GRANITE…IT’S USED ON THE BEST CMMS BUT HOW DIFFICULT IS IT TO GET?

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GRANITE…IT’S USED ON THE BEST CMMS BUT HOW DIFFICULT IS IT TO GET?

Written by Al Hall

Impala black granite is one of the key components for the world’s most accurate Coordinate Measuring Machines. The best granite is to be found in mines in South Africa.

The are only a couple of Coordinate Measuring Machine (CMM) manufacturers that still use black granite for their machine bases, X rails, Y beams and Z rams. One of those manufacturers is Wenzel GmbH in Wiestal, Germany.

Impala black granite has proven to be the most structurally and thermally stable material that will insure the high accuracies required by a coordinate measuring machine. Combine those properties with low porosity, low moisture absorption, superior strength, lower coefficient of thermal expansion, uniformity of texture and a non-glaring surface, and you truly have the ideal material for a CMM.

Heike Wenzel-Dafler, joint managing director of Wenzel emphasizes, “We believe the key to Wenzel’s continued success has been our commitment to building the highest quality and most structurally stable and accurate CMMs in the world. And to do that, it’s a necessity that black granite be used for the bases, rails, beams and rams. For those reasons, as well as the need to become more vertically integrated to better control our manufacturing and production processes and costs, Wenzel purchased a granite processing business in Germany in 2006. We made a strategic decision to not follow the direction of the majority of our competitors who continue to outsource not only granite plates, but also high value parts, sub assemblies and even complete machines to manufacturers in China. Wenzel will not compromise its level of quality and the granite, which is the single largest cost in our CMM produ cts, provides Wenzel CMMs with the precision, dynamics and stiffness that most industries want and require,”

Heike Wenzel-Dafler, joint managing director of Wenzel, left, Keith Mills, president of Xspect Solutions/Wenzel, center, and Peter Heina, Steintechnik granite plant manager, discuss granite slab cutting in the diamond wire sawing area of Wenzel’s granite processing facility.

Heike Wenzel-Dafler adds, “We acquired tremendous granite processing experience and know-how when we purchased Steintechnik GmbH. The company had been processing granite since 1888. We’ve invested over $2 million in facility upgrades and added a new Favretto grinding machine at our Wiestal plant that has more than doubled the production capability for finish grinding or our granite components. Our facilities have the ability to process up to 12 meter granite blocks weighing up to 42 tons.”

Impala Black Granite

Peter Heina, Steintechnik plant manager and great grandson of the founder of the granite business, explains,

“The type of high quality Impala black granite used for Wenzel CMMs is only found in granite quarries in South Africa. Inferior black granite like what is found in China is in question today as the Chinese government continues to evaluate the adverse environmental issues related to the mining of the granite there. South Africa is almost exclusively where the world’s supply of Impala black granite is mined. There are twenty granite quarries in South Africa but not all of those quarries want to make big blocks which are typically considered to start around 3m x 3m x 1m. Wenzel has identified five as having the level of granite quality and the capabilities that are required for CMM manufacture where the base tables can be as large as 9 meters in length, 6 meters wide and 1 meter thick. Wenzel has approximately fifteen standard granite base sizes and we typically have three to five of each of those sizes in stock, I personally visit these five quarries twice a year to look at the quality of the granite being mined and specifically identify the quarry areas from which we want large pieces. It’s essential that we optimize the yield from the granite blocks because they are being transported from such long distances. The granite for Wenzel machines is selected for its dense structure, low porosity and low absorption rate. We want an easy-to-clean surface that resists stickiness, an essential property that insures surface accuracy. The low or tight porosity makes this igneous rock extremely resistant to water absorption or warping from humidity,

Material	Specific Weight (Kg/dm3)	Expansion Coefficient (I/K)	Temperature Diffusion Rate (W/mk)	Elasticity Module (103N/mm2)	Material Metrology Ranking
Steel	7.25	10.4 x 10-6	42-63	90-180	+++++++
Aluminum	2.7	23.8 x 10-6	210	72	+
Ceramic	3.85	8.0 x 10-6	28	370	+++++++
Granite	2.8	6.5 x 10-6	3.5	0	++++++++++

Table refers to the use of Metrological Materials for CMMs

Heina explains further, “Lighter colored granites like gray and pink are less than one-third quartz, with other constituent materials that are softer than black granite. Pink and gray granites will also absorb 3 to 4 times the moisture than black granite. The presence of quartz can cause uneven wear to a surface and even abrade tools and gages that are moved around on its surface. Black granites contain no crystalline quartz, they have very high modulus of elasticity, producing superior load-carrying capacity.”

Processing the Granite

“In the production control process for a new CMM machine order,” Heina says, “We have to allow six weeks for delivery of the raw piece of granite from the quarry in South Africa to our processing facility in Hessen, Germany. After placement of the granite order to the quarry, it has to be mined which usually takes 3 to 4 weeks, and then one week to be delivered by truck to a ship that transports it by water to Antwerp, in Belgium where it is then brought by truck to our processing plant. In order to shorten this time, we have a supply of large blocks that we have pre-purchased and held in stock until they are needed or scheduled for a shipment. With this production control strategy, we have been able to take nearly four weeks out of the delivery cycle. ”

“We have two Pellegrini diamond wire sawing machines that are used to make the desired slab from the “as quarried” piece of granite,” Heina says. “The surfaces of the rough slabs are cut 10 mm larger than what the finish size will be. However, it is at this point, when the first cut on one side of the slab is made, that a visual inspection of the slab determines whether there are any voids, stains or cracks that could scrap the slab from being a good piece. This is a very anxious moment because there has been a lot of time and expense invested to get the slab of granite to this stage of the process, and finding you have a bad piece turns into a very costly disappointment. That’s why we visit the quarries regularly and use our granite knowledge and experience to insure as best we can that only good pieces of granite leave the quarry and are shipped to us. Unfortunately, we still have issues. Sometimes flaws don’t show up until after the surfaces are honed, but we only have that happen about 5% of the time.”

“The next process involves diamond grinding the surfaces of the slab to finish size, see Fig. 5. Then any required slots are cut into the slab, see Fig. 6. Finally the edges are polished and honed, see Fig. 7, and any drilling for mounting holes is added. A final CMM base plate, rail, beam or ram will leave the granite processing facility with a flatness and squareness tolerance of ± 2 mm. A finished CMM base is typically processed in one week, smaller components in a short time frame. ”

Chrisfried Pumm, Wenzel technical director explains further, “After the granite is transported to the Wenzel manufacturing and assembly center in Weistal, approximately 40 miles away, it is allowed to remain static for a number of days to more or less relax any stresses. It is at this point in the manufacturing process that Wenzel sets itself apart from all other CMM manufacturers. Every granite component is laser checked for flatness, see Fig. 8, and hand lapped, see Fig. 9, to insure that any flatness variations are no larger than 5 to 6 microns in a 20 foot length. The skill level of our hand lapping technicians cannot be rivaled anywhere in the world. This level of intrinsic accuracy is what makes a Wenzel CMM a very desirable machine”, see Fig. 10.

Today, Wenzel has over 5,000 CMMs installed worldwide and is 4th largest CMM builder in the world and the 3rd largest in North America. Heike Wenzel-Dafler concludes, “We sincerely believe that the consistent high level of accuracy of Wenzel CMMS is assured through the use of black granite for the base plates, rails, beams and for the most critical component, the Z ram. Our commitment to making a long term investment in this manufacturing strategy is paying off in the level of new machine orders we have secured in the growing aerospace and wind energy markets. We are confident that we are successfully managing our business and continue to look for ways to produce the best and most cost-effective CMMs possible.”