JEI Minibeast Cordless Operator's Manual page 14

The clamping force of the electromagnetic base will be lower if the clamping
surface is thin, rough, not flat, not rigid, the voltage is lower than required, or the bottom
of the base is worn.
The force will be lower if there is rust, paint, chips, or dirt. The workpiece should
be ground with coarse-grained sandpaper and cleaned.
Some types of steel (non-ferromagnetic) do not conduct the magnetic
flux, therefore the machine cannot clamp onto them.
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Minibeast Cordless