Sandia LabNews

Cutting the chatter in milling machines

Sandia helps design chatter-suppression device that would allow US factories to mill metal faster

Machinists dread "chatter," the violent vibration of a milling machine’s rotating tool bit against the piece of metal being milled.

Chatter can destroy a cutting tool or spoil a surface. If the work piece is a precision-machined part such as an aircraft engine, the damage can be unacceptably costly.

More often though, operators of today’s high capacity milling machines avoid chatter altogether by running their machines at conservatively slow tool speeds and shallow cuts.

Now Sandia — working with an industrial group including Lockheed Martin, Intelligent Automation Inc. (IAI), Ingersoll Milling Machine Co., and Active Signal Technology (AST) — has tackled the industrial-age-old problem of milling machine chatter in a 21st century way.

As the tool turns

Using the latest in computational structural dynamics modeling and "smart structures" capabilities, Sandia examined mathematically how chatter happens, then helped the consortium design a vibration control system that actively suppresses chatter as the tool spins at thousands of rpm. ("Smart structures" refers to the use of sensors, actuators, computers, and control algorithms to produce a response in a structure that makes that structure more effective.)

In a demonstration earlier this month in Rockford, Ill., using Ingersoll’s developmental horizontal-axis hexapod milling machine, the new Smart Spindle Unit (SSU) allowed the machine to cut deeper and faster, removing metal at more than five times its original rate.

Its developers say the SSU could enable machinists to operate their machines closer to their design capacities, possibly shaving minutes or hours off the milling of each metal part and dollars off production costs.

"It could expand the envelope of stable cutting into faster and deeper regimes with the same precision," says Terry Hinnerichs (9126), Sandia SSU project leader. "It might drive down the cost of metal removal significantly."

The work was funded by the Defense Advanced Research Projects Agency and led by Lockheed Martin. It began in 1994 as part of a national campaign to bolster the competitiveness of US factories by improving manufacturing technology.

The Sandia project, directed by David Martinez, Manager of Structural Dynamics Development and Smart Structures Dept. 9124, was one of several manufacturing-related smart structures and materials efforts in the mid 1990s by researchers from Dept. 9124 and the Smart Structures Lab in Structural Dynamics Engineering Dept. 9125, managed by Tom Baca.

How it works

Just like the bone-jarring bounce that occurs when your car’s tires roll too fast over a washboard road, chatter happens when a milling machine’s cutting tool bounces off grooves on the metal’s surface left there by the previous cut. Choking back the spindle speed lessens the vibration, just like slowing down your car does.

The SSU essentially is a smart suspension system for the machine’s rotating parts.

Strain gauges mounted to the cutting tool sense bending strains on the tool. These measurements are radioed via a specially designed telemetry system to the SSU’s control processor, which maps the strains into a non-rotating coordinate system and then generates command signals that are sent to four actuators placed around the spindle.

These electrical signals cause electrostrictive ceramic materials inside each of the actuators to expand by just a few microns, instantaneously nudging the cartridge holding the spindle in the direction needed to correct the vibration.

The SSU can update forces on the spindle 16,000 times per second. At 3,600 rpm, that’s 266 corrections per revolution, more than enough to correct typical chatter vibrations, says Jim Lauffer (8727), who helped design the unique telemetry system necessary to "fly" sensor data off the rotating spindle and a control system to filter out transmission signals while minimizing the time delay.

"You can hear the machine squeal from the chatter," says Jeff Dohner (1749). "Then when you turn the SSU on, the chatter is silenced."

Now that a developmental SSU has been demonstrated, says Terry, the developers hope to find a manufacturer willing to adapt it for more universal applications.

"The performance characterizations suggest the SSU approach would be particularly useful for milling hard-to-machine materials such as high-strength steels, titanium, and nickel-based superalloys, or for machining deep pockets in molds and dies," says Terry.

"The digital world allows you to design an intelligent machine that wouldn’t have been possible a decade ago," says Jeff. "I think we can apply today’s technologies to a lot of manufacturing processes to make them better and more economical."

"The idea that you could not just prevent, but in real-time recover from, instability on a milling machine is something not too many people believed we could do," adds Jim. "Here with the economy down, it would really help some US companies if we could put this into a commercial product. I think others will look at this effort and say it’s a significant achievement."

Sandia conducted the detailed computational modeling (coupling cutting forces and structural dynamics), controls analysis, hardware development and implementation, and experimental characterization of the SSU’s performance on and off the milling machine.

Lockheed Martin and AST developed the actuators. IAI developed the controls hardware. Ingersoll provided the hexapod milling machine platform.

Labs SSU team members include Terry, Jeff, Jim, Dave Kelton (9125), and Brian Driessen (9124).