$(25$ Points$)$

Consider the following key$-$keyway assembly. The key typically locks within the keyway to

lock the shaft with the bore.

The basic size of the width of keyway was fixed at $0\mathrm{.}500$ inch $.$ The original specification was made

such that the keyway was allowed to vary by $0\mathrm{.}002$ inch in a unilateral zone. Providing clearance fit

$($between key and keyway$)$ with permissible allowances $0\mathrm{.}002$ inch to $0\mathrm{.}006$ inch, the target width of

key was fixed at $0\mathrm{.}498$ inch.

Since the above specification of tolerances was considered very tight and expensive, the tolerance

specification was relaxed. Both the keyway and key target sizes were then altered: the keyway was now

allowed to vary by $0\mathrm{.}006$ inch in a unilateral zone $($same basic size$)$; the target width of the key was

now fixed at $0\mathrm{.}502$ inch with a lower limit set at $0\mathrm{.}496$ inch.

Make three zones and sort the parts to selectively assemble keys and keyways in order to meet the

original allowances and clearance fit. Comment on why such a selective assembly should be

performed, and also what are the implicit and explicit disadvantages of doing so$.$