Exhaustive Study Notes on Machine Tool Processes and Metrology

General Course Information and Objectives

• Course Title: MECE 3201: Machine Tool Processes and Metrology

• Institutional Context: University of Bamenda, National Higher Polytechnic Institute (NAHPI), School of Engineering, Department of Mechanical Engineering.

• Level: Level 300, 2nd Semester.

• Credits: 4 credits ($40 = 20 + 00 + 20$).

• Course Facilitator: Engr. N.P NIDELLE.

• General Objectives:     - To help students acquire knowledge about the theory of metal cutting, the mechanism of machining, and parameters influencing machining processes.     - To teach operations involved in machines such as turning, shaping, slotting, milling, and grinding.     - To teach gear generation methods and principles of non-traditional machining processes.     - To explain instruments for linear and angular measurements and surface finish.

• Course Units:     - Unit I: Theory of Metal Cutting     - Unit II: Lathe and Basic Machine Tools     - Unit III: Milling and Grinding Machines     - Unit IV: Gear Generation and Non-traditional Machining Processes     - Unit V: Metrology and Instrumentation

Unit I: Theory of Metal Cutting

• Machining Definition: A semi-finishing or finishing process performed to impart required dimensional accuracy, form accuracy, and surface finish to enable products to fulfil functional requirements, provide improved performance, and render a long service life.

• Mechanism of Chip Formation:     - Machining involves the gradual removal of excess material from preformed blanks in the form of chips.     - Chips serve as an index indicating: nature and behavior of work material, specific energy requirements ($ext{energy per unit volume}$), and interactions at the chip-tool interface.     - Factors Influencing Chip Form:         - Work material properties.         - Cutting tool material and geometry.         - Cutting velocity ($V_C$), feed ($s_0$), and depth of cut.         - Machining environment (cutting fluids) affecting temperature and friction.

• Chip Formation in Ductile Materials:     - The uncut layer ahead of the tool tip is subjected to all-sided compression.     - Force exerted by the tool arises from the normal force ($N$) and frictional force ($F$).     - Shear stress develops in the compressed region; when it exceeds the material's shear strength, yielding or slip occurs at the plane of maximum shear stress.     - Piispanen Model of Card Analogy: Explains chip formation through lamellar sliding (shifting of postcards).     - Upper chip surfaces show visible serrations, while lower surfaces are smooth due to plastic deformation and rubbing at high pressure/temperature.     - Deformation Zones: Primary and secondary shear deformation zones.     - Experimental Study Methods:         - Deformation of rectangular or circular grids marked on the side surface.         - Microscopic study of chips frozen by drop tool or quick stop apparatus.         - High-speed camera observation with a low magnification microscope.

• Chip Formation in Brittle Materials:     - Mechanisms involved: yielding (for ductile) and brittle fracture (for brittle).     - A crack develops at the tool tip due to the wedging action of the cutting edge.     - In brittle materials, the crack propagates quickly along the path of minimum resistance, causing total separation and resulting in discontinuous chips of irregular size and shape.

• Types of Chips:     1. Discontinuous (Segmental) Chips:         - Formed by a series of ruptures perpendicular to the tool face.         - Friction is reduced, resulting in better surface finish.         - Favored by: brittle materials (cast iron, bronze), large chip thickness, low cutting speed, and small rake angle.     2. Continuous Chips:         - Formed by continuous plastic deformation without fracture.         - Smooth flow up the tool face with uniform thickness.         - Most desirable for stability and good surface finish.         - Favored by: ductile materials (mild steel, copper), high cutting speed, small chip thickness, large rake angle, and low friction (lubricants/polished tools).     3. Continuous Chips with Built-Up Edge (BUE):         - Similar to continuous but work material welds to the cutting edge due to local high temperature and pressure.         - BUE grows, becomes unstable, and breaks off, part sticking to the chip and part to the machined surface, causing roughness.

