Canadian Electronics DPN PIQ

Unlocking the hidden potential of CMMs

Rob Colman
January 11, 2010
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.

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