QUILL

The purpose of the quill is to get the wheel into the part to reach the surfaces you need to grind.  It is a highly customized and tightly toleranced tool that serves as an extension of the spindle itself.

It would not be economical for spindle manufacturers to produce thousands of different spindle designs to reach into every possible interior geometry.  Instead, they produce a single design with a standard interface to which customers can design quills to suit their individual requirements.  A well-designed quill has a long lifespan, allowing the user to mount grinding wheels to it over and over again.