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Article

Brown & Sharpe

LK Metrology
Systems

Mitutoyo America Corp.

 

 

    Representatives from three leading CMM manufacturers give their answers.

 by Dirk Dusharme

                                                                     

Until recently, coordinate measuring machines (CMMs) were considered an option only for large companies with the financial resources to afford the substantial price. Small companies, such as those supplying parts to automakers, couldn't afford to install a CMM in their own facilities and instead had to rely on other methods for first-article or production inspection or wait for customers to use their own CMMs to do the inspection.

 Then industry began to see an influx of CMMs that were not only affordable--due in large part to the introduction of less expensive aluminum structures--but also stable enough to use on the shop floor. The new machines were faster, cheaper, less bulky and (arguably) more accurate than their predecessors, making the CMM more cost-effective for many more companies.

 With hundreds of CMM models available, prospective purchasers face the daunting task of determining which machines will meet their needs. When selecting a CMM, potential buyers must consider accuracy, throughput, cost and whether it will be used in a lab or on the shop floor.

 One key element that affects the answer to all of these questions is the primary material from which the CMM is constructed. Today, the two most popular materials are aluminum and ceramic. We recently approached three CMM manufacturers--Brown & Sharpe (B&S), LK Metrology Systems (LK) and Mitutoyo America Corp.--and asked them to argue the pros and cons of each material. The following is a collection of their responses to several questions about the CMMs' durability, accuracy, suitability for use in varying conditions and more. We hope that the following information will provide some insight and help you choose the most appropriate CMM for your needs.

 

Q. How do lab conditions or shop floor conditions affect your choice of materials? What accuracies must be maintained? What temperature swings would be anticipated?

A. B&S: CMM structural components made from aluminum are very lightweight, and the CMM can operate at relatively high speed, allowing the inspection operation to keep pace with production.

 Because of temperature variations, however, a CMM located on the shop floor won't provide the same performance as one in an environmentally controlled lab, although performance will be close. Even in a lab, temperature variations--slight though they are--must be compensated for. Because aluminum's thermal expansion coefficient makes the machine respond more rapidly and in a linear manner to temperature changes, aluminum machines are easier to compensate than machines made from other materials. This makes it a better choice for both types of applications.

A. LK: The material of choice for lab machines or shop floor machines is the same: ceramic. It's the degree of the temperature effect that varies. A lab environment may vary only two degrees to five degrees Fahrenheit, whereas a shop floor may have five to seven times that amount of variability. The selected material must be as stable as possible. A stable material has beneficial effects in lab conditions and on the shop floor. Ceramic is that material.

A. Mitutoyo: If a potential user specifies a shop floor application, an aluminum CMM with common structural components and temperature compensation would be the logical choice. Mitutoyo CMMs employ real-time temperature compensation and true closed-loop CNC scale feedback to the machine controller in order to maintain the stated accuracy specification.

 

Q. Is high-speed inspection a priority?

A. B&S: With aluminum, low structural weight improves the CMM's acceleration and deceleration, enhances overall traverse speed and ultimately increases measuring throughput without sacrificing accuracy and repeatability.

A. LK: Today's CMMs are compromises in design: speed vs. accuracy vs. cost. In situations where high-speed inspection is a priority, lightweight, stiff structures are the best design choice. But in most real applications, accuracy can't be ignored. Ceramic, due to its malleability and stiffness, provides the best platform for balancing customers' needs for speed and accuracy.

A. Mitutoyo: If quick throughput is an issue, a lightweight CMM is again the best option. Aluminum CMMs can be driven almost twice as fast as granite or ceramic units and can maintain their specifications even when driven at maximum travel and measurement speeds.

 

Q. How does the expansion coefficient affect your choice of materials?

A. B&S: Aluminum's thermal expansion coefficient makes structural members manufactured from it respond quickly to changes in ambient temperature, greatly reducing temperature-related geometric errors.

A. LK: In fighting the temperature battle, two basic design characteristics can be employed. The first is reducing the thermal expansion and contraction ratios by material choice. The second is using software for compensation. Neither are perfect sciences.

 The actual expansion and contraction characteristics of the material can only be estimated within 10 percent of the actual coefficient of expansion. Aluminum's 10-percent uncertainty cushion is greater than the entire range of ceramic's coefficient of expansion. The ability to provide a thermally stable machine is facilitated by the effort to reduce the magnitude of uncertainty of that material expansion and contraction. Thus, ceramic is the material of choice.

