Fasteners and Thread Types

Purpose of Fasteners

• Screw threads hold parts together using bolts or screws.
• Adjust parts relative to each other, as seen with adjusting screws in drafting tools.
• Apply pressure to secure objects in place using setscrews.

Types of Threads

• Helix: Spiral grooves on cylindrical surfaces.
• External Threads: Located on bolts, cap screws, and wood screws.
• Internal Threads: Found on nuts or tapped inside components.

Thread Measurements

• Major Diameter: Largest diameter measured from crest to crest.
• Minor Diameter: Smallest diameter of a thread.
• Crest: Shallowest part of the thread.
• Root: Deepest part of the thread.
• Pitch: Distance between corresponding points on adjacent threads, typically in threads per inch.
• Lead: Distance a thread travels with one complete revolution.

Thread Configurations

• Single Threads: Lead equals pitch.
• Multiple Threads: Lead exceeds one pitch, allowing quicker advancement (e.g., double threads move twice as far).

Fit Classes

• Class 1: Loose fit for rough work.
• Class 2: Free fit for general purposes.
• Class 3: Medium fit for higher quality applications, like automotive use.
• Class 4: Close fit for tight applications, as in aircraft engines.

Thread Characteristics

• Thread Depth: Measurement from crest to root.
• Thread Angle: Angle between the thread walls.
• Standard Thread Angle: Typically 60 degrees.

Thread Direction

• Right-Handed Threads: Advance clockwise; presumed if not specified.
• Left-Handed Threads: Advance counterclockwise.

Tools for Threading

• Tap: Tool for creating internal threads.
• Small Threads: Often cut by hand.
• Large Threads: Typically cut using a lathe.
• Die: Tool for forming external threads.

Thread Standards

• ANSI: American National Standard for Unified Threads.
• Unified National Coarse (UNC): Most common thread series.
• Unified National Fine (UNF): Stronger than UNC, providing secure fastening.
• Unified National Extra Fine (UNEF): Highest tensile strength, suitable for high vibration applications.
• Metric Thread: International standard based on ISO.

Specialized Threads

• Square Thread: Transmits motion or power, challenging to manufacture due to perpendicular sides.
• ACME Thread: Easier to manufacture with a 60-degree angle, used for rapid screw actions.
• Sharp-V Thread: Provides friction holding; commonly used in brass components.
• Knuckle Thread: Common on products like light bulbs and screw-type bottle tops.