Mechanism of Tool Wear MCQ Quiz - Objective Question with Answer for Mechanism of Tool Wear - Download Free PDF

Explanation:

Multipoint Rotary Cutting Tools:

• Multipoint rotary cutting tools, such as milling cutters, drills, and reamers, are designed with multiple cutting edges that engage with the workpiece simultaneously or sequentially during the machining process. These tools are widely used in manufacturing due to their ability to perform high-speed, high-precision cutting operations.

Factors Contributing to Variable Wear:

The variable wear observed in multipoint rotary cutting tools can be attributed to several factors:

• Cutting Conditions: The cutting speed, feed rate, and depth of cut can vary along the cutting edges, leading to differential wear rates.
• Tool Geometry: The design and geometry of the cutting tool, including the rake angle, clearance angle, and edge radius, can influence the distribution of cutting forces and wear.
• Material Properties: The workpiece material's hardness, abrasiveness, and thermal properties can affect the wear mechanisms and rates experienced by different cutting edges.
• Thermal Effects: The heat generated during cutting can cause thermal expansion and thermal cycling, leading to uneven wear along the cutting edges.
• Mechanical Loads: The distribution of mechanical loads on the cutting edges can vary due to tool deflection, vibration, and the engagement of multiple edges with the workpiece.

Wear Mechanisms:

Several wear mechanisms contribute to the variable wear observed in multipoint rotary cutting tools:

• Abrasive Wear: The hard particles in the workpiece material can cause abrasive wear on the cutting edges, leading to variable wear rates depending on the local hardness and abrasiveness.
• Adhesive Wear: The interaction between the tool and workpiece materials can lead to adhesive wear, where material transfer and buildup occur unevenly along the cutting edges.
• Thermal Wear: The high temperatures generated during cutting can cause thermal softening and oxidation of the cutting edges, resulting in variable wear patterns.
• Fatigue Wear: Repeated cyclic loading during cutting can lead to fatigue wear, causing micro-cracks and chipping on the cutting edges.

Implications of Variable Wear:

Variable wear on multipoint rotary cutting tools has several implications for machining processes:

• Tool Life: Uneven wear can reduce the overall tool life, as the most worn cutting edge may determine the tool's end of life.
• Surface Finish: Variable wear can affect the quality of the machined surface, leading to inconsistencies and defects.
• Dimensional Accuracy: Differential wear can cause deviations in the dimensions and tolerances of the machined parts.
• Cutting Forces: Uneven wear can lead to variations in cutting forces, affecting the stability and efficiency of the machining process.

Mitigation Strategies:

To address variable wear in multipoint rotary cutting tools, several strategies can be employed:

• Tool Material Selection: Using cutting tool materials with high wear resistance, such as carbide, cermet, or coated tools, can reduce wear rates.
• Optimized Cutting Conditions: Adjusting cutting parameters to minimize excessive wear and ensure uniform engagement of cutting edges.
• Tool Design: Designing cutting tools with optimized geometry to distribute cutting forces and wear more evenly.
• Coolant and Lubrication: Using appropriate coolants and lubricants to reduce thermal and adhesive wear.
• Regular Inspection and Maintenance: Implementing regular inspection and maintenance schedules to monitor tool wear and replace worn tools in a timely manner.

Explanation:

Tool Wear in Multi-Point vs. Single-Point Cutting Tools

• Tool wear refers to the gradual degradation of a cutting tool due to regular operation. It is a critical factor in machining processes because it affects the tool's performance, the quality of the finished product, and the overall cost of manufacturing.
• Understanding the wear mechanisms and patterns in different types of cutting tools is essential for optimizing machining operations.
• There are two primary types of cutting tools used in machining: single-point cutting tools and multi-point cutting tools.
• Single-point cutting tools, such as lathe tools, have only one cutting edge, while multi-point cutting tools, like milling cutters and drills, have multiple cutting edges.
• The wear behavior of these tools can differ significantly due to their design and operational characteristics.

