Speeding Inspection and Accuracy in the Future of Quality Assurance

It’s an age-old issue-the quality department versus production. While the coordinate measuring machine assures quality, it slows things down; it clogs shop-floor flow.

“The CMM has always been considered a major bottleneck,” says Denis Zayia, CMM product manager at Hoffman Estates, Renishaw . Company management not only sees the machine as a capital expenditure, but it also doesn’t make anything. And on top of this, it presents a bottleneck. No wonder management often accepts the CMM only as a necessary evil.

For this reason, he says, the future of quality assurance will not only be about speed but accuracy as well.Two technologies dominate the CMM world. Touch-trigger, the older and more established method, measures by taking single discrete points when contacting the surface. It’s less expensive and allows for less complex control and interface systems.

Scanning-a technology continuing to gain in popularity in the contact-measurement field, particularly for high-value work-is capable of collecting hundreds if not thousands of points with a sensor moving along the workpiece surface. More measurement points, of course, give more information about the features. While touch-trigger can suffice when simply looking for location and size information, scanning technology is the preferred methods when it comes to finding out about a features shape or form.

A traditional downside to the scanning method has been the effects of the machine dynamics. This is why throughput has always been a challenge on scanning CMMs.

Much of the issue around the scanning speed centers around the contribution of the machine’s dynamics to the measurement; the faster one scans, the more the dynamic error one introduces into the measurement “Picture a car going around a race track,” he says. “The faster you go, the harder it is for the car to keep going around that oval.”

Picture a typical CMM with a granite table and a bridge that travels on air bearings. With a touch-trigger CMM, the machine moves at constant velocity when taking the measurement, and at a constant velocity the dynamic effects of a machine are, in effect, treated like a static position.

“With scanning technology, staying in touch with the part surface at higher speeds tends to be difficult "because you’re fighting dynamics,” he explains. Moving the stylus requires moving a bridge of relatively large mass. To accelerate it requires moving the entire weight, and if that structure lacks stiffness, it will become even more difficult to maintain constant contact.

Consider a simple ring gauge. “The faster I go the more dynamic effect is seen in the measurements,” Zayia says. “If I go 10 mm per second, I may get 2 microns of form error. If I then crank it up to 100 mm per second, I might get eight microns of form error. That’s due to the dynamics of the machine. Going around the circle, the entire mass must accelerate and decelerate, over and over.”It’s simply the laws of physics as you are constantly accelerating and decelerating a heavy mass that may or may not be a stiff platform.”

With touch-trigger technology, there is acceleration and deceleration during machine moves, however, constant velocity with measurement moves. Again, no dynamic effect in measurement moves.

Speeding Up
The next step in CMM technology, Zayia says, is to overcome the trade off of speed versus accuracy-and allow it to go faster, without error, to open the bottleneck. Recently Renishaw introduced its Revo system that attempts to overcome these difficulties. With this system, the dynamic head adds two additional rotary axes – one in horizontal plane, one vertical – to give infinite rotation and positioning capability in Y and Z axes inside the probe/stylus range.

Where X motion is needed, the bridge can move at a constant velocity to minimize dynamic effects. The Y and Z capabilities of the CMM are used mainly to bring the Revo head into range for making the actual measurement. No motion is required in those axes during measurement.

“By getting the head to do most of the work, you are not as reliant on the machine dynamics,” he says. “The machine still has a role to play, but it’s not nearly what it is with conventional scanning.”

The technology has presented advantages for the aerospace industry, measuring blisks without the need to change out tips and index to reach the myriad angles required, including being able to traverse over the blade’s leading and trailing edges. And high-volume engine-block operations have seen their share of benefit, too.

When the new system inspects parts, the head does a lot of the work that the CMM used to. For instance, when scanning cylinder heads, the head performs a helical-type movements, taking about 4,000 data points a second, while the bridge itself moves only in a single axis at constant velocity to avoid acc/dec errors. (There needs to be X motion – pull back – to create the helical, spiraling motion. Otherwise, the stylus will describe only a circle.)

Throughput in the QA department involves more than just inspection time. As Zayia explains, “When you look at throughput, people always tend to ask, “How long is it going to take me to inspect this part?” What tends to get overlooked are the qualification times with different tips, or tools. Every time you use a different tip in a different position, each one of those different positions has to be qualified.”

He adds that it’s not uncommon for some to spend in excess of 20 hours a month just qualifying the tips and positions before inspection. For each stylus, “I need to know what size ball is at the end of the tip, and I need to know what angle I’m at, because if I have my head at, say, the 0,0 angle, straight up and down, a certain measurement will result. Now, if I index that to 90 degrees, I must account for the effects of gravity; and the longer the stylus is, the greater those effects are”;-hence, the need for qualifying each tip and each angle. The CMM programmer simply reaches into his “tool box” and picks up the tool he needs to inspect his feature.

New scanning technology helps reduce those qualification times to minutes instead of hours. Different tips still need to be qualified but not new positions. First, the tip can stop at any angle, according to Zayia, so this eliminates the need to find new tips to access hard-to-reach part features. Second, every angle need not be qualified, thanks to a laser beam integrated into the Revo head and special hollow stylus.

The laser, enclosed in the system, is directed at a reflector just above the stylus tip. Once the stylus touches the part, it deflects slightly, so the reflector just above the stylus is, in effect, displaced. This alters the laser’s path of return, and that alteration is sensed by a position-sensing device (PSD). This is how the unit knows where the exact tip position is at all times. And the laser beam, having no mass, is immune to inertia effects.

Machine synchronization brings up its own challenges, since the new technology allows the machine and head itself to move simultaneously. This adds immense complexity to motion control. Both the bridge movement and stylus movement work together in five axes to take measurements simultaneously. This requires constant communication between the head, the machine and the controller. Simultaneous motion of the head and machine allows the system to scan faster without big accelerations and decelerations and the resulting inertia effects. To make that happen, Renishaw developed a 5-axis universal CMM controller that allows Revo processing to be integrated into new CMMs or retrofit to existing machines.

Scanning the Horizon
Historically, upgrading CMMs from discrete point taking to scanning has been a challenge due to machine dynamics as so many of these machines were never designed to scan. High speed scanning is now possible on these older systems due to the availability of a sensor that volunteers to do most of the work. Put another way, to get more throughput out of new and existing machines, manufacturers can focus on the sensor side of things, where the rubber meets the road or, in this case, where the probe meets the part.

Editor’s Note: Artwork courtesy of Renishaw Inc.,www.renishaw.com.