Hole centering aid

[mklotz]

Like many HSMs I don't have the headroom on my mill to use a coax indicator to center holes under the quill.

If the job doesn't require the accuracy of the Zero-It indicator, I'll often just use the pointy end of the edge finder (shown at top of picture) to center the hole. Unfortunately, the larger end of the conical tip is only 0.3" diameter so one is out of luck for holes larger than that.

If the holes happen to be aliquot sizes, a set of nesting drill bushings (shown at bottom) can help but, if the hole is slightly over/under size, the fit can be sloppy.

I made these two devices to deal with the problem. Each consists of a slug of steel, accurately center-drilled at one end and having a conical taper at the other. Each has a companion close-fitting ring.

In use, the cone is inserted into the hole and the ring is pressed down on the stock to ensure that the conical plunger is held vertically. Then the cone on the edge finder is inserted into the center-drilled hole.

No, it's not as accurate as using a DTI in the Zero-It but it's more than accurate enough for 90% of the jobs I do.

They're simple to make. Try one.

[kbalch]

Thanks mklotz! I've added your Hole Centering Aid to our Measuring and Marking category, as well as to your builder page: mklotz's Homemade Tools. Your receipt:

Hole Centering Aid by mklotz

tags: centering, hole

[Toolmaker51]

Centering Aids, never thought of using those for clocking, but it works with transfer punches just as well.

[Toolguy]

You could just use the edge finder to get x and y center of the hole within part of a thousandth.

[mklotz]

You could just use the edge finder to get x and y center of the hole within part of a thousandth.

I've heard of using an edge finder to center a cylinder. Since, in general, you start off diameter, an iterative process is required. The Osborne maneuver, which I described here...

Osborne maneuver

is typical. However, I've never heard of using an edge finder inside a hole and would want a proof of the technique.

Can you describe the method you would use in more detail, please ?

[Toolguy]

Hi Marv. Just stick the straight end of the edge finder in the hole, center in X axis by eyeball. Move in Y axis to touch off the back of the hole (when the touch probe kicks off) and zero in Y. Move to the front side of the hole in Y axis and take a reading. Divide this number by 2. That will be the center of Y axis. Move to center of Y and rezero. Touch off the left side of the hole in X axis and zero X. Touch off the right side of the hole and see what that number is. Divide by 2. That is the center of X. Move to the center of X and rezero. You now are in the center of the hole. You don't have to know the hole diameter or do any math besides dividing by 2.

You can find the center of a pin the same way, just on the outside instead of the inside. Carefully approach the edge that the edge finder will touch to get an accurate result. The cylindrical part of the edge finder is .200. If the hole is too small for that to work, you can use the pointed end the same way. In actual use, this takes (me) about 30 seconds to do. My edge finders will repeat to a 1 or 2 tenths. You can check the position with a DTI in the zero-it.

[mklotz]

Yes, I opined that that was the technique you had in mind.

The problem I have is that initially you're not on a diameter of the hole so when you touch off, it's against a slanted surface. Same when you go to the other side. I'd want some assurance that you had actually measured the length of the chord before halving it to establish the middle.

The Osborne iteration solves this problem by iterating, each iteration bringing you closer to a diameter where you're touching off on a non-slanted surface.

There's also the problem of handling backlash. Many home shops lack DROs.

Before using your method, I would still want to see an accuracy check using a Zero-It or coaxial. I'm very pessimistic about achieving "part of a thousandth" centering error.

[Toolguy]

The great thing about it is that you don't have to measure the chord, or worry about it. If you're close to center in X, each end of the chord will be centered in Y. The slightly angled part isn't an issue. I've been doing it this way for a lot of years and checked it out with a dial indicator several times. It's always close enough for my jobs. Maybe it will work for you, too.

[Toolmaker51]

There is merit in both versions; Marv's drills into a decreasing error, I've used it and it works.
Toolguy is correct as well, though I don't use a wiggler or edge finder. This is especially good on rough or not very symmetrical material. I use an endmill shank, dowel pin, any diameter works, being a decent round and straight is sufficient, long as part fits within boundaries of material. A DRO makes it easy.
As described, touch one side and note position or zero axis. Retract and traverse to opposite side without disturbing cross axis, draw up to contact and divide result by 2. Do other axis the same way.
This often avoids surface plate & height gauge work, where locating a scribe line remains subject to interpretation.

Marv mentioned 'aliquot' (which wasn't in microsoft spell check, lol), I recognized term but took wrong definition being a certain portion instead of any portion of a whole. So, went to look, satisfied at not being too far off.
But there was a totally foreign usage too; of interest to musicians in the house, "Aliquot stringing, in stringed instruments, the use of strings which are not struck to make a note, but which resonate sympathetically with struck notes ..." from wikipedia.