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Does watching YT scribing make you cringe?
- Thread starter graham-xrf
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With that length it is getting into the realm of at what temperature is the measurement being taken. It may be able to measure to .001 but I bet it shrinks or grows more than that with temperature change.
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It does make you think about what it takes to get a correct length on anything longer than about a metre.
The 12" steel rule I have says "Standard at 20°C". Suppose it's a bit chilly in the workshop - say 8C, and you are measuring something the whole 12" long. The rule would be 12x(20-8)x11.7E-6) = 0.00112" too short. Kinda defeats the point of bothering to print the standard on the rule.
OK then - say it's freezing! The ruler is 0.0028" too short. Could you see that on a ruler?
Said Starrett vernier might be "Stainless - Hardened" like all the best eBay Chinesium, which has a temperature coefficient that varies some, depending the grade, but if it were 316 with coefficient 16E-6/°C, then a 60 inches measure when it is totally freezing is 0.0192" too short!
In theory, you have to do some arithmetic when you use these things. Also, I don't even know if 316 can be hardened!
So what do you do when the thing is big? Say 7.3m (24ft)? You resort to photogrammetry, where you cover the thing in hundreds of little circle decals that cost about 15c apiece and there are invar bars and bar-code stickers. You take photos with a camera that has had it's imaging sensor "calibrated" so that image software can do "pixel trigonometry" between every pattern recognized centre.
(You have to zoom it).
All the kit plus software can be had for a price comparable to the 72" vernier, but then again, it's not made by Starrett.
Surely the advert is a mistaaike! Anyway - it's too unwieldy to scribe anything except maybe set out a circle for a garden pond!
[Edit - The Batman reference from @Lo-Fi is acknowledged - I love it! ]
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I don't think the price on the 72" caliper is a misprint. I found it on many websites for 14K and this is one of the "best" prices I found.
Amazon.com
www.amazon.com
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- Aug 15, 2013
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Unreasonable!
I recently bought a old Starrett depth gauge for USD equivalent about $14.00 + $7.00 postage. Cleaned up, it is still dead accurate. It looks very well made. So who is still buying Starrett new now?
Maybe a 72" long thing has such a small market, it has to be made up as a special.
Even so - near 15 grand for a caliper is not going to encourage a bigger market.
Even a home hobby project to "extend" a caliper can beat that, and why bother when for small fraction of that ridiculous price, one can buy a glass linear scale, and 1 micron resolution DRO with all the trimmings.
I recently bought a old Starrett depth gauge for USD equivalent about $14.00 + $7.00 postage. Cleaned up, it is still dead accurate. It looks very well made. So who is still buying Starrett new now?
Maybe a 72" long thing has such a small market, it has to be made up as a special.
Even so - near 15 grand for a caliper is not going to encourage a bigger market.
Even a home hobby project to "extend" a caliper can beat that, and why bother when for small fraction of that ridiculous price, one can buy a glass linear scale, and 1 micron resolution DRO with all the trimmings.
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I think a good shop project would be to convert a digital caliper to a purpose built marker with a mini ball bearing and a carbide scribe. If you follow the design I posted before it would be fairly simple. Move the scribe against the bearing, hit zero and measure. I don't see a digital one available. I don't like the vernier marking on mine at all.
Robert
Robert
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They are on eBay - quite expensive for what they are. One kind with the little wheel that woodworkers use, and the other looks like a sciber.
uk eBay links, but I am sure you can find similar in USA
--> Wheel marker - eBay
--> Digital Scriber - eBay