What tool should NEVER be used to cut pipes coated internally with enamel or glass?

The correct answer highlights the use of a wheel pipe cutter, which should be avoided when cutting pipes coated internally with enamel or glass. The primary reason for this is that a wheel pipe cutter operates by applying pressure in a circular motion to score the pipe, ultimately leading to a cutting action. This method can create significant stress on the pipe and may lead to chipping or cracking of the fragile inner enamel or glass lining. Such damage can compromise the integrity of the pipe, resulting in leaks or failure.

In contrast, the other tools mentioned like a hacksaw, pipe nibbler, or metal saw may be used with more control and less risk of damaging the internal surfaces. For instance, a hacksaw allows for a more manual and cautious cutting approach, while a pipe nibbler provides a method of removing material in a more gradual fashion, which can be gentle enough for fragile surfaces. Thus, the risk of damaging the protective enamel or glass coating is significantly reduced with these tools.