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From: Dave Baker
Subject: Re: Measuring devices
Date: 30 Jan 1999
Newsgroups: rec.crafts.metalworking

>From: (dan)
>On Fri, 29 Jan 1999 23:27:32 GMT, (phred3) wrote:
>>On Wed, 27 Jan 1999 09:43:58 -0500, Ted Edwards scribed:
>>Our Digital calipers have a temp range from 0-50c working. I imagine
>>most other digital tools follow that same range. Remember though,
>>temperature greatly affects measuring accuracy...
>HMmm... That's right, stuff shrinks as it gets colder. But.... If the stuff
>you want to measure is
>cold as well as the measuring tool...won't it  work out O.K.???
>Except perhaps if you were working with copper or other non ferrous metals.
>Anyone try experimenting on this??? Put the Mic' and a standard in the
>freezer and see what happens?

It really depends on whether the coefficient of expansion of the measuring
device and the item to be measured are the same. Most micrometers have a cast
iron frame with a coefficient of expansion of about 12 millionths per degree C.
Verniers and micrometer spindles are steel or stainless steel. Steel, copper
and bronze are in the 15 - 18 range and aluminium alloys from 18 to 23. As long
as both items are at the same temperature there is little effect on the
measurement accuracy but especially with aluminium I prefer to stabilize things
for 24 hours at room temperature when I am working to 10ths of a thou. It is
important to do the same with setting slips before calibrating mikes too.

As a guide to the effect of temperature, a 3 inch steel rod will expand about 1
thou every 20 degrees C. I certainly notice the effect of temperature between
winter and summer when I bore blocks and set piston clearances accurately.

Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

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