Without Comparison

Renishaw has introduced its first new metrology product line in over 20 years. To call it a comparator, for that is what it is, is to understate it. The parallel kinematic mechanism-based machine (PKM), called Equator, is targeting a market said to be worth $3 billion/annum and it is just the beginning of a family of related products. Heading up the drive is Dr Kevyn Jonas, director and general manager, who was heavily involved with Equator's development. With near-simultaneous launches in various markets, including the UK, Germany, the US and Japan, global metrology player Renishaw is clearly putting its full weight behind this new product, which actually saw its 'soft launch' at last September's IMTS, Chicago – a biennial manufacturing technology show. Indeed, at launch on Wednesday, 2 March, there were already some 50 production applications (see box below) and about another 70 units engaged in metrology, but not supporting production directly. Equator looks very different, because it is a PKM. PKMs have a number of proper and rather obtuse definitions, but are most easily explained by saying that, as compared to machine tools or co-ordinate measuring machines, they do not have mutually perpendicular axes, while their axes are not connected in a serial chain or stacked one upon the other. Equator has three axes (extending/contracting struts), all of which must be moved in order to position the vertically inclined probe body and attached stylus – which may be vertical or cranked – in any position in X, Y and Z. The platform to which the axes and probe body holding an SP25 scanning probe, for example, are connected is constrained to remain parallel to the base surface by means of two struts associated with each axis – these prevent the platform from twisting or tilting as the axes extend/contract (basically, controlling the other 3 degrees of freedom, a, b, c – rotation about X, Y and Z, respectively). Image: Renishaw's Equator looks different. It is different CLOSED-LOOP FOR STIFFNESS A fundamental property of a PKM is its closed-loop force path – there are no cantilevered forces in the structure itself, as with an open-loop vertical machining centre, for example. They are therefore very stiff structures, with a high stiffness-to-weight ratio. Equator is also described as thermally neutral – meaning that any growth is symmetrical, due to the use of matched materials. Although still type of novel structure, Renishaw has already supplied well over 2,000 similar PKM structures by way of its dental scanning unit, which is why the company has high confidence in Equator. The new development, incidentally, employs no less than 75 Renishaw patents in its construction. The first product launched, Equator 300, has a working envelope of 300 mm diameter by 150 mm high, has measurement uncertainty very conservatively put at ±0.002 mm and is intended to replace bespoke hard gauging in the production environment. At a cost of between £17,000 to £20,000 (depending on software, see later), it is not only claimed to be less expensive than equivalent hard gauging systems, but also is easily and infinitely reconfigurable. Indeed, Renishaw sub-titles Equator 'the versatile gauge'. The only area where there is acknowledged to be a greater investment than with hard gauging is in training, since Equator has associated software technology. The target applications are expected to be in medium to high volume manufacturing processes, such as are found in the automotive (con-rods), white goods and medical fields, although aerospace is also a good prospect, particularly for blade manufacturing. Equator is envisaged as part of an automated set-up, where parts handling is via robot, for example, and where automatic offset update may operate. Equator can also switch between parts fast, while it has a six-stylus rack. Although not revealing the figure, there are plans for the production of Equator 300 in "significant volumes", says Marc Saunders, Renishaw, group marketing services and UK sales, while the company will also produce a unit that has double the working envelope and which will cater for 90% of all applications in the target area, it is said. So how is Renishaw's PKM comparator applied? Well, being a comparator, it has first to be calibrated. Called Mastering, this can be undertaken in either of two ways. First, by means of a calibrated master part, one which has nominal dimensions for all those features to be measured, and the second by using a production part, where dimensions will likely deviate from nominal, and which subsequently becomes the master. In either case, the part is measured on a CMM in a temperature-controlled environment (establishing traceability to national standards), with the results output as an ASCII text file – measured data points. This is then input to Equator software (Renishaw's MODUS) and compared against CAD nominal data (in DMIS format) that has also been fed in. This results in the creation of offsets from nominal, where actual measured data points are not nominal. Equator then measures the master part, at any temperature, having first established its position via datum features, thus 'zeroing' the system. KNOWN UNCERTAINTY After remastering, measurement uncertainty is 0.002 mm for Equator 300. Subsequent part measurement requires that the parts be placed in the same position to the nearest millimetre only, with parts located on a high accuracy pallet system. Remastering takes place at intervals related to process tolerance limits and changes in ambient temperature (part thermal growth). The program to run Equator is generated using Renishaw's MODUS Equator metrology software – Equator plus programming