Duncan counterweights work well out of the box, but some modifications can be made to turn them from nice to deluxe. The counterweight is a little sharp, which can really hurt if you get hit. Nicer counterweights also have bearings to help string tension automatically adjust. I made this almost entirely out of duncan parts I had in my yo-yo graveyard, so if you have parts lying around... go for it!
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Step 1: Items Needed
Duncan counterweight (theoretically this can be done with any cube or other shape of plastic)
You need an old yoyo bearing. Size A works pretty well (5 x 10 x 4mm). If you don't already have one you might be able to find a pack of them on ebay for cheap.
Washers the same diameter as the yoyo bearing
5 Min epoxy(JB Kwikweld works very well)
Drill and Drill Bits
Sandpaper (200 and 400-600 grit paper)
File/belt sander/80 grit paper (something to remove lots of plastic fast)
Step 2: Drill Hole for Bearing
Use a drill bit of a similar diameter to your bearing. The hole needs to be drilled about 3/4 through the die. A 3/8 inch will get you close to an A size bearing (though getting a metric 10mm diameter bit might be better). The 3/8" bit is a little to small for the bearing, so you need to widen the hole. I used a dremel with a sanding drum, but you could also put a bolt in the counterweight and put it in your drill as a makeshift lathe. You can use a chisel or some sandpaper to make the hole just big enough for the bearing to fit in.
Step 3: Shape the Die
This step doesn't need to be done after you drill the hole, but if you're using a drill press it's nice to have a bigger flat face to hold the die in place.
The purpose of this step is to remove the sharp edges that the Duncan counterweights come with. Shape the die however you want. I used a belt sander, but you can use a file, rough grit sandpaper, or a rasp. If you round out the die too much then you will affect the die markings. You can either leave it as is (so they look different sizes) or you can drill the pip holes deeper, and then fill the holes in with something (I used JB weld and sanded it down flush). You could also fill the hole in most of the way, and then use nail polish (whatever color you want) to fill the rest of the way. Use nail polish remover to remove the excess around the holes.
Step 4: Make a Paperclip Spacer
Take the paperclip and bend it to the same size as the outer seat of the bearing. The spacer is to isolate the inner track of the bearing so that the washers won't prevent it from spinning.
Step 5: Make a String Holder
The inner track of the die is too big for the knot of the string to be held without slipping through the counterweight. The solution is to make some sort of cone that will fit into the inner track of the bearing. I grabbed a small piece of plastic and drilled a hole into it (just slightly wider than the yo-yo string) and then used a razor-blade to shape it into a cone. This piece shouldn't extend past the bottom of the bearing. A small sphere or a bead might also be able to be used instead of the cone. You basically just need something that will attach the string to the inner track of the bearing.
Another idea might be to use the end of a ballpoint pen to make the cone. It's already the perfect size to work with the bearing.
Step 6: Create Bearing Unit
Now time to glue stuff. First, throw the die, bearing, paperclip spacer, and washers on the scale. The point of the washers is to get the right weight. You want to be somewhere around 11 grams, so if you're a little light just add a couple more washers. You then need to glue the washers to the paperclip spacer and the paperclip spacer to the outer track of the bearing. Be very careful not to get glue into the bearing or glue the inner track to the outer track. This can easily be done by just applying the glue to the outside of the bearing, spacer, and washer.
After this add your string cone to the middle of the bearing unit.
Step 7: Sand Sand Sand
This is the business step. Just step up your sandpaper until you reach an acceptable smoothness. I went from 80 grit to 200 grit and finished it off with 600 grit paper. You can get to near the original polish after this step, but you can also stop as soon as you feel the die is smooth enough. I'd recommend spending the time to get it to look nice. The yellow is what the die might look like before and the blue is what it should look like after.
Step 8: Finish It Up
Slide the bearing assembly into the die and string it up. You're now ready to throw!