Modern machine tools continue to make technical advances, thanks in large part to ongoing improvements and fine-tuning in computer numerical control (CNC) machining. Over the past 60 years or so, CNC advances have automated even the most complex machine tool operations. At the heart of many machines is the control of a spindle — because spindles often act as a machine tool's main rotating axis.
In many cases, the shaft itself is referred to as a spindle, but in practice, the complete rotating unit — including the shaft, its bearings, and a chuck that holds a drill bit or some other tool — is also called a spindle. A spindle usually contains a shaft in its center; depending on how it is aligned within the machine, both vertical and horizontal arrangements are possible. For example, a drill press vertically plunges a rotating drill into material to create holes, whereas a milling operation uses tools with multiple cutting edges that move over a workpiece. Other milling machines can move both horizontally and vertically to do their work, while horizontal-only machines (including some boring mills) often employ extensions to hold tool spindles and give them deeper reach. No matter the application, machine tool spindles are subjected to similar demands: They must supply high-speed rotation, provide power and torque to a cutting tool, and offer fairly long life. Central to meeting these demands is the strength and quality of the spindle's bearing system.
Engineers at Hurco Companies Inc., a machine tool manufacturer based in Indianapolis, have much to say about what's required of today's spindle bearings. The company designs and manufactures all of its own spindles, and designs their controls and drives, including those for finished machining centers. Hurco uses ceramic hybrid bearings in the spindle.
“Ceramic bearings can run at higher speeds, to 10,000 or even 18,000 rpm, and stay cooler than other bearing types,” explains Hurco engineer Pete Baechle. “Where some other companies might use ABEC-5 bearings to save costs, we use ABEC-9 bearings and guarantee them to ABEC-7 levels. This means the bearings are extremely accurate and also last a long time.”
The ABEC rating refers to the tolerance that the bearings are manufactured to meet, based on standards from the American Bearing Engineers Committee (ABEC), which are also accepted by ANSI (American National Standards Institute) and on par with ISO (International Organization of Standardization).
“In addition, spindle components must be turned, milled, and ground to extremely tight accuracies,” adds Baechle. “Proper tooling balance is also important, as it guarantees a longer spindle life and a quality surface finish.”
Bearing lubrication is another important consideration. Hurco uses sealed grease-packed bearings: In contrast to atomized oil lubrication, grease-packed bearings ensure the exact lubrication necessary for the entire lifetime without needing weekly maintenance. This also keeps the work area cleaner. To further boost cleanliness, Hurco installs an upper and lower air-purge system to keep contaminants away from the spindle.
Keeping things cool
Heat is yet another area of concern. That's because as a machine's workload increases, a spindle can get very hot, leading to “spindle growth” (metal expansion), which ultimately impacts cutting accuracy. Hurco uses a coolant jacket on many of its spindles, which is designed into the frame head casting to reduce heat dissipation and stabilize temperature fluctuations.
However, depending on the duty cycle and spindle speed, a spindle chiller package may also be necessary. Traditional spindles are designed with a fixed bearing preload, so there's no compensation for thermal expansion. What happens next isn't pretty: High temperatures cause bearing raceways to expand and tighten, causing the bearing preload to exceed the original setting. This increases axial and radial loading on the bearing system, which negatively impacts tool accuracy. A spindle chiller answers these concerns and may be a prudent investment for high speed, high duty cycle applications.
For more information, call (800) 634-2416 or visit hurco.com.
Spindle resources
Spindle motors
Single or dual-wound spindle motors are high speed, low vibration motors specifically designed for machine tools. Designed to allow cooling air to enter the motor from the load machine side and exit from the opposite side, they direct heat exhaust away from the workpiece. Motors are available in either 200 or 400 V, foot or flange mount, with power ratings to 30 kW continuous.
Make contact: Yaskawa America Inc., (800) 927-5292, yaskawa.com
Connection module
A new modular program allows machine tools to be quickly automated using a shaft interface. The core module GSW-B joins standardized parallel or concentric grippers with the machine. The module features a gripper interface on one side, a 20-mm shaft on the other, and can be clamped with many conventional toolholders. The same gripper can be used on various machines, independent of the individual spindle interface.
Make contact: SCHUNK Inc., (919) 572-2705, us.schunk.com
Spindle bearings
Angular contact spindle bearings have one ring shoulder partially or totally removed, allowing a larger ball complement than in comparable deep groove bearings — so they have greater load and speed capacity. Deep groove spindle bearings have full shoulders on both sides of the raceways of the inner and outer rings and are available in a variety of sizes and cage types. A 13-minute spindle repair video demonstrates how to replace machine tool spindle bearings, includes a review of tools, and discusses the do's and don'ts of handling precision bearings.
Make contact: The Barden Corp., FAG Aerospace and Super Precision Division, (800) 243-1060, bardenbearings.com
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Large part machining gets accuracy boost
Extra-large parts such as submarine propellers and aircraft frames are often crafted from many smaller components because even the best machining technologies are not yet accurate enough for large, one-piece parts to meet required tolerances. Manufacturers aiming to use ever-larger parts often find themselves stuck with costly workarounds — make parts to fit, shim as needed, and worst of all, rework. Accuracies considered routine for small parts become much harder to achieve over the longer distances in larger parts. In some cases, cutting-tool position and orientation tolerances are a few thousands of an inch — difficult to consistently achieve for huge parts.
A joint project of the National Center for Manufacturing Science called Volumetric Accuracy for Large Machine Tools (VALMT), involving MAG Industrial Automation Systems, Automated Precision, Boeing, and Siemens, recently developed a promising approach to volumetric error compensation (VEC). Created for large and multi-axis machines, the suite of hardware and software reduces the downtime needed to determine necessary volumetric compensations from weeks to a day or less, via a simple automated process that improves a machine tool's volumetric performance by 50% or more.
The method, developed at Boeing, considers the full effects resulting from the kinematic stack-up of all machine tool axes. Where conventional approaches to volumetric compensation focus on the first 21 error sources associated with three orthogonal linear axes, the new method compensates any arbitrary stack of linear and rotary axes, addressing the 43 (or more) kinematic errors associated with a five-axis machine. A specific VEC solution is determined for every tool position and orientation combination inside the work volume. One official from Boeing estimates the breakthrough could save the company $100 million a year by reducing assembly and fitting costs on large programs like the F-18 or 700 series commercial aircraft.
The metrology system uses laser technology from Automated Precision Inc., Rockville, Md. A laser source, the T3 Laser Tracker, is placed in the work piece position. It directs a laser beam to the “active target,” mounted in the machine tool's spindle. These devices interact to maintain a metrology “beam lock” during data gathering. The procedure captures position data for a cloud of 200 statistically randomized multi-axis “poses” within the work envelope. Programmed poses are compared to measured positions over three program runs with two different tool-length dimensions for the active target. Software processes the data to determine the compensation solution.
The volumetric compensation file is then entered into the control and activated by program statements. Next, the CNC's “compile cycle” technology integrates the compensations into the realtime path interpolation algorithms. As the machine tool moves in five-axis space, compensations are applied within the interpolation loop of the CNC. For more information, visit mag-ias.com.
Information courtesy of Jim Dallam, MAG Industrial Automation Systems