$21\mathrm{.}12($Milling$-10$ Marks$)-($SI units$)$ A face milling operation removes $6\mathrm{.}0$ mm from

the top surface of a rectangular piece of aluminum that is $250$ mm long by $75$ mm

wide by $50$ mm thick. Thickness after the cut $=44\mathrm{.}0mm.$ The cutter follows a path

that is centered over the workpiece. It has four teeth and an $85-mm$ diameter. Cutting

speed $=2\mathrm{.}0\frac{m}{s},$ and chip load $=0\mathrm{.}18m\frac{m}{?}$ tooth. Determine:

$($a$)$ machining time, and

$($b$)$ maximum metal removal rate during cutting.