Possible to convert spa pump motor to bench mounted grinder?
A spa pump motor is usually what's called a "C face" design, meaning that the motor mounts directly to the pump, which is what mounts to the spa. When you take it off of the pump, you have no mounting feet, so no way to hold it to your bench. So yes, you can use a spa pump motor for other things, but it will require some substantial mechanical engineering to make it into a grinder. Also, a typical grinder has a shaft on both ends, a pump motor has only one.

From an electrical standpoint, you need to read the nameplate of the motor to know what it requires and if it is adaptable to 120V household current. They often are, but then the next issue might be how MUCH current it will pull. The practical limit for a typical 120V outlet in a residence is 12A (80% of 15A), which means the motor must be 1/2HP or less. Many spa motors are 3/4HP or 1HP, so you would not be able to legally connect that to a standard 120V 15A receptacle.