A center gauge, also known as a fishtail gauge, is a precision tool used primarily in metalworking and machining, particularly in the process of cutting screw threads on a lathe. Its primary function is to aid in the accurate alignment and measurement of the cutting tool relative to the workpiece. The center gauge is typically made of hardened steel and features a distinctive shape with various angles and notches. These angles correspond to the standard thread profiles, such as 60 degrees for Unified and Metric threads, and 55 degrees for Whitworth threads. The gauge is used to set the angle of the cutting tool to ensure it matches the thread profile being cut. In practice, the machinist places the center gauge against the workpiece and adjusts the cutting tool until it aligns perfectly with the gauge's profile. This ensures that the tool is set at the correct angle and height, which is crucial for producing accurate and consistent threads. The gauge can also be used to check the pitch of the threads by comparing the notches on the gauge with the threads on the workpiece. Additionally, the center gauge can be used to verify the accuracy of the lathe setup and to inspect the finished threads for conformity to specifications. By ensuring precise alignment and measurement, the center gauge helps in achieving high-quality threads, reducing errors, and improving the efficiency of the threading process.

To use a fishtail gauge for grinding tool bits, follow these steps: 1. **Select the Gauge**: Choose a fishtail gauge that matches the thread angle of the tool bit you are grinding, typically 60 degrees for standard threads. 2. **Prepare the Tool Bit**: Secure the tool bit in a vise or tool holder, ensuring it is stable and at a comfortable working height. 3. **Set the Grinder**: Adjust the bench grinder tool rest to the desired angle for the tool bit's cutting edge. Ensure the grinding wheel is appropriate for the material of the tool bit. 4. **Initial Grinding**: Begin by grinding the tool bit to form the basic shape, focusing on the side and end relief angles. Use light pressure and frequent cooling to prevent overheating. 5. **Check the Angle**: Place the fishtail gauge against the tool bit to check the angle. The tool bit's cutting edge should align perfectly with the 60-degree notch on the gauge. 6. **Adjust as Needed**: If the angle is incorrect, adjust the tool bit's position and continue grinding. Recheck frequently with the fishtail gauge until the correct angle is achieved. 7. **Fine-Tuning**: Once the angle is correct, refine the tool bit's cutting edge by lightly grinding and honing. Ensure the cutting edge is sharp and smooth. 8. **Final Inspection**: Use the fishtail gauge to perform a final check on the angle and symmetry of the tool bit. Ensure both sides of the tool bit are even and the point is centered. 9. **Test the Tool Bit**: After grinding, test the tool bit on a piece of material to ensure it cuts cleanly and efficiently. By following these steps, you can accurately grind tool bits using a fishtail gauge, ensuring precision and effectiveness in machining tasks.

The common angles on a center gauge are 60 degrees, 55 degrees, and 29 degrees. These angles are used for different threading applications: 1. **60 Degrees**: This is the most common angle found on a center gauge and is used for cutting standard metric threads and Unified threads (UN threads). The 60-degree angle is essential for ensuring the correct profile of these threads, which are widely used in various mechanical and engineering applications. 2. **55 Degrees**: This angle is used for cutting British Standard Whitworth (BSW) threads. The 55-degree angle is crucial for maintaining the correct thread profile for Whitworth threads, which are commonly used in British engineering and older machinery. 3. **29 Degrees**: This angle is used for cutting Acme threads. The 29-degree angle is necessary for the trapezoidal thread profile of Acme threads, which are often used in power transmission and lead screw applications due to their strength and ease of manufacturing. These angles are typically marked on the center gauge, allowing machinists to quickly verify and set the correct tool angles for threading operations.

