Machine control: Long bed CNC gantry mills achieve unprecedented accuracy
A Siemens Volumetric Compensation System and proprietary temperature compensation system combine with laser calibration to achieve +/- 0.003 in. accuracies.
Coast Composites is a vertically integrated supplier of Invar tooling, as well as mandrels and resin transfer molds used for composite lay-up in the manufacture of very large aerospace structures. The company operates two facilities, comprising nearly 80,000 sq. ft. in Irvine, California and a new facility over 115,000 sq. ft. in Santa Ana, California, producing tooling for aircraft structures used by the OEM and primary contractor segments of the aerospace industry.
Coast Composites Inc., part of the UK-based Hampson Industries Plc, is a major supplier of Invar tooling, as well as resin transfer molds and mandrels used in the composite lay-up and manufacture of today's advanced flight-critical aerospace structures. Coast also builds tooling for the construction of end products like satellite reflectors used in the telecom and military markets. On the large, long bed CNC gantry mills used at its main facility in Irvine, CA, the company is achieving unprecedented accuracy on even the longest wing skins as a result of two complementary technologies.
Coast Composites runs many grades of aluminum, plus A36, P20, stainless, monolithic graphite, carbon fiber composites and other materials, in addition to Invar 36 and 42. The largest CNC mill at Coast is a German-built Handtmann five-axis machine that uses twin Siemens Sinumerik 840D numerical controls and accommodates structures to 22 ft. wide x 75 ft. long x 84 in. high. The Siemens Volumetric Compensation System (VCS) acts in tandem with a proprietary temperature compensation system devised and implemented by Coast engineers. Combining these technologies with ongoing laser calibration, Coast achieves and maintains accuracies to +/- 0.003 inches on its longest runs.
The showcase machine tool at the Coast facility in Irvine is a Handtmann five-axis CNC long bed gantry mill, capable of running structures to 22' W x 75' L x 84" H, utilizing a powerful 80 hp, 60 kw spindle up to 15,000 rpm. The 40,000 lb. bridge can move at 2200 ipm. Using the Siemens Volumetric Compensation System (VCS) and its own proprietary temperature compensation system, Coast achieves a better than +/- .0003" accuracy on wing skin tooling and other structures that often occupy the entire machine bed.
Jerry Anthony, Coast Composites founder and "director of best practices," says, "We knew the Siemens Volumetric Compensation System had substantial merit for our operation when it was first introduced to us, precisely because of the workpiece sizes we run and the corresponding thermal growth during machining, even on Invar tools. We had developed our temperature compensation system to adjust the go-to points from the control for real-time temperature on the machine and the workpiece. The data covered all aspects of our machining, from material composition to acceleration/deceleration mapping and the temperature variations over the largest pieces and longest machine cycles we had performed."
Compensating for temp variations
Anthony says the key point of interface for such a system is within the algorithms of the CNC, acting to compensate the signal before sending commands to the motors and drives for adaptive action on the machine tool. In a typical long machining cycle on the very large parts produced at Coast, temperature variations in the workpiece material combine with the naturally occurring thermal expansion in the machine tool to create serious dimensional issues. This is true for whatever material is being run on the machine, even the Invar high nickel-iron alloy used extensively at Coast for producing composite lay-up tooling, fiber tape mandrels and other structures.
While Invar has an extremely low coefficient of thermal expansion, the length of the machine cycle invariably (the word from which Invar got its name) results in sufficient thermal movement of the machine, which adversely affects the tool tip position.
Operator uses one of the twin Siemens Sinumerik 840D CNC stations to control the movements on the Handtmann gantry mill. The two CNC units are linked to communicate seamlessly in parallel, thus allowing a single operator to run this massive machine safely and efficiently.
In operation, Siemens VCS factors all the machine kinematics of its various axes of motion, as well as pitch, yaw and roll, to precisely adjust the orientation of the tool tip to the workpiece. The true workpiece coordinates, rather than the programmed tool tip position, determine the actual cutting path. In conjunction with the highly advanced Siemens cutter path contouring, this compensation process results in a smoother finish on the part surface and a very high degree of machining accuracy.
In the Coast application, they have further enhanced this process by combining its proprietary temperature compensation system with the VCS inside the CNC to produce a highly accurate cutting path that takes into account the thermal expansion rates for the particular material being run as well as the machine itself.
While other companies in the industry typically run such machine tools in a completely air-conditioned environment, Coast Composites has devised its temperature compensation system to overcome these variables in ambient
factory conditions and still maintain an accuracy that often exceeds the customer specification, regardless of the material being machined. The standard NC program assumes a 20nsation is within the machine control-resulting in real-time adjustments to the machine movement and an overall improved surface finish with superior accuracy, according to Anthony.
The Siemens CNC is also used to complete the on-machine inspection of the workpiece. "We have used laser tracking in tandem with our Valisys inspection software for some time now, having had a Siemens CNC engineer in Elk Grove Village, IL write the machine tool inspection interface software for us," says Anthony. "In this way, we were and are now still able to use the power of the CNC to run real-time inspections and data analyses. [These] have been extremely useful as an in-process inspection system for any tool manufactured by Coast."
An operator monitors a high-temperature 1800
Anthony says that the Coast system "essentially turns every machine tool here into its own CMM." The laser tracker is now used primarily for final checking, as the in-process monitoring produces what he says "might be the highest-accuracy machining standard in the aerospace industry." He says Coast has "run enough material types and enough cycles to develop an impressive data base of averages over time. Our algorithms are the key and, when married to the Siemens power onboard the CNC, result in the old adage that says the whole really is greater than the sum of the parts."
Invar tooling and mandrels produced at Coast Composites are used for the production of various commercial and military aircraft.
On the massive Handtmann five-axis gantry mill, application engineering and training assistance were provided to Coast Composites by the machine tool builder's local representative, Bryan Wilson of BD Technology, as well as personnel from the Siemens Aerospace Center of Competence . "The relationship between the Siemens Aerospace Center of Competence and Coast Composites began seven years ago and has progressed to a true partnership, as the two companies have worked together consistently, merging advanced theoretical technologies with experiential knowledge," says Anthony .
- Edited by Renee Robbins, senior editor
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