Probes Push CMMs Forward

May 8, 2003
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Choices

Manufacturers’ demands for versatility, accuracy, higher throughput, greater point density and continuous operation have resulted in the development of coordinate measuring machine (CMM) probes designed for a range of applications. This means a choice of solutions for manufacturers. A number of new styli and probes for coordinate measuring machines have recently entered the market to help manufacturers improve measuring capabilities and overcome stacking, bending and other errors that can hurt the overall accuracy of the CMM.

Choices

Renishaw (Hoffman Estates, IL), one of the leaders in the probing market, introduced at least six CMM products at a recent trade show, not including products for machine tools, calibration and other end uses.

One of Renishaw’s most versatile new products is the SP25M scanning probe. The SP25 is a two-in-one probe that can perform scanning for form and touch-trigger sensing for size and position.

Measuring 25 millimeters in diameter, the SP25M can be mounted to motorized articulating heads, such as Renishaw’s PH10M, and may be fitted to a range of extension bars to allow access to deep part features. It features Renishaw’s isolated optical metrology, which measures probe deflections independently, rather than using transducers mounted to stacked probe axes.

A set of interchangeable scanning probe modules is available, each optimized to suit a range of stylus lengths from 20 to 200 millimeters. Like all Renishaw scanning probes, the SP25M features kinematic stylus changing and bump-stop crash protection.

Modular system design allows the SP25M to be connected to Renishaw’s TP20 probe module to perform touch-trigger probing routines for discrete point measurement or applications in which scanning is not appropriate. The system is compatible with the full range of TP20 probe modules, allowing fast stylus changing and six-way sensing.

A key advantage of the SP25M system is improved component feature access arising from its compact size and the ability to carry long stylus configurations. The probe’s low volume also makes it a suitable contact sensor for vision systems, offering the combined benefits of touch probing and analog scanning.

Renishaw also showcased its SP80 scanning probe. In company tests, the SP80 achieved submicron 2-D and 3-D scanning performance, using section scans as defined within the ISO 10360-4 international standard for acceptance and reverification for CMMs used in scanning measuring mode. These figures are from unknown path scans before any data filtering has been applied.

The SP80 is a passive scanning probe with isolated optical metrology and digital readheads with 0.02mm resolution. The SP80 carries styli up to 500 millimeters in length and 500 grams in weight—including star configurations—without the need for counterbalancing, therefore maintaining maximum working volume.

A repeatable kinematic stylus-changing mechanism enables rapid stylus changing for flexible inspection of complex parts. There is no need for probe requalification after each stylus change, and stylus arrangements can be optimized to ensure the best results for each feature to be measured.

Renishaw is not the only company with new products.

Fred V. Fowler Co. (Newton, MA) has a number of new probe products including the P1-5A, 5-axis touch trigger probe that can be used on most manual or DCC coordinate measuring machines. The P1-5A can be used in any probe body equipped with an M8 threaded probe receptacle.

Another new product from Fowler is the P1-SB5 touch-trigger probe with body and shank for manual and computer controlled CMMs. The universal probe has three LED illuminators that indicate probe hits. It accepts a variety of shank sizes and is compatible with any M2-thread probe styli.

Paul W. Marino Gages (Warren, MI) also has many new products. These include new kits, requalification spheres and cases. One such kit is the 20-piece M2 Styli kit that includes 8 different styli ranging in sizes from 10.5 millimeters by 1 millimeter to 28 millimeters by 6 millimeters, a stylus pointed from 10 millimeters by 30 degrees, 5 extensions, a knuckle joint, stylus holder, and a hexagon and pin key.

These are just some of the new probing products that have recently entered the marketplace. There are many others including those probes and accessories from the CMM suppliers that release products for their particular machines such as Carl Zeiss Inc. (Minneapolis), which plans to release an updated probe head in the next couple of months. Zeiss isn’t alone in its product introductions. Many suppliers report that more probing products are on the drawing board, and these look to be even better, faster and more accurate. For manufacturers looking for solutions, this could mean more versatility and throughput.



Sidebar:
Ruby Balls

Most CMM manufacturers have relied on an assortment of materials over the years and designed their products to work within the community to test the best possible. The criteria have always relied on accuracy, weight restrictions and durability.

All CMM probing systems have a sensing device of one design or another, and nearly all have used the sphere for contact. The synthetic ruby, or Al2O3 monocrystalline, has proved to be the best suited material for hardness, form deviation and surface finish, becoming the standard. This material has been formed and reshaped into many configurations including cylinders and discs for application specific usage. The metallics, such as tungsten carbide, stainless steel and titanium, have proved to be useful in the choice of contact materials but the wear factor has always driven manufacturers back to the synthetics. Ceramics, polycrystalline and carbon fiber have proved to be the choice for controlling the weight factor.