 Also, the environment in the majority of factories gives machines difficulty due to temperature spikes. The low coefficient of expansion in a stable thermal mass allows the elimination of these spikes. Basically, it filters them out, providing a more gradual trend of temperature changes. Gradual changes, rather than spikes, not only make the hardware design easier, they ease the software design burden as well.

A. Mitutoyo: Temperature swings can be more readily controlled if all of the structural components of the CMM are made from a common material--not an aggregate. It's extremely difficult for software engineers to build temperature compensation into structural members of a CMM if those materials have different expansion coefficients.

 

Q. How does thermal conductivity affect your choice of materials?

A. B&S: (Brown & Sharpe did not provide a response to this question.)

A. LK: Because ceramic has a low coefficient of expansion, thermal conductivity is less important. In a perfect world, the best material has a low coefficient of expansion and high thermal conductivity. Unfortunately, the choices we have to make include material compromises. The advantage of a low coefficient of expansion significantly outweighs any benefits that may be realized through thermal conductivity.

A. Mitutoyo: Aluminum alloys reflect about 90 percent of radiated heat and emit only a small percentage of the heat that they do take on. This low emissivity makes it an ideal material for applications where absorbed heat becomes paramount.

 

Q. How does the stiffness-to-mass ratio affect your choice of materials?

A. B&S: Aluminum offers a very high strength-to-weight ratio when compared with other materials used in machine tool and measuring instrument construction. This high strength-to-weight ratio allows large structural components to be fabricated from aluminum without adding excessive weight to the CMM. Because of the lower weight, CMMs built with aluminum structurals use smaller drive motors and less bulky drive systems to move the X, Y and Z axes, which reduces heat generation and improves the machine's accuracy potential. Smaller motors also cost less, reducing overall CMM manufacturing costs.

 However, ceramic material is also stiffer than aluminum, making it a material of choice for thin, heavily loaded elements of CMMs such as the Z axis. The Z axis generally receives the most deflection in a measuring operation due to its thinness and having a heavy probe mounted at its end. The slower response of ceramic material to ambient temperature changes, however, can affect measurement accuracy and must be compensated for.

A. LK: The two materials that are considered to have the best stiffness-to-mass ratios are aluminum and ceramic. But when looking at stiffness, you have to consider the technology of the control system, the smoothness of the drive system and how much technology is placed in the drive technique. An integrated design of a good, stiff structure that allows heavy bearing pressure or a pre-loaded bearing structure, plus a sophisticated drive control system, gives an optimum design for a machine. Ceramic provides these optimum characteristics with the ability to mold the material, particularly in directional stiffness, which maximizes stiffness in the sought- after direction.

A. Mitutoyo: Again, the stiffness-to-mass relationship expressed by aluminum make it a material ideally suited to CMM manufacture. Aluminum has certain metallurgical properties that give it special advantages over other materials. Its specific gravity is only 2.7, and its density is only about one-third that of iron. Despite the light weight, it can easily be made strong enough to replace heavier and more costly materials, such as ceramics.

 

Q. How does the CMM guideway technology affect your choice of materials?

A. B&S: Guideway technology shouldn't be a consideration when choosing a CMM material. As long as the guideways are machined and lapped to a smooth surface finish, performance differences should be negligible.

A. LK: Ceramic is becoming the standard for material choice on high-accuracy bearing surfaces. It can be made as a smooth and accurate surface with air-bearing structures, the bearing structures of choice for today's machines.

 The downside to aluminum is that it doesn't make a good bearing surface: It's typically coated or surfaced with other material, which can lead to problems with quality and cause bimetallic thermal expansion characteristics to become apparent. These add additional uncertainty to the thermal characteristics of the machine itself.

A. Mitutoyo: CMM guideways and air bearings are part of the mechanical integrity of the measurement gage. Aluminum alloy guides and nonporous air bearings should be manufactured to exacting standards. Again, matching materials employed on bearings and guides will help to ensure the mechanical integrity of the measuring device.

 

Q. Is price an issue?

A. B&S: The bottom line is always important. The use of lighter materials in CMM construction allows the use of less complex, and hence less costly, drive systems. Ceramic is more expensive and must be used judiciously in CMM construction.

A. LK: The effect of material choice on the price of a CMM is not always directly related material costs. Obviously, ceramic is more expensive than aluminum or steel structures. However, providing a more accurate and more dimensionally stable surface for temperature and stress relief allows for lower costs down the line. Well-engineered structures are not only valued by pure material cost, but also by future cost savings in installation, warranty and service: the life cost of the machine.

