Final answer:
Abrasive wheel safety guards are crucial for protecting users by limiting the exposure of the grinding wheel periphery, with regulations often specifying the maximum allowable exposure above the horizontal plane of the wheel spindle. However, the exact number of degrees specified is not provided in the question or reference material.
Step-by-step explanation:
The question pertains to the safety measures associated with the use of abrasive wheels on bench stands and cylindrical grinders. The safety guards on such equipment are designed to protect users from the dangers posed by the rapidly spinning grinding wheels. It is a standard safety practice to ensure that the guards do not expose the grinding wheel periphery for more than a specific degree above the horizontal plane of the wheel spindle. Unfortunately, the degree information is missing in the question, and thus, cannot be provided with confidence. However, it is worth noting that various machinery safety standards often regulate this exposure limit to mitigate risks.
The provided reference information, while related to grindstones and rotational motion, does not directly answer the safety guard specification question, but it concerns calculations of centripetal acceleration and linear speed of a rotating object, which is important in understanding the forces at play when an abrasive wheel is in use.