Keychain-convenient Cart Coin (Trolley Token)

Ever get frustrated because you forgot the coin for your shopping cart? We've all been there.

I designed a cart coin that is essentially circular and can fit in the cart that has an inserting tray that allows only coins (and coin-like objects), meaning the designs with handles are not insertable. So how do we attach the cart coin to a keychain (because otherwise we'll just forget it)?

Make a hole on the coin and put it in a split ring? Then you'll hurt your nails and waste time taking the coin out and putting it back in every time you shop, and we all agree that it is super inconvenient!

Or you can use a snap hook instead of a split ring, not a bad idea, but you may not have a snap hook with a small enough diameter for the hole on the coin at your disposal. Even if you do, you may find it cumbersome compared to my design.

So I propose to you my design: convenient, quickly attachable to / detachable from your keychain, is super light (<1g) and fast to print (8 mins only)!

3D printer

PLA filament (basically anything works as long as it is not soft and deformable, like TPU)

(if you don't want to redraw the CAD file, skip till step 4 and use my STL file to directly print!)

In CAD software, create a cylinder with 2 parameters: diameter D and thickness t. Check what coin your grocery store accepts and uses its dimension. In my case, I use D = 24 [mm] and t = 2.4 [mm].

Then we proceed with cutting the coin on CAD. I draw a centerline through the coin and draw the left half side with 'Line' tool and 'Style spline' tool in SolidWorks. Then I mirror it through the center line. Lastly, I use '3-point circle' tool to make a closed loop at the top part. I also use tangent relations where needed to make the spline smoother.

You can draw the spline however you want, just keep 3 things in mind:

1. The thickness of a typical split ring is 1.5 mm so the width of the entrance path should be a bit smaller than 1.5 (i chose 1.2 mm) so when we insert the split ring to the coin, we expect to expand the plastic a bit (but the coin will come back to its initial form after we release it)
2. Make a 'hill' right at the end of the entrance path so that the split ring is most likely to fall into either side of the 'hill' rather than falling back to the entrance path and getting out from it
3. Make 2 (or more) 'valleys' so that the split ring has a place to naturally fall into once it slides off the 'hill'.

Then I use 'Extruded cut' tool and select 'Up to surface' and pick the bottom surface. Then fillet (0.5 or 1 mm) around the corners and that's it, the CAD is done. Now save the file as an STL file and send it to your printer.

Now that we have the CAD file, let's start printing! In the 3d slicer (I use Bambu studio), in case the coin is standing like this, click 'Lay on face' on the top tool bar or press [F] then click either top or bottom face (thanks to symmetry any choice is equivalent).

I use 0.4 mm nozzle with Standard flow and basic PLA filament. I use 15% infill. Other information is in the attached photos.

If you use my SolidWorks file (SLDPRT) you can edit the global variables for 3 parameters: diameter D, thickness t, and width of entrance path x.

OPTIONAL: for a smoother surface finish (and if your 3d printer has the feature), choose 'ironing' on the top surface.

Attach the printed cart coin to your keychain and now you'll always be able to use shopping carts!!