Written By, Karl Hatchett

I’m often faced with fastener-related issues at work. Sometimes it’s fit, other times it’s function. And sometimes it’s poor design. The prime contractor for whom we do the bulk of our assembly work has a dedicated library of specifications. More often than not, design engineers poke their noses into a specification, be it proprietary or military/aerospace, and take the information at face value … which they should, to a large degree. Adhering to design specifications is of paramount importance. It’s the lead dog in every quality standard ever written. But engineers are human. They sometimes overlook or underestimate the impact of their choices.

For one thing, engineers calculate dimensions digitally and then translate them to the nearest fraction. If they do that too late in the design process, you end up having to use twenty 8-32 X 13/16 inch screws where a ¾ inch length might otherwise have done the job nicely. That isn’t much of a difference, digitally-speaking, but on the procurement side it might prove to be a night and day difference, at least initially. Screw lengths generally receive a lot of scrutiny on aircraft because of the weight factor. Protrusion of a fully-seated screw beyond its mating internal thread is specifically limited due to weight consideration, along with other, lesser factors. Every single component or assembly included in the manufacture of an aircraft is judged by its airworthiness. Nobody ever skimps on a military aircraft, but careful assembly design achieves both the strength and trim required while maintaining a minimalist approach to the connecting hardware.

ASME B18.6.3 is the basic American specification for machine screws and tapping screws. The list of available lengths for each screw diameter displayed in the tables pretty much increases at a rate of 1/16 of an inch per line. I suppose it is natural for relatively inexperienced design engineers to presume that appearances represent facts. But in this, as in many things, appearances are deceiving. Some lengths are less readily available than others. The American Society of Mechanical Engineers recognized the potential confusion and carefully hid the following disclaimer:

            “The inclusion of dimensional data in this Standard is not intended to imply that all of the products described are stock production sizes. Consumers should consult with suppliers concerning the availability of products.”

Thanks a bunch, guys. By the way, the people responsible for keeping those fastener specifications relevant are engineers, too, which makes the whole process feel a little incestuous and inbred when it breaks down.

We’ve all heard the term, ”the customer is always right.” And we’ve all rejected the premise more than once. In this instance, if you get stuck with an off-size screw requirement from an end-user, you’re probably not spending enough quality time with your customer’s design team. Engineers are human. They have families. And they will jump at any excuse to stay out for a business meeting and dinner on you. Get them a little liquored up and get them home safe. But before you do that, show them where they can do better, for themselves, their company, and for you, their supplier. It’s a method that always worked before. It seems to have fallen out of favor in our social media age, though I don’t know why. Who doesn’t like to be wined and dined?

 Karl Hatchett is employed in the aerospace/ defense industry as a hardware applications expert. He has also appeared in a  New York Metropolitan Opera 2006 production of Richard Wagner’s The Ring of the Nibelung playing the role of Siegfried.


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