Short Answer:

In engineering drawings, threads and threaded fasteners like bolts, screws, and nuts are represented using standard symbols, notations, and simplified lines. Instead of drawing every thread in detail, engineers use conventional representations such as thin parallel lines, hidden lines, or detailed thread callouts like “M10 × 1.5”.

Threaded fasteners are shown in views with proper head shapes, thread indication, and sectional views if needed. Internal threads (like in nuts or holes) and external threads (like on bolts or screws) are drawn differently and labeled using standard thread designations as per ISO, BIS, or ANSI standards.

Detailed Explanation:

Representation of Threads and Threaded Fasteners in a Drawing

In mechanical engineering, threads and fasteners are essential components used for joining, assembling, and adjusting machine parts. It is very important to represent them correctly and clearly in engineering drawings so that manufacturers and assembly workers can understand the type, size, and function of the thread.

Due to the small size and complex shape of threads, it is not practical to draw every thread line. That is why engineers follow conventions and standards to show them in a simplified way.

1. Types of Threads

Before understanding representation, let us quickly see the types of threads commonly shown in drawings:

• External Thread: Found on bolts, screws, studs, etc.
• Internal Thread: Found inside nuts or tapped holes
• Right-Hand Thread: Most common, tightens when rotated clockwise
• Left-Hand Thread: Less common, used in special applications

Each thread is defined by certain parameters such as major diameter, pitch, length, and thread form (metric, unified, trapezoidal, etc.).

2. Symbolic Representation of Threads

Instead of drawing threads fully, symbols and lines are used:

• External Threads: Shown with two concentric circles in end view. In side view, two parallel lines—one thick and one thin—are drawn on the shaft.
• Internal Threads: Shown with a broken circle in end view. In side view, hidden lines are drawn to represent the thread depth.

This representation is standardized under ISO, BIS, or ANSI.

3. Thread Callouts or Notations

Threads are specified by standard callouts written next to the fastener or hole:

• Metric Thread (ISO): M10 × 1.5
  • “M” = Metric thread
  • “10” = Major diameter in mm
  • “1.5” = Pitch (distance between threads)
• Unified Thread (ANSI): ½ – 13 UNC
  • “½” = Diameter in inches
  • “13” = Threads per inch
  • “UNC” = Unified Coarse Thread

For internal threads (tapped holes), the callout is written on the hole with leader lines or directly on the view.

Example:
M8 × 1.25 – 6H (Depth 20 mm)
This means a metric internal thread with a 20 mm depth, tolerance grade 6H.

4. Threaded Fastener Representation

Fasteners like bolts, screws, and nuts are shown with:

• Head shape: Hex, round, or slotted heads drawn as per standards.
• Shank and threads: Shaft shown with thread lines.
• Sectional views: Used to show thread engagement clearly between nut and bolt.

For example, in a bolt and nut assembly, the bolt threads and the internal threads of the nut are aligned using hatching or hidden lines in sectional views.

5. Thread Depth and Engagement

In drawings, if only part of a hole is threaded, then a thread depth is mentioned.

Example:

• Ø12 drill, M10 × 1.5 thread depth 15 mm

This tells the machinist to drill a 12 mm hole and thread only 15 mm of it.

6. Thread Standards and Tables

To maintain uniformity, thread dimensions and tolerances are based on standard tables from:

• ISO 68, ISO 965 – Metric threads
• DIN 13 – German standard
• BIS IS 4218 – Indian standard
• ANSI B1.1 – Unified threads

Drawings refer to these standards to avoid confusion and ensure accuracy during manufacturing.

7. Thread Representation in CAD and 3D Models

In CAD software, threads are usually represented using cosmetic threads—not actually modeled but shown symbolically. This keeps the model light and easy to process.

However, in detailed 3D modeling, actual thread profiles may be modeled when needed for simulations or printing.

Conclusion

Threads and threaded fasteners in engineering drawings are represented using symbols, standard notations, and simplified lines. Instead of drawing each thread, engineers use conventional methods to clearly indicate external or internal threads. Fasteners like bolts and nuts are shown with proper views and callouts based on size, pitch, and thread standards. This method keeps the drawings clean, easy to read, and suitable for manufacturing. Understanding these symbols is essential for every mechanical engineer and machinist.