Low-cost SQC systems; taking that important first step.
Statistical quality-control methods are a whole new philosophy of manufacturing that nearly everyone will need to learn about. One of the best ways to approach this radical change in QC arithmetic is not in one giant leap, but one step at a time.
The premise for this article was that a lot of you middle managers would like to take that small step by purchasing the smallest possible collection of components that constitutes a complete "system,' i.e., one that will produce some form of statistical analysis from a series of simple data measurements. A cost limit for this system of $10,000 was used because that is typically the limit of expenditures you can authorize without making a federal case out of it. This way, you can try out this little island of measurement automation with your floor people, evaluate it, and move it around as a training device. All without making a major commitment, or a major change in operating procedures.
So the following proposition was made to several leading manufacturers of SQC systems: I want to statistically sample a series of simple parts using a linear mechanical probe accurate to 0.0001' and produce QC charts--histograms, X-bar, R, etc. I want a simple, relatively portable system that does it all--measure, compute, and record--at a total price less than $10,000. What specific equipment can you offer?
The following responses vary widely in cost, sophistication, and flexibility, but they meet the above criteria and represent the leading edge of this rapidly expanding measurement field. The prices quoted may have changed by the time this appears in print (in fact, due to the intense competition, some prices may now be less). For further details on any that appeal to you, use the circle number response.
This system is offered by Quality Measurement Systems Corp, Penfield, N.Y. It consists of a Hewlett-Packard HP85 computer; electronic indicator (0.0001 resolution, 0.5 range) with digital display, footswitch, and stand; and QMS Anstat software, Figure 1. The elemental system cost is $8895 (the two calipers shown in the photo are extra).
In use, parts are placed under the indicator, and tapping the footswitch sends the reading to the computer. If the value is within limits, it is stored and the system waits for the next measurement. If out of limits, an audible warning sounds. Reports, including up to four keyboard-entered variables (date, operator, lot, etc), can be generated at any time.
Reporting has two major options:
1) Lot-sample analysis based on MIL-standard sampling with reports that shows all measured data, identify out-of-spec measurements, print out mean and standard deviation, predict probability of undersize and oversize, and plot histograms. 2) Control-chart analysis with X-bar, R, and Sigma charts, histograms, and data list of measured values, lot variables, lot mean, and standard deviation.
System expands readily to add digital micrometers, caliper and height gages made by QMS or a wide variety of special gages made by others (bore, thread, torque, tensile, optical, etc).
Software is conversational, menu oriented, easily programmed, and designed for use by relatively unskilled people. For more information, circle E65.
Our next system is offered by Digital Techniques Div, Valeron Corp, Madison Heights, MI. They point out that previous statistical systems were elaborate, expensive, bulky, and unsuited to the manufacturing environment. Thus, to apply statistical methods usually meant resorting to manually plotted control charts to train operators in SQC.
Now, with systems like theirs, SQC is both low cost and portable. Their DataQume/8A portable data logger (under $3000), Figure 2, accepts data from gage amplifiers, digitizes it, and stores up to 40,000 bytes of it in battery-supported memory. The companion SPC analyzer (under $1500), also battery-operated and portable, has the software to perform the statistical analysis and produce analyzed data in a format conforming to popular control-chart specifications and histogram analysis. Printouts are 24 characters/line.
For more detailed charts or histograms, you use the RS-232-C serial output to the printer of your choice. The analyzer can also be used to transport the analyzed data to a desk-top analysis system or mainframe computer. Or the DataQume/8A data logger can be tied directly into the center computer of sophisticated production-monitoring systems through its serial output.
For more information, call toll free 800-624-5489 (in Michigan, 313-589-1000) or circle E66.
At Heidenhain Corp, Elk Grove Village, IL, the emphasis is on the gage--digital ground-glass gages photoelectrically scanned--that can add an order of magnitude of accuracy beyond more common mechanical and electronic-analog gage methods. Yet the gage in its simplest form is still inexpensive enough to be incorporated in an elementary statistical system without exceeding our limit of $10,000. With an accuracy of 0.000 02 (10-mm travel), the probe, test stand, 0.000 05 resolution counter, and buffered BCD output, Figure 3, form a basic measurement package for $2210 that can be combined with your choice of computer and software to make a statistical system.
