An engineering drawing is a specialized type of communication tool. It’s also a contract. This particular communication protocol is designed to be complete, closed, and one way. This means follow up questions should not be required for the recipient’s comprehension and contractual agreement. It’s like a message in a bottle. With this, I present to you Dan’s 1st Law of Drawings:
Dan’s 1st Law of Drawings
The drawing shall communicate all information needed for fabrication as intended.
What is a Drawing?
A drawing is a two-dimensional medium used to convey the shape of a three-dimensional object. But where does the missing dimension go? A wormhole, a rift in the space-time continuum, the cracks in the sofa, the lint trap? It’s up to the imagination of the observer. We must use language and visual trickery to create the illusion of the missing dimension.
Shapes have a mathematical ideal, a perfect version that exists only conceptually and not in reality. This is despite my many many letters to governing bodies requesting they mandate real shapes that embody theoretical ideals. While the National Institute of Standards and Technology seems to be making small inroads, I’m not optimistic. Alas, for a drawing to be complete, it must not only describe the ideal but also the allowable variation from that ideal (cringe).
The amount of acceptable variation is called tolerance and impacts the design, function, appearance, cost, manufacturing processes, and lead time. Tolerance can be communicated in a variety of ways using a variety of standards. Tolerancing is a whole discussion topic(s) unto itself. But without tolerances, how nonideal an object can be is unspecified (not communicated) and open-ended. Anything open to interpretation may result in undesired interpretations and is ambiguous and therefore a violation of the first law. Violations of Dan’s codes are punishable by noogies and forced switching to decaf.
In addition to the shape definition, a drawing must contain all the other Product and Manufacturing Information (PMI). Examples of PMI include material, surface finish, coatings, weld information, process imparted artifacts, Bill of Materials, revision history and legal disclaimers.
What an Engineering Drawing Does
Engineering drawings describe a specific outcome. They don’t designate a process to get to the said outcome. One could create a process to create the desired outcome. Ultimately, a specific vendor will need to create an internal process to create the drawing’s resultant objective. Each fabrication process will be unique at some level. Drawings are intentionally process-independent to be general and not limit the vendor pool potential.
Getting the kind of cupcake one fancies could entail describing the cupcake or giving a recipe. When taking the recipe route, if the recipe calls for a special ingredient or utensil the baker doesn’t have then they won’t be able to make it. No cupcake. With a description, having some leeway provides for allowable variance in the ingredients and the process. This, in turn, should result in lots of hot fresh treats that are mostly similar but with minute differences. Some might even be better than your favorite recipe. Let’s not limit our cupcake intake.
Processes Can Be Tested
A nice thing about a process is that it can be tested. Simply follow the steps and check the outcome. Think of how code is routinely compiled to test it. For very complicated processes, one would break out process sections to check that they perform as expected in the context of the whole. Because drawings describe an outcome, not a process, testing must take on a different form. Testing is not as clean and linear as “compiling” and refining.
The testing process occurs ahead of time to validate the design parameters (shape and PMI) and relative importance (tolerances). What parameters are important and their degree of importance may not reveal themselves straightforwardly as a bug, warning, or failure to compile. What’s important may not reveal itself at all until it’s in the field or far along its lifecycle. An imperfect process of hypothesizing leading to prototyping and testing, with a dash of experiential wisdom, is the best we have to isolate variables and ascertain potential failure causes.
It’s All Baked In
The final product is the result of the designer’s variable selection and weighting, the drafter’s translation, and the vendor’s process. It all gets “baked in”.
There may seem a daunting number of important variables to both determine and then consequently spell out. Think about all the ways a person describes, for example, age, height, gender, place of origin, smell, sense of humor, eye color, temperature, identifying marks. Think about how this could apply to drawings. Sure, most parts don’t have a sense of humor or a favorite movie — that is probably why they fare so poorly on dating sites. Parts often serve more than just one functional purpose. In addition to a part’s primary function, it may serve secondary functions and aesthetic purposes. Determining a part’s alloy will impact its strength and also its weight, magnetism, shielding, corrosion resistance, coating, and so on.
Cumulatively, this adds up to a heaping pile of detailed information to layout. And while this may seem obvious, it is possible to omit or miss something. Even experienced writers accidentally leave out a in their writing. The consequences of omission on a drawing though may be dire. While over-specification would seem like the safe option, if everything is marked as important this drives up cost and creates a “drafter who cried wolf” situation.
The corollary to the first law is “assume ignorance”. Don’t assume vendors will do something unspecified because of a belief it should be commonplace or best practice or “obvious”. Beliefs are in the grey zone. Different thinking, experience, and motives will put vendors in other grey zones. These are danger zones of misinterpretation. Beliefs, intents, and wishes are about as useful in hindsight as the pile of scrap parts they caused.
Playtime – Mad Libs
Mad Libs is a game where words of a story are left intentionally blank to be filled in without context by a nascent participant. With the initial gaps filled with contextless fillers, the newly completed story is then read aloud. The nonsensical and unexpected results shirk expectations and spark the imagination to chuckles and delight. Like taking your squid for a walk! Or donning your new robot on your head. Randomness and misinterpretation are great for frivolous fun. Randomness and misinterpretation are sworn mortal enemies of quality. Expected, consistent, and sensical, these are the pillars of quality. Successful manufacturing exists in the world of the mundane, not the fantastic.
The first law is self-protective. The first law protects the creator of the drawing and their organization. If information is missing and the supplied goods are not as intended, the vendor is not at fault. Bad parts consume additional labor and monetary resources and expand timelines. Don’t be a fool, follow Dan’s rule.