Vendor specifications and design intent (Or: How to prevent shoddy parts)

Proper documentation of specifications will improve communications and relationships with your hardware vendors

The Supplyframe Hardware blog is a holistic look at moving a product from engineering into manufacturing. For electrically-minded engineers, the mechanical pieces can seem confusing. For the mechanical engineers, that’s true of the electrical side. This article covers the need for properly specified components and how it can impact an engineer looking at your documentation in the future. Sometimes, that engineer is you!

You have a part designed, you have selected a vendor to produce it. Presumably, you have drawings of some sort prepared. Those drawings or models are how you communicate to the vendor what they are going to fabricate for you. Special attention must be given to these documents.

The fledgling engineer won’t have a lot of experience with documentation. They may have learned how to make mechanical drawings in a college class or may have some hands-on documentation experience from an internship. As important as those lessons are, high-stakes manufacturing requires deeper wisdom. That comes from experience.

At a first job, you might also be getting some sort of feedback from co-workers and management. This feedback can unhelpful and confusing. So we’re going to try to make things clear.

What follows is a short list of concepts, warnings and advice that the rookie engineer can use daily. As an engineer, most of my experience was extremely hard won. The advice I’ve received never really helped or seemed to go anywhere.

The most confusing thing was how jealously they guarded their knowledge. I really would have appreciated someone pulling me aside in my early years and laying it out like this.

The Basics

  • CAD — Knowing how to use CAD does not correlate with being good at it. Nor does it correlate at all with the separate skill of preparing mechanical drawings.
  • 3D models — These are not a substitute for mechanical drawings. The drawings convey information that is almost never documented within 3D models. The 3D model is a geometric ideal. Tolerance lets the manufacturer know how much they can deviate from that ideal.
  • Drawings are used for inspection — After a machinist cranks out a run of your parts, they need to compare what they produced to something, somewhere that lists your specifications. They are not going to go to a computer and load up your 3D model so they can verify figures. In fact, some machinists don’t know how to do this.
  • Injection molded parts Even for these you will need drawings. The mold will almost certainly be manufactured based on your 3D model. The information on the drawings will be used to inspect the parts that come out of that mold.
  • Errors are inevitable — You will make mistakes, and you need to make peace with that. I have been an engineer for about 16 years, and I am still flabbergasted at the things I miss. Remember when you wrote an essay for school and then you went back to review it? How many mistakes slipped past that final check? It is going to be a common reoccurrence for you. That is why drawings are signed off by checkers. It is also one of the reasons for drawing revisions. The day will come when you get yelled at for a mistake. You need to be mentally prepared for it. Always admit to your blunders and either work immediately to correct it. Barring that, ask what needs to be done to correct it. If the yelling doesn’t seem to be letting up, then maybe give them a gentle reminder that yelling isn’t the same as problem-solving.
  • Geometric design and tolerancing — If you don’t already know what it is then go and familiarize yourself with the concept and some of the principles of GD&T. It isn’t something that must be used all the time. Rather, it is used to complement traditional dimensioning methods wherever those methods may fail to effectively communicate intent. If you require two surfaces to be parallel within a certain degree, you cannot assume the vendor will know and act on that.
  • Design intent — Drawings and any other documents you produce help the vendor understand your design intent. Demonstrating how a part is to be used or installed into an assembly gives a vendor deeper insight, and they will be more effective in creating what you envisioned. Sometimes this means taking pictures and screenshots of models or previous iterations of the product. Make short videos in SolidWorks or Inventor if it will help. If you are meeting with a vendor in person, bring everything you can that may help illustrate your point. Not meeting with them? Mail it. This isn’t always necessary. If the part is that simple enough, then forgo this advice.
  • Advice from senior engineers — You might get told something that sounds a lot like this. “Communicate as much as possible with as little as possible” or “if they know what they are doing, then you won’t have to spell it out for them.” Advice like this is way too close to an “ideal world”. In your line of work you have a professional duty to not be confusing. You must communicate clearly and without ambiguity what you expect.
  • Specifications are ad-hoc “contracts” — Mechanical drawings are a contract of sorts between you and a vendor. For example, you design a part, and you specify the tolerance on a feature as +0.005” or -0.000”. If the you receive a shipment of these parts from the vendor and that feature on the parts is not within the specified +0.005” or -0.000” then are within your right to demand that the parts be re-worked or replaced. If you needed the part to meet a dimensional constraint that you never called it out, how can you hold the vendor to that figure? They can’t read your mind.
  • Nomenclature — Drawing nomenclature is an art and a science. Everyone has a different opinion of what is correct as far as drawing conventions. There are often many correct methods. Just try to determine what your employer expects. If you are lucky, you may have a drawing standard. Study the drawings of your predecessors or co-workers if possible. This can backfire, though. The drawings you are looking at for reference might be terrible. In fact, I have witnessed the phenomenon of horribly prepared old drawings time and time again.

Be kind to your future self and co-workers

How are you going to handle opening a drawing 2 years later for reference or revision? What happens when some nuance of the design isn’t listed? At best, you are going to be the source of your own frustration, at worst you are going to get in trouble for your lackadaisical approach to documentation. Your successor might get so frustrated that they will want to hire you again just so they can fire you. I have certainly wanted to do this a few times.

Never assume that the vendor will know what you want. It doesn’t matter if you told them once over the phone or in an email. Everything you expect to see must be documented. Failing to effectively communicate specs and your intent will eventually result in hurt feelings and delayed schedules. That documentation is something that you, your co-workers and your successors will rely upon.

Have you dealt with problematic spec’s in the past? Either mechanical, electrical or otherwise? Let us know below!