Carpet mill vs slat mill
These two get used interchangeably in conversation, but they are different machines with different feels underfoot.
Carpet mill
A carpet mill runs a continuous carpeted surface over rollers. The surface has high friction, so the dog works hard for every stride and the belt slows immediately when the dog eases off. It is the older design and tends to be simpler to build.
Slat mill
A slat mill uses individual slats — usually wood or composite — mounted across a belt or chain running on bearings. It carries momentum, runs quieter and allows a faster, freer gait. It is more involved to build and usually costs more.
Choosing between them
A carpet mill gives more resistance per stride and less speed; a slat mill gives more speed and smoother motion. Dogs that need to build strength are often started on higher-drag surfaces, while dogs doing gait work benefit from the freer-running slat design.