Category: Machine Shop

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…

Continuing the saga from there, this is the etched and plated board.

I mask around the edges with ordinary masking tape and cover the back surface with duct tape. Basically, the less copper you remove, the better and faster the job.

I use ferric chloride etchant, formerly available in nearby Radio Shack stores. These days it’s getting harder to find, so I picked up a few kilos of dry powder on eBay. Most likely that supply will vanish, too.

The usual directions call for heating the etchant, submerging the board, bubbling or agitating, and so forth and so on. The folks at Pulsar suggest simply rubbing the etchant on the board with a sponge and, perhaps not surprisingly, that works perfectly with boards sealed using their green film.

I hold the board horizontally in my left hand, pour a dollop of etchant on it, then rub it with a small sponge in my right hand. The etchant gradually turns into a gel as it removes copper from the board; when the gel becomes too stiff, I just wipe it off with the sponge.

I do this over a small glass tray and scrape the accumulated gunk off the sponge into the tray. Pulsar recommends diluting the residue in a gallon of water, but I’d just as soon not have that much spent solution sitting around.

Wear latex gloves, an apron, and eye protection. Expect that everything within a radius of two meters will accrue small spots of ferric chloride that will instantly produce a vivid, permanent yellow stain. Repeat: everything within two meters will sprout yellow spots.

You have been warned!

Even if you pay attention to the board’s edges and corners, those will still be the last areas to finish etching. You’ll also learn to not run fine lines parallel to the edges right next to the masking tape, as the tape protects adjacent areas from the sponge: no contact, no etching!

Spent ferric chloride (most likely, it’s now copper chloride or some such) disposal occurs on our town’s Household Hazmat collection days, but direct etching leaves very little bulk waste. Although it’s not particularly hazardous, the rituals should be observed.

After eching, rinse the board, remove all the tape, and rinse the board again.

Acetone and paper towels remove the green sealant film and the laser toner from the board, an operation best done outdoors. I have yet to find protective gloves that don’t disintegrate in acetone, so I simply try to not soak my hands in the stuff. Remember: leave the mask on the back side of the board to help protect it from the etchant when you do the front side.

Then I silver-plate the copper with Cool-Amp Silver Plating Powder, which makes the board look and solder better. Looks are important, as the boards sometimes wind up in my Circuit Cellar columns.

That hasn’t stopped me from hand-soldering SMD parts: the bigger the blob, the better the job.