Hold What?

[Post by Chuck Krugh, May 31, 2024]

Picking up where I left off in the blog Tools, Fixtures and Jigs – What?, I thought it would be good to explore fixtures in a little more depth. I believe this is an area that can help us improve our production process, reduce hours and cycle time and increase the quality of our product. So I’m sharing with you my own experiences with tools, fixtures and jigs from my career manufacturing things other than ships – however, the same principles should apply.

In my previous blog, I defined fixtures as those tools that generally hold a part or assembly in a specific place to ensure that the part shape, dimension or location stays where it’s supposed to be per engineering.

A fixture orients a product in a specific position as defined by the engineering and design, and holds it so the product cannot move before the next operation. Holding the product in place so it cannot move assures that the final product is the same every time, meeting the design specifications. Fixtures come in all shapes and sizes, ranging from immense fixtures that may take up a manufacturing bay to small ones that go on the benchtop. It’s easiest to think of fixtures as holding tools.

With this definition in mind, let’s break fixtures down further into two different categories – simple and complex.

Most people would think that simple fixtures are relatively small and almost exclusive to the benchtop-build process. However, the word simple in this case is deceptive. Instead, consider simple fixtures in terms of what the fixture is designed to accomplish. Simple fixtures basically hold or control a part or subassembly in one direction or axis. For example, a fixture that holds one part steady while it is joined with another part is a “simple” fixture that ensures the geometry is correct and matches engineering. This could be a benchtop tool or – depending on the size of the parts it must hold – it could take up an entire manufacturing bay.

In my past experience, simple fixtures have been fabricated by the mechanics at the point of execution, but only up to a certain point. That point is generally around the size and weight of the part being held. The fixture needs to be able to hold the weight and withstand the forces required to do the work safely. Remember, we have to work safely at the point of execution so we can go home the same way we came in!

Don’t get confused by thinking of simple fixtures in terms of size. Size isn’t the defining factor for what makes a fixture simple; it’s really more about the fixture’s function. Another way to think about simple fixtures is to consider them alignment tools or locating tools.

For example, let’s say you have a critical install for a pipe hanger or foundation, and it must be installed exactly in a specific location every time. You can use measuring tools or have the surveyors help locate the part to ensure it’s in the right place. But, in both of those installation methods, it takes time or extra manpower to complete the task.

Instead, imagine that you had a locating fixture that referenced off a fixed location on the unit. That fixture would do nothing other than set the location of that critical part. You would simply install the locating fixture and align it with the reference point – you would be ready to install the part. Pretty quick! Even better than just the speed of the installation, the part will align in the same place on every install, ship after ship, because you used a locating tool.

Let’s consider a real example in our facility – the mocks located in AB. Are they simple or complex fixtures? What do you think? I’ll give you my answer at the end.

I think we covered simple fixtures enough. When I think about complex fixtures, I usually think about a multi-step assembly fixture that you can use for more than one build operation. These are much larger fixtures that can be floor-mounted depending on the build sequence or station, that can be modular and transported by a dolly or some other reusable dunnage for movement, or that can be bench-mounted based on the size. It must be used for more than one build operation for us to think of it as complex.

Another attribute of complex fixtures is that they are typically engineered and designed for the specified work. From my past, this was usually a cooperative effort between manufacturing engineers and tooling engineers. The manufacturing engineers worked closely with Planning and Operations teams to ensure that all of the requirements were well known before starting the design process. Here at BIW, our designers play a key role in this process.

You are probably wondering why engineering and design are involved with complex fixtures. Well, they are complex! One reason why the features must be designed into the fixture is because the fixture will support several build-station operations. Another reason is the size of the fixture. Complex fixtures are usually mounted to the floor in a fixed location along with all of the services required to use all aspects of the fixture during the build station. A third reason is related to the weight of the fixture as well as the part to be worked. The combined weight of both can be substantial, so having the right safety margins designed into the fixture is necessary for safe usage.

Last but not least, the design must ensure the mechanic’s safety! Complex fixtures, by their definition, can be inherently dangerous, are generally required to operate in a specific sequence, have moving parts and need to be as ergonomic to operate as possible. As such, they must be designed with safe operation in mind.

In my past life, assembly fixtures were large. Once a part was loaded, it wasn’t uncommon to conduct five or six different assembly operations using that single fixture.

When I was working in the composite world, a simple fixture or bond tool could weigh almost two tons. The mass was necessary so that the bond tool could withstand the brutal autoclave cure cycles of heat and pressure while maintaining its shape. We manufactured each part to a stringent tolerance, and each part we produced had to be the same.

While different from what we manufacture here, the concepts are all the same for fixtures. It’s just different shapes, sizes, uses and complexity here at BIW.

I hope this deeper dive into fixtures helps you understand more of what I’m talking about when I mention tools, fixtures and jigs.

By the way, the mocks would be considered a simple fixture by my definition. They hold the unit’s shape, location and orientation while mechanics conduct their operations.

See you on the deckplates!

Safely Execute High-Quality Work

President, General Dynamics Bath Iron Works

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