How Do I Find the Center of a hole using an edge finder?

[JWB_Machining]

Heyo,

So I've been googling trying to find out how to find the center of a hole using the old school edge finders, the type with a lower portion attached via a spring. I know to find an edge that when I touch up against the edge the lower portion will "kick" to one side. How do I use the conical end of my edge finder to locate the center of a hole? Does the conical end also kick in some manner or is there no way to do it with this type of edge finder? Unfortunately my probe isn't calibrated and the batteries for my electronic edge finder don't come in till next week, I also don't have a dial indicator that can be mounted in my spindle.

[307startup]

if your machine has a DRO function on it, you jog the X axis to one side til you see the edge finder wiggle, then back off in opposite direction til it stops...note location from there...do the same in Y axis...

measure the diameter of the hole, diameter of the edge finder, subtract, divide by two...and adjust the noted DRO locations for X & Y by this amount to arrive at your newly found center

[307startup]

if it's a small hole and you are using the conical type, you use the method in my previous post, but without the math...when it stops wiggling in X & Y, you are centered. Note DRO positions and adjust your program from there....

[MrWild]

use the finger with the .100 or .05 disk. Touch a wall and watch it deflect. Set "0". Move to other side of circle and watch it deflect. Read the amount. Divide by 2 and move this distance. Set "0" for that axis moved. Repeat for the other axis. Double check the first axis. Done.

For holes under .5, I have a 1/2 inch dowel machined to a point. I then use the hand wheels and lower the Z into the hole and "feel" the restriction as I find center and hit an edge. This gets me within .003 which is usually good enough for bolt holes.

For precision, I always use an indicator.

[307startup]

Mr. Wild...on an outside edge, having the finder wiggle is the acceptable practice for setting zero.
On any internal edge, you make it wiggle, then back off til it stops. This is due to the fact that two objects cannot occupy the same space at the same time.
If it wiggles, you are indeed trying to occupy the same space at the same time.
The center point of a hole is more accurately located by backing off the wiggler than if going by the point at which it does wiggle.
Remember, when you are setting a datum for an outside edge, you are not finding the inverse location of two intersecting lines, as you are with the center of a hole.

[Konrad]

Not sure why you want to use a edge finder...on a round hole I always use a indicator.

Konrad

[Mcgyver]

Originally Posted by JWB_Machining Heyo, I also don't have a dial indicator that can be mounted in my spindle.

but you have an indicator? its not too tough to fashion something - have you a pic of the indicator mounting? being able to mount an indicator in the spindle is fundamental to mill work

as the guys have said, an inside or outside surface is quickly centred using the edge finder....guess at the middle, pic up an edge, go to where the centre should be - go past and back again for no backlash, no go at the other axis and repeat. called the osborne maneuver by lautard. i always do three iterations but the last is just a check, its as accurate as edge finders can be....when it matter put a tenths indicator on it

[Thrasher]

If I were you.... I would make an investment in a sweep gauge. May save you a lot of problems. And get rid of the electronic edge finder. Not very accurate!! Nothing beats a .0001" dial or digital indicator.

[aaron p]

Indicol test indicator holder and a nice 0-15-0 test indicator in .0005" readings. Best 2 tools any machinist can own. I use mine daily at work.
The adjustable Indicol holder is best so that you can pre-load the indicator. The more round holes you indicate, the faster you get at it. I remember 3 years ago when I 1st was learning how to indicate a round object, it took me a few minutes, now its a 30second deal.

Much more accurate than a conical edge finder.

[JWB_Machining]

Thanks for all the help guys, I ended up just waiting till the batteries came in and used my electronic edge finder. Thrasher you said the electronic edge finder won't be very accurate, do you know how accurate they are off the top of your head? I'm just curious.