A complete cutter head assembly designed to be mounted on a TBM is generally classified as a machine or mechanical appliance rather than as a 'part,' because it constitutes a functional unit in its own right. However, individual cutter discs or bearing assemblies are more likely to be classified as parts. The distinction matters because parts and complete assemblies often carry different duty rates. A binding ruling from CBP is advisable for high-value replacement cutter heads.

The manufacturer's technical specification sheet is the primary evidence. It should clearly show that the cutting unit has no integral propulsion system and requires a separate carrier or haulage vehicle to move. Hydraulic or electrical connection diagrams showing that motive power comes from an external source further support the non-self-propelled classification.