The search for new materials that can do all of the above better, faster and cheaper is on.

The new kid on the block, silicon nitride, has some testing to do before it becomes a standard. It has a place in the contact issues when measuring surface materials of the alumina group, but for now it is still a cost issue for most.

The newest technology has been in the selection of lightweight materials for extensions and styli shafts. For years it has been accepted that the threaded mounts and shank sections should be a combination of stainless steel and tungsten carbide. This is true in the small ball probes, and in many diameters it is the only possibility. The larger requirements can now use specialty aluminum composites and alloys that reduce weight and improve rigidity for minimal, if any, deflection.

In summary, the probe stylus is the key component in the successful utilization of the CMM. The ease of availability and lowered costs during the past 20 years for this material has made it the top choice for CMM manufacturers worldwide. There are other methods including the noncontact devices, but none have endured the consistent reliability of the little ruby ball.

—By Linda Marino, Paul W. Marino Gages Inc.



Sidebar:
Ruby Balls

Most CMM manufacturers have relied on an assortment of materials over the years and designed their products to work within the community to test the best possible. The criteria have always relied on accuracy, weight restrictions and durability.

All CMM probing systems have a sensing device of one design or another, and nearly all have used the sphere for contact. The synthetic ruby, or Al2O3 monocrystalline, has proved to be the best suited material for hardness, form deviation and surface finish, becoming the standard. This material has been formed and reshaped into many configurations including cylinders and discs for application specific usage. The metallics, such as tungsten carbide, stainless steel and titanium, have proved to be useful in the choice of contact materials but the wear factor has always driven manufacturers back to the synthetics. Ceramics, polycrystalline and carbon fiber have proved to be the choice for controlling the weight factor.

The search for new materials that can do all of the above better, faster and cheaper is on.

The new kid on the block, silicon nitride, has some testing to do before it becomes a standard. It has a place in the contact issues when measuring surface materials of the alumina group, but for now it is still a cost issue for most.

The newest technology has been in the selection of lightweight materials for extensions and styli shafts. For years it has been accepted that the threaded mounts and shank sections should be a combination of stainless steel and tungsten carbide. This is true in the small ball probes, and in many diameters it is the only possibility. The larger requirements can now use specialty aluminum composites and alloys that reduce weight and improve rigidity for minimal, if any, deflection.

In summary, the probe stylus is the key component in the successful utilization of the CMM. The ease of availability and lowered costs during the past 20 years for this material has made it the top choice for CMM manufacturers worldwide. There are other methods including the noncontact devices, but none have endured the consistent reliability of the little ruby ball.

—By Linda Marino, Paul W. Marino Gages Inc.



Silicon Nitride Balls

Customers report up to five times longer tool life using CMM styli with silicon nitride balls when compared to conventional ruby ball styli, according to Mark Osterstock, president of Q-Mark Manufacturing Inc. (Mission Viejo, CA). Silicon nitride balls, which look like a putty gray, are hard and have a highly polished surface. This means the silicon nitride balls are less likely to accumulate material on their surface and are more likely to outlast ruby balls in demanding applications.

Ruby balls can accumulate tiny bits of material on their surface during CMM probing, especially when probing soft materials such as aluminum and cast iron. When used in scanning CMMs, ruby balls can develop flat spots from rubbing against the part. Even the slightest buildup or wear can cause measurement errors. It is important to note that buildup or wear is not always discovered during routine probe calibrations unless great care is taken to touch nearly all points on the stylus ball.

Q-Mark Manufacturing Inc. now offers CMM styli made with silicon nitride balls. The balls are drilled and bonded to a carbide shaft for maximum strength and rigidity. They are available in diameters from 1 to 10 millimeters. Silicon nitride styli can be used with all touch-trigger and scanning CMM probes.

"The advantage of the silicon nitride is that it is very smooth. It has an unfaceted surface that will not accumulate the material like ruby does," says Osterstock. "What is unique about our styli is that we have successfully drilled a hole in the silicon nitride ball to allow us to bond it to the stem of the probe. Silicon nitride is so hard that it is extremely difficult to drill. Before drilling, glue was used on the end of the stem, but the styli would fall off under shock and impact."

It took the company about a year to produce the drilled silicon nitride styli. While ruby tips are the most often used, Osterstock says that silicon nitride balls are gaining acceptance throughout the community.

—Excerpted from an interview with Mark Osterstock, president of Q-Mark Manufacturing Inc.

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