Turning
Making a long-spin top
Making a top that spins a long time is simple but exacting. There are only three parameters that govern how much energy you can put into the top in the half-second or so you are spinning it up—weight, how far that weight is from the spin axis, and how fast it spins around the axis. Likewise, there are only a few factors influencing how fast that energy bleeds off—air friction, tip friction, and erratic spinning due to the stem and weight not being concentric and balanced. I put the various factors in a graphic here. It came together well in this year's winning top, seen above, which spun more than 27 minutes after a string launch.
WEIGHT
f started with a steel ring, sliced from a pipe. You can see it plainly. It's about 1" high and 1/4" thick, rabbeted on the bottom and polished on the outside. I turned it from a larger piece of pipe on a metal lathe, then parted it off. If you don't have a metal lathe, you can use bearing races, which have the advantage of being hardened. (My ring was mild steel and got a little dinged up in use.)
I experimented to find the right stem diameter. The stronger you are, the smaller a stem you can use and still get the top spinning fast. I'm not as strong as I used to me, so my stem's 7/8" diameter.
BODY AND LID
I bored a hole in a 1/2" MDF disc which serves as the body. I then mounted a piece of scrap in the chuck of my wood lathe and turned a 7/8" stub tenon to temporarily mount the MDF. I turned the MDF's outside diameter to fit the rabbet on the bottom of the steel ring, trued both faces, then dished out the upper face (like hollowing a plate) to leave a thin web over most of the area between the stem and the outer edge. I left the MDF full size in a ring around the stem, and also the inside of the pipe so I could drill balancing holes in it.
I wanted to hide the balancing holes, both for aesthetics and to eliminate air friction. So I veneered a piece of 1/8" plywood, then prepared it as above: drilling a mounting hole, mounting it on my stub tenon, and turning and rabbeting it to fit down into the upper side of the steel ring.
STEM
The stem is dry, straight-grained white oak, about 5" long. I bored a 5/16" hole in one end to mount a ball bearing as a tip, then mounted it between centers. I turned it round, then left a raised collar that tapered down to the tip. The collar would support the body and ring mechanically, when I slid them on from the top of the stem. I glued the MDF body onto the stem and trued it up again on the lathe. Finally, I finished the stem and body, and glued the bearing and ring in place.
BALANCING
Balancing is simple but tedious. I use a machinist's granite surface plate, levelled as best I can do it. I roll the top around and mark the rim where it comes to rest. After a couple dozen marks, I drill a hole in the MDF ring at that point. Then I do it again. And again. When it balanced (or when I gave up), I glued the lid in place.
THE LAUNCHER
I use a simple launcher: just a couple UHMW plates screwed to the top and bottom of an arm made of a piece of 2x4". I drill a 1" hole through both of them, and support the arm on a couple long bolts threaded into an MDF base. The space between the UHMW plates is where I thread the string into the top (you can see the little string hole in the picture). Then I wind the string up on the top and I'm ready to go.