Direct Fastening Technical Guide , Edition 24
3.6.1.2 THREAD SELECTION
Thread length Always choose a fastener with sufficient threads to fully engage in the base metal . For attachments to 1 / 4 " base steel , a self-drilling screw should have at least 1 / 4 " of threads . It is helpful , but not critical , that the threads also engage in the material being fastened . The head of the fastener provides the bearing force for the material being fastened , while the threads provide the clamping force in the base material .
Clamping Material being fastened
Base material
Threading
Thread pitch The thickness of material being fastened and diameter of the screw determine the type of thread pitch to be used . In general , the thinner the fastened materials , the fewer the number of threads . The thicker the material , the greater the number of threads . This principle is due to two primary methods of thread engagement / holding power : Clamping and Threading . In light gauge metal , the materials are actually being clamped together by the upper and lower threads .
Metal gauge
Gauge
Aluminum metal
Sheet
( Approx . thickness in decimal parts of an inch )
8 0.1285 0.1644 9 0.1144 0.1495 10 0.1019 0.1345 11 0.0907 0.1196 12 0.0808 0.1046 13 0.0720 0.0897 14 0.0641 0.0747 15 0.0571 0.0673 16 0.0508 0.0598 17 0.0493 0.0538 18 0.0403 0.0474 19 0.0359 0.0418 20 0.0320 0.0358 21 0.0285 0.0329 22 0.0253 0.0295
Fraction to decimal
Fraction ( in .)
Decimal equivalent ( in .)
1 / 64 0.015 1 / 32 0.031 3 / 64 0.046 1 / 16 0.062 5 / 64 0.078 3 / 32 0.093 7 / 64 0.109 1 / 8 0.125 9 / 64 0.140 5 / 32 0.156 11 / 64 0.171 3 / 16 0.187 13 / 64 0.203 7 / 32 0.218 15 / 64 0.234 1 / 4 0.250
Screw diameter
Clamping |
23 0.0226 0.0269
24 0.0201 0.0239
|
Number |
Decimal equivalent ( in .) |
Thinner base material requires a coarser thread pitch to assure proper clamping . The thicker the material , the finer the threads must be . In very thick metal ( 3 / 8 " to 1 / 2 " thick ), a fine thread is advisable . This will allow the thread to tap into the base material with less installation torque than a coarse thread .
25 |
0.0179 |
0.0209 |
26 |
0.0159 |
0.0179 |
27 |
0.0142 |
0.0164 |
28 |
0.0126 |
0.0149 |
# 6 |
0.1380 |
# 7 |
0.1510 |
# 8 |
0.1640 |
# 10 |
0.1900 |
# 12 |
0.2160 |
1 / 4 |
0.2500 |
5 / 16 |
0.3125 |
Thread engagement
211