Robotic Testing Systems Handle High-Volume Tensile Testing
The Davenport Works of the Aluminum Co. of America (Alcoa) is a major producer of aluminum sheet and plate products used for aircraft components, automobile bodies, ships and many other applications. The quality of round-the-clock production is checked by testing samples from production lots representing as much as 40,000 lb of aluminum each day to ensure mechanical properties meet the specification requirements. This results in a steady flow of test samples to the plant's physical testing laboratory, which, in the past, was difficult for lab personnel to keep up with.
Operators in the lab were performing all tensile tests manually on vertical universal testing machines. The operators measured each sample prior to testing using micrometers, placed it in a machine, measured the length of the broken specimen after the test, reduced the data on hand-held calculators, read the stress/strain information from graphs obtained on drum-type recorders to determine yield and entered test results on data sheets. Tensile testing machines often were used day and night, in three eight-hour shifts, five operators per shift, but the lab still remained far behind testing demand.
Five days could pass from the time the specimen arrived until it was released for shipment according to lab supervisor Dannie Shryack. Because just a few specimens represent as much as 40,000 lb of finished material, a backlog of many specimens resulted in millions of pounds of aluminum held in inventory at all times, creating handling and storage problems and increasing costs. It was necessary to speed flow time on tensile tests, which are the first tests conducted when a sample arrives at the lab. It also was necessary to handle a very uneven demand-a large number of samples are received at the lab when a furnace is unloaded-in a timely fashion.
Automating tensile testing
The Davenport plant is a world leader in computer-integrated manufacturing and other plant automation. Engineers at the lab realized they needed computer-controlled robotic testing to help keep up with the large workflow. The lab introduced a plan to incorporate a robotically loaded tensile machine that could test a wide range of round and flat specimens within a few hours after the samples arrived. Robotic testing became the goal for a carefully planned program of steady advances in automated testing.
The lab selected Tinius Olsen to reach its goal of implementing automated tensile testing because the company offered expertise in computer-controlled tensile testing, which was needed before it would be possible to use robotics. The determining factor was Olsen's expertise in microcomputer-controlled horizontal tensile testing machines, which lend themselves to automation for robotic systems much better than vertical machines. In addition, the automatic extensometer offered with the machines also was an important factor; the extensometer remains on the sample through rupture, providing a good elongation measurement.
Alcoa first established a unified identification system for all samples. Each specimen had its own number, which identified the coil or plate from which it was taken, the test to be performed and the way the test was to be run. This identification system was part of a plant-wide tracking system containing the information on which tests were to be made. The correct flow of information to the proper locations was essential to realize the goal of robotic testing, because a computer is only as good as the information that it receives. After establishing this information base, the lab installed the first microcomputer-controlled Mechanical Horizontal Tensile (MHT) testing system.
The 30,000-lb capacity MHT was a stepping stone to total automation. It included an advanced computer-aided data acquisition and control system, and performed calculations that formerly required several personnel to make using calculators. In addition, the automatic extensometer furnished accurate tensile, yield and elongation measurements based on a 2-in. gage-length sample. The computer-controlled MHT performed tests automatically, processing all information, plotting x-y stress-strain curves and printing out and storing all test results. The MHT primarily was used to test 0.505-in. diameter specimens and gradually interfaced it to the plant-wide tracking system established earlier to eliminate the need for manual data entry.
The lab automated its specimen-measurement process, interfacing laser micrometers (for round specimens) and electronic gages (for flat specimens) to the data acquisition system. Then it implemented a system that generated bar code labels for all required test specimens using information from the plant-wide tracking system, simultaneously opening a file on microcomputer-based local area network (LAN). This file included the minimum/maximum results required from the tests.