• Orthogonal and Oblique Cutting:     - Orthogonal Cutting: Chip flows along the orthogonal plane ($\pi_0$), i.e., chip flow deviation angle ($\rho_c$) is zero ($\rho_c = 0$).     - Oblique Cutting: Chip flow deviates from the orthogonal plane ($\rho_c \neq 0$). Usually occurs when inclination angle ($\lambda \neq 0$).     - Pure Orthogonal Cutting: Typically shown in pipe turning where $\lambda = 0$ and the cutting edge angle ($\phi$) is $90^{\circ}$.     - Effects of Oblique Cutting:         - Positive $\lambda$: Chip flows away from the finished surface (less damage, but inconvenient for operators); reduces tool tip strength; increases temperature and vibration ($P_y$).         - Negative $\lambda$: Enhances tool life by increasing strength and reducing temperature but may impair the finished surface.         - Cross-sections may change from rectangle to skewed trapezium.

• Machining Forces and Merchant’s Circle Diagram (MCD):     - Purpose of Force Analysis: Estimation of power consumption, structural design of machine/fixture/tool, evaluation of parameters, and condition monitoring.     - Turning Force Components ($R$):         - $P_z$ (Tangential): Largest component; main power component. Consumption is $P_z \times V_C$.         - $P_x$ (Axial): Direction of longitudinal feed; least significant/least harmful.         - $P_y$ (Radial/Transverse): Responsible for dimensional inaccuracy and vibration.         - Resultant $R = \sqrt{P_z^2 + P_{xy}^2}$.     - Drilling Forces:         - Tangential forces ($P_{T1}$, $P_{T2}$) at main edges produce torque ($T$).         - $T = P_T \times \frac{1}{2}(D)$.         - Power consumption $P_C = 2\pi \times T \times N$.         - Total axial force $P_{XT} = P_{X1} + P_{X2} + P_{Xe}$ (chisel edge force).     - Milling Forces:         - Tangential force ($P_{Ti}$) governs torque and power.         - Resultant $R$ is composed of $P_T$ and $P_R$ (radial).     - Merchant’s Circle Construction and Equations:         - Valid for orthogonal cutting based on a single shear plane theory.         - Forces involves: $P_s$ (shear), $P_n$ (normal to shear), $F$ (friction), $N$ (normal to rake), $P_z$ (main cutting), and $P_{xy}$ (transverse resultant).         - Dynamic yield shear strength ($\tau_s$): $\tau_s = \frac{P_s}{A_s}$.         - Apparent coefficient of friction $\mu_a = \frac{F}{N}$.

• Evaluation of Power and Energy:     - Cutting Power $P_C = P_z \times V_C + P_x \times V_f \approx P_z \times V_C$ (since $V_f$ is small).     - Specific Energy Requirement ($U_s$): Amount of energy to remove unit volume. Grinding requires significantly higher energy than turning due to negative rake angles.

• Thermal Aspects of Machining:     - Power consumed is converted to heat near the cutting edge.     - Distribution Zones:         - Primary (Shear) Zone: $80\text{--}85\%$ of heat generated by plastic deformation.         - Secondary (Friction) Zone: $15\text{--}20\%$ of heat generated by chip sliding against rake face.         - Tertiary Zone: $1\text{--}3\%$ of heat due to burnishing/BUE action.     - Disadvantages: Tool wear, thermal damage to surfaces, dimensional errors.     - Cooling Methods: Cutting fluids, reduced speed/feed, positive tool rake angles.

• Cutting Fluids:     - Categories: Straight oils (best lubrication, poor cooling), Synthetic fluids (best cooling, no oil base), Soluble oils (emulsion with water, least expensive), Semi-synthetics.     - Desirable Properties: High thermal conductivity, high flash point, stability, non-corrosive, non-toxic, correct viscosity.

• Machinability:     - Definition: Responses of work material to cutting (surface finish, tool life, force).     - Machinability Index ($K_M$): $K_M = \frac{V_{60}}{V_{60R}}$ (where $V_{60R}$ is the reference material's speed).     - Factors: Additives like Lead (solid lubricant) or Sulphur (stress raisers) improve machinability.

• Cutting Tool Materials:     - Carbon Tool Steels: Inexpensive, very heat sensitive, obsolete for commercial machining. Hardness $HRC\,65$.     - High Speed Steel (HSS): Retains hardness at moderate temperatures; most common for drills/taps. Hardness $HRC\,67$.     - HSS Cobalt: Heat resistant, used for titanium. Hardness $HRC\,70$.     - Cemented Carbide: Tungsten carbide + Cobalt. Common for turning bits. Hardness $HRA\,93$.     - Ceramics: Chemically inert, heat resistant, fragile. Hardness $HRC\,93$.     - Cubic Boron Nitride (CBN): Second hardest. Used for hard machining. Hardness > HRC\,95.     - Diamond: Hardest known; unsuitable for steel due to chemical affinity to iron.

