1. SCREWS are basically classified under 3 major types or categories:

A. MACHINE SCREWS -
needs a pre-tapped hole - most commonly used

B. TAPPING SCREWS -
needs the hole to be drilled pierced, but not tapped - typical types are taptite screws (used on soft metal or casting), plastite screws (used for plastic) or High-Low screws (normally used on sheet metal). - need to specify the type of point for leading the screw into the hole e.g. type BT, type C or F „

C. DRILLING SCREWS -
do not need a hole (but for positional accuracy, a pilot point is recommended) - typically used in construction industries - drilling screws consists of a drill bit tip followed by a tapping screw body. - eliminates the need to bring work piece to the drill bench to prepare hole.

A SEM is an assembly consisting of a screw with one or more washers.

2. How to specify a screw (basic requirement)

a. Head type e.g. Pan head
b. Type of drive e.g. Cross Recess
c. Thread used e.g. M3 or 6-32
d. Length needed e.g. 10 mm or 0.5" (note: do not mix metric length and unified thread sizes)
e. Type of screw e.g. taptite or machine screws or a SEM
f. Material used e.g. Low carbon steel (LCS)
g. Plating (if any) e.g. zinc clear

Optional :
h. Standards e.g. JIS or IFI

Note: In all preparation, the precedence priority in terms of quality definition, the following order is adopted:
a. First article approval documentation (which may include deviations)
b. Print or Drawing specifications
c. Part description d. Samples