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  1. Definition and Types:
  • Spiral Grooves: Threads are the spiral grooves found on screws, bolts, or nuts. These grooves allow the fastener to be driven into or onto a surface, creating a secure hold.
  • Helical Groove: Threads can also refer to the helical groove inside a cylindrical hole, such as in a nut, which is formed using a tap or lathe tool. This is known as a female or internal thread.
  1. Thread Classes:
  • Class 1 Threads: Loose tolerance threads that are easy to assemble but may have more play or movement between mating parts.
  • Class 2 Threads: These are the most common and are used in 90% of stainless fasteners. They have normal commercial tolerance and are found in many everyday applications.
  • Class 3 Threads: These threads have stricter tolerance and provide a tighter fit, often used in applications requiring precision, such as socket cap and set screws.
  1. Thread Identification:
  • External Threads (A): Symbolized by ‘A’ (e.g., 2A), these are threads on the outside of a fastener, such as on screws.
  • Internal Threads (B): Symbolized by ‘B’ (e.g., 2B), these are threads on the inside of a fastener, such as inside nuts.
  1. Coarse vs. Fine Threads:
  • Coarse Threads: These threads are deeper and have fewer threads per inch compared to fine threads. Coarse threads are predominant in hex head cap screws and hex nuts sold in 18-8 stainless steel, comprising 90-95% of these products and about 98% of other stainless items including machine screws and socket products. They offer advantages like greater resistance to thread stripping, better tapping in brittle materials, and improved fatigue resistance.
  • Fine Threads: These have more threads per inch and a shallower depth compared to coarse threads. Fine threads are preferred for applications that require a better fit in thin-walled materials, higher torque strength, and increased tightness under vibration conditions.
  1. Application Considerations:
  • Material Suitability: Coarse threads are often better suited for brittle materials due to their deeper grooves, which provide a more secure grip and reduce the likelihood of stripping.
  • Vibration Resistance: Fine threads are typically used in applications where vibration is a concern because their higher number of threads per inch provides a more secure and tight fit.
  • Torque Strength: Fine threads can handle higher torque levels, making them ideal for precision applications where exact tension is critical.
  • Fatigue Resistance: Coarse threads offer better fatigue resistance, making them suitable for dynamic applications where the fastener might experience repeated loading and unloading cycles.
  1. Popular Sizes:
  • 10/32 Diameter: This size is extremely popular and often used in a variety of applications.

Understanding the differences between thread types and their respective classes is crucial for selecting the appropriate fastener for a given application. Each type of thread has its unique advantages and is suited to specific conditions and materials.

See Related Terms:

  • American Standard Pipe Thread
  • Bastard Thread
  • British Standard Pipe Thread
  • British Standard Brass Thread
  • Buttressed thread
  • Chasing threads
  • Class A thread
  • Class B thread
  • Class of thread
  • Coarse thread
  • Cut thread
  • Depth of thread
  • External thread
  • Female thread
  • Incomplete thread
  • Internal thread
  • Left-hand thread
  • Male thread
  • Mating thread
  • Metric thread
  • National coarse thread
  • National fine thread
  • Nut thread
  • Reverse Thread
  • right-hand thread
  • Rolled thread
  • Total Thread
  • UNC thread
  • UNF thread
  • V-thread
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