Cutting Speed and RPM on a Lathe

Definition and Importance of Cutting Speed on a Lathe

The distance the tool tip travels around the workpiece, in meters per minute, is called cutting speed. Cutting speed is of great importance for extending tool life and achieving the desired surface quality.

Factors Affecting Cutting Speed on a Lathe

The material’s hardness, depth of cut, and feed rate affect cutting speed inversely. The tool’s hardness and the machine’s capacity affect cutting speed directly.

In turning, as the workpiece’s hardness, depth of cut, and feed rate increase, cutting speed should be decreased; as the tool’s hardness increases, cutting speed should be increased.

Cutting Speed Formula

Cutting speed (Vc) is calculated using the workpiece diameter and RPM with the formula Vc = π x D x n / 1000.

Cutting Speed by Material

Soft materials like aluminum can withstand high cutting speeds, while hardened steels require low speeds.

Effect on Tool Life

Exceeding the optimal cutting speed can cause rapid wear and premature breakage of the tool tip.

Calculating Cutting Speed on a Lathe

Cutting speed is calculated based on the workpiece’s diameter and RPM.

d: workpiece diameter (mm)
n: workpiece speed per minute: (rev/minute)
v: cutting speed m/min (meters/minute)

with these; the circumferential distance the tool tip travels in one revolution of the workpiece = π . d.
The distance traveled per minute is then π . d . n. Converting the diameter measurement to mm:

Cutting Speed V = π . d . n / 1000 (m/min)

Example Problem: Calculating Cutting Speed on a Lathe

Find the tool’s cutting speed for machining a 20 mm diameter part at 500 rpm.

SOLUTION:
V = π . d . n / 1000
V= 3.14 . 20 . 500 / 1000
V= 31.4 m/min

NOTE:
Cutting speed is taken as V=20m/min for HSS cutting tips,
and V=40m/min for carbide-tipped tools.

Calculating RPM from Cutting Speed

To perform turning at the selected cutting speed, it’s important to set the machine to the correct RPM. Once the RPM that will give the required cutting speed is found, the necessary setting is made using the lathe’s speed-change levers. Since cutting speed is V = π . d . n / 1000:

RPM (n) = 1000 . V / π . d

This will be the result, with units of rev/min.

Example Problem: Calculating RPM from Cutting Speed

Find the RPM for machining a 100 mm diameter part at a cutting speed of 25 m/min.

SOLUTION:
n = 1000 . V / π . d
n = 1000. 25 / 3.14 . 100
n = 79.6, found, and the machine’s RPM is set to the nearest value, 80 rpm.