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PLANT

MAG 5-axis system reduces error compensation


March 31, 2010
by PLANT STAFF

VEC improves machining accuracies for large machine tools that produce large, monolithic and complex-shaped parts.

Photo: MAG

MISSISSAUGA, Ont.: MAG IAS Canada Inc.’s new 5-axis volumetric error compensation (VEC) system reduces the time to determine needed error compensations from days to hours, and integrates both linear and rotary axes into the tool point compensation process.

The machine tool and systems company, with offices in Mississauga, Ont., claims its VEC system analyzes and corrects positioning errors in all machine-tool axes simultaneously to achieve machining accuracies that weren’t possible on large parts.

To perform a VEC event, an NC program positions the active target to a cloud of some 200 points representing a series of statistically randomized multi-axis “poses” within the work envelope.

Measurements are automated within a single coordinate system using laser tracker technology, a simple metrology tool that does not require extensive training to use. Calibration is performed in just a few hours in a single setup, compared to conventional methods that require multiple setups and several days of time, yet fail to capture volumetric axis interactions.    

VEC is offered as a standard option on new MAG machines and is available through MAG’s service group for field upgrade of legacy machines.

Multi-axis VEC collectively treats all of a machine’s degrees of freedom that affect tool point positioning, unlike conventional calibration methods that sequentially examine machine motion one axis at time, said Jim Dallam, MAG’s VEC product manager.

Conventional approaches to volumetric compensation are generally limited to three linear axes and the associated total of 21 potential motion error sources. However, a typical five-axis machine with linear and rotary axes can have 43 potential error sources. He says MAG’s multi-axis VEC system compensates for all these, and more, in machines with unique and more complex multi-axis configurations.

The multi-axis methodology originated in Boeing R&D, and uses innovative laser technology from Automated Precision Inc. A T3 Laser Tracker is placed in the work piece position and directs the beam to the Active Target, mounted in the machine tool’s spindle. These interact to maintain a metrology “beam lock” during the volumetric calibration event.

To perform a VEC event, an NC program positions the Active Target to a cloud of some 200 points representing a series of statistically randomized multi-axis “poses” within the work envelope. Dallam said the same NC program is run three times, first with the Active Target at a long tool length, then twice again at a short tool length.

The 200 commanded and measured positions from the first two runs are mathematically combined to establish each tool axis vector orientation and the third run gives a measure for repeatability. Automated software processes all pose/point data as simultaneous polynomial equations to determine volumetric compensation based on the kinematic error model of the machine.

The compensation solution is then entered into the control, where “compile cycle” technology integrates the compensations into real-time CNC path control algorithms. The volumetric accuracy compensations work in conjunction with, and on top of, traditional, underlying single axis and cross-axis comps, said Dallam.

“Measurements are automated within a single coordinate system using laser tracker technology, a simple metrology tool that does not require extensive training to use,” he said. “Calibration is performed in just a few hours in a single setup, compared to conventional methods that require multiple setups and several days of time, yet fail to capture volumetric axis interactions.”