During the turning of a $$20$$ $$mm-$$diameter steel bar at a spindle speed of $$400$$ $$rpm,$$ a tool life of $$20$$ minute is obtained. When the same bar is turned at $$200$$ $$rpm,$$ the tool life becomes $$60$$ $$minute.$$ Assume that Taylor's tool life equation is valid. When the bar is turned at $$300$$ $$rpm,$$ the tool life (in minute) is approximately

In an orthogonal machining with a tool of $${9^ \circ }$$ orthogonal rake angle, the uncut chip thickness is $$0.2$$ $$mm.$$ The chip thickness fluctuates between $$0.25$$ $$mm$$ and $$0.4$$ $$mm.$$ The ratio of the maximum shear angle to the minimum shear angle during machining is ______________

For a certain job, the cost of metal cutting is $$Rs.18C/V$$ and the cost of tooling is Rs.$$270C/(TV).$$ Where $$C$$ is a constant, $$V$$ is the cutting speed in $$m/min$$ and $$T$$ is the tool life in minutes. The Taylor's tool life equation is $$V{T^{0.25}} = 150.$$ The cutting speed (in $$m/min$$) for the minimum total cost is ___________

In a single point turning operation with cemented carbide tool and steel work piece, it is found that the Taylor's exponent is $$0.25.$$ If the cutting speed is reduced by $$50\% $$ then the tool life changes by ____________ times.