CONTROLLING MACHINE VIBRATION

Last month we compared the use of box ways vs. linear guides in damping machine vibration. Now we want to explore how vibration can be monitored using the newest technologies that are available.

HOW DO YOU MONITOR VIBRATION?
Dennis Nichols, Senior Applications Engineer, Okuma America Corporation:
Typical machine tools are structurally designed to minimize the possibility of vibration. Among these vibration-limiting features is the use of cast iron in the machine beds. Cast iron has much better damping characteristics than the steel weldments that are used by some machine tool builders.

Certain lathes use a technology called “harmonic spindle speed control”. This option does not monitor vibrations. However, the spindle speed is oscillated within a narrow band. This oscillation prevents the vibrations from resonating and developing into full-fledged chatter.

Another newly-developed technology for machining centers is tentatively being called “chatter control,” which uses sensors to detect vibration that indicates imminent chatter. When vibration is detected, this system communicates with the machine tool control to collect data from the vibration sample, calculate the optimum spindle speed required to avoid any chatter, then automatically change the spindle rotation speed and eliminate the chatter.

At that point, the operator can manually choose whether to use that speed or continue with the previously programmed speed.

William Howard, Product Line Manager, Makino:
Machine tools utilize a number of techniques (built into the machine – and added to the machine in the form of options) to monitor and address cutting conditions – namely vibration and chatter. The basic machine tool design incorporates spindle and servo motor monitoring.

This integrated monitoring system has the ability to sense and react to changing cutting conditions in real time. By communicating with proprietary software in the machine tool control, the spindle and servo motor feedback is sampled and used to identify any change in cutting conditions, such as chip load, tool engagement, material consistency and hardness, etc. Any of these changes may produce vibration, causing the system to immediately adjust the conditions so that chatter is minimized or eliminated during the cut.

The spindle also incorporates a number of features based on principles specifically designed to minimize vibration and chatter, including:
? Short, compact spindle length to increase stiffness and rigidity and therefore reduce vibration.
? Appropriate spacing (or distance ratio) between the various bearings (i.e. lower and upper) that support the spindle and suppress spindle vibration.
? Very tight thermal control of the spindle (during high speed operation) which permits close tolerance of the spindle elements – reducing the potential for lost motion, providing greater stiffness/rigidity and resulting in less vibration and chatter.
? Utilize super precision, hybrid bearings that provide higher stiffness and rigidity to the spindle design and feature finer surface finishes to both the inner/outer race and rolling elements of the bearing for less vibration and finer finishes.
? Incorporate higher stiffness/rigidity designs (i.e. HSK) at the spindle/tooling interface that translates the stiffness/rigidity of the spindle to the tool.
? Dynamically balance the spindle to insure that the basic spindle will achieve both high rpm performance and minimal vibration.

Some machine tools incorporate an active vibration monitoring system that uses an accelerometer built into the spindle to constantly monitor vibration resulting from the cutting process. Proprietary software internally compares years of experiential data against the monitored information to make decisions relative to the active cutting conditions.

These cutting conditions are then altered to provide more stable cutting conditions, or the spindle might be stopped and a warning sent to the operator about conditions considered non-stable (those that exceed allowable vibration/acceleration parameters).

Obviously, the advantage of this active monitoring is the ability to provide real-time data that is immediately used to address vibration, make part processing decisions – and prevent damage to the spindle.

Tom O’Brien, Engineer, Setco:
Accelerometers are typically used to monitor vibration in machine tools. These devices are secured by a magnetic base to make periodic measurements, or maybe rigidly stud-mounted to the machine frame for continuous monitoring systems.

Accelerometers are often selected because of their wide range of uses. They are rugged and are largely unaffected by the oil and coolant that is common in the machining industry. They can operate in the harshest environments and are unmatched in both their frequency and amplitude ranges.

Next month, we’ll conclude our series by exploring how machining different types of materials affects vibration. We’ll also compare the use of gear-driven spindles vs. direct drive spindles and hand-scraping mating surfaces vs. other methods.

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Kyndall Brown is the assistant editor of Fabricating & Metalworking magazine. She can be reached at kyndall.brown@cygnusb2b.com.

Okuma America Corporation, 11900 Westhall Drive, Charlotte, NC 28278, 704-588-7000, www.okuma.com.

Makino, 7680 Innovation Way, Mason, OH 45040-8003, 800-552-3288, Fax 513-573-456, www.makino.com.

Setco Inc., 5880 Hillside Avenue, Cincinnati, OH 45233, 513-941-5110, Fax: 513-941-6913, www.setcousa.com.