Why the person making it should be in the room when it is designed

What gets lost when design and manufacture are separated.

The person making a part should be in the room when it is designed because a drawing can be entirely correct on paper, every dimension consistent, every tolerance sensible, and still describe an object that is genuinely difficult or impossible to build in the order the drawing implies, a problem no amount of checking the drawing itself will ever catch since the drawing is not lying, it is simply silent about how the thing actually gets assembled.

Introduction and overview

A design drawing specifies what the finished object should look like and how its dimensions relate to each other, but it says almost nothing about the sequence in which material has to be removed, holes drilled, or subassemblies joined for that finished object to actually come into being. Two designs can look equally correct on paper and differ enormously in how they behave once someone actually starts building them, one allowing each feature to be added with clear access and a stable way to hold the part, the other requiring a hand to reach somewhere it cannot physically reach or a tool to approach at an angle the part's own geometry blocks. Nothing about that difference shows up in a set of dimensions and tolerances, because a drawing describes an end state, not a process, and the process is exactly where a design that has never met a real pair of hands tends to fail.

The flat-pack comparison

Anyone who has built flat-pack furniture from a printed instruction sheet has met this failure directly. A particular step will call for one hand to hold two panels perfectly square to each other while the other hand drives four screws through a third panel into both of them at once, a sequence that looks entirely reasonable as a diagram on a page and turns out to need three hands the moment anyone actually attempts it. The designer of that instruction sheet was not careless, every screw and every panel is exactly where it should be, but the sheet was clearly drawn by someone reasoning about the finished cabinet rather than by someone who had stood at a workbench and tried to physically hold three loose panels square at once. A person who had actually built the thing would have caught the problem instantly and reordered the steps, or added a clamp to the instructions, long before it reached anyone else's living room floor.

Why this knowledge resists being written down

The knowledge a skilled maker carries about what a design will actually be like to build is largely the kind of knowledge that is hard to write into a specification, since it lives in things like which hand naturally reaches where, how a part behaves once it is only half-clamped, or which sequence of operations leaves enough material intact to hold the part steady for the next one. Vincenti's What Engineers Know and How They Know It describes a category of engineering knowledge that is genuinely tacit in this way, learned through direct contact with the work rather than transmitted cleanly through a document, and design-for-manufacture problems sit squarely inside that category. A drawing can capture what the part should be. It struggles to capture what building it will actually feel like, and that gap is precisely what a maker sitting in the design review closes simply by being present and saying, out loud, that a particular feature cannot be reached with the tool that would need to reach it.

The one number worth remembering

A manufacturing problem caught during a design review, while the drawing is still a sketch on a screen or a printout on a table, typically costs a short conversation and perhaps a redrawn feature to fix. The same problem caught only once the part has reached the machine or the bench, after tooling has been ordered or a batch has been started, routinely costs many times that, sometimes an entire redesign and a repeated tooling cycle, because by then the mistake is embedded in decisions that have already been acted on rather than sitting in a drawing that costs nothing to change. The gap between those two costs grows with every further decision made downstream of the original drawing rather than sitting still as some fixed multiple, which is exactly why the same conversation about the same unreachable feature is cheap on day one and expensive by the time a batch is halfway finished.

Why this matters in practice

Separating the person who designs a part from the person who eventually has to build it leaves the same manufacturing knowledge in the process, only later, moved from the cheapest possible moment before a single cut has been made, to one of the most expensive, after the design has already been committed to and tooled for. Inviting the maker into the room while the design is still fluid is not a courtesy extended to the person doing the work, it is the single cheapest manufacturing review available, and it happens to also be the one most often skipped simply because design and manufacture have, in many organisations, ended up as two separate rooms in the first place, sometimes literally, on different floors or in different buildings, with a drawing as the only thing that ever travels between them.

Where this stops being true

None of this argues that a maker's objection should automatically override a designer's intent, since a maker's familiarity with how things have always been built can just as easily resist a genuinely better sequence as it catches a genuinely impossible one. The real value of having the maker in the room has less to do with their judgement always winning than with timing: their objection surfaces early, while it still costs nothing to weigh it against the reasons the design was drawn that way in the first place, rather than surfacing for the first time at the bench, where there is no longer room for that conversation to happen calmly.