Industry evolution requires new accuracy standards. (Technology Notebook).
Understanding volumetric accuracy--how it is measured, as well as machine tool and thermal characteristics that affect it--is an important step toward obtaining shorter lead times and meeting increasing quality expectations.
Industry follows a wide variety of standards and definitions to ensure that a machine tool performs its movements precisely and consistently. Still, one of the more exacting accuracy standards that often goes unmeasured and unnoticed is volumetric accuracy.
The added dimension
Machine tool spec sheets typically guarantee accuracy on a linear or two-dimensional basis. Volumetric accuracy ensures a machining center can precisely position itself in a three-dimensional volume in the x-, y- and z-axes.
"Machine tools come with a guaranteed linear accuracy specification but shops with applications working on contoured, 3D cores and cavities are interested in volumetric accuracy" says Tom Siegel, milling product specialist for the Makino Die/Mold Group, Mason, OH. "Ironically coordinate measurement machines (CMMs), are tested for volumetric accuracy when most CMM applications are 2D. If dies and molds are being checked for 3D accuracy, the machine tool should be held to similar standards."
Siegel also notes that a heightened need for volumetric accuracy is driven by industry factors. "More die and mold shops are customer-driven 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's Die/Mold Group. "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 tool-path generation for a high-speed machine tool capable of near net finishes or better with less human intervention and less chance for error."
Proving volumetric accuracy
To illustrate volumetric accuracy, Makino had Robert (Buz) Callaghan, chief engineer at Independent Quality Labs, Inc., Wyoming, RI, take a randomly chosen V55 high-speed vertical machining center through two tests that measure it. Independent Quality Labs specializes in 3D measurement and Callaghan has a wealth of experience on this topic.
"Volumetric accuracy is a problem avoidance tool," says Callaghan. "A machine tool with proven volumetric accuracy has the capability of producing known tolerances in dies and molds. The operator knows the machine can perform exactly as programmed."
Before the V55 laser diagonal and volumetric accuracy testing began, the machine was checked for linear accuracy. Tests were run without scale feed back. Tests run by Makino have guaranteed linear accuracy for this machine of [+ or -]0.0001". This machine was verified to be well within spec with the x-axis at [+ or -]0.000070", the y-axis at [+ or -]0.000063"' and the z-axis at [+ or -]0.000074".
Under the laser diagonal positioning test, the volumetric performance of a machine 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 table and a reflector in the spindle.
"We run the laser diagonally to the machine axis to measure diagonal displacement, which includes angular machine errors, and record 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. Measurements are taken by the laser at one inch intervals over 40 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.
While linear positioning and repeatability commonly are guaranteed in tenths or millionths of an inch, volumetric accuracies typically result in errors of tenths or thousandths. While at first it can seem difficult to understand how a larger number can represent higher accuracies. Callaghan says volumetric accuracy represents 3D movement and combines the linear movements of each axis with the geometry of the machine.
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 0.000173" with a volumetric accuracy repeatability bandwidth of 0.000071".
Are servos in sync?
A test also is 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 feed rates. 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.
Siegel 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." Most mold shops do not have the resources to constantly check a machine's leveling system.
Says Siegel: "Several factors of machine tool design, construction, and thermal stability impact volumetric accuracy."
"Machine tool symmetry also enhances 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 for example, the x- and y-axes are totally supported from both ends of the machine by the casting.
Other V55 design considerations include rigid, heavily reinforced castings. 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 centers s various components.
Spindle: High-speed spindles can experience growth due to heat from friction running at high rpms and require a saturation period before they stabilize. Too much heat will compromise accuracy and can cause failure. "Makino's core cooling and under race lubrication system cools the spindle from the inside out to minimize heat and control growth," says Siegel.
Ballscrews: Makino addresses baliscrew heat from high feedrate levels by forcing chilled oil through the core. Hot chips, and chip-heated coolant, are kept away by shields installed to protect the machining center.
Support Components: Miscellaneous machine components also become heat sources. The location and design of pumps, motors, hydraulics, 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 sometimes is used to wrap the machining center column.
Environment: Even with safeguards in place, the shop environment must be checked for external heat sources. From sunlight on the machining center to external heat and vibrations on the shop floor, any increase or decrease in temperature can impact accuracy and repeatability.
The future of accuracy
While standards will continue to fight for dominance, volumetric accuracy undoubtedly will become more widely recognized, and more widely followed. As testing becomes simpler and more cost effective, it will be used on even more shop floors to help diagnose an accuracy problem.
Makino Die/Mold Group, Mason, OH, http://www.RSLeads.com/l09tp-252 or circle 252
Quality Labs Inc., Wyoming, RI, http://www.RSLeads.com/l O9tp-235 or circle 235
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|Publication:||Tooling & Production|
|Date:||Sep 1, 2001|
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