Introduction To Metal Cutting Process

Manufacturing Processes

It is defined as, "a process which converts unfinished materials to finished products using machine or machine tools". It is classified as:

1. Casting, foundry or molding process
2. Forming or Metalworking processes
3. Machining Process 
4. Joining Assembly
5. Surface Treatment (Finishing)
6. Heat Treatment 
7. Rapid Prototyping

Cutting

It is defined as, "process of removing material from work piece to obtain require shape and size". It is classified as:

1. Non-Cutting Process  →  Shape is obtained under the action of force and heating. E.g.: forging, drawing, spinning, rolling, extrusion.
2. Cutting Process  →  Shape obtained by removing unwanted metal from work piece. E.g.: turning, drilling, boring, milling.

Factors Affecting Cutting Process

Factors which affects the cutting process is described below:

1. Cutting Speed, depth of cut, feed, cutting fluids
2. Tool angles  →  influence on chip flow direction, resistance to tool chipping.
3. Chip Formation
4. Temperature Rise  →  influences tool life, crater wear and dimensional accuracy of workpiece.
5. Tool Wear  →  Influences surface finish, dimensional accuracy, temperature rise, forces and power.
6. Tool Wear Machinability  →  related to tool life, surface finish, forces and power.

Types of Cutting Tool

Principal aspects of cutting tool are: tool geometry, tool material. Classification of cutting tool on the number of major cutting edges:

1. Single Point Cutting Tool  →  turning, shaping, planning, slotting and boring tools.
2. Double (or Two) Point Cutting Tool  →  drills 
3. Multi-Point Cutting Tool  →  milling cutters, broaching tools, hobs, gear shaping cutters.

Single Point Cutting Tool

• It is used to remove material from work piece.

It contains the following parts:

1. Shank  →  main body of tool and used to hold tool.
2. Flank  →  surfaces below and adjacent to cutting edges.
3. Face  →  surface on which chip slides.
4. Nose  →  formed at the junction of side and end cutting edge. Its radius is called Nose Radius.
5. Cutting Edge  →  edge which removes the material from work piece. It consist of side cutting edge, end cutting edge and nose.

Important Concepts

• Side Cutting Edge Angle (or SCEA)  →  angle between side cutting edge and side of tool shank.
• End Cutting Edge Angle (or ECEA)  →  angle between end cutting edge and line perpendicular to shank of tool.
• Side Relief Angle (or SRA)  →  angle between portion of side of flank below the side cutting edge and line perpendicular to the base of tool. It is used to reduce wear which is in contact with side material surface.
• End Relief Angle (or ERA)  →  angle between end flank and the line perpendicular to the base of tool. It is used to reduce wear which is in contact with material surface.
• Back Rake Angle (or BRA)  →  angle between tool face and the line perpendicular to the base of tool. It is used to reduce cutting force and removes chip.
• Side Rake Angle (or SRA)  →  angle between tool face and a line parallel to the base of tool.
• Clearence or End Relief Angle  →  angle of inclination of clearance or flank surface from the finished surface.
• Rake Angle  →  angle of inclination of rake surface from reference plane. It can be positive, negative, zero.  

Tool Life 

It is defined as, "the span of actual uninterrupted machining time after which the tool needs replacement". There are regions over which we describe tool life, which are:

1. Break-in Period  →  region in which sharp cutting edge wears rapidly at the beginning of its use.
2. Steady-state Wear Region  →  wear that occurs at a uniform rate and is a linear function of time (but deviation in actual machining).
3. Failure Region  →  wear rate begins to accelerate. Cutting temperature increases, machining efficiency reduced.

Tool Life Determination Process 

• A way in which a level of tool wear is set as a safe limit (i.e. allowable wear land). 
• Tool life (T) is a cutting time required for the cutting tool to develop a flank wear of width VB. It depends on cutting velocity, feed and depth of cut. It is given by Taylor's Tool Life Equation.

Tool life can be measured by:

1. Visual inspection of tool edge 
2. Changes in cutting sounds
3. Type of Chip (ribbony, stringy)
4. Surface finish degrades

Cutting Fluids

It is defined as, "a liquid or gas that is applied directly to the machining operation to improve cutting performance".

• Act as Lubricant (Friction Reducer)  →  reduce friction, wear, welding tendency and reduce energy consumption.
• Act as Coolant (Heat Transporter)  →  decrease temperature, tool life increases. Cooling of cutting zone which increases tool life and dimensional stability.

Types of Cutting Fluids

1. Air blast or Compressed air slowly
2. Cutting oils and soluble oils
3. Water
4. Chemical Fluids

Chips

It is defined as, "metals chips are formed due to the shearing from work piece". There are four types of chip formation, which are:

1. Discontinuous Chips  →  result in when brittle materials are machined at low cutting speed, irregular texture.
2. Continuous Chips  →  result in when ductile materials are machined at high cutting speed, good surface finish.
3. Continuous with Built-up Edge  →  obtained by machining ductile material under high local temperature, pressure, friction in tool chip interference.
4. Serrated Chips  →   semi-continuous and non-homogenous chips formed due to non-uniform strain at work piece.

Chip Breakers

It is defined as, "is used with single point tools to force chips to curl more tightly causing them to fracture". There are two design types of chip breakers which are:

1. Groove Type Chip Breaker  →  designed into the cutting tool itself.
2. Obstruction Type Chip Breaker  →  designed as an additional device on the rake face of the tool.

Principles of Chip Breaking 

• Self Chip Breaking  →  accomplished without using a separate chip breaker.
• Forced Chip Breaking  →  chip breaking using tool geometrical features or devices.

Types of Chip Breakers

1. Clamped Type Chip Breaker
2. Chip Curl
3. Groove Type
4. Integrated Obstruction Type
5. Clamped on Obstruction Type

Note:

• Engine Block Casting + CNC Lathe

References:

• Material from Class Lectures + Book named Fundamentals of Modern Manufacturing Materials, Processes and Systems by Groover + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).