What's the silliest mechanical design mistake you've made?

[GarethW]

Here are a few tips for avoiding silly mechanical design mistakes, but please share your silliest ones below!

As I said in the article, one of my silliest mistakes was to put a PCB supporting feature on an injection moulding in the wrong place (3mm out of position). Miraculously I still got away with it...

Another one (recently) was I designed a simple aluminium plate to space a microwave radio module off a casting. The only problem was when I received the parts I discovered that I forgot to include a cutout in the middle for the RF signal to pass through!

 

[s.weinberg]

This should be a fun one. I've certainly made my share of mistakes, but the only one that immediately springs to mind is when I did a short stint on the manufacturing side.
What I was supposed to be doing was the design of a new product line, which would align with what I had done in the past. When management decided a few days after hiring me not to launch the product line after all, I had to find my own place. I ended up slowly taking over most of the manufacturing side of things, despite having very limited experience.
Anyway, mostly I was 'writing' CNC codes (with MasterCAM doing most of the actual code), and creating work instructions for our more custom machines, but in order to keep me grounded in reality, I also ran the machines sometimes. My first time running the CNC mill with no supervision was using a different jig than normal. I knew the origin X and Y offsets were different, but the thought that the z-offset might be different never crossed my mind, nor had I been told.
I turned the program on at about 1/4 speed, to be 'safe', and promptly watched a 3" mill plunge straight into the jig (the expected gap was something like .5mm, and at 1/4 speed, there was no way I was hitting the e-stop in time).
They didn't let me touch the machines again for about a month.

 

[Dana]

Almost my entire first machine when I switched from military/aerospace to automation...

 

[topher5150]

not me but my predecessor, she would call out a drill depth and tap depth for a tap through hole, and the depths that are called out are 2-3x's deeper than the thickness of the material.

 

[GarethW]

I managed to get a mirrored CNC machined part made a few years back. It was a part generated from PCB layout data which happened to be flipped. Not sure how the mistake happened because I was well aware of this classic mistake at the time and usually check the data at least 8 times to avoid it!

 

[Dana]

I managed to get a mirrored CNC machined part made a few years back. It was a part generated from PCB layout data which happened to be flipped. Not sure how the mistake happened because I was well aware of this classic mistake at the time and usually check the data at least 8 times to avoid it!

Hah! I had an entire machine built mirrored from what it was supposed to be. Apparently the decision to reverse the machine was never communicated to me, and nobody at my customer noticed it at the time. They tried to not pay me, and make me pay to have the machine parts remade, until I showed them my original layout that their chief engineer had approved. The engineer was a nice guy and on my side; it was the bean counter president who tried to stiff me.

 

[GarethW]

That sounds like a nightmare situation, Dana. I hope it was resolved satisfactorily in your favour.

 

[K.I.S.S.]

25 years ago, when I was firmly affiliated to the school of experience over knowledge, I made a proper clanger. I had the task of working on a Strongroom for a large corporate business. The preventative maintenance aspect of this work required some grinding to take place.
In full knowledge that this would activate the fire alarm on this floor, the floor was disabled.
I forgot the one smoke detector at the top of the stairs, on the next level.
The resultant murderous looks from 600 plus people caught in a torrential downpour following the fire evacuation have always been a strong motivator for me. It's many years ago now, and a different life, but now that I design and engineer things, It makes me appreciate the consequences of assumption.

 

[Dana]

That sounds like a nightmare situation, Dana. I hope it was resolved satisfactorily in your favour.

Yes, I got paid (including the additional work to make new reversed drawings) and they redid the machine at their own expense... but I never got any further work from them.

Long before that, coming from the aircraft business into machine design, I had a tendency to make everything as lightweight as possible, not the way to design machinery that has to be rigid and stable. That first machine involved a hydraulic press for die punching. I did a stress analysis to verify the press frame wouldn't break, but I neglected to calculate the deflections... when the 2x6 inch cross section aluminum bar started visibly flexing the first time we tried it, people were running for cover! Then there was the oil mist that sprayed all over the shop when we cycled the 9" air over oil hydraulic booster...

 

[s.weinberg]

I remember another one.
This was when I was fresh out of school, with, to put it mildly, not a whole lotta real-world experience.

I was building a pneumatic test fixture that I had designed to run on high-pressure nitrogen from a tank. I checked the pressure rating for all the equipment, and it was well in the safe zone. Then I connected it all up with some hose from a rack on the wall. Thought it was high-pressure hose. It wasn't.

At something like 6000 psi (memory is fuzzy here), the hose failed catastrophically, with a sound like a gunshot, rapidly venting pure nitrogen into the air. My boss came running, thinking that I had likely killed myself. I don't know if anything would have happened anyway, but I was behind a barrier, and had the warehouse loading doors wide open to vent, and was fine.