Dan Skulan, general manager of industrial metrology at Renishaw Inc. (renishaw.com) says there are four stages of the production process that need to be understood for comprehensive quality control. Traditionally, however, he says, companies only paid attention to two of them.
The four are:
- Process foundation
- Process setting
- Process control
- Post-process monitoring
Steps two and four are what most people have paid attention to. That, he suggests, is insufficient, especially as if steps one and three aren’t considered, overall inefficiencies can slip in.
“Machine evaluations that took hours and days can now be conducted in minutes.”
Skulan explains, “‘Process setting’ is how we refer to the setting up of an operation on a CNC machine. ‘Post-process monitoring’ is where the part is inspected. If there is a problem with the part, the settings are adjusted to compensate, and you try again. That’s the old approach.”
Starting at number one, process foundation, means understanding the equipment that is being used for the operation. “This is the step where, for example, machine tools and CMMs are calibrated for volumetric compensation,” Skulan says, adding, “Verifying a machines positioning capability provides the foundation for which all advanced process control techniques are applied.”
And the third step, in-process control, means monitoring sources of variation that are inherent to the machining process such as tool wear and temperature variation, and “providing intelligent feedback to the process and decision-making as machining progresses.”
Skulan offers examples of solutions that engage with the until-recently neglected first and third steps.
Measurements of the Machine
As for process foundation, the XM60, a volumetric laser calibration system, uses laser interferometry to measure all six degrees of freedom of each axis of a machine tool—including linear positioning, vertical and horizontal straightness, pitch, yaw and roll—in a single pass. The data collected can be directly used to provide compensation of the axes as required so as to increase accuracy throughout the working volume of the machine tool. The XM60’s breakthrough is that it can take all six of those measurements simultaneously rather than in sequence, so that “machine evaluations that took hours and days, can now be conducted in minutes.”
Measurements on the Machine
Traditionally, dedicated and hand gages have been used to verify quality at the operational stages of precision machining. The results of these measurements would then be used to make manual offsets to the machining equipment. “These gages are inflexible, require service and calibration, can be affected by the operator and require manual operation,” Skulan says. In the Smart Factory era, “factories are beginning to demand automated measurement with direct feedback of measured offsets from the gage to the machine tool.” In other words, they want it less post-process and more in-process.
Renishaw’s solution is the Equator gaging system, which uses a scanning probe on a highly repeatable (2.0 µm) parallel kinematic design that provides high-speed measurement directly at each operation. The system is programmable, thermally compensating and can be calibrated in minutes. The probe comes in two sizes—300 mm and 500 mm—and can be directly integrated with automated part handling systems. It can provide direct feedback to the cutting machine to automatically update tool and work offsets. Because the system measures so quickly, it can provide 100 percent inspection in most cases. The measurement data can also be directly output to plant and enterprise “dashboards” for management of the machining resources.
The company’s Sprint scanning probes are another example of in-process monitoring technology. The probe is kept in a CNC machine’s tool-changer and can be automatically deployed to the spindle to measure part features and location for in-process control. “This type of measurement has traditionally been reserved for CMMs, but with Sprint, measurement of feature size, location and form and finish—meaning flatness/waviness—can be directly done on many modern metalcutting machines, and without operator intervention,” Skulan says.
Renishaw has launched a simpler, more flexible software package for the Sprint called “SupaScan.” According to Skulan, “The new probe and software combination is designed for easy integration into machine tool applications that require fast workpiece set-up, and where overall cycle time is critical.” Testing on typical industrial components has seen cycle time reductions of over 70 percent when compared to standard high-speed touch-trigger cycles.
Speeding Post-process Inspection
Then there are improvements in the post-process measuring of parts. Renishaw is adding to the capabilities of the Revo five-axis multi-sensor platform for CMMs. The platform provides the ability to perform touch-trigger, high-speed tactile scanning, and non-contact vision measurements. “The system enables measurement speeds up to five times faster than previous systems without loss of accuracy,” Skulan says. Accuracy is maintained at these higher measuring speeds because only the sensor head moves rapidly, while the CMM positions the head at a constant, stately pace, eliminating measurement errors caused by the effects of inertia on the CMM structure.
A new feature for the Revo is the SFP2, a surface finish measurement probe. The ability to combine surface finish measurement and dimensional inspection on the CMM offers throughput advantages over traditional inspection methods requiring separate processes, Skulan says.
“We had a customer that measured a cylinder block for dimensional size and location in one hour and 16 minutes using their existing technology,” Skulan said. “With Revo, we measured all of the same features and added surface finish measurement as well. Total measurement time was 16 minutes. This was not only a savings of an hour per part. It also eliminated the need for secondary surface finish measurement and freed up the floor space that would have been required for a second separate system.”
Although the term “continuous improvement” is generally associated with another company, Honda is certainly pursuing that approach, as is evidenced by the Accord, which is now in its ninth generation.
By James Gaffney, Product Engineer, Precision Grinding and Patrick D. Redington, Manager, Precision Grinding Business Unit, Norton Company (Worcester, MA)
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