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Sorting out electronic parts sorters.

Electronic sorters can eliminate small defective parts and assemblies by combining a parts feeder with an appropriate sensor. A reject mechanism, under command of the sensor signal, is used to discard defective parts. The term inspector sometimes is used in place of sorter since the parts actually are being inspected (i.e., measured or compared to a standard).

Small parts, by the way, are judged small for several reasons. The first is size, e.g., the parts usually are smaller than a matchbox. And since most electronic sorters use vibratory feeding, part size generally is proportional to bowl diameter. The largest bowls are about 36" dia, limiting part size to about 4" in length and perhaps 2" as the maximum second dimension.

Small parts also connote low economic value. In all but some aerospace and electronics manufacturing, defective parts can be thrown away rather than reworked. The inspection job is simplified if a sorter must only prove a workpiece is defective, not where it's defective and by how much.

Filling the need

Traditionally, small parts are assembled by hand. If a manual assembler detects a defect--for example a bolt with no threads--he simply throws the part away and uses another. Even today, 70 percent (in the dollar value) of manufactured products are manually assembled.

Automated assembly equipment isn't as forgiving as manual assemblers, however. Parts feeders often jam if they feed a defective workpiece. In a typical automated manufacturing operation, an attendant is required for every two to three feeders. His job is to ensure that the feeders run continuously, unjamming parts that don't feed properly.

In an assembly machine, the situation is more critical; jams can shut down an entire production line. Often, assembly equipment operates only 75 percent of the time because of jamming.

As throughput and complexity of production equipment increase, jamming becomes intolerable. If one part in a thousand is defective, then a machine assembling one part/sec will have several jams/hr. If the jam takes 30 sec to clear, there's not a big problem; downtime is only a few percent. But suppose the machine assembles 10 different parts, each with the same one in a thousand defect rate. Here, downtime can increase up to 30 percent.

Of course, not every workpiece has the same potential for causing trouble, nor does every defective part cause a jam. On the other hand, few manufactured parts have defect rates as low as one in a thousand, and few jams take only 30 sec to clear. As assemblies have more parts, and the percentage of defects rises, production suffers.

Lost in the shuffle is a second issue: the impact of defective assemblies. Producing quality products has a long-term advantage in the market--consider the positive reputation that Japanese and German products have. Thus, while a defective part may not jam a production machine, it may have long-term affects by reducing the product's quality and the manufacturer's reputation.

Why is there junk?

Why is one part in a thousand defective? In a screw machine, when a cutting tool breaks, few operators notice the error quickly enough to avoid a few bad parts in the note box. Or consider stamping machines were dies or punches occasionally break and produce defects until the machine is stopped. Similarly, taps break in holes, and bar ends produce short parts. Unless an injection machine is properly adjusted, moldings have short shots and flash.

In addition to defective parts, workpieces simply get mixed when they are processed by the same equipment at different times. If fasteners are manufactured, different sizes are degreased and heat-treated in the same washers and furnaces. When stampings are made, different workpieces are plated and deburred in the same machinery. Some of the parts from a previous operation always seem to be left behind, and only one mixed part in a thousand causes problems in assembly equipment. Junk--defects or mixed parts--is inherent to manufacturing.

Given that a certain percentage of defects is present, why aren't these flaws discovered? The answer lies in the statistical sampling methods used for quality control since World War II. A sample of parts, usually less than a hundred, is checked against specs. While sampling is valuable in detecting whether parts match drawings, it seldom detects the one part in a thousand that jams assembly equipment. On the other hand, electronic sorters check all parts, not just a small fraction.

User pressure

Electronic sorters were first used to prequalify parts for assembly. Certain troublesome workpieces were 100-percent inspected by the sorter, then fed into production equipment. The defective parts, however, should have been picked up at incoming quality control on the receiving dock. But for reasons already discussed, statistical sampling doesn't do the job.

For this reason, sorting is shifting away from assembly machines and toward the source of defects. Users are demanding that suppliers bear responsibility for delivering only good parts. Purchasing agents for large auto and aerospace manufacturers are forcing vendors to provide quality parts, without the junk that jams production equipment. Increased automatic assembly, coupled with just-in-time inventory procedures, make high quality parts a necessity.

Vendors that don't provide high quality are dealt with harshly: future purchases are shifted to suppliers with higher quality, suppliers are forced to pay for lost production time, and suppliers whose quality is too low are eliminated. Service vendors such as platers and heat-treaters also are being pressured into providing unmixed parts.

Types available

It's the sensor that varies from one type of electronic sorter to another. There are four: eddy-current, ultrasonic, laser, and video.

