CeramTec uses the advantages of diamond styli in 14 Coordinate Measuring Machines (MMC) by Carl Zeiss

Diamond styli are a production manager's best friend

By Norbert Schmidt

Who would have thought it: Measuring styli suffer from wear and tear. Despite the very sensitive touch of the coordinate measuring machines, the ruby spheres flatten off when scanning flat surfaces or suffer from wear and tear or material build-up when scanning internal and external radii - and the accuracy is gone. The only answer is to throw it away. But that can get expensive. For this reason CeramTec has been employing measuring styli with diamond spheres since the summer of 2007, which have not shown any indications of wear after being used for a year.

"We are installing styli with diamond spheres in all coordinate measuring machines - and there are 14 of them -" Dipl.-Ing (BA) Uwe Kemmer, Production Manager in the Medical Technology department at CeramTec AG, quotes, opposing the impression that he will be using diamond measuring probes exclusively, "but only where it makes sense to do so." That means that we must differentiate. "With measurements on polished surfaces," asserts Dr.-Ing. Frank Richter, Managing Director of Carl-Zeiss 3D Automation GmbH and supplier of the measuring probes, "styli with ruby spheres are virtually wear-free and have a very long service life."

Dipl.-Ing (BA) Uwe Kemmer (right), Production Manager of the Medical Technology department at CeramTec AG and Dr.-Ing. Frank Richter (left), Managing Director of Carl-Zeiss 3D Automation GmbH with a diamond stylus and a dish insert.

And that's exactly what Uwe Kemmer's colleagues are doing: The polished surfaces on ball heads and dish inserts of the synthetic hip joint components made of ceramic material are measured using a ruby sphere stylus. But the joint ball head has unmachined surfaces after pressing and sintering - an end face and an inner tapered hole for mounting the titanium element that is anchored in the thigh. The dish insert is only polished on the inside and its base surface and the external shape are not; however, they provide the base values, in a way, the zero line, of the ceramic parts. For this reason they are important and need to be measured. But these surfaces are rough in places, even if it can only be detected by the eye based on the degree of gloss. "The consistency of these surfaces", Uwe Kemmer explains his measurement problem, "make it necessary for us to change the ruby stylus once or twice every week."

Replacement means throwing it away. The ruby spheres on the styli exhibit such a high level of wear that they can no longer be used for documented measurement. However, the ceramic joint components need to be measured with 100 percent accuracy and the measured values must be traceable to each individual part via the serial number, even years later. What can we do? "We had to check the stylus spheres for damage in short intervals and then we could only replace them", according to Uwe Kemmer. He didn't comprehend that the machine component had changed from an investment item to a wear item. Every year around 70 styli per coordinate measuring machine had to be purchased and replaced. A stylus costs around €30, so that's €2,100 per year - times 14, because that's the number of measuring machines that Kemmer is responsible for.

The diamond styli are used to measure the external contours of the dish inserts for hip joint prostheses.

"This problem was also described to us by other customers of ours", Dr.-Ing. Frank Richter explains the motivation to develop less sensitive styli "the ruby spheres wore out on hard materials, or had material build-up of soft material and changed their diameter." In all cases, they are no longer serviceable and need to be thrown away because the build-up layers cannot be removed again. The developers at Carl Zeiss 3D Automation soon found out in the analyses, that the surface structure of the measured objects is definitive. "On hard materials we observed damage to the ruby sphere when measuring flat surfaces which leads to flattening", Dr. Richter remembers of the investigation of the causes, "and on soft surfaces a layer of build-up developed." We were also able to determine that damage of this type was not seen with point sensing. Scanning was identified as the source of the problem, i.e. when the measuring probe was guided over a surface and slid along it, the surface of a gearbox or the internal and external radii of components.

For many users, including Uwe Kemmer, point measurement was not the solution, "because we did not have adequate information about the entire surface", he explains. And with an increased point density, the measurement would have taken far too long to make it economical. The developers at Carl Zeiss 3D Automation therefore had to find a measuring sphere that is hard enough not to withstand wear and tear from hard and rough materials, and to which soft material does not adhere. "At first we performed tests with silicon nitride", Dr. Richter describes the route to the result, "but with a 30 percent increase in costs, our performance only increased 10 to 15 percent."

