For managers already facing numerous business challenges, large 100-foot plus gantry mills that fail to produce wing spars and other aerospace components to specified tolerances are an especially expensive headache. In less than a year, normal wear can reduce a machine's ability to hold tolerances and when the moving components of a machine tool crash, putting the machine back in tolerance can be enormously expensive. In most situations, calibration can improve a machine's ability to hold tolerances and its overall performance.
Many aerospace OEM's require machine calibration every six months to a year as part of their vendor certification programs to ensure part specifications. In the past, calibration and compensation were expensive processes that took a machine down for three to five days, or more. With new technology and processes, large gantry mills can be 3D volumetrically (not 3-axis) calibrated and compensated in as little as a day.
One such system that speeds the process is the MCV-5000 Aerospace Calibration System from Optodyne. The system allows an extremely fast process for 3D volumetric calibration and compensation of 3-axis and 5-axis machines - even large 150-ft gantry mills.
By way of definition, 3D volumetric calibration does not mean linear measurements along three axes. Linear measurements do not consider straightness or squareness. Volumetric calibration measures linear, straightness and squareness errors. The Sequential Step Diagonal Measurement Method allows 12 sets of data to be collected with the same four diagonal setups.
The static positioning errors include three linear displacement errors, six straightness errors, and three squareness errors. The angular errors include pitch, yaw and roll errors of each axis. In addition to static measurements, the system measures such dynamic errors as servo parameter tuning, effect of feed forward and look ahead, velocity and acceleration. Also, circular and non-circular contouring performance is measured.
Based on this data, the system's Window's based metrolmeasured positioning errors and automatically generates a 3D (not 3-axis), compensation file for most leading controllers.
A profile milling machine approximately 100 ft long with three 3-axis heads for machining large aerospace parts.
Additionally, measured data can be used to determine such servo parameters as loop gains, look-ahead, and feed forward. For example, one company measured 4 in. diameter circular contours at a feed rate of 200 in./min.
The total time of the measurement was less than 13 minutes. When different diameter runs were required, new setups and alignments were unnecessary; only the CNC program was changed. The additional time for each diameter was five minutes per run.
Typically, the system measures the volumetric positioning errors of a working volume of 4m3 to 5m3 in two or three hours. Usually, conventional interferometers require a week to complete volumetric calibration and compensation. With this new system, even with a large gantry machine, the measurements can be made in single day.
AFTER THE CRASH
During a maintenance procedure, the 5-axis head of a gantry mill with a 100ft x 25ft bed crashed into a tombstone while traveling at about 600 in./min. After the crash, the mill was unable to hold 0.0020 in. over 20 in. when cutting across a flat plane. Repairs by the factory were estimated to be $2 million. Because of the high repair cost, the aerospace manufacturer opted for a few minor repairs to make the machine operational and to put it back in service rough cutting parts.
With increasing demand for the large components and a growing backlog, the management decided to take another look at the machine's accuracy issues. With proper calibration and compensation, performance might be improved enough for regular production. The Aerospace Calibration System allowed this manufacturer to take another look at the machine's accuracy issues.
Because of the length of the machine, it was split into four overlapping 3m cubes for volumetric measurement purposes. The measurements were compiled to produce an error map. The MCV-5000's two longrange laser heads were used to automatically synchronize and measure the master and slave axis displacement errors, and pitch and yaw angular errors.
The single-aperture laser and a flat-mirror target measured the volumetric positioning errors, including 3-displacement errors, 6-straightness errors and 3-squareness errors. We have measured the volumetric positioning errors by the laser vector technique. The volumetric positioning errors include 3-displacement errors, 6-straightness errors and 3-squareness errors. The working volume of each of the four cubes the machine had been divided into was 2m x 3m x 1m. The total time of the measurement was less than two hours.
Using two flat-mirrors, two fast interface cards and software, the circular and non-circular contours were measured for servo tuning or dynamic performance. The measured displacement data was used to calculate the actual feed-rate, the acceleration/deceleration, and the machine vibrations.
The non-contact measurement and the circular contouring diameters were continuously varied to reach very small diameters.
The Windows-based metrology software collected the data automatically, on-the-fly. Additionally, the software processed the data and generated error tables that were uploaded to the Siemens control. Because of the size of this machine, the error tables from the four cubes were compiled into a compensation table and uploaded to the controller. On a smaller machine, the software automatically generates compensation tables for most leading controllers.
The volumetric calibration and compensation process improved performance, so the machine now holds 0.0010 in. through a 200 in. 12 in. diameter fly cut, allowing the 5-axis gantry mill back into full production. From start to finish, the entire process was completed in less than two days.