How to Buy a Coordinate Measuring Machine

by David M. Genest

Quality manufacturing is generally defined as the consistent production of parts that conform to design specifications. However, achieving production consistency requires control over the manufacturing process. The most effective way to establish and maintain process control is by accurately measuring work-piece dimensions. When dimensional information is captured, analyzed and fed back into a production operation, it becomes a valuable tool for continuous quality improvement.

Coordinate measuring machines offer one of the most efficient ways of measuring and capturing dimensional data because they can replace numerous surface plate tools and expensive fixed gages, and reduce complex measurement tasks from hours to minutes. The ability of CMMs to quickly and accurately evaluate dimensional data and provide the operator with meaningful information concerning the condition of the manufacturing process is what differentiates CMMs from hand-measuring instruments of all types.

CMMs measure by contacting a part with a sensing device called a probe. The acquired data can be combined into a dimensional description of the part or a particular part feature, such as a bore. From that dimensional description, it is relatively easy to determine if the part or feature is within tolerance. That information also provides a clue to correcting aberrations in the process that may have caused the out-of-tolerance condition.

Putting the CMM to work in your shop

If a CMM is appropriate for your operation, how do you choose the best one? The first decision is which type of CMM to purchase. There are three basic styles to choose from: vertical, horizontal and portable.

Vertical CMMs have the probe attached to the vertical axis. They have the potential to be more accurate than their horizontal counterparts because their bridge structure can be more massive and built with fewer moving parts, making them more rigid and therefore more stable. Vertical CMMs come in all sizes and can handle work pieces ranging from small gears to engine blocks to commercial aircraft bodies.

Horizontal arm CMMs have the probe mounted on the horizontal axis. They are generally used in applications where large parts, such as automotive bodies, must be measured with a medium level of accuracy.

Portable CMMs simplify the inspection of parts or assemblies that can't be transported to a CMM. The portable CMM can be mounted on or even inside the part or assembly. This permits the measurement of interior space and allows users to measure entire assemblies in situ, saving time necessary to remove, transport and measure individual components.

In any stationary CMM design, increasing mass and rigidity by increasing the cross-section of the structural components and lengthening the distance between bearings, increasing the power of the drive motor and optimizing the selection of structural materials based on weight and thermal capabilities improves measurement accuracy and repeatability along with the speed and acceleration. This principle has been applied to a clas s of horizontal shop-floor CMMs. These shop-hardened systems combine the flexibility and capacity of a horizontal CMM with the higher accuracy of the vertical design.

The horizontal direction of measurement makes these CMMs a logical choice for production applications where horizontal machine tools are used. They are particularly well-suited for measuring large gear cases and engine blocks that require high-precision bore alignment and geometry measurements.

Four-axis capability can be obtained by incorporating a rotary table, and dual-arm configurations are also available. Both provide measurement access to all sides of a part. The horizontal configuration also provides accessibility for quick loading and unloading of parts. Small, shop-floor versions are available for high-speed production applications.

Choosing the right one

CMMs are available in both manual and automated versions. The choice depends on the application. If you're inspecting the geometry and tolerances of simple parts, or if you're measuring small batches of dissimilar parts, a manual machine may be the best choice. Software provided with some manual CMMs allows inspection programs to be stored and recalled, facilitating repetitive measuring. For large-batch inspection of similar parts, or where higher accuracy is required, a direct computer control machine is the best choice. DCC coordinate measuring machines automate the inspection process and eliminate operator influence on measurement results. Programmed movement means higher measuring speeds without error.

Tolerances are important, too. Tolerances of 0.0001" or less are difficult to achieve repeatedly on a manual machine. A DCC machine with its consistent touch is better suited to high accuracy and repeatability for close tolerance parts.

DCC machines used to measure parts that require massive amounts of data to accurately define their geometry- gears, cylinders, auto bodies, windshields- can be equipped with an analog scanning probe. These probes provide continuous data collection for parts that are either fully defined mathematically via CAD or completely unknown, as in the case of a component that is to be reverse-engineered from a model or broken part. Very small contoured parts can also be ideal candidates for scanning probes due to their smaller measurable surfaces and the need for large amounts of data to define them.

The location of the measuring operation is also important. Ideally, measuring machines should be used as close to the manufacturing process as possible by the operators who actually make the parts. These shop-floor CMMs are generally equipped with user-friendly operator interfaces ranging from interactive color-graphics terminals to hard panels similar to machine-tool controls.

Different types of CMMs can work together. A laboratory-grade vertical CMM can be used off-line as a master arbiter of part specifications, while a shop-hardened CMM can be used on-line for auditing work-piece measurement, providing real-time statistical process control. Any approach can be justified, but it must fit smoothly within the overall manufacturing process plan.

