Machine Design
A key capable of tilting in a recess milled out in a shaft is known as

Hollow saddle key
Woodruff key
Feather key
Flat saddle key

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Machine Design
Cap screws are

Similar to small size tap bolts except that a greater variety of shapes of heads are available
Slotted for a screw driver and generally used with a nut
Provided with detachable caps
Used to prevent relative motion between parts

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