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.Yeswelder MIG-205DS(3) Angle Grinders at the ReadyJust a hobbyist trying to improve
Reply:Syncrowave 250 w/ coolrunnerMM210 W/spoolgunSP 135Victor O/ARamsond cut 50Titanium Stick 225.
Reply:Assume the dome is reasonably close to being a perfect circle.1) measure the diameter in several places and get an close average2) 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 drilling3) 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 pointPush the domes into the jig and drill.Last edited by John Bartley; 08-03-2020 at 02:54 PM.
Reply: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.https://uploads.tapatalk-cdn.com/202...30ed99d3d0.jpgSent from my SM-G930V using TapatalkMillermatic 252 MIGMiller Dynasty 200DX TIGMiller Spectrum 625 PlasmaAltas 12x36 Metal LatheBridgeport Milling Machinewww.psacustomcreations.com
Reply:Looks like only 1 1/2” dia. Lathe with 3-jaw chuck....Cap should fit flat against chuck body. Drill with tailstock.
Reply:Originally Posted by John BartleyAssume the dome is reasonably close to being a perfect circle.1) measure the diameter in several places and get an close average2) 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 drilling3) 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 pointPush the domes into the jig and drill.
Reply: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?
Reply: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.Ol' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply: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.
Reply:I use lathe and a 3 jaw.If have a 3 jaw Chuck just the center Chuck on drill press.Dave Originally Posted by ShootrDoes 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.
Reply: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.
Reply: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
Reply:I think the problem is no drill press.
Reply:I think the problem is no drill press.
Reply:Originally Posted by Willie BPointed 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.
Reply:Ok Hand drill, center punch and centering head with ruler.https://www.wonkeedonkeetools.co.uk/...-9/3PS-9-1.jpgI 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 DaveI think the problem is no drill press.
Reply:Do you know someone with a lathe? That would be the quickest. Sincerely, William McCormickIf I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.
Reply: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 McCormickIf I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.
Reply: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 McCormickIf I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.
Reply:Polish the domeHang 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.
Reply: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.Yeswelder MIG-205DS(3) Angle Grinders at the ReadyJust a hobbyist trying to improve
Reply: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.Ol' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply:Are those cast Iron or press steel?
Reply:Originally Posted by tapwelderAre those cast Iron or press steel?
Reply: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.Originally Posted by Welder DaveIf 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.
Reply: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.
Reply:Originally Posted by InsanerideShooter, 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.
Reply: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. Attached Images
Reply: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. Attached Images
Reply: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.
Reply: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.Ol' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply:Originally Posted by mla2ofusThis 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.
Reply: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 McCormickIf I wasn't so.....crazy, I wouldn't try to act normal, and you would be afraid.
Reply: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. Sent from my iPhone using Tapatalk
Reply: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
Reply:This works if it's flat.  3D is a bit harder.https://www.mathopenref.com/constcirclecenter.html"USMCPOP" First-born son: KIA  Iraq 1/26/05Syncrowave 250 w/ Coolmate 3Dialarc 250, Idealarc 250SP-175 +Firepower TIG 160S (gave the TA 161 STL to the son)Lincwelder AC180C (1952)Victor & Smith O/A torchesMiller spot welder
Reply:Originally Posted by chewinggumInstead 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
Reply: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-2ZUT4They are made by others for hole set $7.00https://www.harborfreight.com/12-in-combination-square-set-63688.htmlI have all three size from StarrettDaveLast edited by smithdoor; 08-05-2020 at 02:40 PM.
Reply: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.
Reply:Originally Posted by Denis GMount 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.
Reply:Originally Posted by mla2ofusGee, I think I already said this ??
Reply:Originally Posted by shortfuse1. 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????
Reply: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.Ol' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply:Originally Posted by mla2ofusChewinggum, I'm 3 yrs your junior but did the same things when young. Spent hours rubbing pennies in mercury to make them appear silver.
Reply:Originally Posted by 12V71I 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.
Reply: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 . Sent from my SM-G973U using Tapatalk
Reply:Originally Posted by labparamourCan 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?
Reply: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.IMPEACH BIDEN!NRA LIFE MEMBERUNITWELD 175 AMP 3 IN1 DCMIDSTATES 300 AMP AC MACHINEGOD HELP AMERICA!“Freedom is never more than one generation away from extinction.We didn’t pass it to our children in the bloodstream".RONALD REAGAN
Reply: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.Lincoln, ESAB, Thermal Dynamics, Victor, Miller, Dewalt, Makita, Kalamzoo.  Hand tools, power tools, welding and cutting tools.