Commercial Success: Defense Manufacturing Excellence Award to Automated Precision Inc. (API)

In 2012, Automated Precision Inc. (API) was awarded the “Defense Manufacturing Excellence Award – Volumetric Accuracy for Large Machine Tools (VALMT)”. Originally coordinated through the efforts of the National Center for Manufacturing Sciences’ (NCMS) Commercial Technologies for Manufacturing Activities (CTMA) Program, this project was developed through the joint technical collaboration of API, Boeing, Siemens, and Cincinnati MAG (now Fives). The project entailed creating an innovative process for capturing a machine error and creating a calibration process which would use the data and correct for the error in the machine controller, ultimately improving the overall machine accuracy. Since completion of this patented volumetric error compensation (VEC) calibration process, hundreds of machine tools have been calibrated
with excellent results in a fraction of the time (week to hours).

The purpose of this article is to share these commercial successes and show the value of technology devel- opment to enhance readiness for America’s Warfighters.

Large monolithic parts are inte- gral within today’s weapons sys- tems whether they be subma- rine propellers, aircraft skins, airframe structures, or armored vehicle hulls. Best manufacturing processes are not good enough to produce truly interchange- able large parts; this results in the need to make inefficient and costly assembly accommoda- tions such as make-to-fit, drill-at- assembly, shim-as-needed, and rework. The VALMT project devel- oped and demonstrated new technology to overcome these limitations and enable achieving machining tolerances within 0.005” over the entire 20’ x 10’ x 5’ or larger working volume of big machine tools. The method uses a Laser Tracker, an Active Target (tracking target used with the Laser Tracker), and analysis software to derive parametric- equation-based, real-time posi- tion and path compensation for a large machine equipped with a modern machine tool controller.

Background

All machine tools need maintenance, adjustment, and calibration over time. For precision multiaxis machines, scheduled service including machine conditions, performance, and cali- bration are necessary to maintain machine performance, thereby reducing scrap or rework. Regular calibration is also a must for owners who rely on a machine’s probing or measurement capability to inspect their parts. Too often, owners rely on simple checks with a ball-bar or posi- tional lasers which measure only one major axis at a time to get an overall check of the machine’s accuracy. Unfortunately, these conventional devices do not give the owner the total picture of the machine tool health.

A complete calibration of the volume of the working machine is the answer. The calibration and subsequent compensation of a medium to large, three or more axis machine tool throughout its entire machining volume is critical for owners producing complex and accurate parts. True volumetric calibrations give owners the con- fidence in their machine’s total accuracy, from tool tip throughout its entire volume. This includes all axes, including the rotaries, and is the only way to eliminate minor angular error other than mechan- ical adjustment.

Problem Statement/ Traditional Machine Tool Compensation

Volumetric compensation is essen- tially an innovative method of assembling the volumetric map on a given machine. Until now, a typ- ical three-axis machine has used the traditional 21-error-parameter method to measure machine axis errors. Measurements were taken one at a time along each individual axis, (X, Y, and Z). Each of the three axes has six error parameters; linear position, vertical and horizontal straight- ness, pitch, yaw, and roll. Each error parameter required a different set-up and measurement pro- cess. In addition to these 18-error-parameters, three additional error parameters determine squareness of each of the X, Y, Z axis to each other, X to Y, Y to Z, and Z to X, for the total of 21-error- parameters. Even with the most sophisticated laser, this data acquisition is a long process. If the machine tool has addi- tional axes, like a rotary axis, the calibration time increases rapidly. Lastly, and users can perform a diagonal displacement test that, in theory, mea- sures the volume of the machine tool. This test is also very time consuming and is still not a robust indicator of the machine’s volumetric accuracy.

Solution

API’s approach to the vol- umetric calibration solu- tion consists of a preci- sion laser tracker with an interferometer capability (NIST Traceable). The interferometer feature of the laser tracker is the same technology used with traditional laser sys- tems. Coupling the inter- ferometry technology with a laser tracker offers the added capability to track the machine’s tool tip location through extensive axial move- ments throughout the entire volume of the machine.