The first electronic sorters used eddy-current sensing. Here, a fixed-frequency voltage source energizes a coil to set up a magnetic field that induces eddy-currents in a part near the coil. These currents are detected by a second coil whose voltage changes determine an eddy-current signature for the workpiece. By comparing this signature to the signatures of good parts, a defect can be detected.

In practice, two separate sets of coils are used. A custom-tooled feeder transports parts through one coil in the same orientation as a good part placed in a reference coil. The coils are connected in a bridge circuit to emphasize the differences between the coil signals. Electronic circuitry detects whether the match between the tested part signature and a good-part reference signature is close enough to be acceptable.

Eddy-currents are sensitive not only to part shape, but to electromagnetic characteristics as well. For example, and eddy-current sorter can detect material and amount of heat treat, as well as shape. For detecting the smallest defects in parts, eddy-current sorters are volume-dependent: they can detect a small increment of missing volume in a workpiece.

Ultrasonic sorters operate on a field distortion principal similar to eddy-current. Operating in the frequency range above 20KHz, an ultrasonic transmitter floods the region surrounding the part. An array of ultrasonic receivers record an ultrasonic part signature much as eddy-current coils record an electromagnetic signature. Changes in signature reflect variations in a part.

Unlike eddy-current units, though, ultrasonic sensors are sensitive to those part surfaces that reflect sound waves: the bigger the surface the better the measurement. Also, unlike eddy-current equipment, signatures are stored digitally. Hence, parts can be compared to good parts regardless of the orientation in which they pass by the sensors.

For example, the signature of a stamping can be determined in any of four possible orientations in which it could be fed into the sensing region. In addition, ultrasonic devices use multiple sensors, allowing a part to be sensed from many different perspectives simultaneously.

Laser sensing, on the other hand, directs one or more laser beams on a part to detect such profile parameters as diameter and length. Some systems use a scanning laser beam, similar to the ones at a supermarket check-out. Such scanning allows accurate profile measurements of various diameters on a shaft, for instance.

Although most laser sorters are used for determining if a part has the proper silhouette, other measures also are possible. In a nut, for instance, the laser can be aimed through the ID, checking for the presence of threads by noting their characteristics reflections.

Most video sorters use binary algorithms that rely on the part silhouette to determine whether it is good or bad. Typically, the silhouette is formed by back-lighting the workpiece with a strobe to freeze the part's shape or, with linear arrays of sensors, a constant light source. The binary image of the part can be analyzed against many dimensions. On a fastener, for example, length, diameter, head presence, number of threads, and thread pitch all can be used as criteria for part acceptance.

Electronic sorters can be classified according to how they sense parts: field-distortion (eddy-current and ultrasonic) or profile (laser and video). Field-distortion sorters are sensitive to 3-D shapes. They are essentially 3-D comparators, accepting a part by comparing its 3-D field with that of a good part.

Thus, defects that can't be detected in profile, such as a socket in a socket-head screw, can be detected with a field-distortion sorter. Pattern recognition algorithms used in some field-distortion sorters allow 3-D comparison to be made to a statistically good part rather than to a particular good part.

Profile sorters more closely correspond to data on drawings, viz., how wide, how long, how many threads, etc. If actual part dimensions are required--for example, in the aerospace industry where traceability is important--profile sorters can be limiting. Important features, such as bend angle of stampings, can't always be discerned.

New developments

The trend in electronic small parts sorters is improved versatility. Most use custom-tooled vibratory feeders to move parts one at a time into their sensing region. Custom feeders, of course, aren't very versatile; they often must be retooled to feed a different shaped part.

Rather than being tied to a specific custom-tooled feeder, sorters now can use the same sensing methods for inspection as for orienting parts.

Another development concerns ability to learn a new part shape by showing the sorter a few good parts. Manufacturers of both field-distortion and profile sorters have developed this capability. Some have taken a step further: both good parts and bad parts are learned. The sorter's software makes adjustments to optimize sensitivity to the difference between good and bad, while being insensitive to acceptable manufacturing tolerances.

These advances make electronic sorters much more versatile and reliable. Instead of being suitable only for specific high-volume parts, sorters now can be changed over from one part to another in a few minutes. Moreover, sorters are getting better at quickly and accurately removing junk.

For more information about electronic sorters, circle E15.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Buckley, Shawn
Publication:Tooling & Production
Date:Sep 1, 1985
Previous Article:Hot issues in automated assembly: will it ever work?
Next Article:Making a dedicated machine tool flexible.

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