Distribution of Cutting Load: In multi-point cutting tools, the cutting load is distributed over multiple cutting edges. This distribution of load leads to several advantages:

• Reduced Load per Edge: Each cutting edge in a multi-point tool bears only a fraction of the total cutting load, reducing the stress and wear on individual edges. In contrast, a single-point tool must bear the entire cutting load on its single cutting edge, leading to more rapid and uneven wear.
• Heat Dissipation: Heat generated during the cutting process is also distributed across multiple edges in a multi-point tool. This distribution allows for better heat dissipation, reducing thermal stress and wear. Single-point tools often experience higher localized temperatures, accelerating wear.
• Consistency in Wear: Since the load and heat are shared among multiple edges, the wear on each edge in a multi-point tool tends to be more uniform and predictable. This consistency helps in maintaining the tool's performance and the quality of the machined surface over a longer period. Single-point tools, however, often experience uneven wear, leading to unpredictable tool life and variations in the machined surface quality.

Operational Stability:

​Multi-point tools offer greater operational stability due to the presence of multiple cutting edges. This stability contributes to more consistent cutting conditions and wear patterns:

• Vibration and Chatter Reduction: The multiple cutting edges in a multi-point tool help in damping vibrations and reducing chatter during the cutting process. This stability minimizes the adverse effects of vibrations on tool wear and surface finish. Single-point tools are more susceptible to vibrations and chatter, which can lead to erratic wear patterns and poor surface quality.
• Improved Surface Finish: The consistent engagement of multiple cutting edges in a multi-point tool results in a smoother and more uniform surface finish on the workpiece. This improved surface finish reduces the likelihood of tool wear due to abrasive interactions between the tool and the workpiece.
• Extended Tool Life: The combined effect of reduced load per edge, better heat dissipation, and operational stability in multi-point tools leads to extended tool life. This extension is economically beneficial as it reduces the frequency of tool replacements and the associated downtime.

• Margin wear in drill is due to thermal softening.
• Margin wear is the gradual erosion of the margin of a drill bit, which is the thin strip of land between the cutting edge and the flute. It is caused by the high temperatures generated during drilling, which can soften the drill bit material. Thermal softening can also lead to crater wear, which is the erosion of the cutting edge.
• Abrasion, vibration, and diffusion are not the primary causes of margin wear. Abrasion can cause flank wear, which is the erosion of the flank face of the drill bit. Vibration can cause premature drill bit failure, but it is not the primary cause of margin wear. Diffusion is a process of material transfer between two surfaces in contact, but it is not typically a significant factor in drill bit wear.

Here are some tips to reduce margin wear and extend the life of your drill bits:

• Use the correct drill bit for the material you are drilling.
• Use the correct cutting speed and feed rate.
• Use a lubricant or coolant to reduce heat and friction.
• Sharpen your drill bits regularly.
• Inspect your drill bits for signs of wear and replace them when necessary.

Explanation:

Tool Wear: 

• Tool wear is the gradual failure of a cutting tool during regular operation. Every tool will experience tool wear at some point in its life. Major 02 types of tool wear include rake wear and crater wear.

​

​Crater Wear:

• Crater wear happens on the tool face at a short distance from the cutting edge by the action of chip flow over the face at a very high temperature. The crater wear is mainly due to diffusion and abrasion.
• It is mainly observed while machining ductile material due to the continuous formation of chips.
• It is generally observed in high-speed cutting.​

​Flank Wear:

• Flank wear occurs at the tool flanks, where it contacts with the finished surface, as a result of abrasion. The cutting force increases with flank wear.
• It is generally observed in low-speed cutting.

Explanation:

Forms of Tool Wear:

The progressive wear of the cutting tool takes many forms as shown in the figure:

1. Tool wear on the tool face is characterized by the formation of a cra ter, which occurs as a result of the hot chip flowing over the tool face.

2. Flank wear, which is in the form of a wear land that is generated as the newly cut surface of the workpiece, rubs against the cutting tool. Minor chipping along the cutting edge is usually accompanied by flank wear.

3. Notch wear occurs locally in the area of the main cutting edge where it contacts the workpiece surface. It is caused by hard surface layers and work-hardened burrs, especially in austenitic stainless steels (www.widiaindia.com).

