1.1 Mechanical Drawing

1.1 Mechanical Drawing

Introduction to Mechanical Drawing

Mechanical Drawing is the universal graphical language of engineering, used to communicate design intent, dimensions, tolerances, and manufacturing instructions with precision and clarity. It transforms abstract ideas into tangible specifications for fabrication, assembly, and quality control. This unit covers the core elements of this language—from standardized conventions for representing machine components and permanent joints like welds and rivets, to the detailed specification of temporary fasteners, assemblies, and the critical systems of limits, fits, and surface texture that ensure components work together as intended.

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1. Machine Drawing

Machine Drawing involves creating detailed, dimensioned, and annotated 2D or 3D representations of machine parts and assemblies. It serves as the primary communication tool between design, manufacturing, and inspection departments.

1.1 Principles and Conventions

1. Orthographic Projection (First Angle vs. Third Angle):

   • First Angle Projection: The object is placed between the observer and the projection plane. Commonly used in Europe and Asia.

   • Third Angle Projection: The projection plane lies between the observer and the object. Standard in the USA, Canada, and Australia.

   • Key Rule: The view from the front is the most important; other views (Top, Side) are aligned with it.

2. Sectional Views:

   • Purpose: To reveal internal features of an object that are not visible in external views.

   • Cutting Plane Line: Indicates where the imaginary cut is made.

   • Section Lines (Hatching): Lines at 45° used to indicate the solid material that has been cut.

   • Types: Full section, Half section, Offset section, Revolved section, Removed section.

3. Auxiliary Views:

   • Purpose: To show the true shape and size of an inclined or oblique surface that is not parallel to any principal projection plane.

4. Standard Drawing Elements:

   • Title Block: Contains part name, drawing number, material, scale, tolerance, and approval details.

   • Revision Block: Tracks changes to the drawing.

   • Notes and Specifications: General instructions, heat treatment, finishing notes.

   • Bill of Materials (BOM): List of all parts/assemblies in a tabular format.

1.2 Dimensioning

1. Fundamental Rules:

   • Dimensions should be placed on the view that best shows the contour of the feature.

   • Avoid dimensioning to hidden lines.

   • Each dimension should be given only once.

   • Dimensions are given in millimeters (mm) without units, or inches (in) with notation.

2. Types of Dimensions:

   • Functional Dimensions: Critical for the part's operation.

   • Non-functional Dimensions: For manufacturing convenience.

   • Auxiliary Dimensions: Given for reference only, in parentheses.

3. Dimensioning Systems:

   • Chain Dimensioning: Dimensions are placed in a sequence. Accumulates tolerance.

   • Datum Dimensioning (Baseline): All dimensions originate from a common datum. Prevents tolerance accumulation.

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2. Welded Joints

Welding is a process of joining metals by fusion. The joint and weld type must be clearly specified on drawings using standardized symbols.

2.1 Types of Welded Joints

1. Butt Joint: Members in the same plane, joined at their edges.

2. Lap Joint: One member overlaps the other.

3. T-Joint: One member is approximately perpendicular to the other, forming a "T".

4. Corner Joint: Members meet at right angles at the corners.

5. Edge Joint: Edges of two or more parallel members are joined.

2.2 Basic Weld Types

1. Fillet Weld: Triangular cross-section, used for Tee, Lap, and Corner joints. Most common.

2. Groove Weld: Made in a groove between members (e.g., Square, V, U, J, Bevel grooves). Used for Butt joints.

3. Plug/Slot Weld: Weld metal is deposited in a hole or slot in one member, fusing it to the other member.

2.3 Welding Symbols (AWS/ISO Standard)

A welding symbol is a systematic way to communicate all information about a weld.

1. Reference Line: The foundation of the symbol. Essential data is placed on it.

2. Arrow: Points to the location of the weld.

3. Tail: Used for supplementary information (process, specification, etc.).

4. Basic Weld Symbol: Indicates the type of weld (e.g., triangle for fillet, various shapes for grooves). Placed on or below the reference line.

   • Symbol on Reference Line: Weld on the arrow side.

   • Symbol below Reference Line: Weld on the other side.

5. Dimensions: Leg length for fillets, depth of penetration for grooves, length and pitch of intermittent welds.

6. Supplementary Symbols: Indicate weld profile (convex, concave), field weld, weld all around, etc.

Example: A fillet weld symbol with a 6mm leg length on the arrow side and an 8mm leg on the other side, with a tail note "GMAW".

