Ways to measure holes center to center?

Uglydog

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In a separate thread Brockwood inquired how I intended to measure large holes center to center on a VN overarm support.
Well, I'm finally getting around to answering how I might do it.
But, more importantly while I'm presenting some options.
I suspect that there are many more ways to accurately measure.
I'm very interested in how many different ways we might get the job done.

Please submit your solution to the problem.

Daryl
MN
 
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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
 
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This device uses a dial tail indicator.
It's great for accurately comparing holes.
I've found it awkward for this application as lining up the gage blocks is a clumsy process for me .
Likely a better way to do this.

Daryl
MN
 
Daryl, I would first measure the overarms, outside to outside dimension, then inside to inside. Then do the math to determine the center distance. Then make a gauge the exact length you need and use it as a spacer gage for the stop dogs on the mill table, or just measure the distances between the stop dogs as you set them. Set up the first hole while against the first stop, then bore away! Repeat at the other stop.

Edit: for best results, use your jig borer... ;)
 
For lack of a Cordax (CMM), If my mill had a DRO I would sweep the holes and get the c-c dimension through a little trig if I didn't want to bother lining up one axis. That's one advantage to a CMM. It will establish an artificial axis independent of alignment with the machine axis and give a direct reading. One assumption is either required, or averaged into the calculations by the CMM, and that is the roundness of the holes. Holes, similar to planes, can be defined by any 3 points in a plane, and for every 3 points, there is only one circle that includes these three points. If the hole is not round, wherever the 3 points are taken is where the center of that circle is derived. Any points beyond the three simply make the infinitely thin theoretical line defining that circle "thicker" so as to include those additional points. Generally, the more points taken in an out of round hole, the better representation of the actual base circle will be developed and the center of that sort-of-round shape can be determined. Obvious defects such as gouges and intentional sawed expansion slots should be avoided when taking the series of points measurements.
 
Sweet! What width did you come up with?
 
Just a bit of quick input based on my sheetmetal layout experience. The first thing I see is a part with three holes, so my first thought or question is center to center or centers. Do the holes have the same axis? For center to center (if the holes are the same dia. and accurate) I measure outside to inside. If the holes are different diameters then I do the same and add a little math. Of course when the holes do not have the same axis there's just more math because we have center to center and centers to measure. When the part we are measuring is worn the challenge is determining what the hole diameters should be and then their location and if possible we would measure the shaft diameters and centers before the part is removed for repair. As Tony stated the DRO goes a long way in making this easier (not simple) to determine.
 
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