Machine Tool Design (ME-802 (C)) - Unit 4 Short Notes

UNIT 4: MACHINE TOOL DESIGN

I. FUNDAMENTALS OF MACHINE TOOLS & KINEMATICS

1.1 Definition, Classification & Primary Functions

• Machine Tool: A power-driven, non-portable machine used for cutting, shaping, or forming materials (metal, wood, etc.) to produce precision engineered components.

• Primary Functions:

  1. Holding the workpiece (workholding device).

  2. Holding the cutting tool (toolholder).

  3. Providing relative motion (cutting speed, feed, depth of cut) between tool and workpiece.

  4. Transmitting power from source to cutting zone.

• Classification: By method of operation (turning, milling, drilling, grinding, gear cutting) or by control (conventional, NC, CNC).

1.2 Kinematical Structures

• Kinematic Chain: An assembly of links (rigid bodies) connected by kinematic pairs (joints allowing relative motion). Represents the motion transmission path in a machine tool.

• Kinematical Features:

  • Degrees of Freedom (DOF): Number of independent motions a body can have in space (6 for a free body: 3 translations, 3 rotations).

  • Mobility (M): Number of independent input motions required to define all positions of links in a chain. Governed by Kutzbach Criterion for planar mechanisms:

$$ M = 3(n - 1) - 2j_1 - j_2 $$

    Where, `n` = total links (including frame), `j₁` = lower pairs (1 DOF), `j₂` = higher pairs (2 DOF).

*   **Structural Synthesis:** The process of **inventing/designing** kinematic chains with a specified mobility and function. Uses graph theory and structural formulas.

1.3 Need & Importance of Speed Variation

[!TIP] Exam Focus: Direct 7m question (May 2022).

• Need: Different materials, tool materials, and machining operations require different cutting speeds for optimal performance (tool life, surface finish, power efficiency).

• Importance:

  • Achieves optimum cutting conditions across a wide range of operations.

  • Provides flexibility to machine various workpieces on a single machine.

  • Matches motor speed to required spindle speed.

  • Enables stepless or stepped variation within a designed speed range (R = N_max / N_min).

1.4 Selection of Low Speeds, Feeds & Power

Selection is based on a systems approach considering:

1. Workpiece Material: Hardness, strength, machinability.

2. Tool Material: HSS, carbide, CBN, diamond (dictates max speed).

3. Machining Operation: Roughing (high feed, low speed) vs. Finishing (low feed, high speed).

4. Machine Tool Rigidity: Limits depth of cut and feed.

5. Power Requirement: $$\displaystyle P_c = \frac{V \cdot f \cdot d \cdot K}{60 \cdot 10^3} $$ kW, where V=speed (m/min), f=feed (mm/rev), d=depth (mm), K=specific cutting force (MPa). Must be ≤ machine motor power.

6. Surface Finish & Dimensional Accuracy: Tighter requirements → lower feeds/speeds.

7. Tool Life: Higher speeds reduce tool life exponentially (Taylor's Tool Life Equation: $$\displaystyle VT^n = C $$).

II. MOTION TRANSMISSION & SPEED DIAGRAMS

2.1 Ray Diagram

• Definition: A graphical representation on a logarithmic scale showing the speeds of a shaft (usually spindle) as a function of the speeds of the driving shaft (usually motor). It is a plot of speed ratios.

• Construction:

  1. Draw two vertical lines: Driving shaft (Motor) and Driven shaft (Spindle).

  2. Mark logarithmic scales for speed (RPM) on both.

  3. For each gear pair (or transmission stage), draw a ray (line) from a point on the driving scale to the corresponding point on the driven scale. The slope of the ray represents the speed ratio ($$\displaystyle i = \frac{N_{driven}}{N_{driver}} $$).

  4. The intersection of rays from different stages gives the resultant spindle speeds.

• Purpose: To design and analyze multi-stage gearboxes. Ensures:

  • Geometric progression of speeds (R = constant).

  • No speed overlap or gaps between adjacent speeds.

  • Optimum use of gear teeth and bearings.

2.2 Speed Diagram

• Definition: A graphical method to determine the number of speeds (Z) and speed range (R) obtainable from a given number of gear pairs (n) and number of speeds per pair (z). Plots theoretical speed combinations.

• Construction: For n pairs each with z speeds, draw n sets of parallel lines (each set for one shaft speed). Intersections of lines from different sets represent possible speed combinations. The envelope of these points gives the actual achievable speed range.

• Utility: Quick estimation of maximum possible speeds and range for a given gearbox structure. Identifies optimal grouping of gear pairs.

2.3 Critical Comparison: Ray Diagram vs. Speed Diagram

2.4 Design of Speed Variation Systems

• Goal: Achieve a required speed range (R) with minimum number of gear pairs (n) and minimum number of speeds (Z).

• Key Equation: For n pairs with z speeds each, total speeds $$\displaystyle Z = z^n $$. Speed range $$\displaystyle R = z^n $$ (if all rays used).

• Design Steps:

  1. Determine required R and Z.

  2. Choose n and z such that $$\displaystyle z^n \geq Z $$ and $$\displaystyle R \leq z^n $$.

  3. Use Ray Diagram to arrange gear pairs in series or grouped (e.g., 2x2, 3x2) to avoid overlap and ensure geometric progression.

  4. Select standard gear ratios (common factors) to simplify gear inventory.

III. MATERIAL FORMING & PROCESSING

3.1 Cold Working of Metals

• Definition: Metal forming processes performed below the recrystallization temperature (usually at room temperature).

