Some measurement required: Research & Innovation

Machine tool users can access the information contained in a stability lobe diagram to greatly increase productivity. My May column demonstrated that by knowing which speed to select and what DOC is permissible, it is often possible to improve productivity by a factor of four or more. No tooling or machine changes are required, just a knowledge of the right cutting conditions expressed on a stability lobe diagram.

As long as the setup is repeatable, the stability lobe diagram is repeatable, and the best cutting conditions can be selected with no guesswork. When milling, some of the required pieces of information are relatively easy to obtain or are already known.

Impact excitation is used to measure a frequency response function. Image courtesy of BlueSwarf Manufacturing Laboratories, State College, Pa.

You need to know a specific material property called the cutting force coefficient. The cutting force coefficient relates the chip area to the cutting force. This coefficient is also sometimes called the specific power because it relates power to the metal-removal rate. This data is tabulated for many common workpiece materials. (See “Machining Dynamics—Frequency Response to Improved Productivity,” by Schmitz and Smith, published by Springer.) As an example, Aluminum7075-T6 is about 850 N/mm2, which corresponds to about 0.3 hp per in.3/min.

You also need to know how many teeth are on the tool because that factor changes the location of stable pockets in the stability lobe diagram.

In addition, you need to know the radial DOC. A safe choice is to assume full slotting because that sometimes happens in corners, and full slotting provides the smallest permissible axial DOC. If you are stable in a slot, you are stable at other radial immersions, too.

One other needed piece of information is more difficult to obtain—the frequency response function (FRF). If you apply a force to the tip of a cutting tool via machining, the tool deflects. The relationship between force and deflection is called stiffness. However, the force from cutting is rarely constant and the resulting deflection also depends on the frequency of the force. There are particular frequencies (natural frequencies) where the deflection is large compared to the force.