Determining the center of a small hole?

[Frank O]

I have a circuit board that has a hole about 0.140" in diameter. I need to enlarge it fairly precisely to about 0.240". Getting it in the same location (within reasonably achievable error) is important.

I figured I'd do the drilling on the mill. Any advice on the best device/technique to find the center of a hole this size? I've been looking at online videos that use edge finders, wigglers, etc, but they all involve much larger holes.

 

[ErichKeane]

For particularly small holes, the pointed edge finders are really good. You get the point as close as you can, and then iteratively 'lower' it until it touches a side of the hole (and cams out like an edge finder). Adjust it so it no longer touches a side, then lower it a little more.

 

[Frank O]

Thanks, pretty sure I have one of those, and I figured it was probably the device to use. I was just having trouble finding a video or description of how it's used. This should get me started.

 

[chips&more]

I would do it with a tapered reamer typically used in clock repair.

 

[ErichKeane]

I was thinking, a circuit board might be soft enough that spinning it could damage the tool! You might find that you can do it without running, and just feel on the side of the edge finder when it is off-centered.

 

[gradient]

Yeah, the non-spinning pointed edge finder is just the thing to use. I use it all of the time and can get +/- a couple tenths of a thou centering with it. Good enough for most of what I do.

 

[Frank O]

Yes, I was thinking that it could damage the board material. I just finished doing it (found my Mitutoyo 050103 pointed edge finder), and going at a slow speed seemed to work out fine.

Re the tapered reamer ... how would you use that?

 

[ch2co]

This is what I use. Pointed end wiggler.

 

[bkobkobko]

Put a 0.140 pin (or drill bit if you don't have a pin) and use an indicator in the spindle to center up on it.

 

[hman]

+1 on the pointed end "wiggler" that ch2co {formaldehyde?] suggested. My technique is to lower the point all the way into the hole, then minimize/zero out the offset in X and Y. Feeling the transition between the shank of the wiggler and the full diameter section of the point, it's very easy to detect any offset while adjusting the X and Y axes of the mill table. You can easily get it to better than 0.001" by feel.