How to find exact center of a drilled hole???

[Brad Tallis]

Hello All-

I have a bit of a dilemma. I used my circle cutter in my drill press to create a 5 inch diameter circle. The circle cutter has a drill bit in it that drills through the center as it cuts the outside diameter.

Ok. So, now I have a disk the right diameter and I want to drill a 1 inch hole through the center. And I mean the "exact" center. However, the drill bit removed the "center" of the disk, and even with me lining up the forstner bit tip into what I thought was the center, I was too far off.

Due to the fact that the throw of my drill press isn't all that great, I have to lower the drill press table to get the circle cutter out to put the forstner bit in. So, I can't leave the disk in place to redrill the 1 inch hole.

Any ideas on what I can do to find the exact center of a hole? :confused:

Thanks-

Brad
http://home.comcast.net/%7Estarjumper/images/holes.jpg

[Doug Shepard]

Put a drill bit in your DP the same diam. as the one on your circle cutter (or just use the circle cutter itself). Lower the bit into the existing hole then clamp it down to the DP table so it cant move. Then change over to the larger bit and drill the bigger hole.

[Aaron Koehl]

Measure the diameter of your larger circle.
Subtract the diameter of your forstner.
Divide that number in half.

This number will be the distance from the edge of the circle to the edge of the forstner bit. Simply measure a few times and adjust the disk, and you're there. (North, South, East, and West)

------------------

So, 5" circle, 1" Forstner.
Measure the bottom and right edge, they should be 2". Then you're centered (assuming everything's round!)

[Randy Meijer]

Doug: If I understand the problem correctrly. your solution won't work because the table has to be lowered to change drill bits and that will cause the alignment to change even if the work is clamped to the table???

Drill the 1" hole first; but don't drill it all the way through the board. Then use the circle cutter to cut out the 5" circle centering on the center dimple left by the Forstner bit. Then finish drilling the 1" hole.

Or if you want to save the existing disk, glue/tape a piece of paper, cardboard or very thin piece of wood over the existing hole and lay out the center using a compass. It may be difficult to do this precisely enough on the existing disk so the alternative would to be to draw a 5" circle on a piece of paper or cardboard with a compass and then glue the paper to the existing disk.

[Ed Lang]

My typing was too slow. But I will leave the post anyway.

Put the larger bit in the DP FIRST. Then drill about 1/4 the way into the material. Then install the circle cutter and cut your hole using the point in the center. then you can reinstall the larger bit and use the outside of the hole, 1/4 into the material to line up the bit and continue to drill into the material. On my circle cutter I can move the cutter part up and down so you can still get the center pilot bit into the material before the outside cutter starts to cut.

I hope this helps.

[Doug Shepard]

Doug: If I understand the problem correctrly. your solution won't work because the table has to be lowered to change drill bits and that will cause the alignment to change even if the work is clamped to the table???



DOH!
Randy, you just might be right. I keep forgetting that I'm probably one of the few that's using an old antique dinosaur DP. That technique works on mine, but the whole table moves up/down on a giant dovetail way adjusted by turning a big old 8-10" handwheel. There's no table tilt mechanism either, so nothing moves out of alignment by raising or lowering the table. It's a monster DP retired out of an auto parts factory in the 60's. Gotta remember that the next time I weigh in with DP advice.

There's pics of the beast on this thread
http://www.sawmillcreek.org/showpost.php?p=185091&postcount=3
You can see the big dovetail way sticking up about an inch above the fence on the first pic.

[Greg Mann]

Use 2 stops to create a nest for the disc such that you can remove it and replace it exactly. That way you can get the part out of the way to facilitate tool changes, replace the part to its exact location, and finish.


Greg

[Brad Tallis]

Hello-

I knew you all would come up with a better idea than I did for this problem. I think the "drilling with the forstner bit partway through first" idea will work just right for me.

I'll give it a try this weekend and let you know how it worked.

Thanks again. :)

Brad

[Steve Wargo]

Lodge a square piece of stock into the hole and remark the center. Then the forstner will have a place to grab and guide properly. Easiest way I can think of.

[Bill Neely]

Use a 1" hole saw with the same size pilot as the original.

[Dev Emch]

This is a hard problem in the woodshop but cake in the machine shop. The reason its cake in the machine shop is that milling machines have a precise X axis which I can read off the indicator. Woodshops, i.e. drill presses do not.

If you have a X-Y slide for your drill press from say harbor freight, then this job becomes cake. If not, we gotta improvise. Load up your fostner bit and check its outside diameter. Mark it down.

Now, take a scrap of ply or MDF and place it into the drill press. Push the bit down to make a pip mark. Do not have the DP running! Now using a straight edge, mark a line through the pip. Using a drafting square and checking for exact square, repeat the line for the other axis. Be as accurate as you can.

Now take the diameter of your disc and divide by two. Using your axis grid, mark out an outer line on one side of the ply this distance from center. Repeat for the other axis line. These two parallel lines represent your fence locations.

Lastly, attach two fences along these two parallel lines with screws, nails or chewing gum. Whatever you like to do when doing this.

Lastly, position this ply into the drill press with the motor off. Lower your fostner bit such that the pip lines up again and lock the quill for now. Attach the ply to your drill press using clamps.

Now, everytime you place a disk into this setup, the disk will self center to the axis of the drill press.

As i said, this is the hard way. If I were doing this, I would do it this way.

Put an edge finder into the drill press and attach an X-Y drill press sub table to the drill press. Clamp up the disk to be drilled. Turn on the drill press and position the X axis such that the edge finder indicates. Lift the drill press quill, figure your movement and then crank the table over. Repeat this for the Y axis. Replace the edge finder with a fostner bit and have at it. Note that this assumes that I have dials on my X and Y axis on the drill press sub table. Note that not all drill press X-Y tables have this feature. You can still use these but you need to count the revolutions and you need to know how much movement you get per revolution of the axis lead screw.

This is a milling machine procedure that is done all the time in the machine shops. You can get edge finders from all sorts of places including MSC where they sell for $10 to $35 dollars or so. The imports are even cheaper and its a nice gizmo to have for placing your work item directly under the spindle of a milling machine or drill press. The X-Y tables are available from many sources many of which are from overseas. Tables from harbor freight for example are more than accurate enough for wood shops. If you pursue a highly accurate machine shop table with milling machine accuracy, you will spend a small fortune for one of these!!!!!