Sizing up coordinate measuring machines.

Sizing Up Coordinate Measuring Machines

Coordinate measuring machines (CMM's) are being used more and more commonly as quality tools in plastics processing. CMM's can make multiple measurements on parts with contours and irregular surfaces in a fraction of the time required for conventional manual gauges. For over 25 years, these devices have been used for off-line part inspection in a laboratory environment. As dimensional tolerances become stricter in automotive and other industries, CMM's are gaining a foot-hold on the factory floor, and an increasing number of plastics processors are considering CMM's for inspection of tooling and parts.

Rather than discovering and rejecting bad parts in the q-c lab long after they have been made, the trend is toward checking parts right after molding, in hopes of catching an early indication of a shift away from the center of the tolerance band, before any parts must be rejected. Furthermore, increased use of CMM's is creating demands for greater accuracy, speed, and ruggedness.

Here's an introduction to the subject for processors unfamiliar with how CMM's work and the benefits they offer. A list of suppliers appears at the end of the story.

HOW THEY WORK

Typical CMM's consist primarily of a 3D measuring area with a touch-sensitive probe arm attached vertically to an overhead gantry or horizontally to a vertical guidebar. The probe arm can move in two directions, while the gantry or guidebar moves in only one, resulting in three axes of motion for the probe. On some CMM's, the probe rotates on one or more axes independently of the arm for additional flexibility.

When a tool or part is placed within the measuring area, usually on a table, the probe is moved along the object's surface by servo motors, either manually or under automatic control. For automatic control, the user first directs the probe manually, typically with a joystick, and stores the measuring sequence in computer memory for later recall. With a manual model, the user must direct the probe with the joystick for each workpiece. Most manual CMM's can be retrofitted for automatic control, either by the original CMM manufacturer or third-party firms such as M&M Precision Systems (see supplier list).

Whenever the probe actually touches the workpiece, the point of contact is defined in terms of the x-y-z axes of a coordinate system. This is repeated for several points, and with the aid of a computer, successive data are interpolated to determine the distances between points. The distances are then automatically translated into part dimensions for comparison with specifications.

HOW A CMM CAN HELP

CMM's allow plastics processors to greatly increase the number and accuracy of measurements that they are performing on parts and tooling. Regal Plastics, Roseville, Mich., an automotive custom injection molder, acquired a CMM out of necessity last year. Without it, Regal would not have been able to accept a Chrysler prototyping contract for injection molding engine-compartment parts, such as shock-absorber dust covers, in glass-filled ABS. The contract called for 20 parts with up to 100 dimensions to be measured per part, at a tolerance of [+ or -]5 mils on each dimension.

Previously, Regal had relied on angle blocks, micrometers, and optical comparators to measure parts, or it had paid an outside firm to perform some of the measurements. On a previous, less demanding project for Chrysler, Regal spent approximately $35,000 on such measurement subcontracting. Hiring a third party to perform the measurements on the engine compartment parts would have cost over $100,000, while the CMM that Regal elected to purchase cost around $50,000.

The CMM, an automatically controlled contact-measurement model from Federal Products, has long since paid for itself. Although the measuring range of the machine--40 X 20 X 18 in.--exceeds Regal's current needs, the firm appreciates having the capability to handle large parts, such as entire instrument panels, in the future.

According to John McLean, quality assurance manager at Regal, tasks that previously took an hour can be done in less than 6 min with the CMM. Regal uses the CMM during initial production setups, capability studies, and annual measurement layouts required by the auto makers. Although shrinkage is forecast during die modeling, it is not uncommon to go through four to six test shots to optimize the mold. The CMM provides a shot-to-shot history of dimensions during setup, enabling the tool maker to adjust the mold and bring the dimensions within tolerance.

For inspection of parts in actual production, Regal relies on in-process gauging supplied by the auto makers to measure the most critical dimensions, which typically number only around 5% of the total dimensions studied initially with the CMM.

Toledo Molding and Die Corp., Toledo, Ohio, uses a CMM from Numerex to measure tooling and finished parts it molds for polypropylene and nylon ductwork used in automotive air-conditioning systems. With the CMM, Toledo Molding is able to perform measurements that were too time-consuming with manual gauging. The company also uses a noncontact video system from RAM Optical Instrumentation for measurements on materials that are too flexible or dimensions too small for a contact CMM.

NONCONTACT, ON-LINE AHEAD?

Prices for CMM's start at around $25,000 and can go well into six figures for advanced systems. While they have performed well in controlled environments, users have experienced problems with vibration, humidity, and heat. Thermal expansion of the steel used in the structure of CMM's has led to inconsistent measurements in the past, and many of the newer machines have granite measuring tables and ceramic axis guides for greater dimensional stability at high temperatures.

Even with stable materials, however, most CMM's are dependent on the probe physically touching the workpiece. While contact measurement is still the best choice for many applications, parts that are flexible, fragile, tiny, or highly contoured may be better suited to noncontact methods of measurement. These methods use lasers or video cameras, and they have the potential to operate fast enough for factory-floor use. Although contact CMM's provide great time savings over manual gauging, they're not yet fast enough to be used for 100% on-line inspection.

Prism, an optical CMM introduced last year by Videometrix, is said to measure dimensions with a resolution comparable to or better than similarly priced contact CMM's. The company says the machine's open physical structure permits its inspection platform to be placed directly in series with a production line. Even so, it stresses that most on-line applications require extensive computing power and mechanical modifications to obtain accurate and consistent measurements.

Other manufacturers of non-contact optical measuring systems include Federal Products, Optical Gaging Products and RAM Optical Instrumentation.

Some manufacturers of conventional CMM's also sell machines that use laser triangulation probes to perform measurements without contacting the part. These companies include LK Tool and Carl Zeiss. Zeiss also makes a CMM that uses an optical probe in conjunction with a touch-sensitive probe. Several CMM manufacturers without noncontact machines told PLASTICS TECHNOLOGY they expect to develop them in the next decade.

A laboratory model CMM from Moore Special Tool Co. uses laser triangulation to lock onto a point, and analog linear variable differential transformers (LVDT's) to determine the position of the probe axes. The company says that LVDT's are more accurate than measuring scales used in conventional CMM's for determining probe position.

Other companies developing laser-measuring systems include Chesapeake Laser Systems, Hymarc, and Tychoway Bearings Co.

Although noncontact CMM's have much promise for the future, their higher cost should keep them from replacing contact machines outright. Other drawbacks of noncontact machines include their sensitivity to surface finish and lighting conditions, and the inherent difficulties encountered when measuring dimensions inside of a part, such as the length of a bore hole. Technologies that might overcome these limitations in the future include x-ray and magnetic resonance imaging, both of which could not only measure internal dimensions readily, but also detect material flaws and bonding voids in plastic parts.