# Thread: Find Top Dead Center of a Hemisphere

1. ## Find Top Dead Center of a Hemisphere

Does anyone know of a way to determine the top dead center (or bottom center) of a hemisphere?

I need to drill about 50 of these steel pieces with a 1/16" hole to hang them from a 1/16" water hard drill rod, and I want them to hang level. I'd like to think there is a trick out there to find the top center of a hemisphere - but I'm coming up blank.

Thanks in advance for any ideas.

2. ## Re: Find Top Dead Center of a Hemisphere

3. WeldingWeb Craftsman
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## Re: Find Top Dead Center of a Hemisphere

Assume the dome is reasonably close to being a perfect circle.

1) measure the diameter in several places and get an close average
2) build a wood jig flat on a wood surface large enough to hold the dome and still clamp to the drill table. The jig is two pieces of wood (say 1" x 1") at 90 degrees (x and y axis) to nestle the dome into for drilling
3) where the x and y axis meet is 0x by 0y. Fasten the jig to the drill table so that a coordinate of 1/2x diameter by 1/2y diameter is under the drill point

Push the domes into the jig and drill.
Last edited by John Bartley; 08-03-2020 at 02:54 PM.

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## Re: Find Top Dead Center of a Hemisphere

One option might be to put them on a turn table or lazy suzan. Put a sharpie or other marker in a jig to point it at near center.
If you have a magnetic base dial indicator that should be able to hold the marker.
Rotate the hemisphere so it draws a line around the center.
Another option to get close to the center might be to rest a 4.5 inch grinder disc on the top of the hemisphere so it is level. Then use that to draw your circle.

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## Re: Find Top Dead Center of a Hemisphere

Looks like only 1 1/2” dia. Lathe with 3-jaw chuck....
Cap should fit flat against chuck body. Drill with tailstock.

6. ## Re: Find Top Dead Center of a Hemisphere

Originally Posted by John Bartley
Assume the dome is reasonably close to being a perfect circle.

1) measure the diameter in several places and get an close average
2) build a wood jig flat on a wood surface large enough to hold the dome and still clamp to the drill table. The jig is two pieces of wood (say 1" x 1") at 90 degrees (x and y axis) to nestle the dome into for drilling
3) where the x and y axis meet is 0x by 0y. Fasten the jig to the drill table so that a coordinate of 1/2x diameter by 1/2y diameter is under the drill point

Push the domes into the jig and drill.
Now if I just had a drill press lol. Sorry, should have mentioned that at first. I was hoping there was a way to just mark/dot the spot on the hemisphere to drill. I'll have to think about your solution - pretty sure a jig as you describe would indeed work for me, just have to figure out the marking mechanism.

Thank you for the detailed reply.

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## Re: Find Top Dead Center of a Hemisphere

Can you find a pvc plumbing cap whose id is your metal cap od, center drill that. Use that as jig to put over each metal cap, use sharpie to mark/center punch?

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## Re: Find Top Dead Center of a Hemisphere

Clamp your center punch vertical in the vise, then set the hemisphere concave side down on the punch. keep moving it around until level and tap it over the punch with a hammer.

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## Re: Find Top Dead Center of a Hemisphere

A bunch of different ways to do it but if they are standard pipe caps let gravity be your friend. If they are square and flat, block them on a table and level them across the open end at least 3 different ways. A block of wood with a hole smaller than the OD to hold them would work good. Then put a drop of cutting oil in the dome. It will sit in the middle at the bottom so you can center punch them. The oil will help with drilling but any liquid, even water would work but only use a tiny amount to be more accurate. Having a drill press would make it fairly easy to set up a jig but manually drilling might have to mark each one separately.
Last edited by Welder Dave; 08-03-2020 at 04:35 PM.

10. ## Re: Find Top Dead Center of a Hemisphere

I use lathe and a 3 jaw.
If have a 3 jaw Chuck just the center Chuck on drill press.

Dave

Originally Posted by Shootr

Does anyone know of a way to determine the top dead center (or bottom center) of a hemisphere?

I need to drill about 50 of these steel pieces with a 1/16" hole to hang them from a 1/16" water hard drill rod, and I want them to hang level. I'd like to think there is a trick out there to find the top center of a hemisphere - but I'm coming up blank.

Thanks in advance for any ideas.

11. ## Re: Find Top Dead Center of a Hemisphere

Pointed caliper. Adjust to turn an arc from the edge to center. Get very close to center. mark several arcs from the edge, Your point is at center of these marks.