4. Thermal cracks occur in the form of small, running cracks across the cutting edge that are caused by thermal shock loads during inter rupted cutting operations.

Important Points

When machining at high cutting speeds and feed rates, the tool wear occurs mainly at the tool face. At medium cutting speeds, tool wear occurs simultaneously in its flank and face. At low cutting speeds, a substantial tool wear occurs, primarily at the tool flank.

Crater wear occurs when machining ductile materials, whereas machining brittle materials such as cast iron and bronze would normally lead to flank wear.

Explanation:

• The sequence of parameters affecting tool life is:
• Cutting speed > Feed > Depth of Cut
• Tool life is defined as the time interval between two successive regrinds.
• Tool life represents the useful life of the tool expressed generally in time units from the start of a cut to some endpoint defined by a failure criterion.
• Tool wear and hence tool life of any tool for any work material is governed mainly by the level of the machining parameters i.e., cutting velocity, (VC), feed, (f) and depth of cut (t). 
• Cutting velocity affects the maximum and depth of cut minimum.

According to Modified Taylor’s equation:

VTnfadb = C

\(T = \frac{{{C^{\frac{1}{n}}}}}{{{V^{\frac{1}{n}}}{f^{\frac{1}{{{n_1}}}}}{d^{\frac{1}{{{n_2}}}}}}}\)

\(\frac{1}{n} > \frac{1}{{{n_1}}} > \frac{1}{{{n_2}}}\)

Cutting speed has a greater effect followed by feed and depth of cut respectively.

Concept:

Secondary Shear Zone (SSZ):

• It lies between the metal chip and cutting tool.
• In SSZ the energy supplied is converted into heat energy because of the presence of friction at the chip tool interface.
• About 30 to 35% of the energy supplied is converted into heat energy in the SSZ.
• Out of the heat generated the maximum amount of the heat is carried away by the chip, Only a small amount is transferred to the tool.
• This is because the thermal conductivity of the tool is less than the chip.

Primary Shear Zone (PSZ):

• It lies between the workpiece and metal chip.
• In the PSZ when shearing action os taking place, the atomic bond present between the atoms of the material is getting breaking.
• For breaking the atomic bond it needs to supply a certain amount of energy but during the breaking of the atomic bond, they release an equal amount of energy in the form of heat energy.
• Out of the heat generated the maximum (60 to 65%) amount of the heat is carried away by the chip.

Tertiary Shear Zone(TSZ):

• It lies between workpiece and cutting tool.
• In TSZ, the energy supplied is converted into heat energy is due to the presence of friction of the tool work interface.
• About 5 to 10% of the energy supplied is converted into heat energy in this zone.

Explanation:

Tool Life:

• It is defined by the span of actual uninterrupted machining time through which the tool renders desired service and satisfactory performance and after which the tool needs replacement or regrinding.
• It is globally standardized that when the value of average flank wear (V_B) reaches 0.30 mm, the tool-tip is considered to have failed.
• The relation between machining time and magnitude of wear is shown in the figure given below.

where V_B = Principal flank wear, K_M = Location of crater wear, and K_T = Depth of crater wear.

Principal flank wear:

• It occurs more or less uniformly at the principal flank mainly by abrasion and adhesion.
• Diffusion may also occur while machining at high cutting velocity and without cutting fluid.
• It grows systematically with machining time and the wear passes through three stages –
  • rapid break-in wear initiated by attrition
  • a longer span of slowly and uniformly growing mechanical wear
  • again rapid wear accelerated by diffusion, grain pull-out, fracturing etc.

Crater wear:

• It is caused by adhesion, abrasion, and diffusion in different degrees depending upon tool-work materials and the machining condition.
• It is influenced by plastic deformation and pull out grains due to intensive friction at the chip-tool interface.

Concept:

• In the machining process, the wearing action takes place on those surfaces along which there is relative sliding with other surfaces.
• The wear over rake surface is known as crater wear and over flank surface is known as flank wear.

