A lot of young engineers (myself included) turn to the fit and tolerance tables in the “Machinery’s Handbook” or similar specifications from other sources. These tables overwhelm you with different fit options and it is up to you, the engineer, to choose one. I struggled with this early on in my career, and I probably sent out drawings with some really tight dimensions on them. I had no frame of reference for what a “G7/h6” (Sliding Fit) was, or what to call out for roundness tolerance. I had knowledge, but no experience.
My first professional job had a small machine shop for the engineers. I spent a lot of time making parts, measuring them, and really getting a feel for what 0.002’’ or 0.015” clearance really means. As the engineer, your job does not end when you close the book. Specs and standards serve an important purpose, but you have to take the time to understand the practical considerations for each element specified.
A lot of engineering and manufacturing talk is centered on the idea of absolute perfection. But how do you measure perfection? The inspection room is where you really learn what that tolerance that you just called out actually means. In the first five years of my career, I spent a lot of time in the inspection room. Many other engineers in my group would come down briefly, try to say something smart, and then bring the report back to their desk to figure out what went right and wrong. In other words, they would treat the problem as though it were a big puzzle rather than a test with one definitive answer. Believe it or not, I have seen many parts scrapped because a tolerance was set too tight on a feature that didn’t matter very much at all. The job of the engineer isn’t to make every part perfect, but to figure out which parts needs to be precise and which don’t. The inspection room is the one place where you can literally put your hands on the feel of what all those fits and tolerance tables are saying.
The last piece of the puzzle is understanding how your parts are made. You need to be intimately aware of the entire process of manufacturing, from start to finish. This includes fixturing, cutting tools, order of operations — everything. Ask the person who made it what was easy and what was difficult. If you have time to drop by the manufacturing space, bring a drawing you’re working on. You would be surprised what a quick question can lead to. I have had parts that I thought were going to be difficult to make turn out to be simple and vice versa. I can’t tell you how many times I have designed a part, then brought it down to the shop and changed my dimensions or design because of a conversation I had with someone on the floor. I have never met a machinist who was not happy to talk about how they made something. If you show interest in learning about what they do, they will show interest in what you do. Manufacturing is a team effort between engineer, machinist, and inspection room. We didn’t land on the moon by throwing impractical designs up on the wall. The Apollo missions involved a team with skills from all walks of life.
My father taught me something: in order to be a good leader, you need to know how to do the job of everyone around you. You don’t have to be the best at it, but put the effort in and have a good understanding of what you are asking someone to do. Leave your ego at the door, learn to admit when you made a mistake, and then move on. If you talk to people, show interest, and are willing to learn, you will be a welcome sight in the shop — and I promise you’ll make some really great things.
Tim Mallard is a mechanical engineer, machinist, welder, and bicycle mechanic. He is a guy who just can’t refrain from taking stuff apart and making it do something it wasn’t designed to do. He has worked in research and development labs, as a support engineer in machine shops, and even on a professional rally car team. Tim also coaches his local high school’s FIRST Robotics team and is diligently working towards training the next generation of engineers with real hands-on skills. Currently, he is self-employed as a contract engineer and prototype maker using his Tormach PCNC 1100 and Makerbot Replicator 2x.