Quality Digest Article
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Give New Life to Your Old CMMby         

 by Walter Pettigrew

Upgrading your old CMM
is a cost-effective way
to boost quality and

Programmable coordinate measuring machines rapidly are becoming fixtures of today's increasingly digital manufacturing environment. In coordination with customer electronic build information and CNC machine tools, state-of-the-art CMM systems provide speedy part inspections and tight control of manufacturing processes, allowing manufacturers to produce precision parts within a consistent tolerance range in the shortest possible time.

CMMs also are an efficient and reliable source of inspection reports customers now routinely demand along with on-time parts deliveries. Networked to a CAD system, CMMs will input customer electronic data for part inspections and output required product assurance documentation.

These capabilities are impressive, but they don't come cheaply. CMMs represent a sizable capital investment, and manufacturers must consider their inspection requirements carefully before deciding on needed capabilities. In many cases, these capabilities can be acquired by simply upgrading existing CMM equipment. Upgrade services are available for practically all CMM makes and models and can provide virtually any level of desired capabilities. Upgrades are of three types:

  Retrofitting--Selectively adding desired hardware or software.

  Rebuilding--Bringing an older machine back up to original equipment manufacturers' specifications. This would include    repairing and replacing worn or defective mechanical components, which results in a CMM that essentially is as good as new.

  Remanufacturing--Bringing the old CMM up to the standards of today's CMMs, which results in a CMM that essentially is better than new.

Converting manual CMMs to automatic

One significant upgrade, possible with retrofit packages, is to convert a manual CMM to automatic. Automatic CMMs allow users to network digital capabilities into a highly efficient manufacturing system. Basic CMM tie-ins include customer electronic build information and CNC machining operations. Such networks make it possible to produce precision parts to consistent tolerances in the shortest possible time and also generate quality inspection reports. In addition, automatic CMMs can measure complex shapes beyond the capacity of manual machines and may network to analysis and engineering tools.

Retrofit packages that convert manual CMMs to automatic include a microprocessor-based controller with direct computer control capabilities, and advanced operating and programming software. They may also include full servo control and new linear scales for faster, more precise positioning of the measuring probe, a remote digital readout display or even a new angle probing system that provides a fourth axis of contact.

The DCC controller is the heart of the retrofit because it enables efficient downloading of customer-supplied mathematical part data from a computer-aided design system. System software converts this numerical data into a database used to create part inspection routines. The inspection programming produced--generally in conjunction with CNC parts programming--allows efficient, accurate inspection setups and probing routines. In contrast to manual CMMs, their speedy automatic counterparts will perform part inspections that integrate fully with CNC metal-removal operations. Interrupting parts production for inspections is no longer necessary, and this upgrade also eliminates scrap from additional parts made during inspections. The resulting productivity gains can help maintain cost-competitiveness and allow rapid pay-off of a CMM investment.

A retrofit success story

In a recent retrofit project, a DEA Beta CMM used by a well-known designer and manufacturer of gages and fixtures was upgraded to include a new DCC controller and software, servo amplifiers, new axis drives and linear scales. With these improvements, the manufacturer can now run inspection routines two or three times faster than before and has sharply reduced inspection setup times when a program is in place. Once the machine has been taught to check for a given fixture, it will check the same fixture automatically in succeeding setups--or even its symmetrical opposite.

Because the programming system employs simple geometric icons that guide the programmer through every step of the inspection programming process, the manufacturer also can quickly train an operator who has no previous experience with automatic CMMs to do the programming.

A Windows-based package called CAMIO from LK Inc., a leading CMM manufacturer, offers sophisticated software for CMM upgrades. This program allows models created on a CAD system to drive any CMM that supports the American  National Standards Institute's DMIS protocol. The software is actually a comprehensive suite of CAD-based inspection modules that support solid, surface or wireframe data from CAD systems using either standard IGES and STEP protocols, or dedicated links to CATIA and Unigraphics. By using CAD mathematical data for off-line inspection programming, the software frees the CMM to perform actual measurements and effectively eliminates the need for program  debugging.

