Large stump grinders that are pulled behind trucks are used by many professionals for stump removal. Those machines can grind a stump quickly and efficiently and are generally safe if used correctly and carefully.
 
Several models of small, less expensive walk-behind stump grinders are aimed at homeowners, rental use and smaller professional operations (Figure 1). Most are reasonably safe if used carefully, but a few are not safe no matter how they are used.

Types of Small Stump Grinders
Most small walk-behind stump grinders have an elongated frame with wheels and an engine near the center, a handle for the operator at one end and a cutting wheel at the other end. The cutting wheel can be mounted in a horizontal or vertical orientation, but vertical is more common. The cutting wheel is usually belt-driven from the engine. A typical cutting wheel is comprised of a heavy wheel with replaceable tungsten carbide teeth that do the actual work (Figure 2). The cutting wheel does not saw the stump; it grinds the stump. A sharp cutting wheel will reduce a stump to chips.

How a Stump Grinder Is Used
Most stump grinders are designed to be operated in a back-and-forth mode. Moving the handle left to right moves the cutting wheel right to left. As you move the handle back and forth, the cutting wheel will move back and forth across the stump, gradually grinding it down. After each pass across the stump, you either lower the cutting wheel (by raising the handle) or move the whole machine forward an inch or so. Some machines have a brake on one wheel so the machine can more easily be pivoted about that wheel.

Unsafe Designs
Some stump grinders, especially some obsolete models that may still be in service, have the cutting wheel on the same end of the machine as the operator. In this case, the cutting wheel is essentially under the handle. The manufacturers of this type claim better visibility of the cutting wheel, but this design is extremely dangerous and should never be used. There have been dozens of serious injuries from this type of stump grinder. Not only is the cutting wheel too close to the operator's legs and feet, but the wheel rotates backward. Thus if the teeth grab, the machine will jump back at the operator. In testing one of these machines (using a remote handle), I have had a 300+ pound machine literally jump off the ground and back at where the operator would have been standing.

Safety Considerations in Use
Obviously the first consideration is to use only stump grinders with the cutting wheel on the end opposite the operator. A further consideration is not to use a small walk-behind stump grinder unless you are physically fit and reasonably strong. You will have to wrestle it around.
 
Don’t use a stump grinder unless you have been properly trained in its safe use. Keep all bystanders - especially children - far away from the machine. A stump grinder with the blade opposite the operator won’t kick back, but it might jump forward and injure a bystander. Not only is the cutting wheel dangerous, but the machine can throw objects a long ways. Watch out for rocks, buried pipes and electric wires, concrete, etc. Don’t try to take a deep cut; don’t force the machine. Let it feed itself. It is critical to keep the teeth on the cutting wheel sharp; sharp teeth are less likely to grab and jump.
 
On most machines, you should shut off the engine before moving the machine. Most machines use centrifugal clutches, which are notoriously unreliable, so you can’t count on the clutch to keep the blade from turning when you move the machine. Don’t leave the operator’s position behind the machine when the engine is running.

Small stump grinders are handy tools but are too specialized and too expensive for most homeowners to own. Your most practical choices are to rent a stump grinder or hire a professional. If you decide to rent a machine and do the job yourself, be careful. Stump grinders are powerful machines that can chew up feet and legs as easily as they chew up stumps. Always treat a stump grinder with respect, and never become complacent while using one.