[Shop-talk] Micrometers

[David Scheidt]

On Thu, Oct 29, 2009 at 10:45 AM, J. F. Juhas <james.f.juhas at snet.net>wrote:

> I have a book on precision measuring instruments that describes how digital
> calipers work, and although I don't recall it clearly, I believe it
> suggested this technology is simple and inexpensive and so most bargain
> brands can be as good as the name brands.  I'll see if I can locate the
> portion of the book and send it to you tonight.
>

Yes.  No moving parts.  There's a printed pattern on two printed circuit
boards.  A sensor detects the change of capacitance as the two parts move
relative to each other.  With standard PCB fabrication methods, you can get
something like a tenth (one 10,000th of an inch) accuracy in 6" board.  Some
apparently use induction, but the idea is the same.  The circuit involved is
about a second semester project these days.  Only precision machining the
caliper needs is to make sure it runs square.  A cheap digital caliper
(assuming it's not bent or something) is more accurate than an expensive
non-digital one.  Progress is cool, sometimes.

-- 
David Scheidt
dmscheidt at gmail.com