Measurement: CMMs on Target for Productivity
Production is the name of the game for contract manufacturers like Turnamatic Machine Inc. (Richardson, TX). At this supplier of machined medical parts and assemblies, every activity-including quality control-must support the delivery of perfect parts. The company simply cannot afford for its three coordinate measuring machines (CMMs) to hold up the flow coming from the six-axis Swiss-style lathes, five-axis mills and other machines in its shop.
To keep pace with production and relieve bottlenecks, the quality department bought another CMM to help the other three perform in-process and final inspection on hundreds of parts a day. The move is characteristic of a growing group of companies that have been investing in the latest flexible and fast CMMs for high-mix manufacturing. Not only do these CMMs have accuracy commensurate with the capabilities of today’s machine tools, but they also can be programmed offline to perform multiple measurements in one setup.
CMM builders report that this ability is not really due to any major technological breakthroughs over the past few years, but from a steady refinement of existing technology to increase efficiency. “Measurement time has been decreased through optimizing the machines and sensors,” observes Dr. Hubert Lettenbauer, senior director - systems at Carl Zeiss Industrial Metrology LLC (Maple Grove, MN). “In addition, the software has been improved to shorten programming and analysis time.” The software is more intuitive and easier to use.
As important as these refinements have been to justifying the new CMM at Turnamatic, they played second fiddle to the accuracy. “The parts we measure typically have size tolerances of +0.0005 inch/-0.0000 inch and position tolerances of 0.005 inch,” explains Mark Munnerlyn, quality manager. The CMM measures to those accuracies, in addition to providing the flexibility necessary for fitting into the high-mix measuring environment.
The flexibility comes in part from the active scanning sensor. The sensor’s head accepts a variety of probes, permitting the machine to change them in the middle of a routine, much like machining and turning centers do with cutting tools. Because styli can be as long as 800 millimeters and weigh as much as 600 grams, the CMM can exchange a short probe for a longer one to reach into deep features, for example. The sensor also allows the machine to perform single-point measurements and to do contact scanning at 300 millimeters per second.
Another contributor to flexibility is the computer-aided design (CAD)-based metrology software. Using the simulator feature technicians import customers’ CAD data and program the new machine and others offline, thereby freeing the machines to process parts while new programs are being written. “The same program can be used on multiple CMMs,” notes Munnerlyn. Operators are able to control three CMMs with one program running on the communications network.
Munnerlyn and his staff handle all in-process and final inspection data electronically. Using a Merge-to-File feature, they can export data to other software for additional analysis. The software has other features that allow Turnamatic to track those components that have serial numbers and recall the data upon request. For some customers that receive the parts directly into stock without reinspection, the contract manufacturer sends the data with shipments.
Many Sensors, One JobThe ability to change probes is not unique to the sensor on Turnamatic Machine’s CMM. In fact, CMM builders identify multisensor carriers as one of the leading technical trends helping their machines to make inroads into a greater number of smaller manufacturing facilities. These carriers, however, can accept more than just the contact probes that Munnerlyn and his team are using. Many can also wield a variety of noncontact sensors.
Just as continuous-contact scanning probes caught on during the previous decade, it seems that various types of noncontact probes have been gaining a measure of popularity over the last few years. Examples are vision cameras and laser scanners.
“Many users find that their conventionally tooled CMMs are sufficient for measuring the vast majority of their parts,” notes Eric Bennett, product manager at Hexagon. “However, they may have the occasional small feature or flat part that is well suited to measurement via a vision camera. Rather than investing in a vision machine, these users may find an advantage in adding a vision probe to a CMM.”
Speed is another advantage. These optical sensors can collect and process large amounts of data in a much shorter period of time, making the CMMs wielding them more productive.
Because such sensors are usually integrated through the same software interface, not only can CMMs have access to a variety of sensors, but they also can use more than one kind in measuring various features on the same part. Vision cameras, for example, can check features that are too small to accommodate probes. Free-form surfaces lend themselves to laser scanning. Contact probes, however, are usually best for tapers inside bores.
Accuracy: A Hot TopicAlthough Turnamatic Machine houses its CMMs in their own environment, not all CMMs need one anymore. “One of the biggest advances in CMMs over the past decade, especially the last few years, is the continuing trend of pushing dimensional inspection from the quality lab to the shop floor, next to the machines that are making the parts,” says Bennett at Hexagon. “With the CMM on the shop floor, inspection becomes simply another operation in the manufacturing process.” Parts need not be taken into measurement rooms for inspection and returned to subsequent processing.
This move into the shop is possible for a number of reasons. First is that builders are designing their CMMs to be less susceptible to thermal changes and more resilient to shop-borne contaminants. Other reasons include special damping devices that isolate CMMs from vibration transmitted through the floor and software that compensates for thermal expansion based upon ambient temperature readings taken by sensors in the CMM.
Accuracy is not a trivial question these days, especially in precision machine shops. Because many of these shops are asked to hold tolerances to tenths or better on a regular basis, their machine tools can often be more accurate than some low-end CMMs, which might have accuracies of only 3.8 microns, or 1.5 tenths. Even in environmentally controlled rooms, these CMMs are unacceptable for these jobs, if you accept the 10% rule that the accuracy of a measuring device should be within 10% of the tolerance being measured.
“For example, a true position callout of 0.001 inch (diametrical tolerance zone) only allows a total of 0.0005-inch radial deviation,” offers Eric Tingle, national CMM sales coordinator, Mitutoyo America Corp. (Aurora, IL). “In order to verify a callout of 0.001-inch true position, your measuring machine must be 0.00005 inch (50 millionths) or better to qualify as a 10:1 measuring method.”
He points out that these applications need a high-end machine with a measurement uncertainty of 0.35 micron, or 14 millionths. These CMMs, of course, are more expensive because of the materials and mechanisms used in their construction. For example, a fixed-bridge design made of a spheriodal graphite ductile cast iron and has guideways plasma-coated with a ceramic to minimize thermal distortion. To guard against vibration, not only do these high-end machines sit on air-damped springs, but a floating mechanism also isolates each ballscrew from the reader head.
More manufacturers are finding these extras necessary. “We are receiving more inquiries from small to large companies that want to differentiate themselves in measurement uncertainty,” reports Tingle. “Most companies that are AS9000 compliant, or are striving to be, need a 10:1 uncertainty. In most cases, their [current] CMM does not qualify for high-precision measurements.” So, these companies are upgrading to stay on target for greater productivity and profit. Q
For more information on CMMs, visit www.qualitymag.com to read the following:
“Multisensor CMMs Add Flexibility”
“CMM Market: A Look at the Future”
“Identical CMMs, or Not”
Tech TipsCompanies today have been investing in the latest flexible and fast CMMs for high-mix manufacturing.
CMMs can be programmed offline to perform multiple measurements in one setup.
The software has been improved to shorten programming and analysis time.
Application Advice“Probably the most important things users can do to get the most from their CMMs is to keep their software up to date, their machines well maintained, and their CMM programmers and operators well trained,” offers Eric Bennett at Hexagon.
Lettenbauer at Zeiss recommends looking beyond your current requirements. “Today’s environment changes quickly, so tomorrow you may have to measure other parts with other characteristics and other precision requirements,” he says. “The machine you buy should be flexible enough to cope with these changes and to be able to use other sensor technologies.”