PSN-L Email List Message

Subject: magnet pole dimension
From: sean@...........
Date: Thu, 7 Sep 2000 12:48:12 -0500 (CDT)


I have given some thought to making a new coil for a speaker magnet,
but as I said, I haven't done it yet. However, I would not attempt
to dismantle the magnet, mostly because getting it back together 
would be difficult, and also because it would permanently weaken
the magnet (I've had some un-framed, ie no keeper shunts, ceramic
magnets go completely dead hanging by themselves on nails).

The problem is to get the OD of the magnet center pole. Of course,
measuring it with an (electronic) caliper works; I bought an extension
points kit for mine that accurately probes the magnet gap of even
a small (4") speaker magnet. The extension probes are slightly magnetic
but manageable. Knowing the OD, I can create the form by wrapping
layers of 1 mil aluminum tape around a wood dowel, or you could use
a lathe to make the form, or find the size of anything cylindrical
at the store.

Plan B: I wrapped some thin (2 mil) brass foil sheet (ACE hdwe) around
a pencil to make a tight coiled cylindrical spring. Then I gently 
expanded it as I slid it around the magnet pole, and once it seemed 
square and smooth fitting, taped the overlap ends. Later I soldered
both the inside and outside, and added another few layers to build up
to the clearance I wanted of about 0.032" (1/32").

I presume that liquid nail polish works OK for distinguishing the
layers as a coil is wound as long as it stays thin. I used the 
correcting tape because it was handy, since I found that it is easier
to remove from the computer screen than white-out.


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Larry Cochrane <cochrane@..............>