Warning track for stability limit

Maximizing machine tool productivity requires knowledge of the stability limits imposed by metalcutting process physics. Selecting cutting conditions that exceed the stability limits causes chatter. Selecting cutting conditions that are too conservative avoids the danger of chatter, but underuses machine capacity.

It is important to stay in the stable zone, but how close to the boundary should you go? How accurately do you need to know where the boundary location is? The answers depend on the quality of the data and the repeatability of the setup. Computation of the stability diagram requires several pieces of information. Some are well known, such as the number of teeth on the cutter, and some are only approximate, such as the material properties.

There are two important workpiece material properties: the cutting force coefficient(s) and the process damping coefficient. The cutting force coefficient(s) relate the chip geometry to the cutting force. In a first approximation, meaning the approximation catches the major relationship as opposed to smaller effects the second and third appoximations catch, the tangential component of the cutting force is proportional to the frontal area of the uncut chip (chip width × chip thickness). The normal component of the cutting force is smaller, but proportional to the tangential component of the cutting force. Cutting force coefficient(s) are obtained via measurement with a table dynamometer mounted under the workpiece. The cutting force coefficients are adjusted to obtain agreement between the calculated cutting forces and the measured cutting forces.

Courtesy of Manufacturing Laboratories

Figure 1. A stability diagram with “warning track.”

The measured forces are compared to the programmed chip geometry. The data is typically noisy, or inexact, and varies from one workpiece to another. To know the stability boundary precisely, some researchers define as many as eight cutting force coefficients to be identified. In addition to the workpiece material and chip width and thickness, some reseachers relate the cutting force to the cutting edge radius, feed, lubrication and other variables. For a single setup in a laboratory, this may be possible, but it is not practical to require so much data in a production environment. Beyond the first-order approximations, there is diminishing value from additional data.