Machine Tool Design (ME-802 (C)) - Unit 5 Short Notes

UNIT 5: MACHINE TOOL DESIGN

Based on analysis of ME-802(C) Machine Tool Design past examination papers (specifically May 2022), the following short notes cover the frequently tested topics.

I. FUNDAMENTALS OF MACHINE TOOLS

Definition & Classification:

A machine tool is a power-driven, non-portable machine used for shaping, cutting, or forming metal (or other rigid materials) by removing material (machining) or by pressure (forming). They are classified primarily by:

• Method of operation: Cutting (e.g., lathe, drill, mill) vs. Forming (e.g., press, forge, roll).

• Control: Conventional (manual) vs. Numerical Control (NC/CNC).

• Function: General purpose (e.g., centre lathe) vs. Special purpose (e.g., gear hobbing machine).

Kinematical Structures & Features:

• Kinematical Structure: Refers to the arrangement and sequence of kinematic chains (series of links and joints) that provide the required motions (primary cutting motion, feed motion, and auxiliary motions) to the tool and workpiece. It is represented by a kinematic diagram.

• Kinematical Features: These are the inherent motion characteristics of a machine tool, defined by:

  1. Number of Basic Motions: Typically 1 primary (cutting) and 1-2 feed motions.

  2. Kinematic Chain Arrangement: Series, parallel, or mixed.

  3. Range of Speed & Feed: The minimum and maximum achievable values.

  4. Power Transmission Path: From motor to spindle/tool.

[!TIP] Exam Focus: Questions often ask to discuss structures and features together. Be prepared to draw a simple kinematic diagram for a lathe (e.g., motor -> belt/pulley -> spindle) and list its 3-4 key features.

II. KINEMATIC ANALYSIS AND SPEED VARIATION

Need for Speed Variation:

Different work materials, tool materials, cutting operations, and desired surface finish require different cutting speeds (m/min). A machine tool must provide a wide and stepped range of spindle speeds to optimize tool life, power consumption, and production rate.

Ray Diagram (Speed Diagram):

• Definition: A graphical representation (on a logarithmic scale) of the speed range of a machine tool spindle. It plots Speed (N) on the Y-axis and Number of Speeds (n) on the X-axis. Each step is a horizontal line.

• Construction: Speeds are arranged in geometric progression: \( N_n = N_1 \times \phi^{(n-1)} \), where \( \phi \) (phi) is the speed ratio or progression ratio.

• Application: Used to determine the number of speeds (\( n \)), minimum speed (\( N_{min} \)), maximum speed (\( N_{max} \)), and the progression ratio \( \phi \). The ideal \( \phi \) for a 12-speed range is approximately 1.41 (since \( \phi = \sqrt[11]{\frac{N_{max}}{N_{min}}} \)).

Speed Diagram (Feed/ Power Diagram):

• Definition: A similar logarithmic plot used for feed rates (mm/rev) or power (kW) requirements across different speeds/operations.

• Interpretation: Shows how feed or power varies with speed. Often, feed is inversely proportional to speed for constant material removal rate.

Distinction: Ray Diagram vs. Speed Diagram

Selection of Low Speed, Feed, and Power:

The lowest spindle speed (\( N_{min} \)) is chosen based on:

1. Tool Size: Larger diameter tools (e.g., large drills, boring bars) require lower speeds to avoid vibration.

2. Workpiece Material: Harder materials need lower speeds.

3. Desired Surface Finish: Finer finishes often require lower speeds.

4. Rigidity: For long, slender workpieces, low speed reduces chatter.

Feed and Power at \( N_{min} \) are selected considering the maximum material removal rate possible without overloading the machine's structure or drive.

[!TIP] Common Pitfall: Students confuse "Ray Diagram" and "Speed Diagram." Remember: Ray Diagram is specifically for spindle speeds. The term "Speed Diagram" is sometimes used synonymously, but in exam context, "Speed Diagram" often refers to feed/power variation.

III. TOOLING: JIGS, FIXTURES, AND GAUGES

Jigs vs. Fixtures:

Principles of Location (3-2-1 Principle):

To fully constrain a workpiece (prevent 6 degrees of freedom: 3 translational, 3 rotational), use at least 6 locating points in a specific arrangement:

• 3 points on one plane (e.g., surface plate) to restrict Z-translation & X,Y-rotation.

• 2 points on a perpendicular plane to restrict Y-translation & X-rotation.

• 1 point on the third perpendicular plane to restrict X-translation & Z-rotation.

This is the minimum requirement for positive location. Clamping provides the necessary holding force.