Heidenhain doesn't supply the latter but they recommend the HP 85B (computer, CRT, thermal printer, and tape drive) plus the HP82941A interface for an additional cost of $3545. To this you add statistical software from General Statistics ($95) and get a total system price of $5850.
Heidenhain offers a wide range of specialized gage and counter combinations. Gage travels can range up to 100 mm (3.94), can be manually operated or servocontrolled, and gages can be coupled directly to mainframe computers. Counter options include timed display, tolerance (go/no-go) sorting, max/min display, difference/sum of two measurements, and externally reset, preset or reversal of counting direction.
Tor more information, circle E67.
Our fourth system, from MTI Corp, Paramus, NJ, illustrates a much different measurement emphasis--the Japanese SPC philosophy that no defects are acceptable.
They feel measurements should be in small sample sizes (less than ten), made on the shop floor in real time, quickly converted into "variable charts,' and put to immediate use before faulty parts are made. In contrast, "attribute charts' require 30 or more samples to show trends, and whether computed manually or by computer later in the data-processing department, are too late to be useful--recording defects after the fact.
Thus, MTI's emphasis is on the micrometer itself, and the simplification and miniaturization of the measurement system. Their electronic Digimatic micrometer uses a built-in microprocessor to keep track of up to 200 measurements, digitally displaying on command: the highest reading, lowest reading, average value, and standard deviation. Units with measuring ranges of 1, resolutions of 0.0001, and accuracies of 0.0001 are available for $310 to $400. The micrometer is battery powered and constitutes the most elemental "system.'
For printouts, this can be coupled with the DP-1 miniprocessor, Figure 4, to produce a listing of each measurement, the high and low reading, average value, standard deviation, and the total out-of-limit readings and their identification numbers. The DP-1 can also plot a simple histogram showing the data distribution. All on a tiny 1.5-wide paper tape for only $210.
The newer Model DP-2 for $600 offers additional printout variables (identifying parameters, temperature corrections, etc) and plotting options with its four color pens. Other processors are being developed to produce more conventional control charts at price ranges from $1200 to $6000, depending on the computer used.
For more information, circle E68.
The measurement philosophy at Federal Products Corp, Providence, RI, is that the typical operator doesn't understand standard deviation or want the hassle of charting, yet he/she can still be effective in using statistical devices. So their entry in the low-cost SQC race is an in-process analyzer nIPA), Figure 5, for $2500 that attempts to put the QC professional inside the box--a 32K memory program stores a sophisticated algorithm to make all the decisions.
Input is direct from a $200-displacement transducer that replaces the dial gage in a manual measurement station. The operator enters the reading, and if the process is under control, he gets a green light. If not, a red light indicates a failure, and one of eight indicators defines which variable is over limit. If it is X-bar, he can then display by push button the last X-bar reading or series of readings to see the trend. For permanent records nor recording the daily output of several IPA units), a $1500 printer (8 1/2 11 dot matrix) can be added, bringing the total elementary system price to $4200. The IPA's nonvolatile memory will produce a complete X-bar chart of the last 99 subgroup readings, complete with time of day, nature of out-of-spec condition, etc.
But, Federal points out, the printer is not necessary for true real-time statistical process control. Unlike humans, the IPA's computer doesn't have to plot trends to see them. It can monitor a whole list of X-bar/R charting rules and tell you immediately when any have been broken. There is no need to worry that the operator forgot a rule, made a transcription error, or miscalculated an X-bar calculation by mixing up positive and negative readings (a common mistake even with quality professionals). They plan to add a CRT monitor in the future, primarily as a training aid, so the operator can see the charts evolving.
For more information, circle E69.
Photo: Figure 1
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Photo: Figure 5
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|Title Annotation:||statistical quality control|
|Author:||Sprow, Eugene E.|
|Publication:||Tooling & Production|
|Date:||Feb 1, 1984|
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