Short Answer:

Internal threading means cutting threads on the inside surface of a hole, like those found inside a nut. It allows a screw or bolt to be inserted and fastened tightly. On the other hand, external threading means making threads on the outer surface of a rod or cylinder, like on bolts or screws, which are inserted into a hole.

The main difference is their location and purpose. Internal threads receive or hold, while external threads are inserted or turned into a mating part. Both types must match properly for strong and smooth joining in mechanical assemblies.

Detailed Explanation:

Difference between internal and external threading

Threading is a basic operation in mechanical engineering where helical grooves are cut onto a surface to create a screw-like profile. These grooves are known as threads, and they allow parts to be joined together by twisting or rotating motion. There are two main types of threading based on where the thread is made — internal threading and external threading.

Both types are equally important in creating strong, reliable joints in machines, tools, and structures. A nut and bolt assembly is a perfect example where both types work together to form a secure and detachable connection.

Let us understand them in detail.

Internal Threading

Internal threading refers to the process of creating threads inside a hole. These threads are female threads, meaning they are meant to receive or hold an externally threaded part like a screw, bolt, or stud.

Where it is used:

• Inside nuts
• Inside threaded holes in parts like engine blocks, casings, brackets
• In pipe joints and fittings

Tools used:

• Taps (manual or machine taps)
• Threading inserts in CNC machines
• Internal thread mills for high-precision CNC work

Method:

1. First, a hole is drilled slightly smaller than the thread diameter.
2. A tap is inserted and rotated slowly to cut threads into the inside wall.
3. Lubricant is applied to prevent breakage and improve finish.
4. After cutting, the hole can accept a bolt or screw.

Key Features:

• Found on inner surfaces
• Provides space for male parts to screw in
• Needs proper alignment and correct hole size

External Threading

External threading means creating threads on the outer surface of a cylindrical part. These are male threads, like those on bolts, studs, or screws, and are meant to enter a female thread for connection.

Where it is used:

• On bolts, screws, studs, threaded rods
• On pipes and shafts
• On components that go into nuts or tapped holes

Tools used:

• Dies (for hand threading)
• Lathe machines with thread cutting tools
• Thread rolling machines
• External thread mills in CNC machines

Method:

1. A rod is prepared and slightly chamfered at the end.
2. A die or cutting tool is applied around the rod.
3. It is rotated to form threads along the outer surface.
4. Lubrication helps reduce friction and improve accuracy.

Key Features:

• Found on outer surfaces
• Designed to fit into a matching internal thread
• Requires careful cutting to avoid misalignment

Main Differences

Feature	Internal Threading	External Threading
Thread Location	Inside a hole	On the outside of a rod
Common Example	Nut or tapped hole	Bolt or screw
Tool Used	Tap	Die or lathe tool
Purpose	Receives the threaded part	Fits into an internally threaded part
Difficulty	Slightly harder to align and cut	Easier to observe during cutting

Conclusion

The key difference between internal and external threading lies in the location of the threads. Internal threads are made inside holes and accept external threaded parts like bolts, while external threads are made on rods or shafts and fit into internal threads. Both are essential in mechanical assemblies for joining components, enabling movement, and allowing disassembly when required. Understanding their purpose, tools, and method ensures accurate and reliable threading in workshop and machine operations.