• Tool Life Calculation:     - Taylor's Equation: $V \times T^n = C$.     - Factors: Speed (greatest influence), feed, depth of cut, tool geometry, and cutting fluid.

Unit II: Lathe and Basic Machine Tools

• Lathe Classifications:     - Engine Lathes: Bench, speed, precision, toolroom. General purpose.     - Manufacturing Lathes: Engine lathes modified with tracer attachments or digital readouts.     - Production Lathes: For large runs (Turret, Automatic, Numerical control lathes).

• Lathe Operating Parameters:     - Cutting Speed: Speed of work rotating past the tool.     - Feed Rate: Rate the tool advances into work.     - Depth of Cut: Amount of material removed as work revolves.

• Lathe Operations:     - Facing: Machining ends perpendicular to axis to reach desired length.     - Straight (Cylindrical) Turning: Reducing diameter parallel to axis.     - Grooving (Necking): Internal or external V, round, or square furrows.     - Parting Off: Cutting off stock using a parting tool.     - Boring: Internal surfacing/enlarging existing holes.     - Knurling: Forming (not cutting) peaks and valleys for grip.     - Thread Cutting: Helical grooves using a single-point tool.     - Filing/Polishing: Using mill files or emery cloth for finish.

• Main Components:     - Bed: Heavy casting; base foundation with machined ways.     - Headstock: Contains main spindle, speed gears, and motor drive.     - Tailstock: Supports long pieces; holds drills/reamers/taps.     - Carriage: Consists of saddle, apron, cross slide, compound rest, and tool post.

• Tool Nomenclature (Single Point):     - Parts: Shank, nose (radius), face (top), side/flank, cutting edge, base, heel.     - Relief (clearance) angles: End-relief and side-relief allow the tool to feed without rubbing.

• Work Holding Devices:     - Three-Jaw Universal Scroll Chuck: Jaws move in unison; centers work within $0.002\text{--}0.003\,inches$.     - Four-Jaw Independent Chuck: Jaws move separately; used for irregular/eccentric shapes.     - Collet Chuck: Most accurate for small diameter workpieces.     - Lathe Centers: Live centers revolve with work; dead centers are stationary and require lubrication.     - Mandrels: Tapered axles pressed into a bore to support work between centers.

• Shaping Machines:     - Principle: Linear reciprocating tool movement (fast cutting) and intermittent work feed.     - Quick Return Ratio (QRR): Return stroke is faster to reduce idle time.     - Applications: Horizontal/vertical/inclined flat surfaces, slots, T-slots, gear teeth.

• Planning Machines (Planers):     - Difference from Shaper: Workpiece reciprocates (cutting motion) while tool feeds (slow motion).     - Suitable for very large, heavy jobs and simultaneous use of multiple tools (e.g., machining machine beds).

• Slotting Machines:     - Vertical shaping machine; tool reciprocates vertically.     - Applications: Internal flat surfaces, non-circular holes (hexagonal), internal keyways/splines.

• Drilling Machines:     - Originate cylindrical holes (blind or through) ranging $1\text{--}40\,mm$.     - Parts: Speed gear box (SGB), Feed gear box (FGB), spindle, quill with rack.     - Drill Types: Center drills, step/subland drills, gun drills (oil holes), trepanning tools (large holes in soft material).     - Twist Drills: Made of HSS; held via parallel shanks (chuck) or taper shanks (Morse taper sleeves).

• Reaming:     - Process: Enlarging previously formed holes with high accuracy and smooth finish.     - Types: Rose, chucking, taper, expansion, shell, adjustable.

• Broaching:     - Linear tool (broach) with successive teeth removes material in one pass.     - Advantages: Very high production rate, high accuracy ($\pm 0.0075\,mm$), high finish ($0.8\,\mu m$).

• Threading (Manual Bench Work):     - Tapping: Internal threading using Taper (first 8-10 threads), Second (intermediate), and Plug (bottoming) taps.     - Diesing: External threading using solid, spring, split, or pipe dies.     - Faults: Broken tap (hole too small, no reverse), drunken thread (die not square).

