Old CMMs: Recondition vs. Upgrade

by Klaus Ulbrich

It usually costs much less to upgrade a
CMM than buy a new one, and an old machine
can be more accurate than a new one.

Modern coordinate measuring machine technology is more than 25 years old. Yet air bearings, granite and the basics of machine-frame design have not changed. In contrast, electronics, computers, sensors and software have advanced substantially. With these advancements have come changes in manufacturing tolerances. Today, tolerances are tighter, part diversity is greater, and the manufacturer demands more complexity and flexibility of its inspection department. Thus the inspection department demands more from its CMM.

To keep up, metrologists buy new CMMs-usually an expensive proposition. Or, they upgrade their existing CMMs. More and more this is becoming a viable alternative. Many companies offer to bring a CMM into the 1990s. It usually costs much less to upgrade a CMM and, with the latest technology, an old machine can be more accurate than a brand-new one.

That's right, more accurate. The first CMM builders corrected inaccuracies through hard work. If the granite wasn't perfectly flat, it was lapped. If a guide wasn't straight, it was remachined. The CMM's mechanical parts were made as accurately as possible-resulting in a highly repeatable machine.

In the late 1980s, computers were sufficiently developed so that a CMM manufacturer could eliminate any remaining imperfections with computer compensation. With this procedure, a builder maps a CMM's inaccuracies and compensates for them through the computer software. Today, nearly all CMMs use computer compensation.

Unfortunately, some builders, relying on the assumption that mapping overcomes all inaccuracies, have lowered the mechanical precision of a CMM's frame by using less expensive frame materials. Not surprisingly, then, older frames are more mechanically precise than the ones made since computer compensation became an industry standard. In addition, an older CMM, having settled for several years, has higher stability because the initial stresses of assembly are gone. And, when upgraded properly, they are mapped on site, making them more accurate than those mapped by builders at the factory.

TRW recently asked Electronic Measuring Devices Inc. to recondition and upgrade a Zeiss UMM500. Designed in the late 1960s, the UMM500 was still state-of-the-art when TRW bought a new unit in 1981. Using current technology, EMD made the UMM500 more accurate than it was in 1981 for a lot less than the cost of a new one. Plus, they gave it multisensor capability, installing continuous-contact and noncontact 3-D analog scanning probes and software.

Recondition or upgrade?

This is an important question. Some companies will recondition an old touch-trigger CMM with the latest motors, electronics and software, and perform computer compensation, bringing it up to today's standards for that type of machine. But, better yet, consider upgrading to continuous-contact analog scanning.

A CMM with analog scanning capability can collect thousands of data points per minute, compared with the traditional touch-trigger method, which allows for collection of no more than a small fraction of that amount.

Touch-trigger technology acquires data at the precise moment that the probe impacts the part. One impact, one data-point acquisition.

An analog probe measures displacement in space with built-in sensors and a miniature 3-D stage. Analog probes also are an order of magnitude more accurate than touch-trigger probes because they can process-out dynamic oscillation and vibrations occurring in the machine. And analog probes don't suffer the inherent error of lobing, commonly found in touch-trigger probes.

Contour applications

An analog probe excels when numerous data points are required, e.g., with shape or contour applications. Its ability to take continuous data enables it to scan any part, known or unknown, without ever leaving the surface. In effect, the probe feels around the part in a smooth, continuous motion, providing a true picture of the part and the most data-rich inspection information possible.

Trying to make a touch-trigger CMM do the equivalent of scanning, which many CMM makers are attempting to do these days, is actually transforming them into sewing machines instead of precision CMMs. And it's easy to see why: The CMM must accelerate away from the part until the probe lifts off, traverse to the next clearance point, decelerate until stopped, approach the part until the probe senses the impact, then decelerate to a stop. The high-speed repetition of this motion sets up resonant vibrations in the machine and results in reduced accuracy. This kind of motion also deteriorates the CMM's service life.

Analog software

The simplicity of an analog scanning CMM's computer programming offers another advantage. The software to control and operate scanning is much simpler than the software for touch-trigger CMMs. In the case of scanning, all the probe needs is a starting point and a direction to begin passively recording data.

A touch-trigger CMM needs highly specific instructions, requiring huge amounts of programming language code, because it must already "know" the part and be able to tell the probe where to touch the part.

Of course, not every measurement routine requires a scanning CMM. However, with higher computer speeds, scanning's high data-acquisition rates allow an operator to make accurate form and shape measurements. In fact, one manufacturer uses scanning to inspect the edges of end mills-something not feasible with a touch-trigger probe.

Optical vision sensor technology

Most gage labs have a vision-based measurement system. Despite this, companies are beginning to realize the benefits of combining vision inspection capability with existing contact capability on a CMM.

Companies want vision capability on their CMM because a CMM is a more precise staging system, and dedicated image processing can now be done directly in the CMM's computer. CMMs also offer computer-controlled incidental lighting, oblique lighting and back-lit light tables, all critical factors for accurate readings. In addition, a CMM is well-suited for auto-focus range-finding measurements over its full stroke, not just in field-of-view.

Fortunately, just as older CMMs can be upgraded to the latest in analog scanning technology for a lot less than buying a new one, they can be outfitted with vision capability and seamlessly switch between contact and vision.

Final words of warning

Many CMM computer systems have become obsolete and unsupportable. You may no longer be able to get parts or service. A proper upgrade replaces all dated components, using a system that can accept future advances in computers and controllers. If it can't, it's not worth doing. Also, when contemplating a CMM update, look for experienced personnel with a proven track record-ask for references and talk to them.

Getting the most up-to-date CMM capability does not require buying all new components. Many times, a retrofit or upgrade is more cost-effective. The old mechanical frame is often superior to the new one. Take advantage of the precision built into that aging frame and breathe new life into it.

About the author

Klaus Ulbrich is a co-founder of Electronic Measuring Devices Inc., a leading designer and supplier of coordinate measuring systems and high-accuracy probing technology for coordinate measuring machines. He is responsible for the design and development of the company's SCEPTRE System: a continuous-contact analog probe and software for CMMs. Previously, he served as a product manager for American Sip Corp.