The ball head and dish insert for synthetic hip joint parts are made of ceramic and need, in part, to be measured using diamond styli.

Only when a supplier of industrial diamonds was able to make spheres, was there light at the end of the tunnel. Even if the centre of the sphere is the actual measuring point, the shape deviation from it being "100 percent spherical" is very small. And the problem was coming as close as possible to this ideal dimension. But the industrial diamond manufacturer was then able to herald their success, and all the tests at Carl Zeiss produced outstanding results. And as Uwe Kemmer saw the diamond stylus at the Carl Zeiss stand at Control 2007, he ordered it straight away because he was convinced that the team around Dr. Richter had found the correct solution.

And after a year of use, the diamond stylus has fully lived up to this expectation and is now available on the market as the "Diamond!Scan" and even from the web shop. "We use the diamond styli in all of our 14 coordinate measuring machines" Uwe Kemmer provides an intermediate report, "and we have not needed to replace a single one during this period." For CeramTec this means that instead of using more than 70 probes per year per measuring machine, only one will be used. We cannot make a final assessment because we do not yet know the end of the service life of the diamond stylus. Perhaps it will last another year? "Tests using a raster electron microscope have shown that there has been virtually zero wear and tear", Dr.-Ing. Frank Richter says, excited about the good result.

Ruby stylus with flattening as a result of wear from measuring the surface of a ceramic component.

For Uwe Kemmer and CeramTec, the investment in the diamond stylus has paid for itself many times over, even if it does cost 3,000 percent more than a ruby stylus. Even the simple comparison in investments shows that the €1,000 diamond stylus wins out, because this value must be compared to the €2,100 (70 ruby styli at €30 each). This results in an amortization period of less than 6 months. "And this does not include the testing expenditures or the replacement of the ruby styli and the associated standstill of the coordinate measuring machine", Uwe Kemmer points out the error in the profitability calculation "which, combined, amount to around €60." This means that an extra €4,200 must be included in the calculation and the amortization time for the diamond stylus would be decreased to less than 2 months.

This means that, for Uwe Kemmer, the stylus has gone back to being an investment part of the machine. "But our diamond styli can only compete with the ruby styli in extreme measurement situations", Dr. Richter describes the deployment specification scope of use, "where high levels of wear or material build-up can falsify the measurement results." And he harbors a suspicion that there will be many users throughout the country who would prefer to scan, but who, because of the rough surface, won't risk it and settle for point measurement. In many areas of application, the Carl Zeiss representative predicts a high demand and predicts a rosy future for his "Diamond!Scan". Material for a box: The CeramTec company

The curve clearly shows the measurement inaccuracy of the ruby stylus when compared to the diamond stylus.

The ceramics specialists

The medical technology department of CeramTec AG is the global leading manufacturer of ceramic components for hip prostheses. Ball heads, inserts and special solutions are available for virtually all endoprosthesis manufacturers. Around 80 percent of all ceramic implants for hip prostheses come from CeramTec. So far, around 4.4 million ball heads and 900,000 dish inserts (as of 2008) made of BIOLOX ceramic have been implanted throughout the world. CeramTec has more than 35 years of comprehensive experience in this field and is the leader in this technology. Ceramic components made of BIOLOX materials ascribe their outstanding properties to the unique material composition, sophisticated manufacturing technology and a multi-stage, 100 percent quality control of all manufactured components. For this purpose they use PRISMO type 14 coordinate measuring machines from Carl Zeiss Industrielle Messtechnik GmbH with diamond styli that measure 450,000 ball heads and 230,000 dish inserts for hip prostheses per year. Material for a second box: The PRISMO coordinate measuring machine

The lightning-fast all-rounder

The PRISMO navigator is a bridge-type measuring machine based on the multi-application-sensor-system measuring concept which takes the increased demand for multi-functional measurement into account. The PRISMO is number one in the world for high-speed scanning in production. Accuracy, dynamics and outstanding environmental resistance are the defining properties of this measuring machine. The machine concept stands for product proximity and offers a high level of reliability and optimum regulation behavior thanks to the ISC control system. The VAST universal probe head for multi-point or single-point measurement is used on the PRISMO navigator and adapts flexibly to all measurement tasks. It facilitates lightning-fast determination of dimensions, shape and position in a single measuring process. The use of the VAST navigator reduces the measurement time to an absolute minimum.