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## Re: Find Top Dead Center of a Hemisphere

Fasten two pieces of 1/2 plywood to your drill press table one on top of the other and both screwed together . Drill a hole the size of the cap in the top piece of plywood only using a hole saw or forsner bit, without moving the table, replace the hole saw with a small pilot drill and mark or drill thru the cap that you place in the previously drilled hole.................Mike

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## Re: Find Top Dead Center of a Hemisphere

I think the problem is no drill press.

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## Re: Find Top Dead Center of a Hemisphere

I think the problem is no drill press.
Oops, missed that key piece of info LOL............Mike

15. ## Re: Find Top Dead Center of a Hemisphere

Originally Posted by Willie B
Pointed caliper. Adjust to turn an arc from the edge to center. Get very close to center. mark several arcs from the edge, Your point is at center of these marks.
I was thinking of that from another old post years ago where the guy had huge domes. I had forgotten what I described but it's the same as you say. Just finding the center of a circle with a compass/caliper, but 3-D.

16. ## Re: Find Top Dead Center of a Hemisphere

Ok Hand drill, center punch and centering head with ruler.

https://www.wonkeedonkeetools.co.uk/...-9/3PS-9-1.jpg

I have in past works great do not buy or even look at Staerrter there are other brands a lot cheaper.

Dave

Originally Posted by Welder Dave
I think the problem is no drill press.
8
Last edited by smithdoor; 08-03-2020 at 06:55 PM.

17. ## Re: Find Top Dead Center of a Hemisphere

Do you know someone with a lathe? That would be the quickest.

Sincerely,

William McCormick

18. ## Re: Find Top Dead Center of a Hemisphere

Back in the day before we had a drill press, this is what I used to make rather accurate holes in things. That piece on top is a drill bushing, you can interchange them as they wear out, or if you want to use different sized drill bits.

The holder was cut from a solid piece of aluminum on a bandsaw. You can make the adjustment slot and then just weld the end shut if you do not wish to play around with welding the bandsaw blade back together after you put it through a starting hole if you want more adjustment or just leave it open like that one is. There is a countersink head screw under the plate that the wing nut tightens the holder to the plate.

If you make that all you have to do is center one, and the rest you can just lay into a depression that is lined up with the drill guide.

Sincerely,

William McCormick

19. ## Re: Find Top Dead Center of a Hemisphere

If you get a 1/16" drill bushing you could even mount the bushing in a piece of wood, line it up once, and drill away.

Sincerely,

William McCormick

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## Re: Find Top Dead Center of a Hemisphere

Polish the dome

Hang a plumb line. Bob at dome height. Slide dome under bob. It will touch at TDC.

Or plumb/level a bushing on top. Then drill down the center.

For 50...I'd probably make a box/fixture, similar to what has been described. Center of the circle base is center of the top/dome.

21. ## Re: Find Top Dead Center of a Hemisphere

I can't thank you all enough for taking the time to provide these many great ideas. If this next project or two results in sales, top of the list is a drill press!

As arduous as it sounds, the best option I'm coming up with is:

Clamp a center punch in my vise vertically - just high enough to set a dome on it with the edges 1/8" or so above the jaws. Manually tweak the position of the dome until it's sitting level. Hold it steady and whack the dome with a hammer to mark the balance point. Drill from the inside.

It took a couple minutes apiece to do three tests. They came out perfect and sat nice and level on the rod. I think doing it this way, as tedious as it may be, will also account for any imbalance in each individual dome from mis-stamping.

If I can ask another question: The drill bits, as you can imagine, are very fragile. Would it be better to clamp them in my Dremel and drill at low (8000RPM) speed? My 1800rpm drill doesn't seem to want to make the hole in any kind of reasonable time.

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## Re: Find Top Dead Center of a Hemisphere

This is where a bench top drill press would work very well. Since 1/16" bits are pretty much impossible to sharpen, I'm speaking for myself on this. 1800 RPM is still awfully fast IMO for even a 1/16" bit. If your drill is variable speed try to run it very slow. Buy a can of cutting oil and with the concave surface it will hold a small puddle of the oil on the hole while you're drilling.

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## Re: Find Top Dead Center of a Hemisphere

Are those cast Iron or press steel?

24. ## Re: Find Top Dead Center of a Hemisphere

Originally Posted by tapwelder
Are those cast Iron or press steel?
Pressed steel, what type I do not know. 0.080" +/- walls.

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## Re: Find Top Dead Center of a Hemisphere

If you're going to use oil to drill why not use the oil to mark the center? Kill 2 birds with one stone. Depending on how high the dome is might be tricky to use a centering head.

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