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Selecting CMMs - manual or automatic.

Choosing between a manual and automatic or direct computer control (DCC) coordinate measuring machine (CMM) is a bit more complicated than merely buying the most CMM you can afford. While the trend has been toward DCC CMMs the past few years, purchasing decisions have been driven more by the demands of the application than by the depth of buyers' pockets.

Why the growing trend towards investing in DCC CMMs? "The price gap between manual and DCC has closed dramatically in recent years," says Hans P Deller, vice president & general manager, Carl Zeiss Inc. "Also, the ability to use CMMs on the factory floor for real-time inspection has fueled a number of new DCC CMM purchases." Still, there will always be applications where a manual CMM is a more practical choice, such as in job shops where every part being measured is different and the work envelope is relatively small.

"An investment in either a manual or DCC CMM is a large one," says John Knutson, senior application engineer, Mitutoyo. "But keep in mind it is probably a one-time only purchase that will outlast most machine tools. That's the good news. The bad news is that because it's a one-shot deal, you will have to think about your future needs more than you would a typical machine tool," continues Mr Knutson.

Depending on your operation, a return on investment (ROI) for a CMM will stem from one or more factors. These factors can include: increased operator or inspector productivity, lowered scrap and rework time, and averting wasted added value due to working on defective raw materials.

Selection criteria

As we reviewed the many CMMs introduced at the recent Quality Expo in Chicago, we talked with experts from Brown & Sharpe, DEA, L&K, Mitutoyo, Starrett, and Carl Zeiss, asking them what the most important factors are to consider when deciding between manual and DCC CMMs.

The consensus was that several factors should be considered when choosing between a manual and DCC CMM, such as: workload requirements, operation repetitiveness, tolerance requirements, measuring difficulty, and upgradeability.

In general, if you're considering a CMM just to say you have one, or to qualify as a vendor, a manual CMM is probably your best choice. On the other hand, if you're serious about quality control and plan to truly utilize the system for that reason, a DCC system is usually the right step. In fact, most first-time purchasers, after they have owned a CMM for a while, discover many additional benefits that they did not originally anticipate.

Repetitiveness of the work at hand should weigh heavily on the choice of CMMs. For lengthy on-line production applications, where measurements are repetitive, a DCC CMM is the only reasonable choice. With a DCC system, the preprogrammed movements mean higher speeds without error, while requiring minimal operator attention or skill. On the other hand, in a job shop where parts are run in small lots and differ from batch to batch, the programming time imposed by a DCC may make a manual CMM more efficient.

Tolerances of 0.0001" or less are difficult to achieve repeatedly on a manual machine. Human inconsistencies lead to uneven measuring forces and hamper accuracy. An automatic CMM with its consistent touch is better suited to high accuracy and repeatability. If you are certain tolerances will always be above 0.0001", either system is justified.

Feature size and difficulty of measurement will often determine which CMM is better suited to the task. Generally, if features are small and/or inaccessible, a DCC CMM usually works out best, as it can save operator time and possible sequencing errors. A manual CMM lends itself better to workloads characterized by large, easily accessible features. Size of the workpiece features and ease of access are more important as selection factors than the shear number of features.

Price should not be a major consideration in the choice between manual and DCC CMMs. The gap in pricing between manual and DCC has narrowed significantly in recent years. More important is if your expected utilization of a CMM appears to be heavy and the work is repetitive; then a DCC system is the best choice. The cost differential between manual and DCC can be quickly overcome by the cost savings a DCC CMM brings to high volume, repetitive operations.

Upgradeability from manual to DCC operation is another factor that should be considered. Some manual CMMs are upgradeable to DCC operation through the addition of servos, servo drives, and controls. If there is a chance that your needs may dictate a DCC CMM in the future, or you just want to cover all your bases, be sure to ask about upgradeability. Most of the CMM vendors we spoke to at the show had at least one manual model that could be upgraded to DCC operation in the field.

Once you have narrowed down the selection to either manual or DCC, you are ready to compare the various offerings from CMM vendors. And this year's Quality Expo afforded attendees a broad range of CMMs to choose from.

Narrowing the choice

Anyone looking for an opportunity to get a hands-on look at what CMM manufacturers have to offer would have spent their time well by attending the recent Quality Expo in Chicago. Virtually every major CMM manufacturer was at the show, many using the show to introduce new models reflecting the current state-of-the-CMM-art. Whether you were looking for a manual or a DCC; contact or noncontact probing; small, medium, or large work envelopes; or simply more user-friendly CMM programming, there was something of interest for everyone.

Brown & Sharpe introduced a class of compact, microprecision CMMs that combine high precision with easy accessibility. The PMM 432 provides throughput of more than 60 points per minute, occupies about half the space of previous PMM systems, and costs about half as much.

With the PMM 432, an operator can sit next to the machine, line up a probe tip on a tiny feature, and initiate high-speed measurement that yields accuracies of microns and repeatabilities of tenths of microns. Though it occupies only 30 |ft.sup.2~ of floor space and is less than 6 ft tall, it has a measuring envelope of 15.7" X 11.8" X 7.9".