The retrofit software also allows for comprehensive analysis and engineering tools. The manufacturer can now validate its gages for repeatability and reproducibility three times faster than before. It also has used the software for PIST studies of customer prototype parts, which document the percentage of inspection points within a specified tolerance. A cost-benefit analysis of its retrofit upgrade indicates that the investment paid for itself in less than a year. The benefits were obtained, moreover, at only one-third the cost of a new-purchase machine replacement.

Other software retrofits

Though most software retrofits enhance CMM performance, some expand system capabilities. A good example is the networking of multiple CMMs in one or more plants. With networking, statistical process control data collected by each individual CMM controller is deposited and stored in a central database for access from locations throughout the organization. Frequently, comparing process data from one location with a similar process at another location can help correct problem conditions.

A prime example of system enhancement is LK's recently introduced software interface for Valisys, the comprehensive suite of computer-aided production engineering tools from Technomatix Inc. The new interface enhances CMM productivity by permitting users to employ powerful Valisys tools for inspection programming as well as for process control and analysis.

One Valisys tool, for example, provides an alternative to standard off-line inspection programming. While still operating from a PC, a user can actually program the CMM and run it at the same time. With the CAD model on the PC screen, the operator can click on a circle, and the machine will instantly measure it. At the same time, the PC screen displays a simulation of the probe going around the hole. Upon completion of the inspection routine, the new interface will qualify the data and return it to the operator for analysis.

Still other software enhancements can help meet special inspection requirements. In one case, software installed in the VAX-based operating system of dual-arm horizontal machines used to inspect location points on very large truck frames has established a single coordinate-axis system between the machine columns. The software allows the dual arms to measure points on the frame's two sides from a common reference point. In contrast to preretrofit performance, the CMM now generates only one output report, providing immediate, comprehensive and fully reliable measurement data.

Rebuilding and remanufacturing

A higher level of upgrade offered by CMM suppliers is rebuilding machines to original equipment manufacturers' specifications. Generally, such projects include the basic retrofit enhancements as well as repair and replacement of worn and defective machine mechanical components.

A recent remanufacturing project entailed the near-total rebuilding of six competitive-make CMMs. Only the original table and arms on these machines were kept, while all other mechanical components were replaced. Retrofits included new motion-control drives, probing systems, 4-axis CNCs and new computer systems.

During the rebuilding, some of the original machines' components were replaced by state-of-the-art parts with advanced performance capabilities. This additional step, called remanufacturing, represents a third tier of upgrades that usually includes basic retrofits and mechanical rebuilding.

The remanufacturing approach is illustrated in the replacement of worn motion-control drives in the six rebuilt machines. State-of-the-art linear encoders, or linear scales, replaced the original rotary encoders. The linear encoders provide the advantage of reading axis motion directly, rather than the motor revolutions. As a result, probe positioning accuracy is increased by as much as 10 times, from a range of 40 microns with the old-style encoders to just 4 microns. The linear encoders also accommodate advanced error-compensation software, which will map out any kind of error or deviation in the inspection cube and correct it.

Best buys

Of the three approaches to CMM upgrading, retrofits of DNC controllers and advanced operating software offer the biggest paybacks for dollars invested. They provide a tie-in to customer-supplied electronic build information, faster and more accurate inspection routines and quicker setups of inspection routines. They also offer easy-to-use Windows-based programming software, fast training of even inexperienced CMM operators in inspection programming, customer product assurance documentation and compatibility with comprehensive analysis and engineering tools. Given today's trend toward all-digital manufacturing environments, such upgrades should seriously be considered as key investments for maintaining cost-competitiveness and profitability.

About the author

Walter Pettigrew is vice president of LK Inc. in Brighton, Michigan. He has a bachelor's degree in production engineering and master's degree in business administration. He has been employed with LK since 1986 and has more than 20 years of experience in the quality industry. His experience covers a wide range of technologies, including CMMs, specialty designed gaging, vision systems and lasers.

LK Inc. is a leading CMM manufacturer that also does CMM upgrades.


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