Successful applications find an optimal blend of speed and accuracy. But as technology advances, there is less room for compromise. And while machine tool builders steadily introduce machining centers with higher speeds and feeds, linear positioning accuracy and repeatability standards seem static.

The industry follows a wide variety of standards and definitions to ensure a machine tool performs its movements precisely and consistently. But one of the more exacting accuracy standards seems to go unmeasured and unnoticed-volumetric accuracy. An understanding of volumetric accuracy, of how it is measured, as well as machine tool and thermal characteristics that lend to it, is an important step toward obtaining continually shorter lead times and meeting increasingly high quality expectations.

The Added Dimension

Machine tool spec sheets typically guarantee accuracy on a linear, or one-dimensional (1-D), basis. Volumetric accuracy ensures a machining center can precisely position itself in a three-dimensional (3-D) volume in the x, y and z axes.

"Machine tools come with a guaranteed 1-D, linear accuracy specification," says Steve Colston, sales/marketing manager for Makino. "But aerospace companies working on contoured, 3-D surfaces are interested in volumetric accuracy. Ironically, coordinate measurement machines, or CMMs, are tested for volumetric accuracy when most CMM applications are 2-D. If machine tools are being used in 3-D applications, then they should be held to 3-D standards. It just makes sense."

Colston also notes the heightened need for volumetric accuracy is driven by industry factors. "More shops are being driven by their customers to hold closer tolerances and maintain the integrity of the true geometry."

"Today's technology demands higher accuracies," says Jeff Wallace, CAD/CAM application engineer for Makino. "Build-to-fit is extinct. We are dealing with a digital, math-based environment now. We can go from a customer's solid or wire frame models, to toolpath generation, to a high-speed machine tool capable of near net finishes or better. With less human intervention than ever in the process, and less chance for error, it is up to the machine tool to deliver these complex accuracies the first time."

Two Tests Prove out Volumetric Accuracy

To illustrate volumetric accuracy, Makino recently had Robert (Buz) Callaghan, chief engineer at Independent Quality Labs, Inc., take a randomly chosen V55 high-speed vertical machining center through the two tests that measure it. Independent Quality Labs specializes in 3-D measurement and Callaghan has a wealth of experience on this topic.

"Volumetric accuracy is a problem- avoidance tool," says Callaghan. "A machine tool with known volumetric accuracy has the capability of producing known tolerances. The operator knows the machine can perform exactly as programmed."

Before a diagonal laser was used to test volumetric accuracy, the machine was first checked for linear accuracy. The V55 has a guaranteed linear accuracy of ± 0.0001". This machine was verified to be well within spec with the X at ±0.000070", the Y at ±0.000063" and the Z at ±0.000074".

Under the laser diagonal positioning test, a machine's volumetric performance is measured as it simultaneously exercises six degrees of movement, and the squareness of three axes; x, y and z. Roll, pitch, yaw, straightness, squareness, parallelism and positioning affect the results. Special fixturing is used to mount a laser interferometer on the machine's table and a reflector in the spindle.

"We run the laser diagonally to the machine axis to measure diagonal displacement, which includes the machine's angular errors and records the maximum error," says Callaghan. "The test is taken from a cold start, prior to machine warm-up. The laser moves at a 45-degree angle from each corner of the table at 100 ipm. The laser takes measurements at one-inch intervals over 40 inches of diagonal travel to test the deviation in each pass. Six passes are taken from each corner, three up and three down, for a total of 24 passes." (See Figures 1 and 2)

While linear positioning and repeatability are commonly guaranteed in tenths or millionths of an inch, volumetric accuracies typically result in errors of tenths or thousandths. While it can seem difficult at first to understand how a larger number can represent higher accuracies, Callaghan says volumetric accuracy represents 3-D movement and combines the linear movements of each axis with the geometry of the machine.

As Figure 1 details, each line on the diagram is an average of a total of three runs volumetrically in one direction. "It is rare that you find a machining center with a volumetric accuracy that small over that length," says Callaghan. "The laser diagonal positioning test also shows the average reversal error, or backlash, as well as repeatability." This machine's average reversal error of all four diagonals (24 reversals) is .000173", with a volumetric accuracy repeatability bandwidth of .000071".

