Some 'near-jig' ideas

In-process testing is the true path to happiness

The FABRICATOR July 2002
July 25, 2002
By: Gerald Davis

In-process sampling and between-process checks can prevent problems at the production stage. Although using an inspection jig can be costly, some testing approaches that are not as rigorous as using an inspection jig are: stacking and blocking a sheared batch in order to scan the batch for variations; weighing castings; touring a customer's facility; and doing small batch inspections.

I'd like to have a conversation with you about my quality control philosophy. Given the constraints of this media, how about if I type and you read? Not much of a dialogue, but at least half of us are communicating.

In the job shop, a critical area of skill and mastery is the setup. Once that's completed, the routine production is nearly automatic. A natural approach might be to have a specialist do the setups and have an apprentice perform the production.

Of course, you know that when things go wrong, fingers point. A built-in opportunity exists for the production guy and setup guy to blame each other.

An alternative is to invest more in training so that both setup and production are the machine operator's responsibility. To audit the production quality, an inspector might be assigned to do either in-process sampling or between-process checks.

Some blame-assigning still may occur between the inspector and the operator, but with suitable arbitration from the foreman, escalation of the conflict can be avoided.

In my opening paragraph, I tried to make a funny point about communication—that one-way communication is incomplete. This is a constant problem with mechanical drawings as they are used in production. The easiest solution is to test-fit the part. It might not matter that the part isn't to print, as long as the assembly is functional. Let the part speak for itself.

I can't take credit for the idea of test-fitting. The idea of making a jig to be used for in-process sampling is not original to me either. However, the result is so good, it is tragic that it isn't used more often.

"Gerald," you say, "We run a job shop. Every batch is different." I understand that the process of designing, producing, certifying, storing, retrieving, training, and auditing the use of an inspection jig is sometimes tough to justify. How about some "near-jig" ideas—When you shear a batch of parts, stack them all with the burrs oriented the same way. Block them so you can scan the pile visually and note variations in size or thickness. You also can spot consistent flaws in the shear blade.

The same principle applies to stamped or punched parts. Stacking them in neat piles is handy not only for counting; it creates a self-jigging inspection system. Doing this doesn't guarantee that they're right, but it does indicate whether they all are the same.

You're probably thinking that you could weigh the castings to verify that all of the machining has been performed or that you could stage a mating part as the inspection jig. You might be planning to send part of your crew to tour your customer's facility so they can see how the parts are assembled. You're thinking, "What if I took all of the verniers, calipers, and micrometers out of the shop? How would we inspect the parts to maintain the needed quality assurance?"

Another quality idea that is not mine emerges from batch size reduction (read The Goal by Eliyahu Goldratt and consider lean manufacturing techniques). With big batches, you can amortize the cost of setup and gain from passing the learning curve; however, producing big batches clogs production, represents huge opportunities for scrap, and hides quality flaws very easily. Smaller batches are easier to inspect, self-fixture, and route. The path to higher margins lies in fast, reliable setup and self-inspecting processes, not in bigger batches.

You might want to share your written thoughts with me. You know how.

Gerald Davis Design and Consulting

Gerald Davis

Contributing Writer
Gerald Davis Design and Consulting

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The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.

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