New technology reduces errors, speeds part measurement
August 15, 2012
Proving out of parts, scrap reduction, and traceability are all reasons to more closely integrate measurement and production activities.
There are many ways to incorporate a part-measurement system into the production process. Depending on the data that needs collected, options are available to reduce measuring time, increase process reliability, and improve the overall production process.
A trend that has emerged over the past few years is measuring a part with a portable measuring device while it is still on the machine bed. These portable coordinate measuring machines (CMMs) can be brought to the production floor to take measurements while the parts are still on the machine, ensuring that errors are not created through repeated clamping and unclamping of the workpiece.
This verifies that parts are being made to spec long before they reach the quality control area.
Data can be collected by using a touch probe, an analog scanning probe, or a laser scanner. The data is then output and transferred to statistical databases. Manufacturers also can output graphical reports based on the information gathered.
“It’s imperative for a manufacturing company to measure its parts. The question then becomes, does it happen on the tail end of production, during production itself, or a combination of the two?” said Eric Hollenbeck, product manager for portable technology at Hexagon Metrology.
If measurement occurs at the end of the production process, no downtime is incurred taking the part from the machine to the CMM, waiting while it’s measured, then receiving the feedback on the quality of the part. Then, if the part is out of spec, it needs to be remachined. Additionally, since it’s quite common for machining processes to be repeatable, an out-of-spec part will consistently be out of spec, and the entire batch will need to be corrected.
“Portable coordinate measuring machines highlight deviations from tolerances when they might still be corrected for. It essentially gives a more real-time picture of how well the manufacturing process is running,” said Hollenbeck.
While the portable arm-type measuring system is not a total replacement for a clean-room CMM in all applications, it can be beneficial to manufacturers in many circumstances, especially in shops that produce low-volume, high-mix parts.
“A dedicated CMM in a clean room still has its place for final inspection of tight-tolerance items,” said Hollenbeck. “It’s the higher level of accuracy on a clean-room CMM [that] makes it ideal for those parts.”
The proving out of parts, scrap reduction, and traceability of production problems can all be improved when the measuring process becomes more closely tied to production, and all are priorities in today’s manufacturing environment.
With the increased use of high-precision, sophisticated manufacturing systems– especially in the aerospace, defense, automotive, and energy markets – proving out of parts is critical.
“When you’re talking about reducing scrap and tracking problems, you’re talking about lean manufacturing, which companies have been striving for for some time. Every dollar saved is one invested in the bottom line,” said Hollenbeck.
A portable system can be a real time-saver when measuring oversized and heavy parts. Because it inspects the parts in place, it reduces their movement around the shop and keeps them out of the quality control room. Portable CMMs also are suitable for rugged environments as they are designed and built to be used in these areas.
Certain types of portable systems have limitations, however, specifically, the arm-type measuring devices.
“The arm’s limitations are inherent to its construction,” explained Hollenbeck. “Although the ROMER Absolute Arm is our most accurate arm, up 23 percent when compared to our previous version and with repeatability of 0.016 mm, it is still not as accurate as most bridge CMMs.”
Another drawback, which again is inherent in the design of these systems, is that it is not automatic. An operator has to “shoot” points manually during the inspection process.
For shops dealing with a large number of parts, the arm-type CMM typically is not an option. In these cases, laser scanning systems are a better choice.
A laser scanner can inspect large numbers of parts with intricate components are easily. An operator simply “paints” the parts, gathering hundreds of thousands or even millions of points depending on the object’s size.
As measuring devices become more commonplace both on the shop floor and within machine tools themselves, the software that controls these devices also is evolving.
The driver? Being able to more tightly control the process by removing the manual interaction of the operator, while making corrections to the machining cycle and offsets as soon as possible.
This closed-loop style of process control is fully automated and consistent, which eliminates operator errors, provides statistical adjustment, and even tracks the amount of dimensional wear on the tool.
“Many more companies both large and small are moving to a closed-loop control system,” explained David Tandori, Canadian sales manager for Blum LMT. “As automation continues to grow in manufacturing, closed-loop feedback becomes essential. A system like AutoComp makes the operator no longer responsible in the decision and action of changing tool offsets. They can’t make a mistake on the direction, amount, or in keying in the wrong value.”
Tracking the amount of tool offsetting over time also is important in a closed-loop system.
“By tracking the amount of adjustment to each tool, we can determine tool performance in order to predict when a tool should be changed, and either inform the operator or call a redundant sister tool automatically,” said Tandori.
Knowing how a tool is wearing dimensionally allows the operator to perform a tool change before the tool has worn to the point of making bad parts or creating a bad surface finish.
These trends provide important and verifiable performance data that can be used to evaluate and then reduce cycle times, as well as costs. These data can actually help determine if the selected tool is meeting expectations. The data also is automatically reported to anywhere on the network so it can be used to verify part tolerances, even off the shop floor.
“This type of software makes the entire process easier and faster for an operator, so it is more likely that he actually will measure more parts,” said Tandori.
This means that productivity from stock to part – including measurement – can be improved.
“Any customer that is doing some type of production, even 20-part runs, and that measures their parts electronically is an ideal user,” said Tandori. “The alternative is having people make the daily compensation decisions and typing in the changes [at the machine], all resulting in a much higher probability of error.”