• Processes: Rolling, forging, drawing, extrusion, stamping (cold).

• Effects on Material Properties:

  • Strain Hardening (Work Hardening): Dislocation density increases → increased strength & hardness, decreased ductility & toughness.

  • Residual Stresses: Non-uniform deformation induces internal stresses → can cause distortion during later machining/service.

  • Improved Surface Finish: Typically better than hot working.

  • Dimensional Accuracy: Higher due to no thermal distortion.

  • Increased Deformation Load: Requires higher forming forces/press capacity.

3.2 Rolling Mills - Three-High Roll Mill

• Construction: Three rolls arranged vertically. Top and bottom rolls are driven (same direction). Middle roll is idler (friction-driven).

• Operation:

  1. Workpiece passes above middle roll, between top & middle rolls (first pass).

  2. Rollers reverse.

  3. Workpiece passes below middle roll, between middle & bottom rolls (second pass).

• Advantage: Allows thick starting material to be reduced in one setup (two passes) without lifting the top roll. Used for heavy plates/sections.

• Disadvantage: Requires reversing drive; friction on middle roll limits maximum reduction per pass.

3.3 Forging Die Design Principles

• Die Materials: High-alloy tool steels (e.g., H13, H11) for hot forging; high-carbon steels for cold forging. Must have hot strength, toughness, wear resistance.

• Parting Line: The interface between die halves. Should be:

  • On a single flat plane if possible.

  • Located on a surface of the forging (not a critical functional surface).

  • Minimize flash (excess material) and facilitate easy ejection.

• Draft Angle: Taper on vertical die walls (typically 3°-7° for hot, 1°-3° for cold) to allow forging ejection without damage.

• Flash Design: The excess material squeezed out at parting line. Flash land (thin section) controls pressure; flash gutter collects excess. Ensures complete die fill and trims easily.

• Other Principles: Proper radius at corners (avoid sharp corners), rib design (height-to-thickness ratio), boss design, hole placement (avoid deep, small holes).

IV. TOOLING, WORKHOLDING & GAUGING

4.1 Jigs vs. Fixtures

4.2 Core Design Principles: Location, Clamping, Support

1. Location (Positioning): Restricting all 6 DOF of workpiece.

   • 6-Point Principle: Use 3 points to locate in a plane (prevents 3 translations), 2 points in a perpendicular plane (prevents 2 rotations), 1 point along the axis (prevents last rotation).

   • Datum Concept: Use primary, secondary, tertiary datum surfaces for sequential location.

2. Clamping: Applying force to secure workpiece against locators during cutting. Must:

   • Be strong enough to resist cutting forces.

   • Apply force close to machining point.

   • Not distort or damage workpiece.

   • Be quick-acting (e.g., toggle clamps, pneumatic).

3. Support: Providing rigid backing to prevent deflection/vibration. Includes rests, pads, supports.

4.3 Thread Snap Gauges (GO/NO-GO Principle)

• Purpose: Quick inspection of external thread (bolt) dimensions (pitch diameter).

• GO Gauge: Checks maximum material condition (MMC). Must fully engage (screw on) the thread without excessive force. If it goes, thread is not too small.

• NO-GO Gauge: Checks minimum material condition (LMC). Must not engage more than two turns (specified). If it engages, thread is too large (oversize).

• Design Features:

  • Made of hardened, ground steel.

  • Full-form (entire thread profile) or segment-type (3-4 teeth).

  • Tolerance: GO gauge = + tolerance on MMC; NO-GO = - tolerance on LMC.

  • Length: Typically 1.5 to 2 times thread diameter for engagement.

V. SPECIALIZED MANUFACTURING PROCESSES

5.1 Flow Molding (Flow Forming)

• Definition: A cold forming process where a rotating metal tube (blank) is axially compressed by rollers against a mandrel, causing the material to flow and thin-wall into a conical or cylindrical shape.

• Principle of Operation:

  1. Rotating mandrel holds the workpiece (tube).

  2. 1-3 rollers apply axial feed force and radial pressure.

  3. Material flows in the direction of roller feed, reducing wall thickness and increasing length/diameter.

• Typical Applications: Symmetrical, axisymmetric components: rocket motor cases, pressure vessels, automotive wheels, gun barrels, seamless tubes.

• Comparison with Conventional Forming:

  • vs. Deep Drawing: Flow forming produces long, thin-walled parts; deep drawing produces cup-shaped parts.

  • vs. Extrusion: Flow forming is incremental (rollers move); extrusion is single stroke.

  • Advantages: High strength (cold working), exentricity control, no welding (monolithic), material savings, good surface finish.

  • Disadvantages: Slow process, high tooling cost, limited to rotational symmetry.

[!IMPORTANT] Exam Strategy for Unit 4:

1. Kinematics & Ray/Speed Diagrams are highly mathematical/graphical. Practice drawing ray diagrams for common speed ranges (e.g., R=8, 12, 16).

2. Jig/Fixture & Die Design are application-based. Use 6-point principle and draft angle as universal answers.

3. Definitions are critical: Always start answers with a clear, textbook definition (e.g., "Machine tool is a power-driven...").

4. Diagrams are mandatory for: Ray Diagram, Three-High Mill, Forging Die (parting line, draft), Thread Snap Gauge, Flow Forming. Practice neat, labeled sketches.

5. Compare & Contrast questions (Ray vs Speed, Jig vs Fixture, Cold vs Hot) require a tabular or point-wise structure for full marks.