How do I balance a spinning top?
Some spinning tops are round after turning, but do not spin smoothly. This is caused by variations in the density of the wood, meaning that the spinning top's center of gravity is not on the axis. This is particularly common with yew wood, but rarely occurs with tropical woods, which grow evenly throughout the year. This yew spinning top did not run well because the heavy dark parts of the wood meant that the center of gravity was not on the axis.
The spinning top wobbled quite a bit:
Sometimes I balance such spinning tops. To do this, place the spinning top at an angle and lean the stem against the edge of a ruler or in the corner of two edges (here on a caliper) or simply between two fingernails. Then move the edge slightly up and down to reduce friction with the spinning top stem. The spinning top will settle with the heaviest point facing down. This means that there is a lack of mass on the side that is now facing upwards. I mark the upper position with a short pencil line.
I select one from a range of stainless steel balls of different sizes (available online) and stick it provisionally to the spinning top with adhesive tape.
Then lean the spinning top with the ball against the edge at an angle again. If it no longer turns clearly with one side down, the ball size is correct. Try out the spinning top and, if it runs well, mill a dent and glue the ball in place with a little two-component adhesive (“Uhu sofortfest”). Since the ball in the dent is closer to the axle, you may need to use the next larger ball. Caution: If you slip while milling, it will look unsightly!
Lo and behold, the top no longer wobbles:
It's a bit time-consuming, but a top that doesn't run smoothly just isn't as nice.
Statistics on my spinning tops
Spinning tops have various characteristics: they should look good, feel pleasant to the touch and spin for a long time without wobbling. Their appearance is determined by the choice of material and their shape. Their feel is also influenced by the surface finishing. Several factors must be right for a spinning top to run well. The shape determines the air resistance. A hard tip reduces friction with the surface. The density, mass and distribution of the material are important because they determine the gyroscope's moment of inertia. A high centre of gravity is more difficult to spin than a low one. If the centre of gravity is very low, the gyroscope must be spun almost vertically on the table, as even a small tilt angle will cause it to touch the surface of the table. The stem diameter must match the rest: if it is too thin, the necessary torque is lacking when spinning it up; if it is too thick, a high speed cannot be achieved. For finger spinning tops, stem diameters between 5 mm and 7 mm are favourable, depending on the size of the spinning top, and for hand spinning tops, 10 mm is more suitable.
I have measured some of the dimensions of the 417 spinning tops I have made so far and show them here, excluding a few very large ones. The number of spinning tops made per year varies:
In December and January, it's unpleasant outside, so there's more woodturning. By February, the enthusiasm has worn off. In summer, it's too warm and nice outside:
The average spinning top diameter is approximately 43 mm:
The average spinning top height is approximately 51 mm:
The average ratio of the height to diameter of a spinning top is approximately 1.25:
The average spinning top mass is about 15 g:
Spinning top as spirit level
On a horizontal surface, a rotating gyroscope remains in one position. If a gyro doesn't run sideways on a smooth surface, you can conclude that the surface is horizontal. Thus you can use a gyro like a spirit level.
If a gyroscope rotates on a slightly inclined surface, then it moves perpendicular to the "downhill" direction on the surface. If the top rotates the other way round, it moves in the opposite direction.
Fig. 1: A spinning top on an inclined plane; one arrow indicates the direction of rotation of the spinning top, the other the direction of travel of the spinning top.
The reason is that the tip of the spinning top is never quite pointed, but slightly rounded. This means that the point on which the gyro stands is only exactly on the axis of rotation of the gyro if the surface is horizontal. With an inclined surface, the contact point of the gyro lies at a distance r from the axis of rotation. Thus the gyro rolls on a small circle around its axis of rotation and thus rolls a little bit further with each revolution.
The angle of inclination a of the surface can be calculated from the traveling speed v. The travel speed v depends on the inclination angle a of the surface, on the rotational speed n of the spinning top and on the radius RSpitze of the spinning tops' tip.
Fig. 2: The gyro rolls on a small circle around the top of the spinning top.
At each revolution of the spinning top, it rolls over the surface by a distance of circumference Ur of a circle with radius r:
with
The walking speed v of the spinning top is:
Thus the angle of inclination a of the surface is:
Example: A gyro with a tip radius of 1 mm rotates at 20 turns per second and travels at 10 mm/s over a surface. This surface has an angle of inclination of:
Since it is difficult to measure the speed of the spinning top and the radius of the tip of the gyro, it is better to use the spinning top only as a spirit level and not to measure the angle of inclination of the surface. Or you simply place a marble on the surface: if it does not roll, the surface is flat:-)