To accurately measure and track the machine’s tool tip position, API developed a spindle- mounted targeting system called an Active Target. The Active Target is a compact motor- ized target system which allows the laser tracker to accurately measure the machine’s tool tip position from a single instrument’s position. This capability is sig- nificant since the VEC process intentionally exercises the spindle in real time through all of the machine’s axes throughout the mea- surement process. This process would not be possible with a standard laser tracker reflector due to the a line-of–sight issue throughout the machine’s volume. This real-time tracking capa- bility for measuring the precision tool tip posi- tion in space makes the advanced process possible. Unlike conventional volumetric calibration processes, this innovative system no longer has to be aligned to the major axes of a machine tool. This process has dramatic advantages in terms of time and system performance.

VEC using the Laser Tracker and Active Target measures all 21-error-parameters for 200 to 400 points within the machine’s working volume with all pos- sible machine poses, even with the rotary axis. Each point is then an accurate reflection of the machine’s kinematic errors. A complex algo- rithm is used to decouple error sources and build an extremely accurate

volumetric map. Error compensation is done by adding the compen- sation from the entire measured axis to get the best tool tip position.

In addition to the mea- suring system, the API solution also includes a sophisticated propri- etary calibration soft- ware which uses a poly- nomial equation-based, kinematic model of the machine to map errors throughout the volume. In order to compensate the machine in realtime, the API software resides on the machine’s con- troller or an industrial computer that directly interfaces with the con- troller and machine tool.

In the simplest terms, the software uses the

measurements from the Laser Tracker and Active Target, then develops compensation
values to drastically reduce machine errors. Once the machine is put
back in production, the API software running in the background will monitor the
programmed toolpath and automatically compensate the tool tip to the desired
location in realtime.

The Five Step API VEC Process
1. Build a 3D machine model for kinematic cal- culations and collision detection purposes. API provides user-friendly soft- ware which builds the 3D machine model for dif- ferent types of machines. It produces a complex path for the tracker to follow in all of the different set- ups of the machine, care- fully avoiding all obsta- cles during the measure- ment process. Within the machining volume, between 200 and 400 random points are gener-

ated to represent all of the possible machine poses in each axis as the final mea- surement plan. The soft- ware simulation ensures these random points are collision-free and will not break the laser beam from the Laser Tracker to the Active Target which is loaded in the machine spindle during measure- ments. The machine model is typically gener- ated in advance of the actual calibration event.

2. Using the measure- ment plan from Step One, perform the actual mea-

surement of the machine with the API Laser Tracker and Active Target. The Active Target is held in the machine’s spindle by a pre- cision fixture that allows the indexing of the target to coincide with the tool tip position. The fixture has two positions, a long and short tool. One com- plete set of laser tracker measurements are taken from the long tool posi- tion and another from the short tool position. These measurements become the basis of the machine’s calibration and compensa- tion. Typically, this process takes one to two hours.

3. Raw measurements from Step Two are then loaded in the API soft- ware. This is a one-button calibration and optimiza- tion process which nor- mally takes just a few min- utes.

4. The calibration is then validated using the output from Step Three and another set of measure- ment data from the Laser Tracker and Active Target. Once this step is validated, the machine calibration results are loaded directly into the controller for real- time compensation. In a variety of field tests and now through actual machine tool compensa- tion events, this new VEC methodology has proven to improve machine tool accuracy by fourfold or better. The chart (upper left) shows five different types of machine tools which were compensated using VEC. The maximum error used as a benchmark was defined as the largest single error in any error parameter of the machine. In many instances the machine owner was unaware of the exact con- dition of the machine

before the error-mapping process.
Machine 2 for example, had a 1 mm
positioning deviation before VEC but
reduced the maximum error to just
39 microns after VEC.
5. Perhaps the most interesting
case is Machine 3. This machine was
compensated using the traditional
21-error-parameter method immediately
before using the VEC process.
Even in a machine tool that had just
been compensated, VEC improved
tool center point accuracy fourfold.
Commercial Market Success
of VEC
Lockheed Martin was the first company
in the world to employ API’s
VEC solution to test the validity of
its five-axis milling machines. This
solution ensures that the movements
across all six degrees (X, Y,
Z, pitch, yaw and roll) are accurate
and can even actively compensate
through the machine’s controllers
any errors in movement or angles.
VEC represents a massive change
for Lockheed Martin for machine
tool measurement. API was only
selected after a thorough evaluation
which proved the system’s
advantages over common calibration
practices.