This chapter will cover setting RPM, calculating speeds and feeds, lathe cutting tools, and machine operation. Additional resources such as recommended apps and practice tests can be found under Lathe Projects and Resources.

Calculating and Setting Revolutions per Minute

The speed at which the workpiece spins during a lathe operation is called RPM, which stands for “Revolutions per Minute.” RPM is vital in machining because it affects the tool’s cutting efficiency and overall lifespan. If the RPM is too high, the cutter will burn out. If the RPM is too low, the cut will not be efficient. To figure out RPM, you use a simple equation:

RPM = (Cutting Speed x 4) / Diameter of the workpiece

The Cutting Speed is the rate at which the tool moves in surface feet per minute (SFPM), while the Diameter signifies the workpiece size in inches.

The specific Surface Feet per Minute value varies based on factors such as the material being machined and the tool material used. Typically, the tool’s manufacturer provides recommendations regarding optimal cutting speed. Here are some general guidelines for cutting speed:

*Only use this chart as a reference, refer to tool manufacturer guidelines for accurate SFPM*

Below is a 6-minute video on calculating and setting RPM on a lathe. Always refer to the machine’s operation manual for instructions on your specific machine. This is an interactive video that will pause at several points and allow you to provide answers to questions in the video. You can check your answer using the “check” button or press play to continue watching. A summary of your answers will be provided at the end of the video if you click the star icon.

Calculating and Setting Feedrate

The lathe’s feed refers to how far the cutting tool moves along the work’s length for every spindle rotation. For example, if the lathe is set to a 0.010″ feed, the cutting tool will move 0.010″ for each complete revolution of the work. It is suggested to adjust the feed rate separately for roughing and finishing operations. Generally, a higher feed rate is applied during roughing to expedite material removal, resulting in a coarser surface finish, which is ok as it’s not the final size. The feed rate decreases for finishing, aiming for a smoother surface finish.
For most machining operations, roughing feed can be set from 0.010″-0.025″/revolution, and finishing can be set to 0.003″-0.005″/revolution.
Below is a 5-minute video showing how to set a machine’s feedrate using the quick change gearbox.

Below is an interactive picture showing the 2 different feedrates. Click on the “+” to see what the feedrate was set at. There is a noticeable difference in roughing and finishing feedrates

Lathe Tools

A wide array of lathe tools is available to machinists for various tasks. This section will focus on essential cutting tools frequently used on lathes, including turning, inside diameter (ID), knurling, and grooving tools.

Turning tools are employed to remove material from a workpiece’s length or face, reducing diameter or length. As the workpiece rotates, the turning tool moves linearly, eliminating material from the outer diameter or face. ID tools, usually in the form of boring bars, enlarge holes already drilled or cast. Knurling tools create decorative patterns on cylindrical workpieces, enhancing appearance and grip. These tools feature toothed steel wheels (knurls) that imprint the desired pattern onto the workpiece surface. Grooving tools are used to cut grooves into workpieces such as O-ring grooves or  have a blade-like cutting edge and are used to cut materials at a specific length. They plunge directly into the workpiece to separate the finished part from the stock.

OD Turning (Top Right), Facing (Top Left), Knurling (Bottom Left), Parting off or grooving (Bottom Right)

The following 10-minute video explains commonly used lathe tools and demonstrates how to properly set them up in a tool block and adjust for center height.

Below is an interactive element. Click and drag the image to match whether the tool is too high, low, or on center. Your results will show in the bottom left once completed.

Lathe Operation

Below is a 2-minute video describing the levers and handwheels on a lathe. These components are crucial in controlling various aspects of the lathe’s operations. Understanding how to operate these levers and handwheels effectively is essential for achieving accurate and efficient machining results.