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3. Rivets and Riveted Joints

Riveting is a mechanical fastening method using a permanent, deformed shank. Common in older steel structures, boilers, and aircraft.

3.1 Rivet Components

1. Head: Pre-formed on one end (factory head).

2. Shank: The cylindrical body.

3. Tail: The end that is deformed with a tool to form the shop head after insertion.

3.2 Types of Riveted Joints

1. Based on Purpose:

   • Lap Joint: Plates overlap.

   • Butt Joint: Plates are aligned, covered with one or two cover plates (straps).

2. Based on Arrangement of Rivets:

   • Single Riveted: One row of rivets.

   • Double/Triple Riveted: Two or three rows.

   • Chain Riveting: Rivets in adjacent rows are opposite each other.

   • Zig-Zag (Staggered) Riveting: Rivets in adjacent rows are staggered.

3.3 Failure Modes of Riveted Joints

1. Shearing of Rivet: $P_s = n \times \frac{\pi d^2}{4} \times \tau$ (Single/Double shear).

2. Crushing of Plate or Rivet: $P_c = n \times d \times t \times \sigma_c$.

3. Tearing of Plate: $P_t = (p - d) \times t \times \sigma_t$. Where:

   • $n$ = Number of rivets per pitch length.

   • $d$ = Diameter of rivet hole.

   • $t$ = Thickness of the main plate.

   • $p$ = Pitch (distance between rivet centers along row).

   • $\tau, \sigma_c, \sigma_t$ = Allowable shear, crushing, and tensile stresses.

3.4 Efficiency of Joint

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4. Bolts, Nuts & Screw Fasteners

Temporary fasteners allowing for disassembly. Represented on drawings by simplified or schematic symbols, often with a note specifying the standard (e.g., ISO, DIN, ANSI).

4.1 Terminology

1. Bolt: A threaded fastener with a head, used with a nut.

2. Screw: A threaded fastener that mates with a pre-formed internal thread or creates its own thread.

3. Nut: A block with an internal threaded hole.

4. Washer: A flat ring used to distribute load, prevent loosening, or seal.

4.2 Thread Representation

1. Detailed: Shows true helical profile (time-consuming, used rarely).

2. Schematic: Uses alternating thick and thin lines to represent crests and roots.

3. Simplified: Uses continuous thick lines for crests and thin lines for roots (most common on engineering drawings).

4.3 Thread Designation

• Metric (ISO): M10 x 1.5 - 6g

  • M: Metric thread.

  • 10: Nominal diameter (mm).

  • 1.5: Pitch (mm). Coarse pitch can be omitted (e.g., M10).

  • 6g: Tolerance class (6 = tolerance grade, g = fundamental deviation for external thread).

• Unified (UNC/UNF): ¼ - 20 UNC - 2A

  • ¼": Nominal diameter.

  • 20: Threads per inch.

  • UNC: Unified National Coarse.

  • 2A: Class of fit (External thread, medium fit).

4.4 Common Fastener Types

1. Based on Head: Hex, Socket, Button, Flat, Pan.

2. Based on Drive: Slotted, Phillips, Hex Socket (Allen), Torx.

3. Based on Function: Cap Screw, Machine Screw, Set Screw, Stud, Locking Bolt.

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5. Keyways and Keyed Assembly

A key is a machine element inserted between a shaft and a hub (e.g., gear, pulley) to prevent relative rotation. The recesses cut into the shaft and hub are keyways.

5.1 Types of Keys

1. Sunk Keys: Most common type. Fits into keyways in both shaft and hub.

   • Rectangular/Square Parallel Key: Uniform width and height.

   • Gib-Head Key: Tapered on top surface, with a head for easy removal.

   • Feather Key: Allows axial movement (sliding) along the shaft while transmitting torque. Fastened to either shaft or hub.

2. Woodruff Key: Semi-circular disc. Fits into a circular keyway in the shaft, providing good alignment. Used in automotive and machine tools.

3. Saddle Key: Fits only on the shaft (no shaft keyway), relying on friction. For light loads.

4. Pin Key: Simple dowel pin or roll pin. For very light duty.

5.2 Forces and Design Considerations

• A key is subjected to shear and crushing (compressive) stresses.

• Shear Failure: $\tau = \frac{2T}{d \times w \times L}$

• Crushing Failure: $\sigma_c = \frac{2T}{d \times h' \times L}$ Where:

  • $T$ = Torque transmitted (N-mm).

  • $d$ = Shaft diameter (mm).

  • $w$ = Key width (mm).

  • $h'$ = Effective height for crushing = $h/2$ for rectangular key.