Subsequently, a 5,000-lb capacity MHT was installed and a Roell + Korthaus (R + K) robotic testing system (Gottmadingen, Germany) having totally automated specimen handling of flat specimens was added. The R + K system (marketed in the U.S. by Olsen) helped the lab establish its automatic procedures for the fully automated robotic MHTs. These procedures included using a robot arm to move specimens, reading a bar code on the sample for test data, measuring the sample's width and thickness, and automatically conducting the tensile test through rupture. The R + K system tested from 125-175 0.006 to 0.10-in. thick flat specimens per shift.
The lab installed a third MHT having a 30,000 lb capacity and incorporating a Zymark (Hopkinton, Mass.) programmable robot specimen handler. The programmable two-fingered manipulator removed flat specimens (from 0.100 to 0.5 in. thick) for plate products from one of the six two-sided racks, placed them in a measuring station, inserted each into the testing machine's grips for the test, removed the broken pieces after the test was completed and communicated all results to the lab's LAN. The programmable robot system was so successful that a similar system was retrofitted on the MHT 5000 system, replacing the R + K installation. The new system was used to test flat samples 9 in. long by 0.005 to 0.100 in. thick. A fourth MHT was installed, a totally automated robotic 30,000-lb capacity system, to test 0.505-in. diameter specimens from plate products.
The machines work simultaneously during the day shift under the control of a single operator at a control station. In the totally automated operation, microcomputers run the tests and results go into the lab's LAN. Then, after all samples from a particular lot have been tested, information on the lot is sent to the plant's mainframe computer for wider use in the plant-wide tracking system, management reports and the SPC information base.
Benefits of automation
The robotic systems provides the following benefits:
- The lab handles the normal production workflow in two, eight-hour daytime shifts. This often took several days under the previous labor-intensive system in which the machines were operated during all three eight-hour shifts. Each of the three robotic tensile testers can test from 150 to 180 samples per shift.
- The computer-directed robotic systems automatically range (autorange) through the appropriate load ranges for samples of varying thicknesses. This eliminates the time-consuming procedure of manually setting up machines for different ranges.
- Tensile testing usually is completed within eight hours after a sample comes in the door. Peak demands (with a larger-than-usual workflow) are handled by the next day. In special demand situations, the lab can test samples from a production lot in 15 minutes, enabling the lot to be shipped within half an hour. As soon as all tests are completed and the specimens pass, the robotic system releases the lots in real time for immediate shipment.
- The systems keep working through normal shift changes and coffee breaks. If necessary, they can be set to run unattended at night in "lights-out" mode, freeing personnel for other assignments.
- One operator per shift at a control station near the tensile testers separates the samples and loads the specimen racks. Before the robotic systems were installed, four or five persons were needed on each shift to measure samples, load them individually into the machine and perform other tasks.
- Testing costs have been reduced by more than 50%-one of the most significant benefits of the robotic system.
- Return on investment on the robotic systems is about 60% per year because of their manpower and productivity savings.
- Reliability and uniformity of testing have been significantly enhanced. When specimens were measured before a test using a micrometer, each operator had a different touch, so results varied. The machine is operator-independent. The robotic systems are much more precise. One of the problems in the past was a mix-up between lots, resulting in 10 or 12 errors a year on the average. Incorrect test results would be discovered downstream, a big inconvenience because of having to retest. Now there is zero mixing of lots because the testing system is coupled with Alcoa's design software, which allows matching test data with actual samples and lots more accurately.
- The robotic system allows Alcoa-designed statistical process control software to be fully implemented. Collecting statistical data on how products are performing in the tensile test allows further product improvement. The fully integrated computerized system allows close examination of the manufacturing process to identify factors that produce the most success.
All of the robotic testing systems use a Tinius Olsen Model 290 Electroluminescent Digital Indicating System, which provides four display channels for almost unlimited display configuration options. Advantages include autoranging, autozeroing, trillingual display and variable sample break detector. Also, the robotic systems use identical components so parts can be switched if emergency repairs are ever necessary.
The lab now is a showcase for advanced robotic testing technology together with the latest state-of-the-art equipment for fracture toughness and fatigue testing, as well as computerized milling machines and lathes.