software is £20,000; without, the price tag is £17,000, with one programming system able to support multiple Equator units. Although underlying Equator is MODUS Gauge metrology software, its complexity is hidden from operators, as there are just three buttons – Master, Measure, Go – with the programming software located in the production engineering office, for example. Image: Push-button simplicity hides Equator's CMM-like capability It is important to stress that measurements on Equator are compared data point for data point with the original CMM data points, even for swept surfaces. And this is true even where CMMs are not running Renishaw's controller or are not programmed via Modus – they must just run the same measurement "script". This data point level operation is key, says Dr Jonas: "We could have launched this years ago, if all we wanted to do was measure radii and offset radii, or measure heights and planes and offset these." What this means is that, although the system runs as a comparator on the shopfloor, data for each measured point is available for export for further analysis or to support SPC, just as if it were coming from a CMM. Box item Equator replaces costly hard gauges Renishaw's Equator system is replacing numerous hard gauges at aero-engine component maker Meyer Tool, Cincinnati, Ohio, US. The company designs, builds, and maintains dozens of these costly tools every year for in-process measurement. Equator gauge has eliminated at least four costly hard gauges in a new work cell. "And that's just the beginning of the impact the company anticipates for its inventory of custom hard gauges, which can cost up to $20,000 each to design, build and maintain," according to Beau Easton, quality manager at the company, who adds: "Down the line, design changes can add another $3,000-$10,000 to re-configure and qualify an existing gauge." For in-process dimensional measurement, the company principally relies on work-cell-based point contact gauges, using pneumatic digital probes. Hard gauges in the machining cell give very fast feedback, but are expensive. Design/build of the part nest can cost $6,000, plus probes at $500 each, verification studies and maintenance, Mr Easton explains. "If we are producing a make-complete nozzle, there could be six to 10 fixtures, each with six to 20 probes, and, if a feature or tolerance on the part changes, it adds time for the gauge to be altered and verified." When shown Renishaw's Equator comparative gauge and offered a pre-launch trial, Easton and SPC manager Bridget Nolan said they immediately saw the potential. "We got involved with Renishaw's introduction of the system and provided parts. Renishaw programmed them, and the results matched our CMM results," reports Ms Nolan, whose group sets up, maintains and programs the company's gauges, fixtures and instruments. Three to five hard gauges in a Meyer Tool work cell can all be replaced by one Equator – and the Equator can be used for multiple parts, switching between them in seconds, as well as be reprogrammed for many other parts over its life. The Equator system is currently assigned to a lean machining cell in Meyer Tool's shop. Demonstrating its adaptability, it integrates with Meyer's Orion SPC system, maintaining a familiar look for machinists and shortening the learning curve. Orion communicates with the Equator's MODUS software, presenting the operator with results in the form of dimensional data, SPC charts, etc, that allow the operator to determine CNC offsets. "Keep in mind the machinist sees variable data and can compare the current part with recent measurements, so it's not just a pass/fail determination," Ms Nolan explains. The parts must meet tolerances of ±0.025 to ±0.075 mm. Inspection time varies with the part, but typically takes 2 to 6 minutes, well within the TAKT time of the cell, so the system easily keeps pace with machining. "We already gauge five part numbers for two different engine programs, so we have multiple fixturing tiles for the Equator set up and qualified," offers Ms Nolan. "We try to combine as many different operations on one tile as possible to limit our qualifications and we're currently working on one tile that will combine three. We are doing GD&T true positions, notches, hole diameters, profiles, runout and such with touch probing, though we are implementing contact scanning with the SP25M probe. We have the stylus changing rack and use at least four different stylii – frequently star stylii – so it's very convenient not to have to requalify with every change," she says. The Equator measurements at Meyer Tool are correlated with those from a CMM, using a CMM-calibrated master part. "The master part sets the values the Equator expects to find inside its measuring envelope, while the software automatically applies the compensation values from nominal taken by the CMM. It must check within 10% of allowable tolerance from nominal," explains Ms Nolan. There is no need to re-master on every part change; Meyer re-masters on a 3-hour schedule to compensate for changes in the plant's temperature. "The Equator system memorises the master parts and validation scores, so we can switch parts as many times as needed during the 3-hour window and not have to re-master," she adds. With well over 100 hard gauges costing $10,000-20,000 each throughout its facilities, Meyer Tool recognises the potential cost advantages of a flexible, software-driven gauging system. "We are still expanding our knowledge and capabilities with the Equator system, but have high expectations it will ultimately alleviate a large part of our cost burden for hard gauges," concludes Mr Easton. First published in Machinery, April 2011