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Scaling New Heights

25 August 201510:18AM3d-printing

Here's a fact you probably didn't know: Those plastic bits on the side of pocket knives are called 'scales'.

Aside from lending itself to all kinds of puns, these little plastic widgets are great targets for general hackery.

my scaly mate

This is the Victorinox Escort my dad got me when I turned about 13. Aside from a brief period of carrying the Classic I got for free at some trade show a few years later and subsequently lost at the beach, I've had this knife on my keychain since I was old enough to start having a keychain to put it on.

And a couple of weeks ago, it broke. The back-side scale cracked, without me even noticing, and I lost the toothpick and a chunk of the plastic too.

This is about a fifteen-dollar knife, so I could just replace it (and I did) - but this is the age of computer aided design, of object piracy, of three dimensional printing. And I thought - I can fix this.

Not only can I fix this, but I can make it better.

A dig through Thingiverse turned up a handful of scales for larger knives, proving that a replacement was at least theoretically possible. But there were none for the knife that I had, the 58mm size. I could probably scale (ha.) one down, but then I'd lose the correct sizing of the slots for the toothpick and such.

But then, who says I have to put a toothpick in there at all? If I'm going to have to design this thing from scratch, why not put something else in there instead?

What's a tool I use every day, that lives on a keyring, that I might want to carry around with me at all times, and that fits neatly into something about a centimetre across?

not to scale

Hmm. Verbatim make these fairly nifty little flash drives, which just stick in the bottom part of a USB port. They're really thin, amazingly light, and already come with a little keyring hole to pivot around.

I can work with this.

scaled up

Right in the middle of a dinner party, I make a few measurements to the closest millimetre. The whole point of 3D printing is that you can prototype, and something this small will only take about 10 minutes to build. I can tweak the design after each iteration, until it's perfect. I steal some time on a computer, throw together a design in Tinkercad, and go back to my soup.

About a week passes, and when I get back to work, they tell me that the 3D printers aren't working.

No worries. We can iterate on that at the same time.

scalar quantities

Eventually I fix - well, "fix" - them, after about two solid days of frustration, by dropping the extruder temperature by about 10 degrees, and covering the build platform in masking tape. And what do you know, the first one off the bed... doesn't fit. But only barely. The void needs to be about a millimetre longer.

And it turns out, that's the only change I actually needed to make. The drive fits perfectly, but the knife is at home. I walk home via Officeworks, and pick up a new drive in a matching colour. Also, some superglue. I included snap-on style holes for the knife's internal rivets, but I'd like something a little more adhesive actually holding the scale on there.

scaled composites

I get home and lay out my materials...

the scales tip in my favour

...try the scale on the knife...

the scales tip in my favour

...and they fit!

Not just reasonably, but beautifully. Turns out my eyeballing was better than expected.

And a little bit of glue in each of the rivet holes finishes it up.

Some product action shots:

petascale


If you want to give this a shot yourself...

Tinkercad - Solid & USB versions.

And Thingiverse.

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