A common question I see asked in the gym is 'How many reps was that?' When you are pushing your body to the limits of what it can do, it is pretty much impossible to count the number of times you do an exercise, especially near the end of the workout. There are definitely devices that can already measure movements such as bench and deadlifts, but I wanted to make something that would integrate itself into most gym machines, and not require you to buy a £200 Apple Watch.
I wanted to use an arduino for this project, with an accelerometer that could detect changes in velocity and add one to a counter. The issue with this is I needed 5V to run the Arduino, which meant that my product would require 4 AAA batteries to run. In the future, I would like to find a smaller board which only requires 1.5V, so that I could use a smaller button cell and make the whole design a lot more compact.
To start 3d modelling the case, I needed to start modelling the batteries. It would be a lot more convenient to have a premade battery case, as they are already made as compact as possible. My plan was to buy a battery case that could support 4 AAA batteries, which could consistently supply 6V. The 6V could then be cut down with a potential divider circuit to 5V, to the Arduino. The battery case could be modelled as a cube:
Which we could design the bottom half of the case around. I did this by modelling a smooth curved surface around the cube with a few millimetres of space to allow for 3D printing tolerances:
This could be extruded and made thicker with the 'Thin' extrude feature on Onshape.:
I could then model on endcaps to make a shell that housed the battery case well, with ridges for better grip for your fingers.
It was here that I realised I needed a way to actually thread the pin that goes into the weightstack into the case. This would not be possible in the current design, as the highest part of the shell is in the middle. To remedy this, I made the lowest part of the case in the middle, to allow a hole to be drilled that would thread into the pin.
Here, you can see the fit of the battery case in the shell, using a Section View that shows the cross-sectional profile of parts.
I then needed a way to bolt the top and bottom halves together. The reason for this is that the battery case needs to be accesible to change the batteries, and having glued halves would prevent this. I added ridges, with around 5mm of extra space now available for threaded bolts.
I could then model in 4 M3 threaded holes, equally spaced due to the mirror tool, and another hole tapped into the middle of the shell with an M8 hole, which is the standard for a gym weight selector pin.
This is what the bottom half of the shell, with the selector pin threaded in, looks like:
I then split both halves to make it much easier to 3d print without supports:
