Machining Fundamentals & Metal Cutting

Fundamentals of Machining & Machining Operations

• Unit I focuses on the fundamentals of machining and machining operations.
• Course outcomes involve producing jobs using lathe and drilling machines.
• This unit accounts for 14 marks.

Mechanism of Metal Cutting

• Metal cutting conditions require the cutting tool material to be harder than the workpiece.
• Applied cutting force must exceed the workpiece strength.
• Cutting tool geometry is a key factor.
• Key angles in metal cutting include rake angle (µ), shear angle (f), and relief angle (r).
• An uncut chip has a thickness denoted as t, while the chip thickness after the cut is tc.

Types of Metal Cutting Processes

• Metal cutting processes can be categorized into orthogonal and oblique cutting.

Orthogonal Cutting Process

• Is a two-dimensional cutting process
• The tool cutting edge is at 90° to the line of action/path of the tool, and the cutting velocity vector.
• Cutting involves only two forces
• Is simpler to analyze.

Oblique Cutting Process

• Is a three-dimensional cutting process.
• The cutting edge is inclined at less than 90° to the line of action/path of the tool and cutting velocity vector.
• Analysis is more difficult compared to orthogonal cutting.

Types of Cutting Tools

• Cutting tools are classified based on the number of cutting points.
• Single point cutting tools exists
• Multi point cutting tools exists
• Tool construction determines whether they are solid or tipped.

Cutting Tool Geometry

• Aspects include the shank, face, flank, principal flank, auxiliary flank, rake angle, back rake angle, and side rake angle.
• Relief angle also is factor, including side and end relief angles
• Cutting angles include side and end cutting edge angles
• Tool nose radius is another factor

Cutting Tool Geometry - Rake Angle

• Rake angles ensure ease of chip flow during machining.
• A positive rake angle reduces cutting force and power requirements.
• A zero rake angle simplifies design and manufacturing.
• A negative rake angle increases edge-strength and tool life.

Cutting Tool Geometry - Relief / Clearance Angle

• Prevents rubbing between the tool (flank) and the machined surface.

Types of Chips

• Chips formed during cutting process are classified as continuous, discontinuous and built-up edge (BUE).
• Continuous chips are formed from ductile materials at high cutting speeds, fine feeds, with positive, large rake angle and cutting fluid
• Discontinuous chips are produced from brittle materials/ductile but hard with medium-high cutting speeds, large feeds, negative rake angle and cutting fluid is absent.
• Continuous with BUE chips are formed from ductile materials at low cutting velocity, medium/large feed, small/negative rake angle and cutting fluid is absent

Cutting Forces

• Several forces are involved in metal cutting: shear force at shear plane (Fs), normal force at shear plane (Fn), and the resultant force of Fs and Fn (R').
• Tangential force on the tool face (F).
• Normal force on the tool face acting on the chip (N).
• The resultant force of F and N is denoted as R
• Horizontal component of the cutting force (Fc).
• Vertical component or Thrust force (Ft).
• Shear stress on the shear plane (τs).
• Rake angle of the cutting tool (α).
• Coefficient of friction (µ).
• Angle of friction (ß).
• Shear Angle (Φ)

Force Expressions

• Shear force = Fs = Fc * cos(φ) - Ft * sin(φ).
• Normal force = Fn = Fc * sin(φ) + Ft * cos(φ)
• Tangential frictional force: F = Fc * sin(α) + Ft * cos(α)
• Normal force is expressed as: N = Fc * cos(α) - Ft * sin(α)
• Shear Angle can be found by φ =tan^-1( (r * cos α) / (1 -r sin α) )
• Coefficient of friction: μ = tan β = (Fc tan α + Ft) / (Fc - Ft tan α).

Cutting Tool Designation

• ASA (American Standard Association System) and ORS (Orthographic Rake System) are used

ASA Tool Designation

• Includes angles like side cutting edge angle (Cs), end cutting edge angle (Ce), side relief angle (Os), end relief angle (Oe), side rake angle (αs), & back rake angle (αb).

ORS Tool Designation

• Involves side relief angle (γ), side rake angle (α), wedge/lip angle (β), cutting angle (δ), back rake/inclination angle (i), end relief angle(γ1), end cutting edge angle (Ce).

Lathe Machine Fundamentals

• Working principle: The workpiece is held in a spindle-mounted chuck and turned against a cutting tool to remove material in the form of chips.
• Classification is based on configuration, purpose of use, size or capacity, and the configuration of jobs handled.

