122-inch caliper - photos

122-inch caliper.

Fullsize image: https://diqn32j8nouaz.cloudfront.net...r_fullsize.jpg
https://diqn32j8nouaz.cloudfront.net...ch_caliper.jpg

https://diqn32j8nouaz.cloudfront.net...1_fullsize.jpg


Previously:

https://www.homemadetools.net/forum/...694#post220077
https://www.homemadetools.net/forum/...661#post190008
https://www.homemadetools.net/forum/...684#post147799
https://www.homemadetools.net/forum/...343#post122020
https://www.homemadetools.net/forum/...870#post167707

Sadly, S-T doesn't appear to make these anymore. A slightly smaller import is available for $3900 plus probably ridiculous shipping.

coefficient of expansion for steel = 6.45E-7

so for 122 inches we have

6.45E-7 * 122 = 0.000787 inch expansion per degF

In a room big enough to contain an object large enough to need a 122" caliper, plus the equipment/assistants to hold the caliper, I doubt you can maintain the temperature of every thing to 10 degF which indicates an expansion of 0.000787 * 10 =~ 0.008"

What were they measuring that such accuracy would be acceptable?

Today we would use a laser but the (presumably steel) would still be changing size with temperature.

Quote:

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Originally Posted by mklotz

coefficient of expansion for steel = 6.45E-7

so for 122 inches we have

6.45E-7 * 122 = 0.000787 inch expansion per degF

In a room big enough to contain an object large enough to need a 122" caliper, plus the equipment/assistants to hold the caliper, I doubt you can maintain the temperature of every thing to 10 degF which indicates an expansion of 0.000787 * 10 =~ 0.008"

What were they measuring that such accuracy would be acceptable?

Today we would use a laser but the (presumably steel) would still be changing size with temperature.

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If the work piece and measuring tool have the same or similar coefficient of expansion, would the measurement be off by that much? Assuming both were given time to adjust to the temperature and the designer was competent enough to take it into consideration. That's a lot to assume, of course. Not questioning your math at all. Just genuinely curious about your thoughts on the subject.

Quote:

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Originally Posted by ductape

If the work piece and measuring tool have the same or similar coefficient of expansion, would the measurement be off by that much? Assuming both were given time to adjust to the temperature and the designer was competent enough to take it into consideration. That's a lot to assume, of course. Not questioning your math at all. Just genuinely curious about your thoughts on the subject.

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Unless you can get everything - room, tool, object - to the same temperature as existed when the calibrations were applied to the tool, your measurement could be off by as much as thousandths of an inch. That might be problematic. If you're trying to hold a succession of parts to the same size (and absolute size is not so important) than, assuming isothermal conditions are maintained, the calipers are useful. For absolute measurements, you can never be sure of your measurement accuracy.

My point was that you never really know the length of your yardstick. That fact has to be taken into account in your use of the tool.

Given that we now have high accuracy laser interferometer tools, I doubt that this tool has much of a market.