QMT Features: April 2009
10x precision improvement
Improve your machine tool’s precision by up to ten times - in a matter of hours -  with API’s Volumetric Error Compensation (V.E.C.)

When using large machine tools many factors can conspire to reduce the accuracy of the tool throughout it’s working volume. Factors such as machine load, force of cutting, foundational deviations and simple wear over time make the machine less accurate.

Most solutions to this problem involve highly specialised measuring equipment and substantial machine downtime for measurements. Traditional methods often take several days to perform and require repeated set-up changes of the measuring equipment. Accurate results are almost impossible due to temperature fluctuations during this long process.

Many times, even with these complicated techniques, only a fraction of the 21 error parameters in a given machine volume are actually measured. Since so many of the possible machine errors are not accounted for, the final results of this type of machine error compensation are not satisfactory for improved production quality and speeds. With the machine tool being idle for several days during the measurement process, it is not uncommon for tens to hundreds of thousands of production dollars to be lost due to machine downtime.

Volumetric error compensation
The API approach to Volumetric Error Compensation (VEC) addresses all of the shortcomings of traditional methods and makes machine compensation easy and far more affordable. The machine tool’s precision can be increased between four and tenfold in a matter of hours rather than days. API’s VEC measures the machine’s complete working volume and gives a true volumetric compensation for every point.

API’s VEC process uses a T3 Laser Tracker and a patented spindle-mounted API Active Target which is loaded into the machine tool for the T3 to follow. The first step of the process is to build a 3 Dimensional Machine Model for kinematic calculations and collision detection purposes. API provides user-friendly software that builds the 3D machine model for different types of machines. It produces a complex path for the tracker to follow for all of the different setups of the machine, carefully avoiding all obstacles during the measurement process. Within the machine’s working volume, between 200 and 400 random points are generated to represent all of the possible machine poses in each axis as the final measurement plan. The software simulation ensures these random points are collision-free and will not break the laser beam from the T3 to the Active Target during measurement.
Once the Measurement Plan is loaded into the machine tool controller the actual measuring can begin. The T3 Laser Tracker is placed in a fixed position and the Active Target is mounted to a long adapter loaded into the machine tool for the first measurement run.

The T3 and Active Target interact to maintain constant measuring contact as the machine tool is positioned to each of the random points, forming a point cloud within the machine work envelope. As the machine reaches each designated point, it stops for a few seconds to stabilize. The Tracker 3 measures the point 200 times in less than 3 seconds. These measurements are then averaged to give the most accurate positioning of the point in all 6 degrees of freedom.

After the first run the Active Target is placed on a short adapter and the process begins again and runs through the same points a second time. By using the long and short adapters every point is measured not only in the linear X,Y, Z positions, but the system also captures each point’s orientation in space, as well, with pitch, yaw, and roll.

Once the measurement runs are complete the data is then contrasted with the actual commanded or desired location. API’s proprietary software then develops the compensation values to drastically reduce the machine tolerance errors. These values are validated by the software as correct and accurate for volumetric compensation before they are downloaded to the machine’s controller. The software then determines the volumetric compensation using a polynomial equation-based, kinematic error model of the machine. Once these values are validated by the software, they are uploaded directly into the machine tool controller. After the calibration, when the production machine is commanded to a desired location, the volumetric compensations are automatically applied during the movement of the machine to improve the path and actual location of the machine tool.

API claim that the  VEC Process offers the fastest and most accurate compensation of machine tools available. Since all of the measurements are done so quickly, there is very little thermal drift due to temperature fluctuations. API’s special software allows the compensation tables to be uploaded in a matter of minutes.l

email: john.rohde@apisensor.com

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