Machine Design
Machine screws are

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

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Machine Design
The contact ratio is the ratio of

Length of arc of approach to the circular pitch
Length of arc of contact to the circular pitch
Length of arc of recess to the circular pitch
Length of path of contact to the circular pitch

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