Newsgroups: comp.robotics
Path: brunix!uunet!elroy.jpl.nasa.gov!decwrl!rtech!amdahl!netcomsv!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: What *IS* that stuff?
Message-ID: <nagleCALAJr.6wK@netcom.com>
Keywords: metal, switch, contact
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
References: <22mtcdINN3ro@uwm.edu>
Date: Thu, 22 Jul 1993 23:28:38 GMT
Lines: 29

rick@ee.uwm.edu (Rick Miller) writes:
>Rather than shell out lots of money for micro-switches, I'd like to make
>some simple contacts like you'd find in small cassette-players.
>Just two strips of copper, parallel to each other and fairly close together.
>The thing is, there are these little bits of metal welded to the two strips
>right where they contact.  I'd imagine that this is to make a more reliable
>connection and perhaps to prevent arc-damage or corrosion...
>WHAT METAL ARE THOSE CONTACTS MADE OF?

      Varies.  Phosphor-bronze and silver are common.  Gold alloys are 
sometimes used.  The springy metal parts are usually plated steel, not 
copper; copper tends to bend permanently all too easily.

      Reliable contact design is hard.  Corrosion is a big problem.
Where currents are very low, and there is no wiping action on the contacts,
very thin layers of oxidation can prevent current flow.  Periodic
contact burnishing is then required.  (Use Jensen tool 63B082).
The snap-action of microswitches forces the contacts together fast enough 
and hard enough to (usually) overcome this problem.  

>Where can I get some? or what might I use as a cheap substitute?

      Surplus telephone relays are a good start.  Telephone equipment
has usually been well-designed in the contact area, since that was the
weak point of the system for decades.  

      There's a lot to be said for buying surplus microswitches.

					John Nagle