Flank wear occurs mainly on: 

• The flank or the relief face of the tool
• The nose part of the cutting tool

Flank wear occurs due to: 

• Abrasion by hard particles and inclusions in the work piece.
• Shearing off the micro welds between tool and work material.
• Abrasion by fragments of built‐up‐edge ploughing against the clearance face of the tool.
• At low speed flank wear predominates.
• If MRR increased flank wear increased.

• Crater wear occurs on the rake face.
• For the crater wear, the temperature is the main culprit and tool diffuse into the chip material and tool temperature is maximum at some distance from the tooltip. So crater wear starts at some distance from the tooltip.

Concept:

The mechanisms that cause wear at the tool-chip and tool-work interfaces in machining are

• Abrasion: Mechanical wearing action caused by hard particles in the work material gouge and removing small portions of the tool. Abrasive action occurs in both flank wear and crater wear. It is a significant cause of flank wear. 
• Adhesion: When two metals are forced into contact under high pressure and temperature, adhesion occurs between them. These conditions present between the chip and the rake face of the tool. As the chop flows across the tool, small particles of the tool are broken away from the surface, resulting in attrition of the surface.
• Diffusion: This is a process in which an exchange of atoms occurs at the tool-chip boundary, Tool surface becomes depleted of the atoms responsible for its hardness. Diffusion is believed to be a principal mechanism of crater wear.
• Chemical reactions: The high temperatures and clean surfaces at the tool-chip interface in machining at high speeds result in chemical reaction oxidation, on the face of the tool. The oxidized layer being softer than the parent tool material is sheared away, exposing new material to sustain the reaction process.
• Plastic deformation: The cutting forces acting on the cutting edge at high temperatures cause the edge to deform plastically, making it more vulnerable to the abrasion of the tool surface. Plastic deformation contributes mainly to flank wear.

Types of wear 

• Flank wear
• Cater wear
• Chipping

Explanation:

Forms of Tool Wear:

The progressive wear of the cutting tool takes many forms as shown in the figure:

1. Tool wear on the tool face is characterized by the formation of a cra ter, which occurs as a result of the hot chip flowing over the tool face.

2. Flank wear, which is in the form of a wear land that is generated as the newly cut surface of the workpiece, rubs against the cutting tool. Minor chipping along the cutting edge is usually accompanied by flank wear.

3. Notch wear occurs locally in the area of the main cutting edge where it contacts the workpiece surface. It is caused by hard surface layers and work-hardened burrs, especially in austenitic stainless steels (www.widiaindia.com).

4. Thermal cracks occur in the form of small, running cracks across the cutting edge that are caused by thermal shock loads during inter rupted cutting operations.

Important Points

When machining at high cutting speeds and feed rates, the tool wear occurs mainly at the tool face. At medium cutting speeds, tool wear occurs simultaneously in its flank and face. At low cutting speeds, a substantial tool wear occurs, primarily at the tool flank.

Crater wear occurs when machining ductile materials, whereas machining brittle materials such as cast iron and bronze would normally lead to flank wear.

Concept:

• In the machining process, the wearing action takes place on those surfaces along which there is relative sliding with other surfaces.
• The wear over rake surface is known as crater wear and over flank surface is known as flank wear.

Flank wear occurs mainly on: 

• The flank or the relief face of the tool
• The nose part of the cutting tool

Flank wear occurs due to: 

• Abrasion by hard particles and inclusions in the work piece.
• Shearing off the micro welds between tool and work material.
• Abrasion by fragments of built‐up‐edge ploughing against the clearance face of the tool.
• At low speed flank wear predominates.
• If MRR increased flank wear increased.

• Crater wear occurs on the rake face.
• For the crater wear, the temperature is the main culprit and tool diffuse into the chip material and tool temperature is maximum at some distance from the tooltip. So crater wear starts at some distance from the tooltip.

Explanation:

Tool Wear: 

• Tool wear is the gradual failure of a cutting tool during regular operation. Every tool will experience tool wear at some point in its life. Major 02 types of tool wear include rake wear and crater wear.

