Tools

Bench Work

Procedure

Department: Mechanical Workshop
Subject: MEM103 - Workshop Practice

1. Objective

To perform precision layout, fitting, and inspection of a mechanical component, verifying dimensions and geometry using standard metrology instruments (Vernier Calipers, Micrometers, Height Gages) and layout tools.

2. Equipment & Tools Required

  • Measurement: Steel Rule, Vernier Caliper, Micrometer (0-25mm), Depth Micrometer.
  • Layout/Marking: Surface Plate, Vernier Height Gage, Surface Gage, V-Blocks, Angle Plate, Scriber, Dividers/Trammels, Layout Dye (Prussian Blue).
  • Marking Permanence: Dot Punch, Center Punch, Ball Peen Hammer.
  • Checking: Engineer’s Try Square, Bevel Protractor, Dial Indicator.
  • Hand Tools: Files (Flat, Bastard, Smooth), Hacksaw (if material removal is required).

3. Procedure

Phase 1: Material Inspection & Preparation

  1. Raw Material Check: Use the Steel Rule to check the raw stock dimensions against the drawing to ensure sufficient material allowance for facing and squaring.
  2. Deburring: Remove any sharp edges, burrs, or rust from the workpiece using a file to ensure it sits flat on the surface plate.
  3. Cleanliness: Wipe the Surface Plate and the workpiece with a clean cloth. Dust or grit can compromise measurement accuracy (Tutorial 14).

Phase 2: Establishing Datum Surfaces (The Reference)

Bench work accuracy relies on a perfect "Datum" (reference edge).

  1. File Reference Edge A: File one broad surface flat. Check flatness using a Straight Edge or by placing it on the surface plate.
  2. File Reference Edge B: File an adjacent edge perfectly straight.
  3. Squareness Check: Use the Try Square (Tutorial 12) to check the angle between Surface A and Edge B. Light should not be visible between the blade and the workpiece.
    • Correction: If light is visible at the top or bottom, refine filing until perfect 90° squareness is achieved.
  4. Mark the Datum: Mark this corner as the reference datum (O) from which all measurements will be taken.

Phase 3: Layout & Marking (Scribing)

  1. Apply Layout Dye: Coat the surface with a thin, even layer of layout dye (wet chalk or Prussian Blue) to make scribed lines visible (Tutorial 11).
  2. Setup: Place the workpiece on the Surface Plate. Use an Angle Plate if vertical stability is needed.
  3. Horizontal Markings:
    • Set the Vernier Height Gage (or Surface Gage) to the required dimension relative to the datum base.
    • Hold the base firmly against the surface plate and scribe the horizontal lines across the face of the work.
  4. Vertical Markings:
    • Turn the workpiece 90° so the second datum edge rests on the surface plate.
    • Reset the Height Gage/Surface Gage and scribe intersecting lines to locate hole centers or slot widths.
  5. Geometric Features:
    • Use Dividers to mark radii or circles. Place one leg in a punch mark (center) and scribe the arc.
    • Use Hermaphrodite Calipers to scribe lines parallel to an edge if high precision is not strictly required.
  6. Angular Marking:
    • Use a Bevel Protractor (Tutorial 10) to mark any required angles (chamfers or tapers) relative to the datum edge.

Phase 4: Witness Marking (Punching)

  1. Prick Punching: Use a Prick Punch (30° point) to lightly mark the intersection of layout lines. This ensures the lines remain visible if the dye washes off during machining.
  2. Center Punching: Use a Center Punch (90° point) at the intersection of hole locations. This provides a seat for the drill bit to prevent wandering (Tutorial 16).
  3. Accuracy Check: Verify the distance between punch marks using Dividers or a Rule before proceeding to cutting/drilling.

Phase 5: Machining/Fitting (Material Removal)

(Standard Bench Fitting operations)

  1. Perform sawing, drilling, or filing operations as per the layout lines.
  2. Use Safe-edge files when filing near shoulders to protect adjacent surfaces.

Phase 6: Final Inspection & Metrology

  1. Linear Dimensions:
    • Use the Vernier Caliper (Tutorial 4) for general external/internal dimensions (±0.02mm).
    • Use the Outside Micrometer (Tutorial 5/6) for high-precision external thicknesses (±0.01mm/0.001").
  2. Depth Inspection:
    • Use the Depth Micrometer (Tutorial 7) to check blind holes or slot depths. Ensure the base sits flush on the reference surface.
  3. Geometric Tolerance:
    • Flatness: Check surfaces using a Dial Indicator mounted on a stand while moving the workpiece beneath it.
    • Squareness: Re-verify corners with the Master Square or Cylindrical Square.
    • Parallelism: Use Adjustable Parallels or measure width at multiple points.
  4. Bore Inspection:
    • Use a Telescoping Gage (Tutorial 8) for internal bores. Lock the gage inside the hole and measure the spread with a micrometer.

4. Maintenance & Storage

  1. Clean: Remove layout dye from the workpiece using solvent.
  2. Oil: Apply a thin film of anti-rust oil to all steel tools (micrometers, height gages, surface plate).
  3. Store: Separate cutting tools (files) from measuring instruments to prevent damage to precision surfaces. Store Micrometers with the spindle slightly open (not locked).