[How do I?] Measuring Hole Spacing Over Irregular Surface

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Hermit
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I need a tool to measure holes or other features over irregular surfaces. Measured distance is frequently over 24 inches but usually less the 48 inches. Surface irregularities could be around 3" tall above the feature I want to measure.
An example would be an exhaust manifold that can't be removed, but I need to pick up the mounting stud centers. Precision need to be somewhere between .005 and .015 inches. The only tool I could come up with would be trammel points like starrett 50A then transfer to a precision rule.
Is there a better solution?
 
Trammel points are what I thought of, reading your requirements. Instead of measuring to a guessed at center, measure to the left edge, or right edge, or some other common, and available point.
 
Trammel points would be a good choice. However, I think it would be optimistic to expect to be able to measure to .005". To begin with, it would be difficult to see both points at the same time. IOf you are looking to measure center to center of stud locations and need the stated accuracy, you might consider making a jig to do so. A rigid bar could have extensions to accept the studs. The bar could be a length of threaded rod with jam nuts on either side or a set screw could be added to either leg. To use, position the legs on the studs and adjust for a good fit and lock the legs in place. Remove the assembly and measure the spacing or use to transfer to your work. If multiple stud diameters are needed, a set of bushings could be added to provide a tight fit to the studs.
Trammel Jig.JPG
 
Make tips for your trammel point that has a cone end, to self center in the bolt hole. Imagine a pipe center in a piece of tubing on the lathe. If the cones have sharp points you should be able to read to .005" by eye. Of course you need an accurate long scale and good eyes.
 
I like Andres idea!
If you can remove the bolts you might be able to center punch the holes and then pick up the punch holes by feel.
You can also make long extensions which will clamp to a vernier caliper. Slots in the extensions would fit attach with set screws.
Or, how about a threaded rod with the radius milled off and screwed in to the bolt holes. Check to ensure they are straight.

Daryl
MN
 
You could use the ball ends on the trammels they are used for that type of measuring . Should be a way to use one of those laser hand measure with your trammel set up.
 
Insert gauge pins in the holes and measure them at the same height....
 
Thanks guys. The header is just an example. Sometimes it's holes or edges, but usually not something "2d" the lends well to direct measure. Looks like I'll be shopping for some points.
 
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