Find Top Dead Center of a Hemisphere

[Shootr]

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.

It's amazing how big a little drop of oil looks against a 1/16" drill bit. When I try that, the oil seems to have enough "stiction" that it doesn't readily flow exactly to center. When I drilled it, the hole was off enough that the dome sat at an angle.

[Insaneride]

Shooter, what were those domes? I have some similar in size that are titanium. Always wondered what they were for. Someone suggested part of a fuel pump.

As for method, good suggestions have been made. I was thinking level and a drop of mercury but oil makes sense for lube. The center finder tool would be good if you have one.

[Shootr]

Shooter, what were those domes? I have some similar in size that are titanium. Always wondered what they were for. Someone suggested part of a fuel pump.

As for method, good suggestions have been made. I was thinking level and a drop of mercury but oil makes sense for lube. The center finder tool would be good if you have one.

Just decorative ones from a local supply house.

[Insaneride]

The one I measured is 0.150" thick 3.45" diameter. I believe it's 6/4 Ti. 6% Vanadium 4% aluminum or vice versa. The stuf in the bowl that looks like rocks is Ti sponge used to produce Ti. It's like pure Ti.

[Insaneride]

I think the method Willie B and Oldendum mentioned is with a compass. See pic. My example isn't precise but look at my half assed compass.
The other pic is the tool Smithdoor mentioned. You can get one from harbor fart especially if your doing a lot of these. There's also a thread way back that goes over this. A member Denrep had a way to do it but I never understood it.

Pics are out of order.

[Welder Dave]

Try water instead of oil and try to put it close to center. You'll want to center punch them so the bit doesn't wander.

[mla2ofus]

Start the bit dry in the center punch hole first then add oil or water. You have the perfect form to contain your coolant provided you have the concave side facing up and not sideways.

[William McCormick]

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.

For a 1/16" bit you need very high speed. 3,200 or higher.

Sincerely,

William McCormick

[William McCormick]

I was looking quickly and I could not find them but they make double-ended drill bits that are short and so cheap, they have a short flute which is great for stability and if you are center punching the domes it would work great.

Too bad you do not have a friend with a lathe, you could do that at high speed on the lathe in under 30 minutes and nothing would be more accurate.

Sincerely,

William McCormick

[mcspeed]

As mrmikey pointed out in post #12, use a hole saw same size as dome OD. Penetrate first layer of plywood but not second. pilot hole in second layer can be used as jig for center hole. Use hand powered drill. Easy breezy.


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[chewinggum]

Instead of water or oil, put a very small drop of mercury in the bowl, level the top, It will settle to the center. Center punch it right there thru the mercury.
CG

[Oldendum]

This works if it's flat. 3D is a bit harder.

https://www.mathopenref.com/constcirclecenter.html

[shortfuse]

Instead of water or oil, put a very small drop of mercury in the bowl, level the top, It will settle to the center. Center punch it right there thru the mercury.
CG

1. Mercury is TOXIC to handle!
2. Mercury is usually not available in the average person's shop.

Why not suggest a safer method to find the center????

[smithdoor]

The Starrett is great brand need if ever do work for the Federal Government. But cost is high. Starrett make 3 sizes this at Grainger is the middle size.


https://www.grainger.com/mobile/prod...ter-Head-2ZUT4

They are made by others for hole set $7.00

https://www.harborfreight.com/12-in-combination-square-set-63688.html

I have all three size from Starrett

Dave

[Denis G]

Mount a center punch in a hole on a level surface with the tip of the punch facing up. Balance the hemisphere on the tip of the center punch so that the edge of the cup is the same all around as measured from the level surface. Strike the top of the cup. That will leave a punch mark on the inside of the cup that should be the exact center for you to drill.

[mla2ofus]

Mount a center punch in a hole on a level surface with the tip of the punch facing up. Balance the hemisphere on the tip of the center punch so that the edge of the cup is the same all around as measured from the level surface. Strike the top of the cup. That will leave a punch mark on the inside of the cup that should be the exact center for you to drill.

Gee, I think I already said this ??

[Denis G]

Gee, I think I already said this ??

You're right. It was such a good idea, I thought that it was mine.

Sorry, I had just missed it.

[chewinggum]

1. Mercury is TOXIC to handle!
2. Mercury is usually not available in the average person's shop.

Why not suggest a safer method to find the center????

I grew up playing with mercury. It was common to take it to school and play with it. Lots of boys did it. We didn't know any better.
I also used to hunt with lead pellets in my mouth. Amazing I'm still kicking at 76.
CG

[mla2ofus]

Chewinggum, I'm 3 yrs your junior but did the same things when young. Spent hours rubbing pennies in mercury to make them appear silver.

[12V71]

Chewinggum, I'm 3 yrs your junior but did the same things when young. Spent hours rubbing pennies in mercury to make them appear silver.

I had almost 3/4 of a pint of the stuff in a mason jar as a sorta kid. It came from the mercury switches from scrapped out Valmont water drive irrigation machines.
Later I found out it was great for de-leading old rifle barrels. I'm sure the lead/mercury amalgram was bad stuff. It went to a battery recycler.

[ronsii]

I had almost 3/4 of a pint of the stuff in a mason jar as a sorta kid. It came from the mercury switches from scrapped out Valmont water drive irrigation machines.
Later I found out it was great for de-leading old rifle barrels. I'm sure the lead/mercury amalgram was bad stuff. It went to a battery recycler.

Used to have lots of the stuff around to play with but eventually the amount dwindled when too much got spilled on the carpet and would disappear should still have a small bottle with reclaimed from switches like yourself I found this one pound bottle in my stock room a minute ago but who knows where the rest is... if it is.

[l0w n slow]

Set it on a flat surface , locate a framing square and a torpedo level. Set one side of the square on the top with the other side touching the side . Level it with the square. Then mark where the swuare is touching. In two spots there's your center .

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[VaughnT]

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?

This is the easiest way, though the flat-bottomed PVC cap doesn't need to have an ID that's the same as your metal cap's OD. As long as the plastic cap is smaller than the OD of the metal cap, it'll want to ride level on the metal cap like a ball-n-socket joint.

You can use a center-finder square gizmo from Harbor Freight or wherever to find the center of the flat-bottomed PVC cap. Drill through it, de-burr, and you're good to go.

Put your metal cap flat on the drill press table. Set the PVC cap on top of it. Use a small bubble level to get the PVC cap level, and you will have magically aligned the through-hole in it with the Top Dead Center of the metal cap.

You can verify this by checking the outside perimeter of the PVC cap to see that it's evenly raised above your work surface. If the bottom edge is parallel to your table, you're spot on.

[CAVEMANN]

Tack a piece of angle the same height as the cap to the table, slide the cap into the "V" of the angle and use the centering head on the combo machinists square to marke the center from several angles by rotating the cap.

[N2 Welding]

So many ways to skin this cat. Now I think I have to go get an el cheapo combo square kit from HF. Would like a Starrit but I’m too poor.