Canadian Metalworking
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Unlocking the hidden potential of CMMs

Measurement is vital to any manufacturing business, providing essential information to control processes and verify products. But older co-ordinate measuring machines (CMMs) can become bottlenecks if they fail to keep pace with changing measurement needs. Advances in sensor, metrology software and controller technology now offer the opportunity to transform existing CMMs, providing greater accuracy, faster measurement, more automation and new capabilities.


Recent advances in sensors, controllers and metrology software have
transformed the performance of new CMMs. As these advances are not
dependent on the CMM frame, they are also available on older CMMs
providing more capability with simple, fast and intuitive CAD-driven
programming and graphical reports that are easier to interpret; more
throughput using the latest scanning sensors and motion control
technologies; more information about parts with automated multi sensor
measurement; and more profitability with reduced maintenance and
support costs.

The introduction of five-axis scanning provides unprecedented
performance on both new and existing machines. Measuring using five
axes of simultaneous motion enables even old machines to measure faster
at speeds of up to 500 mm/sec; measure more points at up to 4,000
points per second; measure more accurately by eliminating dynamic
errors using five-axis techniques; measure more features using infinite
sensor positioning for unparalleled flexibility; and measure without
compromise by using shorter cycle times to enable 100% inspection for
optimum verification and process control.

Consequently, the operational benefits include reduced measurement
cycle times; elimination of CMM bottlenecks and rapid process feedback;
increased uptime due to high speed head and sensor calibration; and
less time spent indexing and stylus changing.

The first generation of computer-controlled CMMs were developed to suit
touch-trigger probing, in which discrete points are acquired at key
locations on the component. The measurement process involves driving
the probe’s stylus onto the surface of the part at a constant speed, so
the CMM’s structure is not accelerating when a measurement is taken.
This means that machines did not need to be particularly stiff to
measure accurately.

The arrival of three-axis scanning drove changes to the design of CMM
structures. Scanning involves moving the stylus of the probe across the
component, following its surface contours. For instance, measuring a
feature such as a hole requires the probe to be moved in a circular
path, causes the moving elements of the CMM structure (the bridge and
quill) to undergo accelerations during measurement. These structures
are large and heavy, so accelerating them requires significant forces
resulting in inertial deflections at the stylus tip. These inertial
forces twist and deflect the machine structure, resulting in
measurement errors.

Even the fastest modern machines are limited to scanning speeds in the
region of 80 to 150 mm/sec, depending on the nature of the feature
being measured. But practical concerns about accuracy mean that most
scanning in production applications is done in the 10 to 25 mm/sec
range.

Five-axis scanning breaks through this barrier by avoiding the problem
of machine dynamic errors. Instead, the scanning head is able to
acquire surface data while moving its two rotary axes at up to three
revolutions per second, enabling scanning speeds of up to 500 mm/sec,
far beyond the capability of even the fastest CMMs. The problem of
dynamic errors is alleviated by not asking the machine to accelerate
during measurement, or at least minimising such acceleration. This is
achieved by using five axes of simultaneous motion, with the bulk of
the workload of moving the stylus tip falling to the REVO head.

The motion controller is a vital factor governing the performance of
any CMM. An outdated controller can limit the range of measurements
that can be taken, and may eventually become unsupported and unreliable.

One family of controllers has been specifically designed to suit
retrofitting with ‘plug and play’ installation on most models of CMM.
Versions supporting touch-trigger, three-axis scanning and five-axis
scanning provide an upgrade path for customers as their needs evolve.
In addition to seamless integration of a wide array of sensors, the
controllers feature advanced motion control techniques that allow rapid
movement around the part and smoother, faster scanning.

The same family of controllers is also I++ DME compliant, enabling them
to operate with any I++ compliant metrology software. This leaves users
free to choose the software that best suits their needs, and even
change software to suit different inspection tasks.

Software is perhaps the biggest single reason why many CMM users choose
to upgrade, as it is the aspect of the CMM that they interact with most
often. Slow programming, clunky program execution, unfriendly reporting
and outdated operating systems can all lead to frustration and harm
productivity.

Modern metrology software is designed to make full use of the power of
CAD, enabling rapid off-line programming with nominal and tolerance
data coming straight from the model. Reporting is now graphical rather
than tabular, making it far easier to interpret measurement data and,
more importantly, take appropriate action as a result. One of the
latest software offerings is also built on industry standards such as
the DMIS programming language and I++ DME communication protocol with
the CMM controller.

Five-axis scanning is best suited to the measurement of complex parts
where inspection cycle times are currently long, or where large volumes
of components must be measured and throughput is a principal concern.
In other instances, upgrading to a 3-axis scanning sensor or to a
trigger probe with stylus changing can provide significant productivity
and automation improvements at a lower cost.

In the current economic climate, making the most of existing assets
makes sense. Cost-effective upgrades to key elements of a
machine-sensors, controller and software-can reveal a CMM’s hidden
potential.

This article was written by Philip L Smith, technical sales manager with Renishaw (Canada) Limited.

renishaw.com


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