Unlock the hidden potential of your CMMs

Measurement is vital to any manufacturing business, providing essential information to control processes and verify products. But older coordinate 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, whilst taking full advantage of CAD-driven programming. In the current climate, upgrading makes good economic and ecological sense.

Breaking the dynamic performance barrier 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 3-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 whilst the measurement is taking place. These structures are large and heavy, so accelerating them requires significant forces and inevitably results in inertial deflections at the stylus tip that are not seen by the machine's position encoders which are located, in the case of the Y axis, on the bed of the machine.

These inertial forces twist and deflect the machine structure, resulting in measurement errors that can quickly become larger than the measurement tolerance. Sadly, the laws of physics dictate that, when measuring holes, the inertial forces increase with the square of the measurement speed, so going faster becomes increasingly difficult, despite the best efforts of machine designers.

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 (illustrated above) mean that most scanning in production applications is done in the 10 to 25 mm/sec range. 5-axis scanning breaks through this barrier by avoiding the problem of machine dynamic errors. Instead, the innovative REVO? scanning head is able to acquire surface data whilst 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 apparently insurmountable problem of dynamic errors is alleviated by not asking the machine to accelerate during measurement, or at least minimising such acceleration where it cannot be avoided altogether.

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 (illustrated above) mean that most scanning in production applications is done in the 10 to 25mm/sec range.

Controlling factor
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. In short, it can cripple a perfectly good CMM frame, or at least limit its ability to meet your changing needs.

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