The Grinder bleeds into the Kuipler belt. the only way to really tell one from the other is the nature of the materials. As asteroid belts go the grinder is fairly thick with under an average 1 million mile separation between objects. Ships are advised to avoid that region, even if the real chance of collision is remote.
Old spacer tales tell of the hidden high technology tugs used to move the El Nanth planets around being abandoned in the Grinder. A fortune to anyone that could find them. It is just that a tale. No such tugs exist.