The Carl Zeiss booth typified the wide range of CMM offerings at the show. At one corner of its booth, the company introduced the C 400 benchtop CMM offering XYZ measuring envelopes of 15" X 18" X 12" or 15" X 32" X 12" and a linear accuracy to 0.000 15". Available in both manual or DCC versions, manual versions are field upgradeable to DCC operation with the addition of servos, servo drives, and control.

Just a few feet away from its low-end C 400 machine, Zeiss was demonstrating its leading edge UMC 850/1200 CARAT systems that are able to measure hundreds of points in the time it takes conventional CMMs to measure four to six points.

The high-speed scanning probe remains in contact with the workpiece and gathers measurement data as it moves along the part surface, thereby eliminating time-intensive, point-to-point probing. By measuring up to 100 times more points, the UMC greatly increases the confidence level that the measuring results represent the true dimensions of the part--very important in form- and position-tolerance measurements, as well as in contour measurements.

DEA Company introduced a new concept in manual bench top CMMs. Called Mistral, the machine has been tailored to serve jobshop applications and sports unique Slant Bridge Technology (SBT) and underway guidance. Its 45 deg inclination is said to provide added stiffness, low mass, and ensure the main guideway is low and close to the measuring volume. This reduces the center-of-gravity coupled with low inertia to produce smooth, precise operation.

The machine is also available in a DCC version, which offers a maximum velocity of 26 m/min, and features pneumatically operated disengagable drives.

The Mistral offers an XYZ measuring envelope of 28" X 26" X 18" with a CMMA accuracy of B89.1.12 = 0.0004" with the inclusion of 0.5 micron resolution glass scales.

Each of the three axes are positioned by the operator using the single-inertia handgrip philosophy. To accommodate the long experienced CMM operator, DEA has introduced a thermal collar, which allows the operator to follow traditional techniques by holding the base of the quill without degrading the accuracy of the machine through heat transference from his hand to the probe.

For people desiring to move their CMM out of their metrology lab onto the shop floor, LK Tool was displaying its HC-90 Micra CMMs.

Boasting an operating temperature range from 32 F to 104 F, the HC-90 Series offers three touch-per-second inspection with volumetric accuracy of 0.000 25" and repeatability of 0.000 08".

The machines come in three standard sizes providing XYZ measuring envelopes of 40" X 16" X 24", 60" X 24" X 40", and 100" X 32" X 48".

An example of field-upgradeable CMMs could be found at the LS Starrett Co booth with its new HGC Series compact CMM, available in both manual and DCC versions. The new machines combine patented Starrett hollow granite technology with a ceramic Z axis to provide rigidity and consistent performance in demanding shop floor environments.

Featuring an XYZ measuring volume of 20" X 18" X 16", the HGC offers a very generous working height to accommodate a variety of probes. The manual HGC provides system linear accuracies of 0.0002" per axis, a bandwidth volumetric accuracy of 0.0004", and a resolution of 0.000 02". Repeatability specifications for the HGC are: unidirectional of 0.000 08", bidirectional of 0.0001", and three-dimensional of 0.000 12".

At the Mitutoyo/MTI booth, show attendees saw several CMMs ranging from the very economical BH 300 Series manual CMM, with an XYZ measuring envelope of 13" X 12" X 12", to a variety of full-featured DCC CMMs such as the BHN 700 Series, which offers a measuring envelope as large as 28" X 60" X 24". A general purpose, high-speed, moving bridge machine with DCC, the BHN 700 series was developed to measure small to midsize parts in the lab or on the factory floor.

While all CMM manufacturers offer noncontact probes in addition to traditional touch probes, Wegu Inc drew a lot of attention with what it has dubbed its Multi-Sensor coordinate measuring machine (MSCMM). Its MMC 800 offers four axes (X, Y, Z1, Z2) and three data acquisition sensors: optical, laser, and touch probe. The operator selects the probe or probes best suited to the inspection task. Whether the part is soft and flexible or hard and rigid, the MMC can provide the best suited probe or probes for the task at hand.

Its CCD video system with coaxial linear laser and 0.000 001 9" subpixel resolution can magnify and inspect micro features up to 400X magnification, while the laser, with a visible spot size of 0.000 39", is used to obtain single data points or dynamically scan/digitize at a rate of 5000 points-per-second with a resolution of 0.000 003 9". Additionally, the touch probe sensor can be used to inspect micro features requiring a stylus of 0.012" or larger features with larger stylus sphere diameters and extensions.

The MMC 800 has a measuring envelope of 35.5" X 31.4" X 23.6" X 23.6" (X, Y, Z1, Z2) and traverse rates of 9.9"/s and acceleration of 78.7"/|s.sup.2~.
COPYRIGHT 1993 Nelson Publishing
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:coordinate measuring machines
Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Jun 1, 1993
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