Final answer:
Safety guards on machines with abrasive wheels protect the operator by covering critical parts, but certain areas like the manual controls may not be enclosed to allow for operation of the machinery. This ensures both safety and functionality.
Step-by-step explanation:
The safety guards on machines that use abrasive wheels are designed to protect the operator and surrounding areas from flying debris and potential hazards. These guards cover essential parts of the machinery, which can include the grinding wheel, the point of operation, and any moving parts associated with the process.
However, the safety guards are not designed to cover all the parts of the machinery. Excerpts from the description hint at a production environment with machinery for processing materials, including using a lever to activate a belt-driven knife for shaping metal shells.
Hazards include flying pieces that could reach the operator's face and the use of chemicals in the process. Therefore, it's imperative that most parts are guarded for safety, but parts like the lever for manual operation may not be fully enclosed as this is the machinery's control point that requires access by the operator.