To set a threading tool using a center gauge, follow these steps: 1. **Prepare the Lathe**: Ensure the lathe is turned off and the workpiece is securely mounted in the chuck. The tool post should be clean and ready for tool installation. 2. **Install the Threading Tool**: Insert the threading tool into the tool holder on the tool post. Tighten the tool holder to secure the tool in place, ensuring it is positioned at the correct height relative to the workpiece centerline. 3. **Position the Center Gauge**: Place the center gauge (also known as a fishtail gauge) against the workpiece. The center gauge has a 60-degree notch that corresponds to the angle of the threading tool. 4. **Align the Tool**: Adjust the threading tool so that its cutting edge aligns perfectly with the 60-degree notch on the center gauge. This ensures the tool is set at the correct angle for cutting threads. 5. **Check Tool Height**: Verify that the tool's cutting edge is at the same height as the workpiece centerline. Adjust the tool height if necessary using the tool post adjustment screws. 6. **Secure the Tool**: Once aligned, tighten the tool post to secure the threading tool in its correct position. Double-check the alignment with the center gauge to ensure accuracy. 7. **Test the Setup**: Perform a test cut on a scrap piece to verify the tool is correctly set. Check the thread profile with a thread gauge to ensure it matches the desired specifications. 8. **Make Adjustments**: If the test cut is not satisfactory, make necessary adjustments to the tool angle or height and retest until the desired thread profile is achieved. This process ensures the threading tool is accurately set for precise thread cutting.

Center gauges are typically made from the following materials: 1. **Steel**: High-carbon steel or tool steel is commonly used for center gauges due to its durability and resistance to wear. These materials provide the necessary hardness and strength to withstand repeated use in industrial settings. 2. **Stainless Steel**: Stainless steel is another popular choice for center gauges. It offers excellent corrosion resistance, making it suitable for environments where moisture or chemicals are present. This material also maintains its structural integrity over time. 3. **Hardened Steel**: Some center gauges are made from hardened steel, which undergoes a heat treatment process to increase its hardness and wear resistance. This makes them particularly suitable for precision work and ensures a longer lifespan. 4. **Tempered Steel**: Tempered steel is used to enhance the toughness of the center gauge while maintaining adequate hardness. This material is less brittle than hardened steel, providing a balance between durability and flexibility. 5. **Alloy Steel**: Alloy steel, which includes additional elements like chromium, vanadium, or molybdenum, is used to improve specific properties such as strength, toughness, and resistance to wear and corrosion. 6. **Brass**: In some cases, brass is used for center gauges, especially in applications where non-magnetic properties are required. Brass is softer than steel, which can be advantageous in preventing damage to delicate workpieces. 7. **Plastic or Composite Materials**: For less demanding applications, center gauges may be made from high-strength plastic or composite materials. These are lightweight and resistant to corrosion but may not offer the same level of precision or durability as metal gauges. These materials are chosen based on the specific requirements of the application, including factors like precision, durability, environmental conditions, and cost.

No, a center gauge cannot be used for both internal and external threading. A center gauge, also known as a fishtail gauge, is primarily used for setting the angle of the threading tool and checking the thread angle during the cutting of external threads on a lathe. It helps ensure that the threading tool is aligned correctly with the workpiece and that the thread angle is accurate, typically at 60 degrees for standard threads. For internal threading, the process and tools differ. Internal threading requires a different setup and often uses specialized internal threading tools or boring bars that can reach inside the workpiece. The center gauge is not designed to assist with the alignment or angle verification of these internal tools. Instead, internal threading relies on other methods and tools, such as thread micrometers or plug gauges, to ensure accuracy and proper fit. In summary, while a center gauge is a valuable tool for external threading operations, it is not suitable for internal threading tasks. Each type of threading requires specific tools and techniques to achieve the desired results.

To read the graduations on a center gauge, follow these steps: 1. **Identify the Type**: Determine if the center gauge is for 60-degree or 55-degree threads, as this affects the graduation markings. 2. **Position the Gauge**: Place the center gauge against the workpiece or tool to ensure proper alignment. The gauge should be perpendicular to the axis of the workpiece. 3. **Locate the Graduations**: Examine the gauge for the engraved lines or markings. These graduations are typically found along the edges of the gauge. 4. **Understand the Scale**: The graduations represent specific angles or thread pitches. For a 60-degree gauge, the markings help verify the angle of the cutting tool or the thread form. 5. **Align the Tool**: Use the graduations to align the cutting tool or verify the thread angle. The tool's edge should match the angle indicated by the gauge's graduations. 6. **Check Symmetry**: Ensure that the tool or thread is symmetrical by comparing both sides against the gauge's graduations. 7. **Verify Thread Pitch**: If the gauge includes thread pitch markings, use these to confirm the pitch of the thread being cut or measured. 8. **Adjust as Necessary**: If the tool or thread does not align with the graduations, adjust the tool angle or workpiece setup accordingly. By following these steps, you can accurately read and utilize the graduations on a center gauge to ensure precision in machining and threading tasks.