Machine Tool Design (ME-802 (C)) - Unit 2 Short Notes

Unit 2: Design of Machine Tools and Associated Systems

I. Fundamentals of Machine Tools

Definition and Classification

• Machine Tool: A power-driven, non-portable machine used for cutting, shaping, boring, or otherwise removing metal to produce precise, interchangeable parts.

• Classification:

  • By Function: Turning (lathe), Drilling, Milling, Grinding, Gear cutting, Boring, Sawing.

  • By Control: Conventional (manually operated), Semi-automatic, Automatic (mechanical/ hydraulic/ electronic control).

  • By Purpose: General-purpose, Special-purpose, Modular/ Flexible (CNC).

Cold Working of Metals

• Process: Deformation of metal below its recrystallization temperature (typically room temperature). Processes include rolling, forging, drawing, extrusion, stamping.

• Advantages:

  • Improved surface finish and dimensional accuracy.

  • Increased strength and hardness due to strain hardening.

  • No oxidation or scaling (no heating cost).

  • Better surface integrity (no thermal distortion).

• Design Implications:

  • Requires higher forming forces/pressures.

  • Machines must be rigid and robust to withstand high loads.

  • Tooling must be made from high-strength, wear-resistant materials.

  • Part geometry is limited by formability and ductility of the cold-worked metal.

[!TIP] Exam Focus: Be prepared to contrast cold working with hot working (above recrystallization temp) regarding forces, properties, and machine design requirements.

II. Kinematic Design and Speed Transmission Systems

Kinematical Structures of Machine Tools

• Kinematic Chain: A series of links (rigid bodies) connected by joints (pair elements) to transmit motion from the input (motor) to the tool/work.

• Kinematical Features: Number of degrees of freedom, type of motion (rotary/reciprocating), motion sequence, and transmission ratio.

• Classification of Chains:

  | Type | Description | Example Application | | :--- | :--- | :--- | | Simple Chain | Single path from input to output. | Basic lathe spindle drive. | | Compound Chain | Multiple paths; motion combines. | Milling machine table feeds. | | Revolute Chain | All joints are rotary (hinged). | Most gear trains. | | Prismatic Chain | All joints are sliding. | Some shaper mechanisms. | | R-R Chain | Mix of rotary and prismatic joints. | Most machine tool structures. |

Need for Speed Variation

• Operational Requirements: Different materials, tool materials, and operations (roughing vs. finishing) require different cutting speeds (Vc).

• Design Considerations:

  • Wide Speed Range: $$\displaystyle N_{max}/N_{min} $$ should be sufficient (often 4:1 to 16:1 or more).

  • Steps: Discrete speeds (geared) or continuous (variable speed drives).

  • Power Availability: Power $P \propto T \times N$. Speed change must maintain adequate torque at low speeds.

  • Structure Rigidity: Low-speed, high-torque transmission must not deflect.

Ray Diagrams

• Definition: A graphical method to determine the range of spindle speeds and feeds obtainable from a given gearbox or transmission system.

• Construction:

  1. Draw vertical lines for each shaft (motor, intermediate, spindle).

  2. Plot speeds (rpm) on each shaft line, usually on a logarithmic scale.

  3. Connect all possible speed combinations with straight lines (rays) from the motor shaft to the spindle shaft through intermediate shafts.

• Interpretation:

  • The envelope of rays on the spindle line shows the achievable speed range.

  • Overlapping rays indicate duplicate speeds (inefficient design).

  • Optimal design aims for a continuous envelope without large gaps.

• Application: Primary tool for synthesizing and analyzing gear train layouts in headstocks and feed boxes.

Speed Diagrams

• Distinction from Ray Diagram: A speed diagram plots the actual rotational speeds (rpm) of all shafts in the transmission system against shaft position. It shows the speed step-up/down at each gear pair.

• Practical Usage:

  • Used for detailed design and calculation of gear ratios.

  • Helps in selecting gear sizes and checking for interference.

  • Complements the ray diagram: Ray diagram shows what is possible, speed diagram shows how it is achieved.

Selection of Speeds, Feeds, and Power

• Criteria for Low-Speed Selection:

  • Must provide sufficient torque for heavy roughing cuts.

  • Avoid chatter (resonance) in the machine structure.

  • Maintain minimum recommended cutting speed for tool material (e.g., HSS, carbide).

• Feed Selection:

  • Based on tool geometry, workpiece material, depth of cut, and surface finish requirement.

  • Must be within the capacity of the feed mechanism (power, rigidity).

• Power Requirement:

  • Calculate maximum power for the heaviest planned cut: $$\displaystyle P_c = \frac{V_c \times f \times d \times K}{60 \times 1000} $$ kW, where $K$ is specific cutting force.