  • $L$ = Length of key (mm).

5.3 Drawing Representation

• Keyways are shown in sectional views of the shaft and hub.

• The key itself is typically shown in its installed position in an assembly section view.

• Dimensions include: key width ($w$), height ($h$), length ($L$), and chamfers.

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6. Tolerance, Limits and Fits

No component can be manufactured to an exact nominal dimension. Tolerance defines the permissible variation, and fits define the relationship between mating parts.

6.1 Terminology

1. Basic Size: The nominal size from which limits are derived.

2. Limits of Size: The two extreme permissible sizes.

   • Upper Limit: Maximum allowable size.

   • Lower Limit: Minimum allowable size.

3. Tolerance: The total permissible variation in size (Upper Limit - Lower Limit). It is always positive.

4. Deviation: The difference between a size and the basic size.

   • Upper Deviation (ES, es): Upper limit - Basic size.

   • Lower Deviation (EI, ei): Lower limit - Basic size.

5. Zero Line: In a graphical tolerance chart, the line representing the basic size.

6.2 Fits

The degree of tightness or looseness between two mating parts.

1. Clearance Fit: The shaft is always smaller than the hole. Always positive clearance.

   • Types: Loose running, Free running, Close running, Sliding, Locational clearance.

   • Application: Bearings, gears on shafts where free rotation is needed.

2. Interference Fit: The shaft is always larger than the hole. Always negative clearance (interference).

   • Types: Shrink/Force fit, Heavy drive, Light drive.

   • Application: Permanent assemblies like gear rims on wheels, crank webs.

3. Transition Fit: May provide either clearance or interference.

   • Types: Similar, Fixed, Press fit.

   • Application: Where accurate location is required, with possible disassembly (e.g., pulley on shaft).

6.3 ISO System of Limits and Fits

1. Fundamental Deviation: Letter symbol (lowercase for shaft, uppercase for hole) that defines the position of the tolerance zone relative to the zero line (e.g., h for shaft, H for hole where the fundamental deviation is zero).

2. IT Grade (International Tolerance Grade): Number that defines the magnitude of the tolerance zone (size of the tolerance). Smaller number = tighter tolerance. (e.g., IT7, IT8).

3. Fit Designation: Hole Basis System (common): 50H7/g6

   • 50: Basic size (mm).

   • H7: Hole tolerance. H = fundamental deviation (hole, zero line), 7 = IT grade.

   • g6: Shaft tolerance. g = fundamental deviation (shaft, above zero line), 6 = IT grade.

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7. Surface Finish

Surface finish (Surface Texture) specifies the characteristics of a part's surface, which affects function, fatigue life, wear, corrosion, and appearance.

7.1 Surface Texture Parameters

1. Roughness ($R_a$, $R_z$): Fine irregularities from the production process.

   • Arithmetic Average ($R_a$): Most common. The average absolute deviation of the roughness profile from the mean line. Expressed in micrometers (µm) or microinches (µin).

   • Average Peak-to-Valley Height ($R_z$): The average distance between the highest peak and lowest valley over five sampling lengths.

2. Waviness ($W_a$): Wider-spaced irregularities (longer wavelength) due to machine deflection, vibrations, or heat treatment.

3. Lay: The predominant direction of the surface pattern (e.g., parallel, perpendicular, circular, radial, multi-directional).

4. Flaws: Isolated irregularities like scratches, pits, cracks.

7.2 Surface Finish Symbols

A check mark-like symbol is used on drawings to specify surface texture requirements.

1. Basic Symbol: The check mark. Indicates a surface requirement exists.

2. Material Removal Required: A horizontal bar added to the basic symbol. Machining is required.

3. Material Removal Prohibited: A circle added to the basic symbol. The surface must be left in the as-cast, as-forged, etc., state.

4. Symbol Layout: Information is placed around the symbol in specific positions:

   • Position a: Roughness value ($R_a$ in µm).

   • Position b: Production method/technique (e.g., grind, turn).

   • Position c: Lay direction symbol.

   • Position d: Waviness criteria.

   • Position e: Machining allowance.

Example: A symbol with $R_a = 1.6$ µm in the a position and a lay symbol for parallel lay in the c position indicates the surface must be machined to an average roughness of 1.6 µm with tool marks parallel to the indicated direction.

Conclusion: Proficiency in Mechanical Drawing is foundational for any mechanical engineer or designer. It is the discipline that ensures a design concept is not only conceived but can be unambiguously communicated, manufactured, inspected, and assembled into a functioning product or system, bridging the gap between idea and reality.