Key aspects to look for

Once you've determined how and where you want to use a CMM, there are key specifications to look for in selecting a particular model: accuracy, repeatability and throughput.

Accuracy is indicated by the B89 or VDI specification in the CMM manufacturer's statement of accuracy. By comparing the B89 (single accuracy number) and VDI (accuracy formula) specifications of machines, you can determine the level of confidence to expect from the measuring operation. The lower the number, the more accurate the machine.

Repeatability is the consistency with which a CMM can measure an object having known dimensions. This is determined by the B89 and VDI standards. The degree to which manufacturing operations can be fine-tuned depends on the repeatability of the CMM and the machine tool.

The number of data points that can be measured at acceptable levels of accuracy and repeatability determines the machine\rquote s throughput. Some CMMs can accurately handle throughputs of more than 100 data points per minute with "very near" lab-grade accuracy.

CMMs can be justified in today's modern manufacturing operations because they can replace surface plate tools, fixed or custom gages, and precision hand measuring tools. Their flexibility in handling different manufacturing-related jobs makes them a bargain. In addition to providing dimensional data for process control, CMMs offer the added advantage of being used for incoming material inspection, machine tool runoffs, customer quality-certification requirements, gage qualification, tool-wear studies and optimizing machine tool setups. Justification of any piece of capital equipment is determined by a number of factors, but when considering productivity improvements, cost reductions and process control, CMMs are a good choice for your measurement and inspection requirements.

Selecting the Right Measurement Software

Software enables CMMs to fulfill their potential for speed and accuracy. Today's CMM software is refined to the point that no computer programming knowledge is required to run even the most sophisticated programs. All CMM software today is menu-driven, i.e., it asks operators what they want to do and even prompts the most likely choice.

Software programs are available for statistical process analysis and control. Programs for sheet-metal applications facilitate the location and measurement of parts containing hemmed edges, weld nuts and studs, and similar features. A myriad of contour p rograms permit the CMM to rapidly and accurately define complex, nongeometric shapes without straight edges such as turbine blades, screw and scroll compressors, gears, pistons, cams and crankshafts.

Soft gaging packages make it possible to graphically construct gages that are mathematically inserted into part features to verify dimensions. Some programs offer modules for the measurement of nonprismatic parts, such as sheet metal and plastic assemblies, windshields and exhaust systems. This eliminates the need for expensive fixed gages. Other modules perform 2-D and 3-D best-fit routines and contour comparisons.

Data transfer standards have been developed and accepted by industry, such as Dimensional Measurement Interface Specification (DMIS) for communication from CMMs to CAD systems and Initial Graphics Exchange Specification (IGES) for CAD-to-CAD data exchange. Such capability makes it possible to use CMMs in applications such as reverse engineering, where specifications are derived from the measurement of a model or broken part. Data transfer also allows for the development of measurement routines directly from CAD data, off-line if appropriate, to relieve the burden placed on "production" CMMs. The emerging STEP standard will expand the capabilities of these two standards and eventually replace both.

Selecting the appropriate software package for your operation is a critical decision in getting the most value from a CMM.

When you've decided on a CMM that is right for you, the CMM manufacturer will suggest the appropriate software for your machine and application, and you should follow that recommendation. However, there are some general guidelines you should keep in mind when making the decision.

Measurement software should be easy to operate. The easier it is to use, the faster the inspection and measurement throughput. The fastest, simplest software uses a graphic user interface (GUI) rather than keyboard commands.

Select a recognized PC-based computer and operating system to run measurement software. They are the common denominators in the industry. Training, service and upgrading won't present a problem.

A software package should give operators of all skills levels an interface compatible with their ability and needs. For example, a shop-floor interface for execution of inspection programs and a programming interface for technicians to input routines for one-off parts, change DCC commands, integrate special canned application programs and develop measurement routines for complex parts.

Measurement software should generate clear, concise graphic representations of measurement results. Data presented in this way can be more easily used by operators to correct out-of-tolerance conditions and by customers who require documentation of the inspection process.

Make sure your software can be upgraded and that new versions are compatible with older versions of the same program. If your software doesn't have upward compatibility, you'll experience disruptive system changeovers.

Measurement software should be able to communicate with CAD/CAM programs and systems. Even if you don't use CAD/CAM now, you may in the future. Having an integrated measurement program can significantly improve manufacturing throughput and quality.

About the author . . .

For the past 20 years, David H. Genest has been involved in product engineering, development and marketing at Brown & Sharpe Manufacturing Co. in North Kingstown, Rhode Island. Genest is currently director of marketing and corporate communications for the company.

Copyright 1995 by QCI International. It is unlawful to reprint, retransmit, or otherwise reproduce this article, except for personal use, without the written permission of QCI International (which gladly grants permission when asked). Call (800) 527-8875 or fax (916) 527-6983.

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