Demagnetizers

It removes unwanted residual magnetism from metal parts after machining or inspection; it prevents measurement errors and surface quality problems. Tabletop and tunnel types are available.

Why residual magnetism is a problem

After machining, grinding, clamping with a magnetic holder or magnetic particle inspection, an unwanted magnetic field can remain in metal parts. This residual field is invisible but causes problems in three places: swarf and dust stick to the part, measuring instruments give misleading values, and components attract each other during assembly.

How it works

The demagnetizer applies to the part an alternating field whose direction changes constantly and whose strength decreases. With each cycle the magnetic domains inside the material become a little more randomly oriented; as the field approaches zero, the net magnetism also drops to zero. That is why it is important to pass the part through the device slowly and without interruption; pulling it out suddenly leaves the process half-finished.

Two types

• Tabletop (plate) type — the part is placed on it or passed over it by hand; for small parts and individual jobs
• Tunnel type — fed by conveyor, it runs continuously on the line in series production

Where it becomes essential

On parts that require bearings, gears, precision measuring instruments or surface preparation before welding. In welding, residual magnetism causes a deflection known as arc blow and spoils the seam. In metrology laboratories, on the other hand, demagnetization before measurement is a standard step.

For more on magnetic applications, see the magnetic hand tools page.

Correct use

It is essential to pass the part through the device slowly and without interruption; pulling it out suddenly leaves the process half-finished and residual magnetism remains. Large parts may need several passes. Using a simple magnetic field meter to verify the result is the only way to see that the process has really been completed.