how to drill wooden hemispheres

[LCHIEN]

Following on to the thread how to cut wood balls in half, that's solved.
Now I'm looking for the way to drill a perfectly centered hole, perpendicular to the flat face of my hemispheres.

problems with the obvious:
To start, I could lay the flat face on the Drill press table and make the hole perpendicular to that face. But the issue to me is how do i locate and drill the exact (top dead center) center of the top of the round ball?

Or, I could use my center finder and mark the exact center of the face, but to drill that it needs to be face up, I'd use a board with a hole drilled in it to keep the hemisphere from moving but then I can't guarantee in any reasonable fashion that the face is parallel to the DP table.

Another thing.. with my sawing jig, after I've cut the ball in half and its still trapped in the jig, I can lay the jig on its side and drill the ball still in the jig on the DP and this will guarantee a hole perpendicular to the flat face And straight through both halves... but I can't locate the top dead center of the ball except by eyeball.

I tried taking the hemisphere and laying it on the table - then take a sanding block holding it a parallel to the table as I can, and sanding the top while I rotate the ball. So I end up with a scuff mark on the top that's about 1/4" in diameter (since I can't ensure the sanding block is parallel to the table). I then eyed the center of the scuff and drilled there but I seem to get no better than about 1/32" of center.

Please don't suggest a cube box to hold the ball and drill the center of the box. That only works if the balls are of perfect and consistent size.

So who's got an idea for this one?

[All Thumbs]

On a piece of scrap, use a compass to draw a circle a tiny bit larger than the diameter of the hemispheres you want to drill.

Use a center finder in your drill press to locate the center point you used to draw the circle, and clamp the scrap to the table.

Now place the hemisphere on the piece of scrap and center it in the circle you drew with the compass, and drill your hole.

[LCHIEN]

Gosh, that sounds simple enough... I must have been thinking too hard.

[cabinetman]

Please don't suggest a cube box to hold the ball and drill the center of the box. That only works if the balls are of perfect and consistent size.

Some questions:

Are you suggesting to drill through holes (TH's) in each half?

Or, are the holes preferred to be partially drilled through (PDT)?

Are the halves to be used in a project that their center drilling is a requirement for the project to be successful?

What is the project?

What is the probability that each half ball is identical to each other?

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[LCHIEN]

Some questions:

Are you suggesting to drill through holes (TH's) in each half?

Or, are the holes preferred to be partially drilled through (PDT)?

Are the halves to be used in a project that their center drilling is a requirement for the project to be successful?

What is the project?

What is the probability that each half ball is identical to each other?

.

actually thru holes is what I want. Preferably straight through both halves.
works best if its well centered
since you ask, its to hold toilet paper rolls on a drill to spool off old fishing line from reels. More concentric = faster take up speeds possible. Put a piece of 1/4-20 allthread, a nut and a wingnut, through the middle to spin with the drill. The "taper" of the ball holds the cardboard tube and compresses it just enough to be rigid.
each half ball is fairly close to the other half

[chopnhack]

Are you reusing the line that is on the reels?

[LCHIEN]

Are you reusing the line that is on the reels?

usually its old line, so not for fishing as they recommend change line every year.
once in a while I want to change line diameter e.g. 10 lb to 15 lb test so I'll save relatively new line.

Anyway, toilet paper spools are a cheap way to store probably not to be reused line if i do save it. Can throw away the spool and the line later and not fret about it!

[cabinetman]

It might be just as easy to turn a taper (cone) on two different pieces. The toilet paper tube is approximately 1⅝". Drill the hole while in the lathe first. Taper the stock from larger than that, and you'll have a center.

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[LCHIEN]

Don't have a lathe.

Here's the result:
Hex nut on the right, thru-threaded star knob on the left.

[chopnhack]

Very cool Loring, now give it a spin!

[cabinetman]

Instead of using toilet paper tubes, you could use the core to the rolled wrapping paper, comes in 30" lengths, about the same diameter, and has about 3/32"-1/8" wall thickness. Less likely to be subject to the ends crushing. PVC could be another option.

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