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.
