Finding the Equator On a Ball/Bead (Making A Knob)

[tsbrownie]

Need a wooden knob. That requires drilling a hole on the equator of the wooden ball / bead I am using. Here are 2 quick, easy DIY methods of doing that. No special tools required. I will post a video on how I mount this ball on a shaft.

MATERIALS 1 Ball / Bead
- Light colored thread
- Pen / fine marker

MATERIALS 2 Balls / Beads
- Shaft
- Dirty surface (or carbon paper or similar)

[TrickieDickie]

I know everyone does not have one but I would chuck up the ball in my lathe and drill a hole

[mklotz]

I know everyone does not have one but I would chuck up the ball in my lathe and drill a hole

That guarantees that you've found a diameter but the OP wants to find the equator relative to an existing, on-diameter, through hole. His string method is loaded with error sources but his second method is spot-on.

[tsbrownie]

Marv, You're a smart guy, it scares me when you say the first method is loaded with error sources. Assuming a round hole and round ball, what can go wrong?

[mklotz]

Marv, You're a smart guy, it scares me when you say the first method is loaded with error sources. Assuming a round hole and round ball, what can go wrong?

My favored medium is metal, not wood, so I tend to evaluate measurement techniques with a machinist's eye.

First, the method depends on the existing hole being the same diameter throughout, no chamfering. Then there's the problem of accurately marking a string and finding and marking the midpoint of the marks with a scale and transferring it to the ball. It would be impossible to maintain a tolerance of 0.050, totally unsuitable in the machinist's world.

But you're working in wood where such a tolerance is totally acceptable. I should have noted that in my response. I apologize for not doing so.

Your second method is much better. The close-fitting shaft averages the hole location, avoiding the problems of edge finding and marking in the first method. Rolling on a marking medium auto-locates the equator without the need to fuss with making measurements with/on a string. The second ball has to be the same size as the workpiece but, again, that's an error source with which only a machinist would concern himself.

In fact, your second method comes close to one of the methods I would use if it was a precision metal sphere and I needed to minimize errors in locating the hole.

[tsbrownie]

Interesting stuff, thank you for the explanation. As you noted, much of what I do is not high tolerance (to say the least). This is for the knob on a crystal radio wiper rod. (

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