I'm always looking for ways of reusing the plastic screw tops of soda bottles, as they've got a nice grip and seems perfect for use as some kind of knob. I've already published an instructable on how to use them for adjustment knobs in the workshop.
Now I'd like to show you a cheap and easy way of using them as volume controls, or for other uses when fitted to potentiometers.
Step 1: The Idea
The idea is quite simple: Cut out a 25 mm disc of wood of a suitable thickness, drill a 6 mm hole right in the centre, and glue it to the inside of a plastic soda bottle top. Paint it to your liking, and push it onto the shaft of your potentiometer.
It's simple if you've got a 25 mm hole saw with a 6 mm drill bit.
If not, like me, please proceed with the next step.
Step 2: Choose and Prepare Your Bottle Top
The first picture shows the three kinds of bottle tops used mostly for soda bottles in the region where I live.
I prefer the first two kinds, because the third kind seems to always have a small ridge around it, making it less attractive.
The first step is to remove the little studs (second picture) on the rim of the bottle top, which was used to attach it to the plastic locking ring around the neck of the bottle. Simply sand them off (picture 3).
If you want to paint you top, you can do it now. The last picture shows a selection of colours I've used.
Step 3: Making the Wooden Disc
This is the most difficult part, but not very difficult all the same. You should use a drill press or drill stand for this part of the work.
Both the types of bottle tops I use have a 25 mm diameter inside, and most potentiometers use a shaft of 6 mm diameter. You get two main types of potentiometer shaft: A splined one (left one in first picture), and one where the knob is fixed on the shaft with a small screw from the side (right hand one in first picture), but luckily both types are 6 mm in diameter.
So for our knob we need a 25 mm wooden disc with a 6 mm hole in the centre.
The thickness of the disc will depend on the type of bottle top you're using. For the one on the left in the second picture a thickness of 6 of 7 mm will work, and for the deeper knob on the right a disc thickness of 9 or 10 mm is needed.
I use 6 mm hardboard for the shallower knob, and 9 mm plywood for the deeper one.
The easiest way to make your disc is with a 25 mm hole saw. My Bosch hole saw kit luckily had one of that size (last picture). Unfortunately my hole saw's drill bit is 5 mm, so the hole in my discs needs enlarging to 6 mm.
If you're lucky and have the right size of hole saw with the right size drill bit, you can just go ahead and drill your disc, perhaps also drilling a millimeter or so from beneath before finishing to minimize breakout when drilling through.
Step 4: If You Need to Enlarge the Hole in the Disc
To enlarge the hole seems simple, but you have to work carefully if you don't want a wobbly knob action due to an off centre or uneven hole.
I do it the following way:
First drill about 2 mm of the disc with your hole saw (first and second picture). Then turn the wood around and drill about 1 or 2 millimeters in from the bottom side (third picture) to minimize breakout of the wood when the hole saw breaks through in the final step.
Turn it right side up again, and now fit your 6 mm drill bit to your drill (fourth picture). Holding the wood lightly in place, slowly lower the drill bit into the hole in the disc, so that the bit can centre itself in the hole by moving the wood slightly. Now firmly grip the wood, and lower the drill bit further, but still slowly, until the drill bit has passed al the way through the wood. If you don't work carefully and slowly, the drill bit will grab the wood and ruin the hole.
Now you can fit the hole saw to the drill again, and fully cut out the disc (last two pictures). Remove the worst of the roughness from the outside of the disc with sandpaper.
Step 5: Glueing and Fitting
If your potentiometer has a splined shaft, you can now glue the disc to the bottle top, and after the glue has dried, push it onto the potentiometer shaft (first picture). You can use almost any kind of glue, because the thread on the inside of the cap and the wooden disc's sides provide excellent grip for glues to cling to. Hot glue, epoxy, contact glue and even ordinary white glue will probably all work fine.
If you find the knob fits too loose on the shaft, you can use a screwdriver to bend the two prongs of the shaft slightly apart (second picture).
If your potentiometer's shaft has a flat face for a screw (third picture), you can use a smidgen of silicone for a firm fit, or fit the wooden disc with a little dowel to stand in for the screw (this must however be done before glueing the disc to the inside of the bottle top).
Do it as follows:
Drill a 3 mm diameter hole in the side of the disc (second last picture). Fit the disc temporarily to the potentiometer shaft, and push a glued 3 mm dowel (a 3 mm cocktail stick is perfect) through the hole in the side of the disc until it stops against the flat part of the shaft (last picture). Remove the excess piece of dowel on the outside, and glue the disc to the inside of the bottle top. When all the glue is dry, fit the knob to the potentiometer.
Happy knob making!