Unit III: Milling and Grinding Machines

• Milling Principles:     - Rotating cutter with multiple teeth; work feeds against cutter.     - Types: Knee-type (Vertical/Horizontal), Universal (swivel table), Ram-type.

• Milling Cutters:     - Pitch: Angular distance between teeth.     - Rake Angle: Path for chips; defines cutting edge.     - Primary Clearance: Prevents rubbing against work.     - Cutter Types: Plain (slab), Side, End mills (can drill own holes), Shell end mills (large facing), T-slot, Woodruff keyslot, Gear hobs.

• Direction of Feed:     - Standard: Work feeds against rotation to take up backlash and cut under surface scale.

• Milling Operations:     - Face Milling: Surfaces perpendicular to cutter axis.     - Straddle Milling: Two side cutters machining two sides simultaneously (e.g., cutting hexagons).     - Gang Milling: Multiple cutters on one arbor.     - Fly Cutting: Single-point tool; economical for special forms.

• Indexing (Universal Dividing Head):     - Used to divide circumference into equal divisions (e.g., gear teeth).     - Ratio: $40:1$ (40 handle turns = 1 spindle revolution).     - Direct Indexing: Using a plate attached to the spindle for standard angles ($30^{\circ}, 45^{\circ}, 90^{\circ}$).

• Grinding Process:     - Abrasive machining using grains (grit) held by a bond.     - Features: Sharp cutting points, high hot hardness, wear resistance.     - Grinding Wheel Interaction: Grit-workpiece (chip formation), chip-bond, chip-workpiece, bond-workpiece.

• Grinding Wheel Specifications:     - Abrasives: Aluminum Oxide (A), Silicon Carbide (C), Diamond (D), Cubic Boron Nitride (B).     - Designation Example ($51\text{--}A\text{--}60\text{--}K\text{--}5\text{--}V\text{--}05$):         - $51$: Manufacturer id.         - $A$: Aluminum oxide.         - $60$: Grit size (mesh size).         - $K$: Grade/Hardness ($A = ext{softest}, Z = ext{hardest}$).         - $5$: Structure/Porosity ($1 = ext{dense}, 20 = ext{open}$).         - $V$: Vitrified bond.

• Superabrasive Concentration: Number '100' indicates $4.4\,carats/cm^3$ ($25\% ext{ by volume for diamond}$).

Unit IV: Gear Generation and Non-Traditional Machining (NTM)

• Gear Types:     - Spur: Parallel shafts; straight teeth.     - Internal: Stronger drive; less space.     - Helical: Smooth/quiet; produces end thrust (requires thrust bearings).     - Herringbone: Double helical; eliminates end thrust.     - Bevel: Intersecting shafts (usually $90^{\circ}$).     - Hypoid: Non-intersecting $90^{\circ}$ shafts.     - Worm and Worm Gear: Large speed reduction.

• Gear Terminology:     - Addendum: Height above pitch circle.     - Dedendum: Distance to bottom of tooth from pitch circle.     - Module (m): Pitch diameter divided by number of teeth ($PD/N$).     - Diametral Pitch (DP): Number of teeth per inch of pitch diameter.

• Gear Manufacture Methods:     - Pre-forming: Sand casting, Die casting (Al/Zn alloys), Investment casting, Powder metallurgy.     - Forming (Machining): Form milling (HSS disc cutters), shaping/slotting (for repair).     - Generation (Machining): Hobbing, Sunderland method (rack cutter), Gear Shaping (circular cutter).     - Finishing: Gear Shaving (serrated cutters for soft gears), Burnishing (rolling under pressure), Grinding (generation/forming for hard gears), Lapping.

• Non-Traditional Machining (NTM):     - Mechanical Energy:         - Ultrasonic Machining (USM): Abrasive slurry + vibrating tool ($20\,kHz$). Used for brittle materials (glass, ceramics).     - Electrical/Chemical Energy:         - Electrochemical Machining (ECM): Anodic dissolution; reverse of electroplating. Metal is deplated from work to electrolyte.         - Electrochemical Grinding (ECG): Wheel responsible for $5\%$ removal, deplating for $95\%$. High longevity of wheel.     - Thermal Energy:         - Electrical Discharge Machining (EDM): Spark erosion.         - Wire EDM: Wire electrode for through-hole cutting.         - Ram (Sinker) EDM: Spark erosion for blind cavities.