Tool Wear and Failure MCQ Quiz - Objective Question with Answer for Tool Wear and Failure - 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:

Rake angle: 

• The angle between the face of the tool and a plane parallel to its base. If this inclination is towards the shank, it is known as the back rake angle and if measured along with the side is known as the side rake angle.

Lip angle: 

• The angle between the face and flank of the tool. As the lip angle increases, the cutting edge will go stronger.

Clearance angle (α):

• The angle of inclination of clearance or flank surface from the finished surface.
• The clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface which causes loss of energy and damages to both the tool and the job surface.
• Clearance angle is a must and must be positive (3° - 15°)  depending upon tool-work materials.​

Side Cutting Edge Angle:

• The angle between the side cutting edge and side of the tool shank is called the side cutting edge angle.
• It is also called lead angle or principle cutting angle.

End Cutting Edge Angle:

• The angle between the end cutting edge and a line perpendicular to the shank of the tool is called the end cutting edge angle.

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.

Explanation:

The surface finish in the machining operation depends on

1. Type of chip formation
2. Tool profile and geometry
3. Cutting speed

For a tool of finite nose radius r, the peak–to–valley roughness can be evaluated as

\(h = \frac{{{f^2}}}{{8r}}\)

Also \(h = f\tan {\gamma _e} + \frac{r}{2}{\tan ^2}{\gamma _e} - \sqrt {\left( {2fr{{\tan }^3}{\gamma _e}} \right)} \)

From the above equation nose radius, cutting–edge angles, and feed rate have an influence on the surface finish.

 

• Cutting speed is the most important factor in tool life and not in surface finish.

Explanation:

Wedge angle:

• Lip angle or wedge angle or relief angle or cutting edge angle depends on the rake and clearance angle provided on the tool and determine the strength of the cutting edge.
• A large wedge angle permits machining of harder metals, allow heavier depth of cut, better heat dissipation, and increase tool life.

• A larger lip angle increases the tool’s cross-sectional area, making it more robust against cutting forces and wear.

• Directly determines the tool’s ability to withstand mechanical stress without chipping or fracture.

• 

• From the diagram:

• Wedge angle (ϕ) = 90° - (rake angle + clearance angle)

Rake angle:

• The angle between the face of the tool and a plane parallel to its base. If this inclination is towards the shank, it is known as a back rake angle and if measured along with side is known as side rake angle.
• Side rake angle is the angle between the face of the tool and the base of the shank or holder.
• It is usually measured in a plane perpendicular to the base and parallel to the width.
• It controls the chip flow direction.
• An increase in the side rake angle reduces the chip thickness in turning operation.
• Rake angle is a cutting edge angle that has large effects on cutting resistance, chip disposal, cutting temperature and tool life.
• Increasing rake angle in the positive (+) direction improves sharpness.

Cutting Force:

• The rake angle influences the cutting force acting on the tool. A larger rake angle reduces the cutting force, which leads to better tool life and improved surface finish.

Chip Formation:

• The rake angle also influences the chip formation process. A positive rake angle generates a thin chip, while a negative rake angle generates a thick chip. A thin chip is easier to remove from the workpiece surface, resulting in a better surface finish.

Tool Life:

• The rake angle also affects the tool life. A larger rake angle reduces the tool wear, which leads to longer tool life.

 

Clearance angle (α):

• The angle of inclination of clearance or flank surface from the finished surface.
• The clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface which causes loss of energy and damages to both the tool and the job surface.
• Clearance angle is a must and must be positive (3° - 15°) depending upon tool-work materials.

​

​There are two type of clearance angle in the turning tool:

(1) End clearance or front clearance angle

(2) Side clearance angle

Plan Approach angle:

• Complimentary angle of lead angle is called approach angle.
• Approach angle = 90° - Lead angle
• Approach angle is the angle between a plane perpendicular to the cutter axis and a plane tangent On the surface of revolution of the cutting edges.

​Clearance angle:

• The angle of inclination of clearance or flank surface from the finished surface.
• The clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface which causes loss of energy and damages to both the tool and the job surface.
• Clearance angle is a must and must be positive (3° - 15°) depending upon tool-work materials.​

Explanation:

Cutting tool geometry is described and designated in several systems.

• Tool-in-hand system.
• Machine reference systems (ASA).
• Tool reference system
  • Orthogonal rake systems (ORS).
  • Normal rake systems (NRS).
• Work reference system.

Tool signatures have 7 elements in both ASA and ORS systems.

American Standards System (ASA):

Back rake angle (α_b) - side rake angle (α_s) - end relief angle (γ_e) - side relief angle (γ_s) - end cutting edge angle (C_e) - side cutting edge angle (C_s) - Nose radius (r).

Orthogonal Rake System (ORS): 

i (inclination angle) - αn (Normal rake angle) - Side relief angle - end relief angle, end cutting edge angle - approach angle - nose radius (r).

∴ in both systems, the nose radius comes at last.

Rake angle:

• Rake angle is the angle of inclination of rake surface from reference plane.
• It is an important parameter in various cutting and machining process.
• There are three types of rake angle i.e. positive, negative and zero.

Positive rake angle: It is used when there is requirement of less cutting force and used for low strength materials like mild steel etc.

Negative rake angle: It is used for hard metals which requires high cutting force and machined at very high speed.

Zero rake angle: It is used for gear cutting or thread manufacturing. Brass and cast iron are machined with zero rake.

Threading is done by a form tool. The forms tool ideally have zero rake angle. 

In Taylor's tool life equation

VTⁿ = C where, T = tool life in min,

V = velocity in m/min, n = tool life exponent (depends on the material of the tool)

and C = machining constant (depends on both tool and workpiece).

The value of n for different materials is mentioned in table.

Tool material	Cutting speed (m/min)	n
High-speed steel	30	0.08 to 0.20
Cemented carbide	150	0.20 to 0.50
Coated carbide	350
Ceramic	600	0.5 to 0.7

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:

The Taylor’s tool life equation for flank wear on the tool is

VTⁿ = C also V₁T₁ⁿ = V₂T₂n

where, V is cutting speed in m/min, T is tool life in min, C = machining constant (depends on both tool and workpiece material), and n = tool life exponent (depends only on the tool material).

Calculation:

Given:

n = 0.25, V₂ = V₁/2 

From, the Taylor’s tool life equation

VTⁿ = C 

\(\frac{{T_2 }}{{T_1 }} = (\frac{{V_1}}{{V_2}})^{\frac{{1}}{{n}}}\)

⇒ \(\frac{{T_2 }}{{T_1 }} = (2)^{\frac{{1}}{{0.25}}}\)= 2⁴ = 16

⇒ T₂ = 16T₁

For machining high strength material, tool stability is the major criteria. If the tool material is brittle like ceramics and carbides negative back rake angles are provided. While machining soft metals there is not much requirement of strength so positive rake angle is preferred. But while machining brass and other Cu alloys zero degree rake angles are used.

Rake angle: It affects the ability of the tool to shear the work and form a chip. After plastic deformation chips flow over the rake face and heavy drag exists between chip and rake face.

Relief angle: It minimize rubbing contact between machined surface. It helps to eliminate tool brakeage and increase tool life. It prevents side flank of tool from rubbing against the work.

Concept:

Cutting speed used for different tool materials are:

HSS (minimum speed), 10 - 60 m/min < Cast alloy < Carbide, (30 - 140 m/min) < Cemented carbide, 150 m/min < Cermets, (150 - 350 m/min) < Ceramics or sintered oxide (maximum speed), more than 500 m/min

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.