A. Mitutoyo: Obviously, price is always an issue. Lightweight CMMs are more affordable, therefore making them more accessible to smaller shops or potential users who traditionally found CMMs cost-prohibitive.

 

Q. How important is precise positioning accuracy--for example, when the user has to measure small closed features, such as bores?

A. B&S: Aluminum structurals use smaller drive motors and less bulky drive systems to move the X, Y and Z axes. This reduces heat generation, which improves the machine's accuracy potential.

A. LK: Attaining high positioning accuracy in prismatic parts, where there are flat surfaces, bores and features (for example), isn't as critical as when you're trying to find a feature on a complex contoured shape that's driving down a vector--any machine in the industry can drive and stop at a single position. Speed, lack of stiffness, and quality or technology of the drive system are all features that affect the outcome. Having a complete closed-loop drive system actively tuned to maintain an accurate tunnel vector within its drive characteristics is important. The technology of the drive system itself is designed for the characteristics of the machine to provide an important balance.

A. Mitutoyo: Precise positioning and tunneling accuracies are achieved by utilizing CNC controllers. These controllers can position a CMM within plus or minus one graduation of the scale resolution.

 

Q. How does aluminum compare with ceramics as a CMM construction material?

A. B&S: Ceramic material is heavier than aluminum, plus it has a lower thermal coefficient, which means it doesn't react as quickly to temperature differentials. On the other hand, ceramic material is also stiffer than aluminum, making it a material of choice for thin, heavily loaded elements of CMMs such as the Z axis. The slower response of ceramic material to ambient temperature changes, however, can affect measurement accuracy and must be compensated for.

A. LK: Ceramic is the material of choice. It's a more expensive material but possesses most of the desirable characteristics for a CMM. Comparing cost against performance is a true balancing act. Ceramic carries a good stiffness-to-mass ratio, it's a good bearing surface without any additives, it's malleable and it has superior thermal characteristics.

A. Mitutoyo: Aluminum is a material that lends itself ideally to CMM construction. Its low mass and relatively high stiffness make it a material that can be engineered into compact and geometrically sound designs. The material can easily be analyzed by the finite element method for optimal structural design and low vibration.

 

Q. What does the future hold in the way of material structure for the CMM industry?

A. B&S: Aluminum appears to be the material of the future. Any material available today that is lighter and has the same or better thermal characteristics than aluminum is far too expensive to use in construction. The CMM technology of the future is in improved software and sensor technology. Not only does the next-generation software provide refined thermal compensation capabilities, it also provides advanced analysis capability and the ability to seamlessly link the measuring system to a CAD database. New noncontact sensor technology will allow dimensional data to be gathered quickly to take advantage of the aluminum CMM's faster speed.

A. LK: During the last 10 years, there have been developments in materials such as carbon fiber, carbon-structure-type materials and matte-type materials that provide a lot of characteristics that would be very beneficial in structural design, stiffness-to-mass ratio, thermal stability, and so on. Unfortunately, with the market driving machine prices down, it's very difficult to afford bringing in any new materials that adversely effect the basic cost structure of CMMs. As these materials are used in other industries in greater volume, they will bring cost down and begin to migrate into the CMM industry.

A. Mitutoyo: The future of CMM structural components seems to lie with high-tech aluminum alloys. Its high strength, good formability and, perhaps most important, its excellent shock resistance make it the material of choice. These qualities make aluminum very useful in load-bearing applications. Other materials, such as ceramics, are compounds of metals bonded to a nonmetal, which may produce high strength but unfortunately are expensive and brittle.

About the participants

 Quality Digest would like to thank the following individuals for their participation in this discussion:

 David H. Genest, director of marketing and corporate communications at Brown & Sharpe Manufacturing Co., North Kingstown, Rhode Island. For the past 20 years he has been involved in product engineering, development and marketing, including the introduction of Brown & Sharpe's Process Control Robot and other systems for shop floor measuring and inspection applications. Contact him at dgenest@qualitydigest.com .

 Walter Pettigrew, vice president of LK Metrology Systems, a premier manufacturer of CMMs and integrated software solutions for metrology. Pettigrew has been with the company for more than 15 years. Contact him at wpettigrew@qualitydigest.com .

 John Knudson, sales engineer for Mitutoyo America Corp. He has more than 10 years' experience with Mitutoyo as an application and sales engineer specializing in CMM solutions. Contact him at jknudson@qualitydigest.com .

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