wrote in message
...
I'm locating the center of some holes .500" dia with a Starrett edge
finder.
The tip of the edge finder is .200. I'm just not sure if I'm doing it the
BEST way.
Maybe I'm missing something. Basically, I lower the tip of the edge finder
just inside
the hole. I then move the X axis until It touches. I then move the Y axis
until it's
not touching. Then back to the X until it touches. I keep this up until
rocking the Y axis
no longer provides any clearance. Then I move the X in .150 and zero the
DRO.
Then I repeat this process again for the Y axis. Basically, I'm finding
the
tangent of the 2 axis.

Is this the way it's normally done? This takes me a long time
(inexperienced)
The only instructions I've found on line, tell me how to locate a straight
edge.

Any PRO suggestions appreciated.

RonL

You've got to be bloody joking. That's the most half arsed way of finding a
centre I've ever heard of. The geometry of chords says you'll be miles out
because it takes a large movement in the X plane to make a small one in the
Y plane as you approach the true diameter of a circle. I know I've explained
that extremely badly but I know what I mean.

Do yourself a favour. Turn a couple of short lengths of 1/2" bar. Stick one
in the hole (it must be a tightish fit) and one in the chuck. Bring the two
into line and eyeball them up until they are closely aligned. Put a straight
edge across the two and you can see a gap of less than a thou. Move the bed
until there is no gap in either plane. You are now aligned over the hole.

Option 2. Stick a tight fitting 1/2" bar into the hole and fix a dial gauge
to the milling head. Clock the bar up until you're concentric.

Option 3. If you insist on using an edge finder (horrible bloody things at
the best of times) eyeball it up until it's fairly central in the hole. Move
X right until it touches and zero the DRO. Move X left until it touches and
note the reading. Split the difference. Move back to the centre and repeat
for the Y axis.

Option 4. Stick a length of 1/2" bar in the chuck. Mount a dial gauge on the
bed. Move the bed until the 1/2" bar goes in and out of the hole when you
move the quill down. Touch the dial gauge against the bar. When the bar is
perfectly centred the dial gauge won't move as the bar moves into the hole.
If you're off centre the bar will still go in by forcing itself a bit one
way or the other but the dial gauge will show up the error. Jiggle until
you're centred in both planes.

There's a bunch of other methods, any of which is better than the one you've
been using which is close to useless.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)