[Shop-talk] Micrometers

[Doug Braun]

The $19.95 HF 6" digital caliper has the interesting property
that if you turn
it off, the display blanks, but it otherwise keeps working.  for example, if
you turn it off, move it, and turn it on again, it correctly indicated the new
position.  In fact, I think the battery will last just as long if you leave it
on all the time.

I have one, and it's incredibly handy.  Everybody ought to
own one.

Doug

--- On Thu, 10/29/09, David Scheidt <dmscheidt at gmail.com>
wrote:

> From: David Scheidt <dmscheidt at gmail.com>
> Subject: Re: [Shop-talk]
Micrometers
> To: "J. F. Juhas" <james.f.juhas at snet.net>
> Cc:
shop-talk at autox.team.net
> Date: Thursday, October 29, 2009, 12:06 PM
> 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
>
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