3 Silly Mechanical Design Mistakes To Avoid

  • There are so many simple mechanical design mistakes that can easily be avoided. Here are a few simple tips
  • Dodgy dimensioning or sloppy drawing standards are a tell-tale sign of the inexperienced or lazy, and can cause confusion or manufacturing errors
  • Check, check and check again! Get into the habit of checking your design thoroughly before getting it made
  • Listen to the right people and ensure you get a watertight, approved mechanical specification written

One of the silliest mechanical design mistakes I made was to mis-position a PCB supporting feature on an injection moulding by around 3mm. Despite this humungous error, it still provided just enough support and the product miraculously passed its drop test. Engineering by luck!

I made this error when I was a graduate trainee, and it was something avoidable that could have easily been corrected.  It’s not a terrible thing to make mistakes. We’re only human and sometimes you have to make a few to learn.

That said, there are plenty of incredibly silly mistakes that just needn’t be made in the first place. Here are 3 types of silly mechanical design mistakes that engineers often make, and tips for avoiding them.

Bad dimensioning / drawing standards

Dodgy dimensioning or sloppy drawing standards are a tell-tale sign of the inexperienced or lazy. This is the engineering equivalent of writing with bad grammar or spelling, and can lead to all sorts of misinterpretations and manufacturing errors.

With just a little bit of attention to detail, effort and the application of basic logic it’s so easy to get this right, and will result in a drawing that looks professional, clutter-free and confusion-free.

Below are a few examples of unforgivable draughting naughtiness that should be avoided at all costs:

Too many decimal places: I’ve lost count of drawings that I’ve seen with ridiculous dimensions like “5.487684747mm” on them. Sadly, I’ve even witnessed professional contract mechanical engineers and consultancies do this. Round things up please! This dimension should simply read “5.5mm”. All CAD systems have template settings where you can preset dimensions to a default number of decimal places. Find it and use it.

Awful engineering drawing - mechanical design mistakes

Awful engineering drawing

Missing dimensions – this is extremely annoying, and can result in endless phone calls with questions, particularly from a manufacturer who may be working from a hard-copy of the drawing.

Over-dimensioning or conflicting tolerances: Ensure that you haven’t dimensioned something twice on different views. Don’t over-dimension parts with unnecessary extra dimensions. Use the minimum amount of dimensions needed to manufacture the part.

Dimensions or notes overlapping drawing views: This is an easy one to get right. No excuses.

Dimension arrows the wrong way around: Just look at the lowest 40mm dimension on the image (right). They’re the wrong way around! Doesn’t it look rubbish? It’s only one mouse click to correct this on most CAD systems, folks.

Not enough checking

There aren’t many worse feelings than finally getting hold of a prototype to discover that it doesn’t fit for the most obvious and silly reason. You’re sitting there holding two parts and can’t believe that you overlooked such a ridiculously avoidable thing that could have been put right on CAD in two seconds: maybe you unnecessarily made a screw boss 5mm too long and it clashed with another part, or something equally silly.

This phenomenon is highly embarrassing when surrounded by eager colleagues who are hovering around to see how it all goes together for the first time. Standard protocol under such circumstances is to slip (hopefully) unnoticed into the workshop and take a file to the offending feature before too many people see it.

Save yourself from such horrific circumstances by checking your design thoroughly before you get it made. When you’ve finished doing that then check everything again …and maybe once or twice more again. Even better if you can find a willing colleague to give it a once-over too. This gives the added bonus of being able to spread the blame if it all goes horribly wrong!

Listening to too many people

Project team members will often have conflicting views on how something should be designed. Sometimes it’s simply not possible to produce a design that satisfies every man and his dog, especially if there are opposing requirements. Bending over backwards to satisfy everyone might result in a severely compromised design.

As a mechanical designer you’ll occasionally need to diplomatically moderate others to ensure that a design can progress by filtering out silly ideas and superfluous contributions. Be on guard for office busybodies who might push for all sorts of idiotic suggestions on a whim. Remember that everyone thinks they’re a mechanical engineer and will delight in meddling if unconstrained. However, keep an ear out for gold nuggets. Sometimes people – even the seemingly idiotic ones – might offer excellent original suggestions.

Finally, make sure you write a mechanical design specification and get it signed off by the core members of the project team. Ideally you should have the backing of your project manager who is capable of defending it and can wield influence.

Got any silly mechanical design mistakes to share? Tell us about your obvious mistakes here. What was the silliest one you made and how did you fix it?

About: Gareth Weeks

Chief clicker of engineeringclicks.com, Gareth holds a BSc (Hons) in Product Design, followed by an MPhil in Engineering. 20+ years in consumer products, bio-science, telecoms industries. Likes electric guitar, piano, sci-fi and horror, designing stuff, listening to and lifting heavy metal (not necessarily at the same time).

3 Responses to 3 Silly Mechanical Design Mistakes To Avoid

  1. K.I.S.S. says:

    Not entirely my fault, but I was complicit in it… Pressure die casting.Parasolid files provided to the tool maker, but no drawings.Tool maker inserts file into SW program and creates cavity. Assistant tool maker accesses program and extracts features via Mastercam.The cavities as opposed to the negative electrode shape end up being machined.So I end up with the complete opposite of the part I wanted.Drawings are important, people…

  2. jagengrg says:

    I don't recall how a drawing came to us from another NASA contractor. Perhaps NASA said our Engineering Support group had checkers and requested help. It was not a particularly complicated part. The ID was bigger than the OD. Not an easy thing to do with a CAD drawing.

  3. K.I.S.S. says:

    Sometimes it's surprisingly easy to get it wrong in CAD. A few years ago, I created a dxf in SolidWorks and another user imported it into sigma nest.  The file was then exported to another SolidWorks user who then created another dxf. My part was twice the intended size, despite having a specified reference dimension within the dxf. Investigating this showed me that although I'd specified a radius dimension, when I also called out a diameter, I got exactly the same number on the same dxf.It was a bit of a head in hands moment, but it taught me to never open files in sigma nest and then send onwards to another SolidWorks user.

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