Today, it is possible to measure component features, using a combination of zoom optics and video analysis; automatically scan surface profiles with an in-built laser; and deploy a touch probe to measure three- dimensional features. All of the probes work within the same axis system, are programmed from a single software source, and are controlled automatically by the machine, without the need for operator intervention.
While it has been possible for some time to add laser or touch probe technology to camera-based measuring systems, the inevitable compromise to the working distance of the optics has left many users dissatisfied. For this reason, conventional 'bolt-on' systems have never fulfilled the potential of true 'multi-probe' measurement that was promised when the first models arrived on the market.
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Similarly, some builders of co-ordinate measuring machines have tried to add camera technology, although the portal motion of most CMMs means that the lighting changes with each machine movement make reliable video measurement challenging.
But a new technology is set to establish multi-probe measuring systems as the metrology devices of preference for manufacturers everywhere. For the first time in industry, the latest SmartScope CNC measuring machines from OGP (01283 585933) feature a combination of probes that work to the maximum of their individual capability, without compromise.
COMBINATION OF STRENGTHS
This has been made possible through a combination of OGP's strength in the development of optical non-contact measuring systems; its close collaboration with Renishaw on the design of the compact SP25 touch trigger/scanning probe; and the development of its own TeleStar TTL (through-the-lens) laser.
Importantly, TTL technology means there is no offset between the laser and the optics, providing a long working distance that allows users to measure recesses up to 200 mm deep, without obstruction issues. In fact, TeleStar can still measure when its path is blocked by up to 60 per cent.
For this reason, TeleStar is well suited to measuring small holes and channels. In one particular obstruction test, the TTL laser managed to collect data successfully from a 0.025 mm gap set between two 25 mm tall gauge blocks. TeleStar is an interferometric system, whereby accuracy is not a function of front lens diameter and working distance, unlike most triangulation lasers.
Image: the Rainbow probe makes possible the optical measurement of transluscent objects, like glass; below, left, the Star probe in action
By supporting fast and repeatable measuring routines using a single system, set-up time and cycle time are reduced significantly, while accuracy is enhanced because all probes are calibrated to a single point of reference – the centreline of the optics. Additionally, there is less capital expense, fewer calibration/service intervals, and lower labour and operator training requirements.
Other probes can also be integrated into the platform, such as the Feather probe, to which OGP has exclusive rights, which can measure small features that are inaccessible to video measurement or conventional touch probes, or too sensitive to withstand the forces delivered via traditional probing.
A Rainbow probe can also be integrated. This is an innovative electro-optical device that measures surfaces without contact by analysing changes in the optical spectrum as a function of part-to-probe spacing as it scans, making it suitable for measuring glass or translucent surfaces.
Dual rotary tables within the same frame of reference can be employed, particularly useful when measuring complex parts, such as nozzle guide vane (NGV) components used in aero engine applications. Here, a touch probe can be used to establish the axis system before a camera probe is deployed to measure the side-facing seal slots. Once this is complete, a laser probe measures the profile of the aerofoil before the system reverts to the optical system to measure the cooling holes. Because of the common optical path, the switch between video and laser is instantaneous and at no point is there any need to re-establish part datums.
VIDEO MEASUREMENT: IT'S IN THE LIGHTING
And because OGP's video-based, non-contact measuring machines incorporate advanced lighting to provide high resolution optical clarity, problems often encountered elsewhere do not show their face. For instance, when NGV cooling holes are spark eroded, there is inevitably some break-out around the edge of the hole. Other machines will measure the break-out and record an oversize hole, but OGP's in-house manufactured TeleStar zoom optics are diffraction limited, colour corrected and fully telecentric for superior imaging, which allows enough light to get inside the hole (beyond the break-out) to measure the diameter correctly. As a result, any kind of micro-component or micro-feature will benefit from this technology.
But the technology is not only suitable for small parts. The latest multi-probe metrology platforms can also handle large components, such as aerospace blades and automotive cylinder heads, previously the preserve of portal-type CMMs.
Another area that can benefit is the medical sector, largely because measurement data is pulled straight from the machine and put into PDF format, so it cannot be changed. This provides complete integrity, in accordance with CFR 21 part 11 (of the Code of Federal Regulations), which deals with the FDA's guidelines on electronic records.
Around 40 per cent of the systems sold by OGP now feature both camera and touch probes, while laser is also being increasingly integrated. It's a trend set to gather pace.
First published in Machinery August 2009