Locating already drilled holes

Someone asked how to find the center of an already existing hole?

If the hole is small enough, you can use the pointy end of an
edge/center finder in the way described in my earlier post(with punch
marks).

If you're not needing a lot of accuracy, you can turn an appropriate
"bullnose" and use the quill lock to hold it tightly in the existing
hole, while you tighten things down.

If you DO need accuracy, then you can try to find the fattest part of
the hole (described by another person, earlier), or..

When a right angle is inscribed in a circle (vertex on the circle,
legs extending to, or through it) the intersection of the "legs" of
the angle with the circle will be the DIAMETER of the circle. Since
half way along this diameter is the circle center, we can use trig to
find the hole center.(our unlocated hole, remember)

We can use our mill axis travels to create the 90 degree angle we
need.

So fasten the part to the mill table. No need to get the mill to any
known position. Now place a center/edge finder(pointy end or wiggler
into the existing hole and clamp the quill to preserve the Z
location. It is best to work as near to the point as possible to get
accuracy(in other words, only go into the hole as far as necessary to
get a deflection of the point).

Now the fun stuff.

Turn(jog) EITHER axis(x/y) EITHER DIRECTION (it doesn't matter!)
until you "indicate"(means when you observe the deflection of the
point)the edge of the hole. Reset the counters/handwheels to Zero.

Now jog "across" the hole with x or y (still doesn't matter) until
the second point of contact with the circle(hole) is indicated.
Strangely enough, we don't care what the readings are here, they just
get us to the next ones(the ones we need).

Next, jog "across" the hole 90 degrees from the direction we just came
(if you used x to get to the second point, use y now: If you used y
to get to the second point, use x now.) When you "indicate" the third
point of contact with the hole, write the co-ordinates down and get
out a calculator.

The first point and third point are ON the DIAMETER of the hole!
(error will be minimal if you were very close to the tip of the
"center finder" while indicating.)

The hole center is the MIDPOINT between the coords of point 1(which
we set to 0,0)and the coords of the third point(which you have
written down). Now if we just AVERAGE the X coords, and AVERAGE the Y
coords, we'll have the coords of the CENTER of the hole!(this
"average-ing" is called the midpoint formula and looks like this:

(X1+X2)/2=Xmidpoint (Y1+Y2)/2=Ymidpoint

Since we "zeroed out" on the 1st point we only have to divide the
point3 values in half and we're done! Pretty cool.

So there you have a way to find the center of a hole already drilled.
Some things to watch for:

1)non-round holes.
2)chamfers or burrs at the top of the hole where we're indicating.
3)The plane of the hole is not perpendicular to Z.
4)errors will creep in if you're not AT the pointiest point of the
indicator. CMM's(co-ordinate measuring machines do the same thing,
but use additional math to account for the diameter of the probe tip.
You can do the same thing and use a .200 edge finder, but the method
above, done carefully will really be "enough accurate" for most.

Hope this helps.

Ballendo