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In today's competitive manufacturing environment, end-users of metalworking fluids seek to maximize their productivity by manufacturing metal parts ever faster. One approach being taken is to utilize high-speed, high-feed machining.

High-speed machining (HSM) was originally developed by German inventor Dr. Carl Salmon in the 1920s. He determined that for a specific workpiece metal, the heat generated at the interface between the cutting tool and the workpiece would peak at a certain critical spindle speed. This critical cutting speed is different for each metal alloy being machined. Salmon also determined that on either side of this peak there was a specific spindle speed range at which the cutting tool could not remove metal.

Research on HSM was picked up by Vaughn at Lockheed Aircraft in 1959. Additional research in the 1980s and 1990s, particularly in the aerospace industry, showed that HSM could, in a practical fashion, provide benefits as compared to conventional machining. Faster metal removal can be realized with a combination of lower machining forces and reduced power exerted by the machine tool.

WHAT IS HSM?

The answer to defining HSM would first appear to be relatively straightforward. STLE (Society of Tribologists and Lubrication Engineers) member Gary Rodak of Machining Efficiencies, Inc. (Gregory, MI) says, "There are several definitions of HSM, but the most common is based upon the rpm of the machine tool spindle. Some machinists consider 8,000 rpm to be the starting point for HSM, but with current machine capability anything over the 15,000 rpm should be considered high speed. Operating above that spindle speed requires special attention to details such as spindle balance, machine setup, coolant application, tool paths and wear patterns."

Dr. Yung Shin, a professor of mechanical engineering at Purdue University, believes the spindle speed that designates high speed is dependent on the workpiece material. "There is no universal definition of HSM," he remarks. "For metals such as aluminum and cast iron, HSM can occur at surface speeds of 2,500 fpm. In contrast, such high speeds cannot be attained with titanium. HSM of titanium can occur at speeds of 400 fpm or slightly higher."

Dr. David Dilley of D3 Vibrations Inc. (Royal Oak, MI) looks at HSM from a frequency perspective. He says, "Every tool/holder/machine combination has a characteristic dominant, natural frequency. HSM can be defined as the point where the tooth passing frequency of the cutting tool approaches the dominant natural frequency." The tooth passing frequency is defined as:

Tooth passing frequency (Hz) =rpm/60 x number of teeth

This definition means the shape and size of the cutting tool also plays a significant factor in determining if HSM can be attained in a specific operation. Dilley explains, "A cutting tool with eight teeth might approach the machine tool's dominant frequency, while a tool with two teeth might not. The selection of rpm, number of teeth and depth of cut (DOC) are the most important parameters for machine tool vibrations. Cutting tools that are longer in length have lower natural frequencies, thus reaching high speed at a lower rpm. For example, a long line boring operation can be in a HSM environment at a cutting speed as low as 150 rpm."

Dilley indicates that this definition of HSM differs from others, as most refer to high speed spindles that relate to the DN number of the spindle bearing. He states, "HSM often takes place even at low DN numbers." STLE member Tom McClure, vice president of TechSolve Inc. (Cincinnati, OH), also believes that HSM has many definitions. He adds, "The key concept is the need to manipulate speed, feed and DOC to increase metal removal rates while lowering cutting forces."

THE NEED FOR SPEED

Sufficient work has been done on HSM to show it provides several benefits. Rodak says, "End-users can realize faster cycle times and better finish on the metal parts. In fact, cycle times for specific machining operations can be reduced by as much as a third. The finish improves part quality due to less residual stress remaining in the part surface. A better quality finish also has a beneficial impact on subsequent assembly and coating processes."

Shin points to three unique advantages end-users can have in utilizing HSM. He says, "HSM generates a much higher level of metal removal. The end-user can increase the depth of cut during machining. Less heat is conducted into the workpiece in HSM, which can lead to less thermal distortion, reduce the forces needed and also reduce the surface roughness. Finally, less heat into the workpiece also reduces the level of residual stress."

CHATTER AUDIO ANALYSIS

One of the biggest problems encountered in increasing spindle speeds is the onset of chatter. Dilley says, "Machinery systems can encounter free vibration, forced vibration and self-excited (chatter) vibration during use."

Free vibration and forced vibration are typically less destructive compared to chatter. Dilley explains, "Chatter occurs when the depth of cut and specific cutting energy required during a specific machining operation exceeds the dynamic stiffness of the cutting tool or workpiece." Chatter creates large cutting forces that can accelerate tool wear and even cause tool failure, as shown in Figure 1.