​

​Crater Wear:

• Crater wear happens on the tool face at a short distance from the cutting edge by the action of chip flow over the face at a very high temperature. The crater wear is mainly due to diffusion and abrasion.
• It is mainly observed while machining ductile material due to the continuous formation of chips.
• It is generally observed in high-speed cutting.​

​Flank Wear:

• Flank wear occurs at the tool flanks, where it contacts with the finished surface, as a result of abrasion. The cutting force increases with flank wear.
• It is generally observed in low-speed cutting.

When HSS tool is used at higher cutting temperature, then due to plastic deformation and crater wear, it wears very rapidly. At high temperature the hot hardness of HSS tool is low which leads to plastic deformation. Crater wear occurs at some distance away from tool tip on tool face. It is mainly due to high temperature where solid state diffusion can cause rapid wear.

Super HSS is a molybdenum-series high-speed steel alloy with an additional 8% or 10% cobalt. It is widely used in metal manufacturing industries because of its superior red-hardness as compared to more conventional high-speed steels, allowing for shorter cycle times in production environments due to higher cutting speeds or from the increase in time between tool changes.

Explanation:

Multipoint Rotary Cutting Tools:

• Multipoint rotary cutting tools, such as milling cutters, drills, and reamers, are designed with multiple cutting edges that engage with the workpiece simultaneously or sequentially during the machining process. These tools are widely used in manufacturing due to their ability to perform high-speed, high-precision cutting operations.

Factors Contributing to Variable Wear:

The variable wear observed in multipoint rotary cutting tools can be attributed to several factors:

• Cutting Conditions: The cutting speed, feed rate, and depth of cut can vary along the cutting edges, leading to differential wear rates.
• Tool Geometry: The design and geometry of the cutting tool, including the rake angle, clearance angle, and edge radius, can influence the distribution of cutting forces and wear.
• Material Properties: The workpiece material's hardness, abrasiveness, and thermal properties can affect the wear mechanisms and rates experienced by different cutting edges.
• Thermal Effects: The heat generated during cutting can cause thermal expansion and thermal cycling, leading to uneven wear along the cutting edges.
• Mechanical Loads: The distribution of mechanical loads on the cutting edges can vary due to tool deflection, vibration, and the engagement of multiple edges with the workpiece.

Wear Mechanisms:

Several wear mechanisms contribute to the variable wear observed in multipoint rotary cutting tools:

• Abrasive Wear: The hard particles in the workpiece material can cause abrasive wear on the cutting edges, leading to variable wear rates depending on the local hardness and abrasiveness.
• Adhesive Wear: The interaction between the tool and workpiece materials can lead to adhesive wear, where material transfer and buildup occur unevenly along the cutting edges.
• Thermal Wear: The high temperatures generated during cutting can cause thermal softening and oxidation of the cutting edges, resulting in variable wear patterns.
• Fatigue Wear: Repeated cyclic loading during cutting can lead to fatigue wear, causing micro-cracks and chipping on the cutting edges.

Implications of Variable Wear:

Variable wear on multipoint rotary cutting tools has several implications for machining processes:

• Tool Life: Uneven wear can reduce the overall tool life, as the most worn cutting edge may determine the tool's end of life.
• Surface Finish: Variable wear can affect the quality of the machined surface, leading to inconsistencies and defects.
• Dimensional Accuracy: Differential wear can cause deviations in the dimensions and tolerances of the machined parts.
• Cutting Forces: Uneven wear can lead to variations in cutting forces, affecting the stability and efficiency of the machining process.

Mitigation Strategies:

To address variable wear in multipoint rotary cutting tools, several strategies can be employed:

• Tool Material Selection: Using cutting tool materials with high wear resistance, such as carbide, cermet, or coated tools, can reduce wear rates.
• Optimized Cutting Conditions: Adjusting cutting parameters to minimize excessive wear and ensure uniform engagement of cutting edges.
• Tool Design: Designing cutting tools with optimized geometry to distribute cutting forces and wear more evenly.
• Coolant and Lubrication: Using appropriate coolants and lubricants to reduce thermal and adhesive wear.
• Regular Inspection and Maintenance: Implementing regular inspection and maintenance schedules to monitor tool wear and replace worn tools in a timely manner.