What is high-speed machining?

Author Cutting Tool Engineering
Published
July 01, 2010 - 11:00am

Although many people talk about high-speed machining, many machine tools are advertised as high speed and there are conferences devoted to HSM, the term is not well defined. Some definitions focus on surface speeds, others on spindle bearing rotation and still others on spindle speed along. However, a better approach may be to define HSM based on stability phenomenon.

One common definition is based on the surface speed—the relative speed between the tool and workpiece. This is essentially a thermal limit because many of the wear mechanisms are temperature dependent. 

The surface speed definition is favored by toolmakers and metalcutting researchers. In a classic paper from the Annals of the CIRP from 1992, H. Schulz and T. Moriwaki produced a figure similar to Table 1, indicating the ranges for conventional machining (green), a transitional region (yellow) and a high-speed region (red) based on surface speed. Surface speed in m/min. is determined by v=πdn, where d is the diameter of the milling tool or workpiece, and n is the spindle speed. Conventional metric units for cutting speed are m/min. For example, a 25mm-dia. milling tool rotating at 15,000 rpm has a surface speed of 1,177.5 m/min.

A second common definition uses the DN number, which is based on the rotation capability of the preloaded spindle bearings. Spindle builders and machine tool designers usually prefer the DN number as an expression of high speed. The DN number is the product of D, the diameter in mm of the main bearing bore, and N, the maximum spindle speed in rpm. For example, a spindle with a 60mm main bearing bore and a 20,000-rpm maximum spindle speed would have a DN number of 1.2 million. 

MachineTechTable.ai

Although laboratory spindles have achieved much higher numbers, commercial high-speed spindles have remained below 2 million DN for a number of years. One of the main reasons is heat in the bearings. The bearings are preloaded (the bearing is arranged so the balls are always in compression, because that increases the bearing stiffness and accuracy). The preload mechanism must fight against the centrifugal forces caused by the balls circulating at high speed. Because ceramic balls are stiff and have a low mass, they are often preferred to steel ones. As the balls rotate in the race of the bearing, they are squeezed and released, and that work shows up as heat. 

There is also heat from the rotor in an integral spindle and from the drivetrain in a nonintegral spindle. Therefore, the preload mechanism must allow for thermal growth of the spindle while maintaining preload—a significant design challenge. 

Toolholder makers often look at spindle speed alone when defining HSM. Toolholders and cutting tools are rated to be safe up to a certain spindle speed, meaning the tool and holder bodies will stay together, and all of the mechanical connections will remain intact. Such a rating usually means the tool or holder has already been spun up to twice the rated spindle speed. 

The definition I prefer is related to the appearance of stability phenomenon. The “stability lobes” appear when the tooth passing a frequency reaches a significant fraction of the dominant natural frequency. In low-speed machining, the selection of the spindle speed does not significantly change the dynamic performance of the machine. The stability limit, the DOC above which chatter occurs, seems like a constant. When the tooth passing frequency reaches a substantial fraction, say ¼, 1⁄3, ½ or 1⁄1, then the selection of a correct speed can easily improve the metal-removal rate by a factor of two or more. 

For example, when milling aluminum with a 2-flute tool and a dominant (most flexible) natural frequency of 1,200 Hz, the stability lobe effects are strongest at 1,200 × 60 ÷ 2 = 36,000 rpm. Other significant stable zones would occur at 18,000 rpm, 12,000 rpm and 9,000 rpm. 

For tool systems with lower dominant natural frequencies, the spindle speeds where these effects appear can be much lower. When titanium milling with a 6-flute tool and a dominant natural frequency of 250 Hz, the most stable pocket would appear at 250 × 60 ÷ 6 = 2,500 rpm and still be HSM from the dynamics point of view. CTE

About the Author: Dr. Scott Smith is a professor at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.

Related Glossary Terms

  • chatter

    chatter

    Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

  • cutting speed

    cutting speed

    Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • metal-removal rate

    metal-removal rate

    Rate at which metal is removed from an unfinished part, measured in cubic inches or cubic centimeters per minute.

  • metalcutting ( material cutting)

    metalcutting ( material cutting)

    Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • stiffness

    stiffness

    1. Ability of a material or part to resist elastic deflection. 2. The rate of stress with respect to strain; the greater the stress required to produce a given strain, the stiffer the material is said to be. See dynamic stiffness; static stiffness.

  • toolholder

    toolholder

    Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.