Design Features of Thread Snap Gauges:

A snap gauge is a GO/NO-GO gauge for checking external threads (e.g., bolts).

• GO Gauge: Checks Maximum Material Condition (MMC). It must snap over the thread completely. Its pitch diameter is equal to the minimum limit of the thread's pitch diameter.

• NO-GO Gauge: Checks Minimum Material Condition (LMC). It must not engage more than 2-3 turns. Its pitch diameter is equal to the maximum limit of the thread's pitch diameter.

• Construction: Typically has two anvils (for GO) or one anvil and a V-block (for NO-GO) hardened and ground to precise thread form.

IV. METAL FORMING PROCESSES AND EQUIPMENT

Cold Working of Metals:

• Process: Deformation of metal below its recrystallization temperature (usually room temperature). Includes processes like cold rolling, cold drawing, cold forging, bending.

• Effects on Properties:

  • Increased Strength & Hardness: Due to strain hardening (dislocation density increases).

  • Decreased Ductility & Toughness.

  • Improved Surface Finish.

  • Dimensional Accuracy.

• Applications: Manufacturing of sheets, strips, wires, precision fasteners (bolts, nails), and components requiring high strength and good surface finish.

Rolling Mills: Three High Roll Mill:

• Construction: Consists of three rolls arranged one above the other in a triangular frame. The middle roll rotates in the opposite direction to the two outer rolls.

• Operation:

  1. The workpiece (e.g., ingot, slab) passes first between the top and middle rolls.

  2. It is then returned (by hand or conveyor) to pass second between the middle and bottom rolls.

  3. This allows two reductions in a single pass-through of the mill, as the workpiece is turned over between passes.

• Advantage: Higher throughput compared to two-high mills for the same roll diameter, as it achieves two passes per cycle.

• Disadvantage: Requires turning mechanism for the workpiece.

Forging Dies: Design Principles:

1. Parting Line: Should be on a single, flat plane to simplify die making and forging. Avoids flash on functional surfaces.

2. Draft: Sufficient taper (1°-5° for steel) on all vertical surfaces to facilitate easy ejection of the forged part.

3. Fillet & Corner Radii: Generous radii to reduce stress concentration in die and improve metal flow.

4. Flash: A thin excess metal (flash) is allowed in a flash gutter around the parting line. It controls excess metal, improves die filling, and acts as a cushion.

5. Balance: Die cavity should be balanced to ensure even filling and reduce die wear.

Flow Molding (Flow Forming):

• Process: A rotational forming process where a pre-formed tube or blank (often from a pressed or spun shell) is held between a mandrel and rollers. The rollers apply localized pressure, causing the workpiece material to flow axially and thin over the mandrel, taking its shape.

• Key Feature: The mandrel determines the internal diameter/surface, while rollers shape the external surface.

• Applications: Manufacturing of seamless, high-strength, thin-walled cylindrical components like rocket motor cases, pressure vessels, automotive wheels, and artillery shells.

V. DRIVES AND CONTROLS

Electrical Drives for Machine Tools:

An electrical drive system consists of an electric motor, a control system, and a transmission to the spindle/axes.

• Types & Characteristics:

  1. DC Motor Drives:

     • Characteristics: Easy speed control (wide range, smooth) via armature voltage/field flux. High starting torque.

     • Applications: Historically used for main spindle drives requiring wide speed range (e.g., large lathes, milling machines). Now largely replaced by AC drives.

  2. AC Motor Drives (Induction Motor with VFD):

     • Characteristics: Constant torque up to base speed, constant power above base speed (field weakening). Speed controlled by Variable Frequency Drive (VFD) which changes supply frequency. Rugged, low maintenance.

     • Applications: Dominant today for both spindle drives and feed drives in CNC machine tools.

  3. Stepper Motor Drives:

     • Characteristics: Open-loop control. Rotates in discrete steps. Position is determined by number of pulses. Low to medium torque, can lose steps under overload.

     • Applications: Feed drives in simple, low-cost CNC systems and open-loop positioning.

  4. Servo Motor Drives (AC or DC):

     • Characteristics: Closed-loop control with feedback (encoder/resolver). High precision, high dynamic response, high torque at low speeds.

     • Applications: High-precision axis drives in CNC machine tools and robotics.

• Selection Criteria: Based on required speed range, torque profile, precision, dynamic response, and cost.

[!TIP] Exam Trend: Questions often ask to "Discuss electrical drives" or "What are the types?" Focus on the modern context (AC VFD + Servo) and contrast with older DC systems. Know the key application for each type.