How the Perfect Becomes the Enemy of the Good
"Quality" has been the holy grail of manufacturing for more than 40 years now, and it is a worthy goal. But, somewhere along the line the concept of "quality" became inextricably tied to the concept of "perfection." The result has been a cascade of unintended consequences, not the least of which is the dawning realization that with perfection as a goal, the cost of success may well be greater than the cost of failure.
The good news is that it doesn't have to be that way. The bad news is that turning things around is going to take a great deal of courage and innovation on the part of the world's manufacturers.
What's wrong with quality? In a word, nothing. Quality isn't the problem. The problem is the way manufacturers have chosen to achieve quality by single-mindedly pursuing perfection in all of the components that go into an assembled product.
It's only a slight exaggeration to say that manufacturers today routinely specify tolerances that couldn't even be measured in the '70s when the quality revolution got started. Machine builders and process designers have responded to this demand with tools and systems that can meet those tolerances and the even tighter ones that are sure to follow.
But, someone has to pay the price for all that precision, and the price tag grows larger each time tolerances become smaller. That someone, of course, is the consumer, who will ultimately decide just how much "quality" is worth. It seems a safe bet that the customer won't be willing to pick up the tab for perfection, even if it were possible to produce it.
Function vs. Perfection
Fortunately, assemblers don't have to fall into the perfection trap. The first step is to consider just what "quality" means to the consumer. In most cases, it means the product works as expected and lasts long enough to deliver reasonable value. That's a long way from perfection, and it's as far as a successful manufacturer needs to go to keep its customers happy.
Getting there will require rethinking many of the procedures and processes used in manufacturing today. The consumer doesn't care if all of the components in a product were machined to submicron tolerances before assembly. All that matters is that the product functions as expected and doesn't wear out or break prematurely. Function is the ultimate measure of quality, and consistent, reliable function ought to be the goal of manufacturing and, especially, assembly operations.
Today, manufacturers spend a lot of money to produce near perfect components so they can be assembled with simple, "dumb" systems using inexpensive tools like hydraulic and pneumatic presses and still yield an acceptable level of quality. The alternative is to spend a lot less on components that are "good enough," and invest part of the savings in "smart" assembly systems and tools that can use those components to achieve the same level of quality at a significantly lower overall cost.
The first requirement is a press that can be controlled precisely and instrumented easily to provide feedback on the operation it is performing. For several reasons, that's not easily done with hydraulic or pneumatic presses.
It is, however, relatively easy to do with an electromechanical press using a servomotor to drive a ballscrew ram. The servomotor is connected to the ram with gears, or, more often, a timing belt so there is little or no backlash in the system. The encoder on the servomotor will then provide a precise measurement of the ram's position.
My company, Promess Inc., makes electromechanical assembly presses (EMAPs) in 16 sizes, with outputs ranging from 25 to 100,000 pounds and strokes to suit most assembly operations. These units, and similar presses made by other manufacturers, are widely used as the basis of intelligent assembly systems in the automotive, appliance and other high-volume manufacturing industries.
To see how a smart press can improve even a simple operation, consider a system used to install studs into automotive wheel hubs. Due to manufacturing variables, the force needed to seat the studs varies from stud to stud and hole to hole. Consequently, the conventional assembly method is simply to set a hydraulic press to the highest force needed to insert a stud in the worst case, and hit them all that hard. The result is a lot of overstressed and bent hubs in the scrap pile because the dumb assembly system treated them all the same.
The traditional solution to this situation would be to tighten the tolerances on both the studs and the mounting holes and thereby avoid much of the scrap. But, the increased cost of the components quickly outweighs the savings gained by reducing the scrap rate-a classic case of success costing more than failure.
The alternative is a smart system using an EMAP equipped with force and position sensors to monitor the insertion process, and a controller that can decide when the operation has been completed successfully. By setting upper and lower limits for the process variables, the manufacturer can reject assemblies likely to fail in customer hands, while producing a much higher proportion of good assemblies-even with less expensive components with much looser tolerances.
To make this work, a load cell must be installed either on the EMAP's ram or in the fixture to measure the amount of force exerted on the assembly as the stud is inserted. In addition, an encoder must be installed on the servomotor to measure the ram's position in real time. With these two pieces of information, force and position, the controller can then create a "signature" of the process by plotting one against the other.
If the baseline operation is performed with parts known to be within specification, the signature will define the parameters of a good assembly operation, and any subsequent operation producing the same signature will also produce a good assembly. In such a system, quality assurance simply means accepting all assemblies that match the signature and rejecting all those that do not.
Smart Assembly Systems
Obviously, the real world is a lot more complicated than that. To make this approach practical, a significant number of operations both good and bad must be measured to establish upper and lower limits for acceptable signatures. Then, the EMAP must be connected to a controller able to both generate and evaluate the signature in real time while the operation is performed.
Fortunately, such presses and controls are available off the shelf, and the balance of the system consists of the same fixturing and material handling components that would be used in a traditional dumb system. Once all the elements are integrated, the only "custom" aspect of the system is the work required to establish the signature benchmarks and program them into the control. Again, these are very simple operations.
What works for wheel studs also works for assembly operations on products as diverse as automotive hood latches, catheters, pacemakers and hydraulic valves.
In the case of the hood latch, an EMAP is used to peen the rivet that holds the latching hook to the baseplate while a motorized torque system cycles it. When the force needed to move the hook is just right, the peening operation stops. Here, the smart system replaced a hydraulic press that simply peened all the rivets with the same force. Then, employees manually wiggled the assemblies, discarding any that were totally immovable and "working in" any that were salvageable. In fact, when the smart system was setting the parameters for the latch assembly, the experience of the manual assemblers helped to establish the baseline for the amount of force that was "just right."
In the catheter assembly operation, an EMAP-based system virtually eliminated separations of a critical crimped joint with a history of failures, sometimes inside patients. This system replaced a hydraulic press that the manufacturer found impossible to calibrate with sufficient precision to produce good assemblies reliably. Here, the signature included data on both force and tool position during the process, because crimps that are either too deep or too shallow are prone to failure. Thanks to this data, 100 percent of the catheters assembled on the EMAP meet the manufacturer's functional test requirements.
A similar EMAP-based system is used to crimp electrodes to the leads of cardiac pacemakers. Another EMAP is used to calibrate hydraulic valves by adjusting the preload on the spring that holds the poppet down against system pressure.
None of these assemblies, or the hundreds of others produced on smart systems, are perfect by any measure. However, they are all 100 percent functional and reliable in the hands of customers who measure quality in terms of performance delivered for the price paid. Measured that way, these assemblies are probably as close to perfect as anything made by human beings is likely to get.