Zeiss CMM Tips

A high-performance measuring machine is the prerequisite for fast and efficient measuring. Yet many operators are not achieving maximum efficiency with their coordinate measuring machine. Nadine Schwab from Zeiss Regional Sales in Germany has come up with tips to help coordinate measuring machine users increase their profitability.

Tip 1: More stable stylus systems increase the measuring speed According to Nadine Schwab, using suitable stylus systems is perhaps one of the most important conditions for expediting the measuring process. The lighter and more rigid the styli and stylus extensions are, the faster the operator can measure without negatively impacting the precision of the system through bending or centrifugal forces. That is why, to take one example, the stylus material is such an important factor. Carbon fiber is ideal for measuring technology: it is lightweight, has high flexural strength and, because of its thermal stability, it eclipses other materials like aluminum, stainless steel, steel, titanium and ceramic. ZEISS uses a special carbon fiber, combined with titanium adapters, for its styli and stylus extensions. The high rigidity of these ZEISS ThermoFit systems is especially important when measuring with high throughput because a higher scanning speed also increases the accompanying bending forces which deform the system. At a speed of 8 millimeters per second, a form error of 7.3 micrometers can arise with titanium stylus extensions. If the coordinate measuring machine scans at 12 millimeters per second, the error is already at 13.0 micrometers. Stylus extensions from the ThermoFit carbon fiber-titanium combination, however, only deviate by 1.7 or 2.4 micrometers under the same conditions.

Tip 2: Diamond styli shorten downtimes Along with the material used for the shaft, Schwab indicates that the stylus tip material – which glides over the workpiece during measurements – is of central importance. If the sphere is made of ruby or silicon nitride, it loses its roundness over time when it comes into contact with very hard materials like ceramic or very light ones like aluminum alloys. Little by little, scanning hard workpieces wears down the surface of the sphere. The ruby or silicon nitride sphere strips away small amounts of material from soft workpieces. This material then accumulates on the surface of the sphere. Both effects distort the measuring results over time. Because of this, the operator has to regularly clean styli made of ruby or silicon nitride and often replace them with new styli when working with extreme materials. The measuring machine cannot be used productively while changing the stylus and redoing measurements. This is why many operators forgo continuously scanning their components; measuring single points means less stylus deformation. Yet in comparison to the scanning process, which generates a large number of measurement data in a short time, the significantly slower measuring of single points has other disadvantages. Operators are faced with a dilemma: they either capture fewer measuring points at the expense of higher throughput and accept greater measurement uncertainty – or the operator measures a lot of single points, reducing measuring uncertainty but increasing the measuring time.
ZEISS offers a way out of this dilemma with the Diamond!Scan brand stylus tip made of diamond or, alternatively, with a special diamond coating. What is the benefit? There is no need to inspect and clean the stylus tips because no material remains stuck to the sphere's surface and they do not wear down. Schwab explains: “You don't even need to replace them because the styli retain their original shape for years. There's also no need to capture the measuring points individually because scanning workpieces no longer adversely affects precision. The end result: increased throughput.”

Tip 3: Pallets reduce changeover times Schwab knows that quality assurance can become a bottleneck even if you are using the most high-performing measuring machine with the most suitable styli. One possible cause is that the coordinate measuring machine is always stopped for minutes on end while the workpieces are loaded. No matter if it is a series of cellphone housings or sensors, loading can take several minutes, especially when multiple workpieces need to be mounted at the same time. It also takes a lot of time and effort when different components require the operator to frequently change the fixtures. “Pallets are a simple but often underrated solution,” says Schwab. “While the measuring machine is still measuring, the operator stages the next set of workpieces onto a pallet outside of the measuring machine. Once that's done, it's easy to load the pallet onto the machine.” Pallets can be correctly positioned in seconds without the need for realignment. For example: by using a pallet, the operator can save around 15 minutes of downtime during loading and calibration every time the fixture is changed. If you assume that measuring, loading and/or calibration take the same amount of time, it is possible to measure twice as many workpieces per measuring machine by using pallets. In Schwab's experience, a calibration pallet can save additional time – even prior to the first measurement. On these pallets, there is a fixed reference sphere mount with a sphere and, if necessary, a calibration ring and a glass artifact. To calibrate, the operator just has to place the pallet onto the machine and then start the measuring process. Since all calibration instruments are a part of the pallet, it only takes a few seconds to clear away these aids.