Introduction: "Humbug" - Your Personal Shopping Coin

Hi, I am a 17 year old tinkerer who love creating designs to help out others wherever I can, and this was made for my mum, to stop her ever forgetting that elusive £1 coin for her shopping trolley!

the idea behind the design is to create a small and unobtrusive keyring, used to store a coin for unlocking a trolley at the supermarket. The name "Humbug" comes from the traditional british sweet, with the two colour design being reminiscent of the brightly coloured striped sweets.


PLA filament(white) - I recommend AMZ 3D 3D printing filament

PLA filament(red/blue) - I recommend AMZ 3D printing filament

10mm keyrings -


hobby knife(for removal of imperfections)

Step 1: 3D Printing

You are going to want to print the 2 stl files separately to get different colours for the 2 halves. I used cura as my slicing software but any slicing software will work.

The slicer settings used are very important as the screw threads have a tight tolerance, so I recommend:

-0.1mm layer height

-20-45mm/s printing speed(depending on your printer)

-infill 100%

I recommend using 100% infill as this is a piece which is going to experience quite a bit of jostling on your keyring and the added plastic makes the part much more sturdy. The keyring has space to fit different sized coins in in as well, like the €1 and the ¥1, and if you wish you can fit up to 2 coins in the space.

Step 2: Finishing + Bonus Design

To get the screw thread smooth, it can be really helpful to gently go over and remove any blobs formed during printing using a sharp hobby knife. Once this is done, twisting and untwisting the cap a few times should help smoothen the screwing and unscrewing motion. If after all this the printed part is still not working, there is likely a printing issue (over extrusion being a possibility), and re printing the pieces will likely be necessary. Once this is done, all that is left is to thread the 10mm keyring through the loop on the bottom piece, and the keyring is ready to go!

whilst I was very happy with my end result, I wanted to see how far I could take the idea, and so using one of my favourite celtic symbols (the triskelion) I designed a slightly different model incorporating the symbol. However, although I liked how it turned out, I found it not to be as sturdy as the original model, and this is why I have included it as a bonus design.

I have really enjoyed working on this project,so thank you for reading and happy making!

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