  • Select motor power 20-30% higher than $$\displaystyle P_c $$ to account for losses and overloads.

[!TIP] Common Pitfall: Students often confuse Ray Diagram (graphical speed range synthesis) with Speed Diagram (shaft-by-shaft speed calculation). Remember: Ray = Possibility Map, Speed = Calculation Sheet.

III. Workholding and Locating Principles

Principles of Location

• Degrees of Freedom (DOF): A rigid body in 3D space has 6 DOF (3 translations, 3 rotations).

• Locating Points/Constraints: A fixture/jig must restrict all 6 DOF to prevent movement during machining.

  • 3-2-1 Principle: Common method using 3 points on one plane (restrains Z trans, X & Y rot), 2 points on a perpendicular plane (restrains Y trans, X rot), 1 point on the third plane (restrains X trans).

• Datum System: The reference surfaces/points on the workpiece against which all other dimensions are measured. Must be stable, accessible, and repeatable.

Jigs and Fixtures

• Design Aspects (Common): Locating accuracy, clamping force (non-damaging), rigidity, chip clearance, operator safety, ease of loading/unloading, modularity.

IV. Design of Specific Machine Tools and Gauges

Rolling Mills: Three-High Roll Mill

• Construction: Three rolls arranged vertically: top, middle, bottom. Middle roll is driven, top and bottom rolls are idler (rotate due to friction).

• Operation:

  1. Workpiece passes first between top & middle rolls (reduction).

  2. Then between bottom & middle rolls (second reduction).

  3. Rolls rotate in same direction (top & bottom opposite to middle).

• Advantages over Two-High: Can roll thinner sections in one pass (two reductions). No need to reverse workpiece.

• Design Feature: Middle roll is smaller diameter than top/bottom to allow for roll gap adjustment.

Forging Dies

• Design Principles:

  1. Parting Line: Should be single, flat plane for easy die separation. Avoid steps.

  2. Draft: Sufficient taper (1°-3°) on all vertical surfaces for easy ejection.

  3. Fillet Radii: Use generous fillets to reduce stress and improve metal flow.

  4. Ribs & Bosses: Should be low and wide to avoid filling issues.

  5. Flash: Allowance for excess material (flash) to ensure cavity fill. Flash gutter must be designed.

• Material Considerations: Tool steels (e.g., H13, H11) for high-temperature strength, toughness, and wear resistance. Heat treatment is critical.

Thread Snap Gauges

• Purpose: Go/No-Go gauge for rapid inspection of external thread pitch diameter.

• Design Features:

  • Go Gauge: Checks maximum material condition (smallest pitch diameter). Must fully engage with thread.

  • No-Go Gauge: Checks minimum material condition (largest pitch diameter). Must not engage more than a few turns (typically 2-3).

  • Made from hardened, ground steel. Thread form is truncated (not full V) to be sensitive to pitch diameter variations.

• Types: Solid (single piece) or split (adjustable for wear).

Flow Molding (Flow Forming)

• Process: A rotational cold forming process. A cylindrical blank is held between a mandrel (spindle) and a rolling tool. The tool axially advances, thinning and elongating the blank wall while maintaining constant diameter.

• Design Integration:

  • Machine requires high spindle rigidity and precise axial feed.

  • Mandrel shape determines internal geometry of the final part.

  • Used for axisymmetric, thin-walled components (e.g., pressure vessels, automotive wheels, rocket motor cases).

V. Drives and Control Systems

Electrical Drives for Machine Tools

• Types:

  • DC Drives: Ward-Leonard (constant torque, wide speed range), DC Motor with Thyristor Converter (precise, fast response). Advantage: Easy speed control. Disadvantage: Commutator maintenance.

  • AC Drives: Squirrel Cage Induction Motor with VFD (Variable Frequency Drive). Advantage: Rugged, low maintenance. Disadvantage: Historically less torque at low speed (improved with vector control).

  • Servo Drives: AC/DC Servo Motors with feedback (encoder/resolver). For positioning accuracy (CNC axes).

• Selection Criteria:

  1. Speed-Torque Characteristic: Match to machine requirement (constant power vs. constant torque).

  2. Duty Cycle: Continuous vs. intermittent operation.

  3. Precision & Response: Needed for CNC axes vs. simple spindle drive.

  4. Environment: Dust, coolant, explosion-proof requirements.

  5. Cost & Maintenance: DC higher maintenance; AC/VFD lower.

• Integration: Drive must be synchronized with kinematic chains (e.g., spindle drive motor coupled to gearbox). CNC systems integrate drives via closed-loop feedback.

\boxed{\text{Key Exam Topics: Ray Diagram Construction, 3-2-1 Locating Principle, Jig vs Fixture, Three-High Roll Mill Operation, Forging Die Draft, Thread Snap Gauge Types}}