May 21, 2017

[2.70] Seek and Geek #14: Switchable Permanent Magnets

During the course of this project, I came across several switchable magnet toolholders and vise accessories.


These things! From a generic google image search

Unlike electromagnets, these require no external power to switch on and off. Instead, they have a pair of permanent magnets. Turning the control knob physically rotates one of the magnets to allow or oppose magnetic field.


When the two magnets are aligned (both poles face the same direction), the magnetic field circulates through the casing and into the workpiece. When the magnets oppose each other, magnetic field travels directly through the magnets (and the casing) without reaching the workpiece. Thickness of the casing therefore has to be tuned to the magnets to prevent flux leakage.

Diagram of magnetic flux circulating through the workpiece
From kjmagnetics.com


Implementation in MagVISE-brand workholding blocks

Learning prototype made by K&J Magnetics
Holding strength is directly related to the thickness of the workpiece. A thicker workpiece allows more magnetic flux - pull force specifications on the manufacturer datasheet come from testing on large thick steel plates in ideal conditions. In practice, the magnets have lower pull strengths since they are holding onto thinner things.

Surface treatment, direction of pull, and temperature also affect pull-force. Rough/irregular surfaces complicate the magnetic field, and usually make the magnets less effective (also if the piece is not very wide, generally <3x a="" any="" contact="" easier="" in="" is="" magnet="" moment="" or="" p="" perpendicular="" pulling="" shear="" surfaces.="" than="" the="" to="" width="" with="">

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