This idea has already been well done in the industrial sector.. but I needed something on a way smaller scale for my shed, to make light work of the shavings and filings after day of metal working...

What I used..

Large house hold or car speaker. Tear it apart until you find the magnet sitting on the pole piece (Picture 1 (a)
Plastic screw top jar with lid – wide enough to just take the magnet. (Picture 1 (b))
Length of steel pipe (part1)that will fit tightly over the pole of the magnet (Picture 2)
Same length in smaller rod.(part2)/ Pipe ( I used a bit of alloy tv antenna)
Circle of wood just small enough to fit in side the jar
Couple of wood screws and washers

Step 1: Step One

Attach the magnet to one end of the long pipe(part1) – I welded mine on – but you can fix it however suits you.
Push the magnet in to the plastic jar

Step 2: Step Two

Place the circle of wood inside the plastic jar lid and drill a centered hole through both which is just slightly bigger then the pipe (part1)
Feed first the wood circle and then the jar lid onto the pipe and let them fall to the magnet
Screw the jar lid on.

Step 3: Step 3

Put a 90 degree bend in the end of the second piece of pipe(part2) so that there is at least 20mm flat.

Place the flat on the top of the plastic jar lid above the wood disc - drill and screw them together.(Picture 3)

Step 4: Step 4

Holding the big pipe(part1) in your hand.. use your other hand to pull the second rod/ pipe(part 2) up towards you, which has the affect of pushing the magnet firmly onto the base of the jar.

Sweep it over the metal, then take it to the bin and while holding the first pipe again, this time push the second one down = the magnet clears the bottom of the plastic jar and magnetism is lost.. it all falls in the bin...