Ways to measure holes center to center?

My understanding is that you were just looking to bore the two overarm holes. If there are three, that complicates things a bit, but the approach is the same.
 
Perhaps I don't fully understand the issue. Problem: making overarm support needing holes to be correctly located respective to each other. How to measure the distance between the center of each hole to match the mill in question.

quick and dirty: the round overarm has a center drilled into it from the factory. Place cutter head in horizontal position, using a small collet or something similar chuck up a small pointy center in the cutter head. Slide the over arm to be on the same plane as the end of the pointy thing in the cutter head. Measure between the center of the overarm and the center of the cutter head. Will this provide the level of accuracy needed to layout the center of each hole and then drill then bore the large hole on the overarm support for the overarm side? Mount the overarm support on the overarm and then drill/bore the arbor/cutter head side in place.

I know that I am missing something, please enlighten me 'cause I'm confused, not an unusual situation
 
My intent on this entire thread is who to measure the center of holes. This might be a daunting task for some of us. It wasn't that long ago when I would have been overwhelmed by the problem.
I did not intend this thread to be a conversation about VN overarms. However, if we find a VN which this might fit. Then GREAT!!

The third hole is the broken site glass for the integral overarm oil reservoir.

Tony Wells, That an interesting idea!
I've been refusing to "upgrade" to DRO. I'm also using a cast surface plate. etc... This is my eccentricity. I don't necessarily recommend it...
I'm guessing I could do the same thing with handwheels. Would have to watch for backlash.

Reeltor,
Perhaps I communicated poorly. As I understand the process you are exactly correct.
This overarm is only valuable as a model on which to base a pattern prior to pouring an aluminum or cast iron copy.
It is my thought that we can bore the support arm and the arbor needs to be marked and bored on the machine for which it will be used.

Again, if we find accidentally find someone who might have an overarm with a center-center distance of 9.150 then a happy dance might result.

Brockwood,
I got 9.150 with the Sorenson. I only took one measurement.
I want to do multiple measurements on both sides of both the A & B and watch for repeatability.
Then it's my intent to build a pattern of these A and B overarms.

Daryl
MN
 
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I might be inclined to machine plugs out of some scrap material and leave a 1" diameter stub on the plugs. Put plugs in bores, measure outside to outside of the stubs and subtract 1".
Result is your center to center distance.
 
For making castings, just leave enough room for the overarm and arbor rough holes in your pattern, and the usual over sizing. Don't forget the draft, on both holes and outside dimensions. Then machine the casting to fit. There are plenty of foundries around for doing good iron castings. If you mistakenly cut the holes oversize or in the wrong places, well, that is why bushings were invented!
 
This uses a vernier caliper and some telescoping gages.
Gotta do a little math. Not difficult.
The vernier tips do not have a not radius therefore I'm not sure that it is as accurate as I'd like.

Daryl
MN

View attachment 235572

If you don't like the square external measuring edges on the vernier, use the ID side and measure the distance between the far sides of the holes (I believe those sides have a radius on them anyways). Using this technique, is most likely the method I would use to determine the center to center distance.

Ted
 
View attachment 235569 View attachment 235570

This Sorenson Center Mike is my go-to center to center mic. (Yes, they call it a "Mike").
It somehow calculates the actual centers of the holes.
Note the directions.

Daryl
MN
Daryl,

Thanks for the tip. Just bought a 13" Sorenson off eBay for $50. Will be much nicer than using those slotted adapters that screw to vernier caliper arms.

Bruce
 
I might be inclined to machine plugs out of some scrap material and leave a 1" diameter stub on the plugs. Put plugs in bores, measure outside to outside of the stubs and subtract 1".
Result is your center to center distance.

Or any size dowel pin or drill rod etc you might have.
However, fitting the plugs might be problematic as it could introduce error (free running, slip fit, press fit).
Options are great!!

Daryl
MN
 
BGHansen,
I've been facinated that it does the calculations without electronics!!

Daryl
MN
Daryl,

There is a "quaintness" to old school. Though I did just break out the surface plate and a digital height gauge to scribe some lines on the horizontal center. Have to admit to finding the center of a 1.375" round mounted in a V-block was easier "going digital"; zero out on the top of the round, then come down 0.6875". Looking forward to trying out the Sorenson.

Bruce
 
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