The standard size of common screw threads is divided into 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9 and so on more than 10 grades, of which 8.8 and above bolts made of low-carbon alloy steel or medium-carbon steel and heat-treated, commonly referred to as high-strength screws, and the rest are commonly known as common screws.
Common specifications (metric screws, American screws, British screws):
Metric machine screws: Metric.
Ex: M3 x 6 - P P B : M3 machine screws, 6mm long, Phillips, rounded flat head, black plated.
Finish Code : Exterior finish specification.
Head Code: Head appearance.
Screw head profile: cylindrical head, semi-countersunk head, countersunk head, spherical cylindrical head, disk head, half-round head, hexagonal head.
Inch screws:
Labeling will be the denominator of 8, and then directly called the numerator of the number.
Ex: 1/8 x 0.50 - PPB : 1 minute screw x 0.50" long PPB.
Ex: 5/16 x 0.50 - PPB = 2.5/8 x 0.50 - PPB : 2 minute screw x 0.50" long PPB.
Ex: 5/32 x 0.50 - PPB : 2 minute screw x 0.50" long PPB.
Ex. : 5/32 x 0.50- PPB = 1.25/8 x 0.50 - PPB : 1 minute 2 1/2 centimeter screw x 0.50" long PPB.
Ex: 1/4 x 0.50- PPB= 2/8 x 0.50 - PPB : 2 minute tooth screw x 0.50" long PPB.