Archive for February 12th, 2009
Those “nonmagnetic” tweezers remind me of a story and a useful gadget.
Two years ago a lightning strike blasted a football-sized chunk of concrete out of the garage door apron, blew out a bunch of networking gear, magnetized every ferrous object in the house (including the nails in the hardwood floors), yet didn’t do any damage to anything else.
Including us: we were sleeping about 20 feet from the crater. Whew & similar remarks.
Anyhow, all my machine-shop equipment and tooling was magnetized, too. Suddenly, lathe bits attracted swarf like, well, magnets, endmills sported fur coats, scales snapped onto the workpieces they were supposed to measure, and tweezers picked up screws without any pressure. Not a good situation.
Fortunately, I’d built a demagnetizer loosely modeled on one described in the Sept/Oct 2000 Home Shop Machinist. It got plenty of power-on minutes after that strike, returning my tools to their normal condition.
Those flooring nails will be magnetized forever.
The general idea is pretty simple: recycle the motor from a can opener-class gadget. Strip off all the shading coils and other frippery, saw enough from the pole pieces to position tools in the air gap, plug it straight into the wall outlet, and shake the magnetism right out of your steel.
It has another nice trick: a relatively low DC voltage that magnetizes your tools. The transformer has a 35 VAC center-tapped secondary, a pair of stud diodes yields about 24 V DC, and that honking big cap whacks the bumps off the full-wave rectified DC waveform.
Absolutely nothing is critical, but the original article suggests measuring the AC current into the motor winding, then choosing a DC voltage to force that current (Ohm’s Law: E=IR!) through the coil’s DC resistance. I picked a transformer that was close enough to work; anything in the 10-20 VAC range would probably be fine, too.
The small DPDT toggle switch routes either AC or DC to the winding. If I were doing this again, I’d use a bigger switch, but that’s what I had in the junk box at the time.
Use a momentary pushbutton for the main power switch, as you do not want this thing on for more than a few seconds. The motor windings get warm from the abuse; it was designed to run with the back EMF from the now-missing rotor, making the currents far higher than the design spec. Use fairly husky wire, not doorbell stuff, inside the box.
I used 100% junk-box parts for this project and bolted everything to the outside of a recycled aluminum box because the inside was pretty crowded with that husky wiring.
Demagnetizing: feel the buzz, then pull the tool a goodly distance from the pole pieces before you release the pushbutton.
Magnetizing: stroke the tool over one of the pole pieces, repeat as needed.
That should handle any residual magnetism in those tweezers…