Coils and magnetics as related to tattoo machines

Tattoo machines work off of an electromagnetic coil. It is usually
agreed upon by artists that the coil has to run in a horseshoe
configuration to achieve synchronized pull. This can only
be achieved if the cores inside the wire wrapped coils share a
conductive base. Only certain types of metals will allow for the
maximum effective conduciveness to occur. The general rule
of thumb is that if your machine is made of anything other than
iron or steel, then you are required to use a yolk. A yolk is the
base-plate that the coils will rest on. The yolk is fastened to the
frame by means of the screws or bolts that hold the coils on
the frame. The bolts will pass through the bottom of the frame,
through the yolk, and into the coil core. The coil cores are typically
pre-tapped. This is very important to be aware of because
a lot of cheaper frames are not steel or iron, and do not come
with a yolk. The machine will run without the yolk, but it will run a
lot better with a yolk installed.