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I am involved right now in the fabrication of steel structures on the order of 5-50 feet long. Presently, we measure these structures with garden variety commercial tape measures. Most of the time, we work to a tolerance of about +/-1/16" and don't have any problems. Recently, we're trying to make some items to a very high tolerance (at least +/- 1/32", approaching 1/64" or .016") This tolerance is due to visual not mechanical criteria, but it is still very important to our management.

My question is, how can we reliably measure these sorts of distances with that level of precision? I'm prepared to order some NIST traceable tape measures, but it's not clear to me if they'll really improve the situation. Are there other technologies or techniques that could practically be applied in a fabrication setting? Are any surveying tools accurate enough to solve the problem? Cost is obviously a factor, but we're more concerned with repeatability and robustness than price.

I realize this tolerance will sound ridiculous to most, but I imagine that some other industries may have to perform similarly, perhaps large engines for ships or power plant components?

dcorking
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Ethan48
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3 Answers3

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Even the best metal tape measure is susceptible to significant thermal expansion over large distances.

Try a laser measurement device ('electronic tape measure') instead: http://www.engineersupply.com/Laser-Measurers.aspx

The laser distance measure, flat plates clamped to the object, and some shims of known thickness, should be all you need to precisely locate something over a large distance.

user2790167
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To get that sort of accuracy over that scale is not trivial and probably won't be cheap.

For smaller size up to a few meters a portable CMM would be an option (here's an example). These have accuracy on the order of 10 $\mu$m and are used for things like high end/F1 car manufacture.

However, CMM type instruments wouldn't be useful for anything larger than a few meters as it is limited by the arm length and must be fixed in place when measuring to get a sensible result. For larger pieces the best performing option would be so sort of laser tracker (example). These also have pretty good repeatability (~20$\mu$m).

The versions I've seen are phase shifting interferometers, which fire a laser onto a retro-reflector in a metal ball which is placed on the surface. They are used by people like Boeing/Airbus to check tolerances on plane manufacture.

These aren't cheap the laser trackers are approximately $80,000. Conventional surveying laser rangefinders are much cheaper but I'm not sure they would have suitable accuracy.

nivag
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For high accuracy over long distances it's typical to use general surveying techniques. You use a total station (like this) which will get you 1.5mm accuracy in a single shot. They're no laser tape measure. Repeated set ups /readings with some statistical corrections should get you easily below 1mm. Note too that these are immune to thermal expansion of any tape, but of course the subject will be. This too can be allowed for though with come mathematical compensation.

The good thing is that you don't have to buy a total station. You can rent it for a few days. Better still (contingent on budget) is to get a survey guy in to do it for you. This may be the way to go considering the experience required to use this kind of kit. It'll take a beginner a day just to stand it up accurately over a mark. I think that the accuracy you're asking for can be achieved at reasonable cost.

Paul Uszak
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