Classification by Configuration

• Horizontal lathes
• Vertical lathes

Classification by Purpose of Use

• General-purpose lathe machines handle all types of operations.
• Single-purpose serve one type of operation
• Special-purpose lathe machines repeat specific operations.

Classification by Size or Capacity

• Small/low duty handles small to medium jobs.
• Medium duty lathes are versatile and commonly used.
• Large/heavy duty lathes handle large jobs.
• Mico/mini handles small size precision work

Classification by Configuration of Jobs Handled

• Bar type lathe machines hold jobs in collets.
• Chuck type lathe machines hold jobs in chucks.
• Housing type lathe machines handle odd-shaped jobs on a face plate.

Classification by Precision

• Ordinary lathe are standard machines
• Precision lathe machines gives high accuracy

Classification by Number of Spindles

• Single spindle lathe machines standard single operation machines
• Multiple lathe operations does fast and mass production

Classification by Type of Automation

• Fixed automation lathe machines utilize conventional single spindle automats.
• Flexible automation lathe machines use modern CNC lathes and Turing centres.

Classification by Degree of Automation

• Non-automatic lathe machines are conventional.
• Semi-automatic lathe machines are copying lathes.
• Automatic lathe machines are single spindle automat.

Lathe Machine Specification

• Includes maximum job diameter and length.
• Power of the main drive
• Range of spindle speeds and feeds
• Space occupied by the machine.

Lathe Machine Parts

• Consists of a headstock, tailstock, bed, carriage, cross slide, compound rest, tool post, and lead screw.

Lathe Machine Accessories - Centre Style

• Mounting jobs in Centre lathes is done without additional tailstock support.
• Chucks (3-jaw self-centering, 4 independent jaw)
• Face plates
• Jigs and fixtures.
• With tailstock support includes
• In-between Centre
• In-between chuck and Centre
• In-between headstock and tailstock

Lathe Machine Accessories - CNC Style

• Coventry concentric chucks
• Air operated chucks,
• Quick acting soft jaw chucks
• Collet chucks

Lathe Machine Accessories - Tool Mounting

• HSS tools (shank type)
• HSS form and threading tools
• Carbide and ceramic inserts
• Drills and reamers
• Boring tools

Lathe Machine Accessories - Automatic Style

• Radial slides (front and rear)
• Turrets (mostly hexagonal) for tool mounting

Lathe Machine Operations

• Straight turning
• Taper turning
• Profiling
• Turning and external grooving
• Facing
• Face grooving
• Cutting with a form tool
• Boring and internal grooving
• Drilling
• Cutting off
• Threading
• Knurling

Lathe Machine Cutting Parameters

• Cutting Speed (V): Is the speed at which metal is removed, expressed in m/min or mm/min; calculated as V = πDN / 1000.
• Feed (f): Relative motion of the cutting tool in one workpiece revolution; expressed in mm/rev.
• Depth of Cut (t): The total metal removed per pass, expressed in mm, varies with tool type and material.

Drilling Machine Fundamentals

• Working principle involves producing holes by using a rotating cutter called a drill.
• The machine tool exerts vertical cutting force to create a hole.

Drilling Machine Classification

• Portable drilling machines, accommodating 12 to 18 mm drill sizes.
• Sensitive drilling machines, available as table top (0.5 kW, 10 mm) and floor mounting (1.5 to 15.5 mm).
• Upright drilling machines. Pillar (0.55 - 1.1 kW, 50 mm). Box column construction

Drilling Machine Classifications

• Gang drilling machines use 2 to 6 spindles with variable drills simultaneously.
• Multi-spindle are central with a planetary gear system using variable drills on a telescopic part
• Turret-type which is column type with a turret that holds no drills

Drilling Machine Specifications

• Specifications include the maximum drill size (diameter), hole size and taper in the spindle.
• Also the range of spindle speeds and feeds
• Power of the main drive
• Axial travel range of the spindle/bed
• Floor space occupied

Drilling Machine Accessories - Job Mounting

• Angle, drilling, and diameter jigs.
• Plate jibs
• Vices for clamping work

Drilling Machine Accessories - Tool Mounting

• Drill sleeves
• Drilling chucks
• Tapping tool mountings

Drilling Machine Operations

• Reaming
• Tapping
• Counter-boring
• Counter-sinking
• Centre drilling
• Spot facing