This edition of the Fastener Training Minute with Carmen Vertullo was originally published July 21st, 2022 as “Do drawings always reduce the risk of producing a good fastener?” during episode 178 of Fully Threaded Radio.
Well, hello everyone this is Carmen Vertullo with the Fastener Training Minute coming to you from AIM Testing Laboratory, in beautiful El Cajon California, and the Fastener Training Institute.
As usual, today’s topic is precipitated by some questions or actually an incident that occurred with one of our clients regarding fasteners here at AIM Testing Laboratory, we often find ourselves in the role of reviewing drawings or blueprints for fasteners and tuning them up to take out the risk that might be associated with them. We also produce technical drawings for the production of fasteners and even though we hope when we produce a drawing and use that drawing as part of the procurement process for the fastener, that it will make things more clear, reduce risk, and increase the chances of a good outcome in that fastener transaction, however when the drawing is not properly executed the opposite can happen. We can actually introduce risk into the transaction because we are using a drawing or a drawing that is not well thought out. When I return, I’ll tell you about a few ways that you can properly use technical drawings, or blueprints, or whatever you want to call them, to enhance the chances of a good outcome in your fastener transactions.
Well welcome back everybody. This is Carmen Vertullo with the Fastener Training Minute continuing on about drawings.
Now I think because I’m a manufacturing engineer, and I have some training in the area of documentation and drawings, I just love fastener drawings. They’re relatively simple, there’s not too much information for most people (even if you don’t have an understanding of engineering principles), and they’re readable in that they help us understand what’s going on in the use and or the production of the fastener. So notice I said “the use of the production“. In my experience, making hundreds maybe thousands of fastener drawings for my clients, I see them fall into two categories.
One category of drawing is that where it simply describes the product. In other words, it would be useful to the engineer who is using the product to know the envelope that it might fit in. (How to properly describe it, gives it a part number, maybe some performance requirements of the application and so on.) But that drawing would not be sufficient to manufacture the item: it doesn’t have all of the technical and quality information that you would need to manufacture the item.
Other types of drawings are designed to be able to make that part. If you have the drawing, you have enough information to produce the part, and maybe even in some cases, to test the part. This is one good reason to have a drawing that indicates it’s suitable for a particular application. And that’s where sometimes we get into trouble.
Oftentimes, a part will be ordered to a specification or standard. Most fasteners, even many of the fasteners that I see that have drawings with them, don’t need a drawing. You can perfectly well describe it without a drawing. A 1/4-20 x 3 Grade 8 Hex Cap Screw, made to SAE J429, zinc-plated ASTM F1941 Fe-Zn 5Z, whatever you want. And that’s one hundred percent of the information that you can write down, and in many cases if they have a code system, a part number. And that’s all the information that you need to produce the part. The issue comes about when that part now has to function in a certain application and now maybe that application had something about it where that part won’t work in the farthest extremes of all of what the specification allows.
We see this for example with flat head screws. Flat head screws could be socket screws or machine screws, both inch and Metric. And they have a very large tolerance for the head height. The head height is actually not something that we measure with these screws, but it is the effective feature when we install it. If the head height is too tall, the part may live above the countersink. That’s not good if the whole object of the game with that part is to keep it below the surface. If the head height is too low maybe it’s not going to look very well because it’s below the surface a little bit. And the nice engineer usually wants their product to look really good and they want that head to be flush with the surface. So the only way to do that, well I should say there are two ways to do that.
One way is you have a drawing and so that drawing is going to dial in the head height so that it’s going to match the countersink that’s going to be produced in the part. Another way, is to wait until you get the parts and then make the counter sink to match those parts. That’s the more common way, as a matter of fact. As it turns, out in my experience, we have had instances in our machine shop for example where a client made a special let’s say 3/8 I think it was a really short part like 3/8 x 3/4 stainless steel flat socket screw. They had a hundred thousand of these parts and they came back and they did not work in the application because the head height was too tall. And we literally made a jig and put those in our mill and we machined about 20/1000 off the head so they would work, and get that part to the customer in time.
So that’s an instance of where a drawing can help reduce risk. Sometimes, though, a drawing will increase risk. We have a situation right now with some thread rolling screws where there’s a very nice drawing, and all the drawing says is that it’s got the size, it’s got the shape, it calls for a TT thread which means that its a TapTite technically ,but it’s a thread rolling screw. But there is no specification or standard called out on the drawing anywhere. Now if the drawing would have added ASME B18.6.3 for material and performance, then the fact that this screw does not work in the particular application and in fact it doesn’t test to ASTM B18.6.3 it fails on a few points, we would be able to hold the manufacturer accountable. But we cannot hold the manufacture accountable because there is nothing on the drawing that tells us how we have to test the part.
So at AIM Testing Laboratory, we are reviewing drawings all the time. I would encourage you, especially if you’re taking drawings directly from your customers, because I hate to tell you this but most fastener users when they make fastener drawings are not particularly competent. They make a lot of errors and they leave a lot of things out. They also over specify, which causes the part to be more expensive than it needs to be, or harder to acquire.
So that’s kind of the lesson of the day. Use drawings to your benefit, but if you’re not careful they can do things that we don’t like, such as increased risk and increased costs, and potentially lessen the number of qualified suppliers who are willing or able to quote or make the part.
Well, that’s your Fastener Training Minute for today. This has been Carmen Vertullo. Thank you for listening.