Tools

Bench Work

Course: MEM103 - Workshop Practice
Module: Bench Work Fitting & Metrology

1. Introduction to Metrology

Metrology is the science of measurement. In a mechanical workshop, accurate measurement is the foundation of mass production, quality control, and interchangeability.

1.1 Fundamental Concepts

  • Interchangeability: The ability of parts to be substituted for one another without the need for custom fitting or adjustment during assembly. This concept enabled the Industrial Revolution and mass production.
  • Accuracy: The degree of closeness of a measured value to the true or accepted reference value.
  • Precision: The degree of repeatability; how close multiple measurements of the same feature are to each other.
  • Least Count (Resolution): The smallest difference between two indications that can be detected by the measuring instrument.
  • Parallax Error: An observational error caused when the scale and the object being measured are not on the same plane, or when viewed from an angle.

2. Linear Measurement Tools

2.1 The Steel Rule

The most basic line-graduated measuring tool.

  • Construction: Made of spring steel or stainless steel.
  • Graduations: Typically 0.5 mm or 1 mm (Metric) and 1/64 inch (English).
  • Usage Rule: Never measure from the end of the rule (which may be worn). Start from the 1.0 cm or 1-inch mark for accuracy.

2.2 Vernier Caliper

A precision instrument based on the principle of alignment between two scales: the main scale and the sliding vernier scale.

[Image of vernier caliper parts diagram]

  • Functions: Measures Outside Diameter (OD), Inside Diameter (ID), and Depth.
  • Least Count Calculation (Metric):
    • Value of 1 Main Scale Division (MSD) = $1.0 \text{ mm}$
    • Value of 1 Vernier Scale Division (VSD) = $0.98 \text{ mm}$ (typically 49mm divided into 50 parts)
    • Least Count = 1 MSD - 1 VSD = $1.0 - 0.98 = \mathbf{0.02 \text{ mm}}$
  • Least Count (English): Typically $\mathbf{0.001 \text{ inch}}$.

2.3 Micrometer (Screw Gauge)

A device providing higher precision than calipers, utilizing the screw and nut principle for magnification.

  • Construction:
    • Frame: C-shaped body.
    • Anvil & Spindle: Measuring faces (often carbide-tipped).
    • Sleeve (Barrel): Stationary scale.
    • Thimble: Rotating scale.
    • Ratchet Stop: Ensures consistent measuring pressure to prevent error and damage.
  • Least Count Calculation (Metric):
    • Pitch of Spindle Screw = $0.5 \text{ mm}$
    • Divisions on Thimble = $50$
    • Least Count = Pitch / Total Divisions = $0.5 / 50 = \mathbf{0.01 \text{ mm}}$
  • Types:
    • Outside Micrometer: For external thickness.
    • Inside Micrometer: For internal bores (uses extension rods).
    • Depth Micrometer: For hole depths (Scale reads in reverse).

2.4 Vernier Height Gage

A vertical layout tool used on a surface plate to mark horizontal lines at precise heights.

  • Mechanism: Similar to a vernier caliper but mounted on a heavy base.
  • Usage: Often equipped with a tungsten carbide scriber for marking or a dial indicator for inspection.

3. Angular Measurement Tools

3.1 Engineer’s Try Square

Used for checking the squareness ($90^\circ$) of a workpiece.

  • Parts: A thick Stock (beam) and a thin Blade.
  • Checking Method: The "Light Gap" method—placing the square against the work and looking for light passing through.

3.2 Bevel Protractor

A graduated circular protractor with a pivoting arm used for measuring angles.

  • Universal Bevel Protractor: Includes a vernier scale to read angles to an accuracy of 5 minutes (1/12th of a degree).

3.3 Sine Bar

A precision tool used for setting angles based on trigonometry.

  • Construction: A steel bar with two precision ground rollers at a fixed distance ($L$).
  • Principle: It forms the hypotenuse of a right-angled triangle. Gage blocks are used to build the height ($H$).
  • Formula: $\sin(\theta) = \frac{H}{L}$

4. Transfer & Comparison Tools

These tools do not provide a direct reading but transfer a dimension to a standard scale.

  • Calipers (Joint Type):
    • Outside Caliper: Transfers external dimensions.
    • Inside Caliper: Transfers internal dimensions.
    • Hermaphrodite (Jenny) Caliper: Has one bent leg and one straight point. Used to find the center of round bars or scribe lines parallel to an edge.
  • Telescoping Gage: A T-shaped tool with spring-loaded plungers used to measure the internal diameter of deep bores. The plunger is locked inside the bore and then measured with a micrometer.
  • Small Hole Gage: Uses a split-ball mechanism expanded by a wedge to measure small holes.
  • Feeler Gage: A set of steel blades of specific thicknesses used to measure gap widths or clearance between mating parts.
  • Dial Indicator: Converts small linear displacements into rotary motion of a pointer. Used for checking run-out, flatness, and parallelism.

5. Layout (Marking Out) Theory

Layout is the process of transferring drawing dimensions onto the workpiece surface prior to machining.

5.1 The Datum

A reference surface or edge from which all measurements are taken. A common practice is to file two adjacent edges flat and square ($90^\circ$) to create a datum corner.

5.2 Layout Tools

  • Surface Plate: A massive, precision-flat plate (usually Granite) that serves as the primary horizontal reference plane. Granite is preferred over cast iron because it is thermally stable, rust-proof, and does not burr when chipped.
  • Layout Dye (Prussian Blue): A blue ink applied to the metal surface to provide contrast, making fine scribed lines visible.
  • Scriber: A sharp, hardened steel tool used to scratch lines into the dye.
  • Surface Gage: A base with an adjustable spindle and scriber, used to scribe lines at a specific height parallel to the surface plate.
  • V-Blocks: Precision steel blocks with $90^\circ$ V-grooves, used to hold cylindrical workpieces.
  • Angle Plate: An L-shaped plate used to hold workpieces perpendicular to the surface plate.

6. Punches & Hammers

Scribed lines are temporary and can wash away. Punches create permanent physical indentations.

  • Prick Punch: Has a sharp $30^\circ$ point. Used for "witness marks" along layout lines to make them permanent.
  • Center Punch: Has a $90^\circ$ point. Used to create a large indentation to seat a drill bit, preventing it from wandering during the start of drilling.
  • Ball Peen Hammer: The standard metalworking hammer. The flat face is for striking punches; the hemispherical (peen) face is for riveting.

7. General Procedure for Bench Work

  1. Preparation: Clean workpiece and surface plate. Remove burrs using a file.
  2. Datum: File two adjacent edges perfectly flat and square ($90^\circ$) to each other. These are the reference edges.
  3. Coating: Apply layout dye.
  4. Marking:
    • Use Height Gage or Surface Gage to mark horizontal and vertical lines relative to the datum.
    • Use Dividers for arcs/circles.
    • Use Bevel Protractor for angular lines.
  5. Punching: Use Prick Punch on lines and Center Punch on hole centers.
  6. Verification: Double-check all dimensions with a Steel Rule or Caliper before cutting.