In an orthogonal cutting process the tool used has rake angle of zero degree. The measured cutting force and thrust force are $$500$$ $$N$$ and $$250$$ $$N,$$ respectively. The coefficient of friction between the tool and the chip is ________________

Under certain cutting conditions, doubling the cutting speed reduces the tool life to $${\left( {{1 \over {16}}} \right)^{th}}$$ of the original. Taylor’s tool life index $$(n)$$ for this tool workpiece combination will be ______________

If the Taylor's tool life exponent $$n$$ is $$0.2,$$ and the tool changing time is $$1.5$$ $$min,$$ then the tool life (in min) for maximum production rate is ____________