Screw Thread
Definition
A screw thread is a helical ridge formed on the outer or inner surface of a cylindrical or conical body. It is widely used in engineering to convert rotary motion into linear motion or to fasten components together.
Uses and Applications
- Fastening machine components such as bolts and nuts.
- Power transmission in machines.
- Adjusting mechanisms in machine tools.
- Clamping devices and vices.
- Motion control in lead screws and jacks.
Types of Non-Standard Screw Threads
Besides the common V-thread, several special thread forms are used for particular engineering applications.
Square Thread
Square threads have a square-shaped profile. They provide high efficiency and are mainly used for power transmission.
Applications
- Lead screws of machine tools
- Screw jacks
- Press mechanisms
Acme Thread
The Acme thread has a trapezoidal shape and is stronger than square threads. It is widely used in machine tools.
Applications
- Lead screws
- Lathe feed mechanisms
- Industrial machinery
Buttress Thread
Buttress threads are designed to handle heavy loads in one direction. One side of the thread is nearly perpendicular to the axis.
Applications
- Heavy-duty presses
- Jacks and lifting equipment
- Gun breech mechanisms
Worm Thread
Worm threads are used in worm gear systems where rotary motion is converted into rotational motion at a right angle.
Applications
- Worm gear drives
- Conveyor systems
- Gear reduction mechanisms
Principle of Cutting Screw Thread on a Centre Lathe
Screw threads are produced on a lathe by synchronizing the rotation of the workpiece with the movement of the cutting tool. The lead screw of the lathe controls the movement of the carriage so that the cutting tool follows a helical path on the workpiece.
The cutting tool removes material gradually to produce the required thread shape.
Principle of Chasing the Screw Thread
Chasing is the process of cutting threads using a threading tool that follows the helical groove formed during the first pass. The cutting tool must move at the correct speed relative to the rotating workpiece so that the thread pitch remains accurate.
Use of Centre Gauge
A centre gauge, also called a fishtail gauge, is used to correctly set the threading tool at the proper angle relative to the workpiece.
Uses
- Aligning the threading tool correctly.
- Checking the angle of the cutting tool.
- Ensuring accurate thread cutting.
Setting the Tool for Cutting Threads
External Thread Cutting
- The cutting tool is mounted on the tool post.
- The tool is aligned with the center line of the workpiece.
- The centre gauge is used to ensure correct tool angle.
Internal Thread Cutting
- A special internal threading tool is used.
- The tool is mounted on a boring bar.
- The tool is positioned inside the hole and aligned with the workpiece axis.
Use of Screw Pitch Gauge
A screw pitch gauge is a measuring tool used to determine the pitch of a screw thread.
Method of Checking Thread
- Select the appropriate blade from the screw pitch gauge.
- Place the gauge teeth against the thread.
- If the teeth match the thread profile exactly, the pitch is correct.
Uses
- Identifying thread pitch.
- Checking thread accuracy.
- Measuring threads during inspection.
Conclusion
Screw threads are essential mechanical elements used for fastening and power transmission. Different thread forms such as square, acme, buttress, and worm threads are used depending on the application. Proper use of tools such as centre gauges and screw pitch gauges ensures accurate thread cutting on a centre lathe.