CMM claims "best in class" title

In a standard ISO10360-4 test in which various CMMs performed an identical analogue scanning cycle on a sphere, LK's Evolution machine was found to be "best in class" in both accuracy and speed.

In a standard ISO10360-4 test in which various manufacturers' CMMs (co-ordinate measuring machines) performed an identical analogue scanning cycle on a sphere, LK's Evolution machine was found to be "best in class" in terms of both the accuracy of results obtained and speed of measurement.

It is an important world lead for the Derbyshire manufacturer, as analogue scanning is becoming increasingly important in metrology, a trend driven by design and manufacturing departments demanding ever more data to be fed back about a product at all stages in its life cycle.

It was noteworthy that all of the major players were demonstrating analogue scanning at the Control exhibition in Germany in May 2004.

Measurement using a touch-trigger probe on a CMM is a very inefficient way of collecting data, as every time a measurement is taken the process stops momentarily.

By contrast, scanning is a dynamic process as the probe is in continuous contact with the component, feeding back a wealth of data.

It means that much more can be learnt about the component, so designs can be continually improved, particularly when dealing with complex surfaces and tolerance-critical features.

For example, a 50mm diameter bore inspected with a touch probe will typically have six point measurements taken, whereas scanning will yield information on around 2000 points.

Overall, one can expect a 60% improvement in measuring productivity using the scanning technique.

A noncontact laser head feeds back similarly large amounts of data, and LK has recently partnered with Metris to develop this technology, but in real applications it can only deliver repeatability of the order of 10um, compared with submicron results using analogue scanning.

Attention to detail in four main areas has enabled LK to take the lead, based on using the SP25M scanning probe from Renishaw.

The areas are rigorous characterisation of the probe; CMM design to impart stiffness and smoothness of motion; new controller algorithms to improve speed, smoothness and profile tracking; and software.

Calibration was important to characterise the probe, ie to predict accurately how it will behave during scanning of different features and at different speeds.

The CMM could then be programmed to ensure constant deflection of the probe during measurement, which translates into constant force on the CMM and hence maximum accuracy.

Renishaw's previous scanning probe, the SP600, had a +/-2mm linear range, whereas the latest improved model is more compact with +/-1mm nonlinear travel.

In principle, the smaller the deflection, the more accurate the results, but in practice one needs to work with a reasonable bandwidth.

LK has undertaken research to establish and control the minimum deflection possible for its range of machines.

The ISO sphere was therefore scanned using various deflection figures in order to characterise the probe in operation.

More than one man-year was devoted to this exercise alone.

Inertial forces are practically zero when touch-trigger probing, due to the "touch" taking place at constant velocity.

Conventional low speed scanning begins to induce small inertial forces in the moving elements of the machine.

It does not lead to significant error, but measuring productivity is limited.

High speed scanning, however, results in inertial forces that can be a couple of hundred times higher.

With this in mind, LK has made changes to the design of its latest CMMs to ensure stiffer and smoother operation, maximising the accuracy of data feedback from the probe when scanning at elevated speeds and rates of acceleration.

Air bearings result in smoother motion in the orthogonal axes than linear guideways.

One improvement made by LK has been to modify the ratio of the air bearings that support the bridge.

Spread-to-travel is now typically 3:1 for the Evolution compared with between 4:1 and 6:1 for other machines on the market.

In addition, the extraordinary flatness obtained on the alumina ceramic guideways used by LK have allowed the air gaps of the air bearings to be reduced by up to 50%, resulting in stiffer bearings.

Conventional air bearings generally have several orifices distributing pressurised air, placed in a pattern across the otherwise smooth face of the bearing "pad".

However LK found that a single central orifice supplying grooves across the face creates a dramatically more efficient bearing that is stiffer, runs with a smaller air gap for the same load and consumes far less air.

Viewed from the stiffness perspective, a typical grooved bearing delivers 140,000N/mm compared with the equivalent plain-pad bearing's 40,000N/mm.

In the context of a CMM, apart from increasing the accuracy and responsiveness of feedback from the probe, greater stiffness allows moving elements to accelerate faster, thereby shortening cycle times and reducing measuring costs.

Crucial also is the selection of materials for the machine elements, both moving and static.

Materials with a high stiffness to weight ratio tend to minimise many of the mechanical errors inherent in CMM design.

Stiff, light materials reduce the moving masses, permitting smaller drives, less heat, shorter measuring cycles and lower costs.

LK use high quality, 96% alumina ceramic for the crossbeam and vertical spindle.

Alumina has a stiffness-to-mass ratio of 8.18 compared to steel's 2.56 and aluminium 2.52.

It is over 4.3 times stiffer than aluminium.

It also has a thermal expansion coefficient half that of steel and a quarter that of aluminium.

This means LK machines can be designed with much lower probe deflections for given axis accelerations, thereby allowing faster scanning for a given accuracy.

LK uses steel for elements of the machine with complex stress patterns such as the carriage and legs.

It has a slightly better specific stiffness than aluminium but half the expansion coefficient.

Drive technology is similarly important.

Precision drive bars and belts are used in preference to rack and pinion drives, minimising transmission of noise and vibration to the machine structure which can transmit to the probe tip and reduce accuracy.

Most CMMs use a simple acceleration/deceleration (acc/dec) algorithm that produces a trapezoidal plot, the strategy being to accelerate to maximum velocity as quickly as possible, maintain that velocity as long as possible, and then decelerate to zero as quickly as possible.

At first glance, this might appear to deliver optimum cycle times, but in a CMM it produces a substantial wait at the end of the motion while the structure settles down and dissipates the stresses and vibration generated by acceleration and deceleration.

LK uses the superior strategy of controlling the motion to produce a pair of gentle "S-curves".

In practice, this approach may add a fraction of a second to any given movement, but at the same time it reduces the initial and final jerk.

The benefit is that a diameter, for example, can be measured without any over-scan, whereas typical systems introduce a productivity penalty by over-scanning by some 45 degrees to compensate for poor process control when ramping up and down.

Software Both known-path and unknown-path analogue scanning techniques are available, and LK has written appropriate algorithms to process the captured data.

In the case of known-path scanning, the drive algorithms are able to adapt to the contours of the part, changing the scanning speed to suit the rate of curvature and adjusting the speed to capture more data where greater resolution is needed.

Collection rates are up to 1000 points per second at speeds up to 10m/min.

The nominal scanning path target data and actual return measurement data are compensated for in real time in the PC controller, both parametrically for mechanical error and thermally to take account of temperature variation.

The latter compensates not only for the machine axes, but also for the probe assembly right to the tip.

Additionally, the latest release of LK's Camio Studio software, version 4.0, has new features to assist in programming analogue probing cycles directly from the CAD model.

They include "path" measurement strategy programming tools such as spirals in cylinders and continuous paths on planes.

The provision of measurement strategies is part of continuing developments within the LK Camio Studio product designed both to improve the efficiency of generating part programs and to aid the consistency of part programs created by different programmers.

Version 4.0 also features a fully scaleable SQL reporting database to accommodate the large amounts of measurement data generated by the analogue scanning approach.

Fully customisable reports can be generated in paper or electronic format (XML) for transfer to remote sites via the Internet.

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