge breaking out and cracking. These types of wear occur in different ways for different tool materials – figure 2 (for example, carbides, high-speed steels, ceramics, diamond, etc.)
- Flank wear
- Crater wear
- Primary groove or wear notch
- Secondary groove
- Other metal chip notch
- Inner chip notch
- VB – average flank
Tool life is as illustrated in Figure 2: (a) – the tool is within the normal required process parameters between points O and M. Following point M, KT and VB have reached the allowable limit.
Wear usually refers to gradually increasing wear without any visible scratching and furrowing, and damage is usually referred to notching with breaking off of particles from cutting edges. The ratio of occurrence of the two types of wear in a particular type of processing operation depends on load conditions at the tool-workpiece interface. In ceramics, for example, plates and tools operating under vibration and used to machine fire-resistant and alloy steels usually break off along their cutting edges. Apart from normal tool wear (flank wear) there are a number of other factors that influence tool wear: insufficient tool strength characteristics and available internal cracks. When the applied pressure (as a result of Pz) exceeds the ultimate strength limit this results in sudden breaking off along the cutting edge. High speeds and temperatures cause diffusion – interpenetration and rubbing of tool-workpiece materials. Friction causes abrasive and adhesive wear. To summarize, the most significant factors include:
- Cutting conditions: speed, feed, cooling, geometry
- Tool and workpiece materials (physical and mechanical properties, chemical composition, inclusions, density, )
- The characteristics of the machine-tool-workpiece system (stability, output, )
- Other factors – operator, qualification, processing
Since cutting speed is among the most significant factors determining tool life (T), it is usually calculated using Taylor’s relation:
VTn = C (2),
Where V is the cutting speed 9m/min], T is time [min] and n is a constant value which depends on cutting conditions and C is a constant value.
Above expression is a synthesized relationship between a number of factors that influence wear. From the diagrams illustrated in figure 3 we can observe how T is influenced by V depending on tool- workpiece materials.
Workpiece material Tool material
- High-speed steel
- Cast alloy
- Ceramic (diamond)
α- the slope angle: determines the value of “n”
It is obvious here that tool life increases when the speed V is reduced and the hardness and toughness of the material being processed are reduced, too. If we apply the expression (2) we can calculate that under certain conditions (fixed n and C), tool life T increases by 300% when V[m/min] is reduced by only 50%. It is a proven fact that for a constant tool life to be maintained, the speed is reduced when the feed f and the depth d are increased and vice versa.