Are the Servos in Sync?

A circularity test is also conducted to ensure circular interpolation is accurate. High-speed machining applications rely on high feedrates on one axis with a light chipload. This dynamic behavior requires strong servos to maintain mold contours. To determine how well the servos are tuned, the test follows a circular path in one plane.

"The circular path can decrease as the feedrate increases," says Callaghan. "It is important to maintain servo control over a wide range of radii and feedrates. We measure the path to determine where control is lost so the servos can be adjusted and errors eliminated."

If the volumetric accuracy is out of tolerance on a machining center with multiple leveling points, it can be realigned to adjust the volumetric accuracies-to some degree. But in most cases, volumetric accuracy is determined when a machine is designed and manufactured. "It is built into the machine tool," says Colston. "Several factors of machine tool design, construction and thermal stability impact volumetric accuracy."

Machine Design is Key to Volumetric Accuracy

Built to Last

Based on their experience in the field, both Colston and Callaghan point to leveling systems as the design characteristic usually impacting volumetric accuracy. "Multi-point leveling requires hours for setup and it tends to be unstable if set up incorrectly," says Callaghan. "If the foundation cracks, settles or is impacted by the shop environment, the multi-point leveling accuracy will change. And if a multi-point machine tool is moved, it needs to be realigned."

"Makino high-performance machining centers utilize a three-point leveling system and incorporate much thicker castings to carry the weight and maintain the accuracy between the leveling points," says Colston. "They are easier to install, easier to move, and maintain better volumetric accuracy over the life of the machine because they rely less on the foundation than a multi-point system."

"Machine tool symmetry also lends to volumetric accuracy," says Callaghan. "If a machine tool is fairly square, with no cantilevered or overhung loads, it provides more support to the axes." On the V55, the x and y axes are totally supported from both ends of the machine by the casting. The z axis also has a very short overhang due to the design of the column.

Other V55 design benefits include extremely rigid, heavily reinforced castings. As some machining centers weigh more than 20,000 pounds, they bring rigidity to an application, can hold accuracies over longer periods of time and resist heat more effectively due to their large mass. But no matter how well a machining center is designed and constructed, there is no escaping heat at high speeds.

The Heat is Off

Speed creates heat, which impacts accuracy and repeatability. This simple equation becomes more problematic with the longer cycle times and higher speeds and feeds of high-speed applications. However, most applications focus so much on keeping heat away from the workpiece that other areas impacted by thermal distortion are overlooked. Thermal stability is maintained by improved heat dissipation throughout the machining center's various components.

• Spindle: High-speed spindles can experience growth due to heat from friction running at high rpms and require a long saturation period before they stabilize. Too much heat will compromise accuracy and can cause failure. "Makino's patented core cooling and under-race lubrication system cools the spindle from the inside out to minimize heat and growth for a shorter saturation period," says Colston. "The cooling system circulates Makino's spindle oil through the center of the rotating spindle. At high rpms, centrifugal force draws the lubricant outward through the spindle circulating through holes in the inner bearing races to lubricate and chill the bearings."

• Ballscrews: Makino addresses ballscrew heat from high feedrate levels by forcing chilled oil through the core. Hot chips, and chip-heated coolant are also kept away by shields installed to protect the machining center.

• Support Components: Running normal operations, miscellaneous machining center components can also become heat sources. The location and design of pumps, motors, hydraulics and magnetics are key. Some machining centers are designed so these components are mounted at the rear of the machine with a dead air space isolating them. To ensure this heat cannot impact the machining center, a radiator cooling system is sometimes used to wrap the machining center column.

• Environment: But even with these safeguards in place, the shop environment must be checked for external heat sources. From sunlight on the machining center to external heat on the shop floor-any increase or decrease in temperature can negatively impact accuracy and repeatability.

The Future of Accuracy

While standards will continue to fight for dominance, volumetric accuracy will undoubtedly become more widely recognized, and more widely followed. As testing becomes simpler and more cost effective, it might even be used on your shop floor to help diagnose an accuracy problem. Makino is continuing its own research and development on volumetric accuracy to ensure the standard continues to evolve along with the rest of the machining center.

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