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Searching for the small-shop CMM?

Searching for the small-shop CMM?

Since the '60s, the coordinate measuring machine (CMM) has caught on big in the larger manufacturing companies. Over 13,000 CMMs are currently in use, and their number is expected to grow at rates of 15 to 20 percent per year over the next five years. One reason for this current growth rate is the accelerated need to improve product quality and speed production time for smaller parts made by small-and medium-sized job shops.

Yet, prior to 1985, the available CMMs were much too large and, even more significantly, too expensive for the the small-shop owner to justify their cost. Recognizing this need, the CMM manufacturers have since developed smaller benchtop models, targeted for the first-time user, yet offering the high accuracy, flexibility, and speed typical of the much larger, more expensive CMM.

In addition to tailoring the hardware to the small-shop environment, they have developed easy-to-use software systems to control CMM movements and gather measurement data. Touch-screens, for example, enable the novice user to control CMM functions directly by touching preprogrammed measurement options depicted graphically on the computer screen. This reflects the fact that most first-time CMM buyers and operators are not generally computer literate.

Basic characteristics

It takes CMMs of all sizes to inspect parts as they are manufactured to ensure that they meet specifications. A probe is passed over each part feature to find its displacement along each of the machine's three measurement axes, and the measurements are transmitted to a central processing unit for comparison to programmed specifications. Compared to manual methods, such as height gages and micrometers, the CMM not only improves the accuracy and reproducibility of the individual measurements, but also reduces the time required to inspect, record, analyze, and document them.

CMMs are available as manual systems or motor-driven, direct computer control (DCC) units. Manual systems are fine for inspecting less complicated parts--the data is sent to the processing unit as described above. The DCC machines--often used on-line in manufacturing cells--automatically inspect parts after the user has programmed their features into the system. The cost difference between manual and DCC machines is significant and varies widely, so it is important that the user evaluate both options thoroughly before committing to either approach.

Machine size is another significant variable. Large CMMs, such as those measuring automotive and aircraft parts, are free standing and can typically measure parts up to 54" wide, 128" deep, and 54" tall. Much smaller benchtop models measure parts 15" wide, 18" deep, and 10" high, and are ideal for medical and electronic parts. The smaller CMMs can be installed throughout a manufacturing facility to quickly inspect parts as they are made on the shop floor. Between these extremes are a wide variety of medium-sized CMMs suited to the more common intermediate part sizes.

Small-shop features

A common misconception is that CMMs are designed only for large companies with huge, highly automated manufacturing operations. True, large companies have historically been the high-volume buyers of CMMs, but today's CMMs offer many advantages for the small- to medium-sized shop: Cost. Often, potential CMM users view its purchase as simply increased overhead, when in reality, the opposite is true: it is a productive piece of machinery that can cut overall manufacturing costs by reducing direct labor.

A number of studies support this claim. For example, manually inspecting a machined casting might normally take 4 hr, whereas a CMM can do that same job in 8 to 10 min. Multiply that difference by the operator's burden rate, and the savings quickly mount.

Jim Smith, founder of James A Smith Co, Minneapolis, MN, bought a CMM for his 10,000-sq-ft job shop four years ago. One repeat job order that took 3 days to inspect manually, now takes 3 to 4 hr. "The CMM was a big investment for us (approximately $65,000)," he says, "but it's a real time saver. It took about 9 months for the CMM to pay for itself in time savings and quality of work." Quality. CMMs yield consistent measurement accuracy, and the ability to guarantee part quality--documented by CMM printouts--has become a competitive advantage. C Machine Co, a Minneapolis, MN, job shop makes custom parts for computer, aerospace, and medical applications. Says Craig Dickison, president, "Our customers demand quality. In order for us to keep up with this demand, we felt the need to purchase a CMM that would ensure product quality. Our old equipment just couldn't measure up."

For the small shop, the CMM is almost a necessity. Quality demands more than the use of a caliper, micrometer, and hand-written notes. Since installing their CMM, C Machine has increased their accuracy rate to 99 percent, improved turnaround time with less time needed for inspection, and now makes measurements 50 percent faster and 40 percent more accurate. Competitive edge. Kurt Manufacturing, a high-precision machine shop with plants in three states, began using CMMs in the early '70s. According to Rocky Schmidt, vice president, corporate quality, Kurt's 18 CMMs helped them improve their competitiveness on complex work. "The time we would spend on the surface plate with complex parts would not allow us to operate the business cost-effectively. With the CMMs, we can control our costs. On complex parts, we can reduce inspection time by 80 percent."

Kurt uses its motor-driven CMM with a CAD/CAM system. The parts are measured, then the data points given to the engineering department for any necessary corrections. A printout of the specification parameters informs customers that quality levels are being met.

CMMs also provide a psychological edge, Smith adds. "The CMM impresses people more than anything in the shop. Accuracy and precision are important to our customers." And Dickison agrees. "When quality assurance managers notice the CMM in our facility and see it in operation, they are amazed with what it can do. With a CMM, we are better able to compete for business." Process improvement. Regular inspection with a CMM allows a shop to learn more about its operations and capabilities. Strengths can be exploited for marketing purposes. Weaknesses can be corrected. Manufacturers cannot correct problems until they are identified, and a CMM can provide important clues. "Statistical studies of the accuracy of our operations have improved our process at Kurt," says Schmidt. "We use printouts to reduce our final inspection time." Training. One of the perceived pitfalls of a CMM for the smaller shop is the time employees must be away from their jobs for training. To combat this, CMM manufacturers have developed easy-to-use software and hardware that allow people unfamiliar with computers to operate the CMM.

The touch-screen system is one example. Numerex's Uni-Touch system allows the user to calibrate the probe, align the part, and build computer-generated constructions by touching preprogrammed operating functions displayed on the screen. The average training time is several hours compared with several days or weeks with traditional software programs.

The construction of smaller, manual-driven CMMs also has made it easier to inspect a part. Friction-free air bearings, improved designs, and lighter instrument materials such as ceramics allow the axes to be moved with much less effort and operator fatigue.

PHOTO : CMM operator at C Machine Co measures slots, angles, and hole locations in a computer-card

PHOTO : guide.

PHOTO : Typical of the new small-shop CMMs, the Numerex Benchtop model is available as a manual or

PHOTO : motor-driven system, and features air bearings; a measuring envelope of 15" X, 18" Y, and

PHOTO : 10" Z; granite beam; ceramic Z-axis shaft; and simplified software.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:coordinate measuring machines
Author:Meyer, Steven J.
Publication:Tooling & Production
Article Type:product announcement
Date:Jul 1, 1989
Words:1251
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