# centering a hole with a drill press

I have a drill press, and I need to center a hole in the face of a cylinder. I would be perfectly happy to one hundredth of an inch
accuracy. An X-Y table attached to the drill press stage would do the job, but I suspect that for the level of accuracy that I need, there is a simpler solution--and I just don't know it.
I've tried scribing diameters across the face, and picking where they intersect--but the results have been less than impressive. Does someone have a trick for this?
Clayton E. Cramer snipped-for-privacy@claytoncramer.com
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What kinds of tools do you have?
What I would do is this. Measure a diameter. This you can do precisely (like with a micrometer or good caliper). Take a steel plate with flat side (think about a ruler if the cylinder's diameter is small enough). Clamp a C clamp with a round leg to it so that the distance from the edge of the plate, to the point of the leg nearest to the edge, is exactly half the diameter. Again, a caliper would be your great friend.
Now you have a tool for drawing diameters on the circular side of the cylinder. All you have to to is put the flat piece on the face of the cylinder, so that the leg of the C clamp touches the side of the cylinder, and draw a line with a hard needle of some sort. That would be a diameter of the circle. Draw another diameter at close to 90 degree angle to the old diameter (exactly 90 degrees is not necessary). The point of the intersection of the two diameters is the center.
You may be fine with the approximation that the intersection is the center.
If not...
If you draw four of such diameters, they would, in the real world, due to measurement errors, define not one point, but a tiny quadrangle, vanishingly small. Its center is the true center.
i
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What did you use for positioning the diameters? A V centering head on a combination square should do a pretty good job, as long as your scribe has a very skinny point, and you angle it so the point is right along the edge of the scale.
Then, you need a spotting drill and good magnification to get the hole started as close to the intersection of the lines as possible. Once that is done, you can move up to a larger bit. But 0.010" may be pushing it if you don't have experience in doing this.
Better, of course, would be a lathe of sufficient size with a 4-jaw chuck, and a good indicator, and a center drill in the tailstock chuck once you get the end running true.
Good Luck,         DoN.
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Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
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DoN. Nichols wrote:

Here's a very simple, but surprisingly accurate method. Place a small pointed rod in the drill chuck, a countersink bit will do at a pinch as long as it comes to a point. Place the bar in position as close as you can by eye and lay a flat metal strip horizontally across it - a 12" steel rule will do nicely. Lower the drill chuck so that the pointed end of the rod gently pinches the rule between it and the bar. The chances are that this pushes the rule off the horizontal because the point is not on the highest part of the bar. Move the bar so as to get the rule as horizontal as you can get it. Replace the pointed rod with a slocome and start the hole.
If you do the maths you will find that the rule being off-horizontal by 1/2 degree translates into an accuracy of approx. 0.004 x the diameter of the bar. Half a degree from horizontal is very easy to spot when you compare it to other horizontal things around, so this method can easily give an accuracy better than 2 thou on an inch diam. bar. I hope this helps.
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Regards, Gary Wooding
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Just use a dial indicator in the chuck and indicate around the periphery of the shaft, once it's all dialed in you will be bang on the center.
Any type of dial indicator will work, just detach the post form the mag base and put it in the drill chuck, if you have a scrap one, chop it down if there is a height restriction.
-S
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Wooding wrote:

To avoid confusing the poor OP, I gotta stick my .02 in and point out that you've just given him a good way of drilling a hole through the SIDE of a round bar so that it comes out close to being exactly on a diameter of the bar.
But the OP said he wanted to drill the hole in the center of a "face" on the bar, which most machinists would read as being the "round" end of the bar.
Jeff
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Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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Jeff Wisnia wrote:

Whoops, mea culpa - I must learn to read posts properly. I must learn to read posts properly. I must learn to read posts properly. I must learn to read posts properly.
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Regards, Gary Wooding
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I was a little confused by his post, to me the "face" of a cylinder is the end of the bar, and the cylinder itself would be to put a hole through the periphery of the cylinder. Perhaps i have been machining too long for laymans terms.
-S
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Depends on what tools you have available. Assuming you don't have things as surface tables and height gages, or even a good caliper.......... I would use a divider or compass. Just set it at your best guess at half the diameter, and scribe arcs from the outside. If the arcs from opposite sides don't intersect, increase the size of the arc. If three or four arcs define an area that is too big, decrease the size of the arc. Should be able to get to your 1/100th of an inch, but not much closer.
Dan