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  1. #1
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    Find Top Dead Center of a Hemisphere

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    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.
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    Re: Find Top Dead Center of a Hemisphere

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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

    Quote Originally Posted by John Bartley View Post
    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

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    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

    Quote Originally Posted by labparamour View Post
    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.

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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.

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    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

    Quote Originally Posted by Shootr View Post
    Name:  IMG_20200803_110233-01.jpg
Views: 349
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    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.

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    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.
    An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.

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  18. #12
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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

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    Re: Find Top Dead Center of a Hemisphere

    Quote Originally Posted by Willie B View Post
    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.
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    Re: Find Top Dead Center of a Hemisphere

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


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    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.

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    Sincerely,

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    If I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.

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    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
    If I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.

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    Re: Find Top Dead Center of a Hemisphere

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

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

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

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    Re: Find Top Dead Center of a Hemisphere

    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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    Re: Find Top Dead Center of a Hemisphere

    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

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    Re: Find Top Dead Center of a Hemisphere

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

    https://www.mathopenref.com/constcirclecenter.html
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    Re: Find Top Dead Center of a Hemisphere

    Quote Originally Posted by chewinggum View Post
    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????

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    Re: Find Top Dead Center of a Hemisphere

    Quote Originally Posted by shortfuse View Post
    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

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  35. #25
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    Re: Find Top Dead Center of a Hemisphere

    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.
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