SMS (Tapping) Screw

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An introduction: A 40 year evolution followed. It can be traced through four distinct phases of tapping screw design- thread forming, thread cutting, thread rolling and self-drilling.

I. Thread forming screw are Types AB,.B, BP A, and C. Type C, with its machine threads and high-driving torques became obsolete with the introduction of thread rolling screws. It is no longer recommended for new designs. The only difference between Types AB, B and BP is their point design. AB is a threaded gimlet, Type B has a blunt point, and Type BP has an unthreaded gimlet. Gimlet points center into preformed holes and facilitate thread start, but they are longer and need more blind side clearance. Also gimlet points tend to be a sharp and may damage other components in the assembly if they contact.

Thread Forming Tapping Screws - Type AB

Table 6 Dimensions of Threads and Points For Type AB Thread Forming Tapping Screws

Nominal Size or Basic Screw Diameter Threads Per Inch D d L
Major Diameter Major Diameter Minimum Practical Screw Lengths
Max Min Max Min 90° Heads Csk Heads
0 0.0600 48 0.060 0.054 0.036 0.033 1/8 5/32
1 0.0730 42 0.075 0.069 0.049 0.046 5/32 3/16
2 0.0860 32 0.088 0.082 0.064 0.060 3/16 7/32
3 0.0990 28 0.101 0.095 0.075 0.071 3/16 1/4
4 0.1120 24 0.114 0.108 0.086 0.082 7/32 9/32
5 0.1250 20 0.130 0.123 0.094 0.090 1/4 5/16
6 0.1380 20 0.139 0.132 0.104 0.099 9/32 11/32
7 0.1510 19 0.154 0.147 0.115 0.109 5/16 3/8
8 0.1640 18 0.166 0.159 0.122 0.116 5/16 3/8
10 0.1900 16 0.189 0.182 0.141 0.135 3/8 7/16
12 0.2160 14 0.215 0.208 0.164 0.157 7/16 17/32
1/4 0.2500 14 0.246 0.237 0.192 0.185 1/2 19/32
5/16 0.3125 12 0.315 0.306 0.244 0.236 5/8 3/4
3/8 0.3750 12 0.380 0.371 0.309 0.299 3/4 29/32
7/16 0.4375 10 0.440 0.429 0.359 0.349 7/8 1 1/32
1/2 0.5000 10 0.504 0.493 0.423 0.413 1 1 5/32
See Notes 3,4              

Notes:

  1. No extrusion of excess metal beyond apex of the point resuiting from thread rolling shall be permissible a slight rounding or truncation of the point is desirable.
  2. The width of flat at crest of thread shall not exceed 0.004 inch for sizes up to and including No.8, and 0.006 inch for larger sizes.
  3. Where specifying nominal size in decimals,zeros preceding decimal and in the fourth decimal place shall be omitted.
  4. Sizes shown in bold type are prefrred.
  5. For determining the effective grip of screws see Appdx. Vll, page H-58.
  6. For additional requirements , see introductiry Notes and General Data , page H-11.

Thread Forming Tapping Screws - Types B And BP / Thread Cutting Tapping Screws- Types BF And BT

Table 7 Dimensions of Threads and Points for Types B and BP Thread Forming and Types BF and BT Thread Cutting Tapping Screws

Nominal Size or Basic Screw Diameter Threads Perinch D d P S L
Major Diameter Major Diameter Point Diameter Point Taper Length Minimum Practical Nominal Screw Lengths
Types B, BF, BT Type BP
Max Min Max Min Max Min Max Min 90° Head Csk Heads 90° Head Csk Heads
0 0.0600 48 0.060 0.054 0.036 0.033 0.031 0.027 0.042 0.031 1/8 1/8 5/32 3/16
1 0.0730 42 0.075 0.069 0.049 0.046 0.044 0.040 0.048 0.036 1/8 5/32 3/16 7/32
2 0.0860 32 0.088 0.082 0.064 0.060 0.058 0.054 0.062 0.047 5/32 3/16 1/4 9/32
3 0.0990 28 0.101 0.095 0.075 0.071 0.068 0.063 0.071 0.054 3/16 7/32 9/32 5/16
4 0.1120 24 0.114 0.108 0.086 0.082 0.079 0.074 0.083 0.063 3/16 1/4 5/16 11/32
5 0.1250 20 0.130 0.123 0.094 0.090 0.087 0.082 0.100 0.075 7/32 9/32 11/32 13/32
6 0.1380 20 0.139 0.132 0.104 0.099 0.095 0.089 0.100 0.075 1/4 9/32 3/8 7/16
7 0.1510 19 0.154 0.147 0.115 0.109 0.105 0.099 0.105 0.079 1/4 5/16 12/32 15/32
8 0.1640 18 0.166 0.159 0.122 0.116 0.112 0.106 0.111 0.083 9/32 11/32 7/16 1/2
10 0.1900 16 0.189 0.182 0.141 0.135 0.130 0.123 0.125 0.094 5/16 3/8 1/2 19/32
12 0.2160 14 0.215 0.208 0.164 0.157 0.152 0.145 0.143 0.107 11/32 7/16 9/16 21/32
1/4 0.2500 14 0.246 0.237 0.192 0.185 0.179 0.171 0.143 0.107 3/8 1/2 21/32 3/4
5/16 0.3125 12 0.315 0.306 0.244 0.236 0.230 0.222 0.167 0.125 15/32 19/32 27/32 31/32
3/8 0.3750 12 0.380 0.371 0.309 0.299 0.293 0.285 0.167 0.125 17/32 11/16 15/16 1 1/8
7/16 0.4375 10 0.440 0.429 0.359 0.349 0.343 0.335 0.200 0.150 5/8 25/32 11/8 1 1/4
1/2 0.5000 10 0.504 0.493 0.423 0.413 0.407 0.399 0.200 0.150 11/16 27/32 11/4 1 13/32
See Note 3       4      

Notes:

  1. Points of screws shall be tapered and fluted or slotted as illustrated above for the respective types .The flute on Type BT screws shall have an included angle of 90 to 95 deg and the thread cutting edge located above the axis of screw. Tapered threads shall have unfinished crests within the point taper length. The flutes or slots shall extend through first full form thread beyond taper except for Type BF screws on which tapered threads may be complete at manufacturer's option and flutes may be one pitch short of first full form thread. Other details of taper and flute design shall be optional with manufacturer provided the screws meet the specified performance requirements.
  2. The width of flat at crest of thread shall not exceed 0.004 in. for sizes up to and including No. 8 and 0.006 in. for larger sizes.
  3. Where specifying nominal size in decimals zeros preceding decimal and in the fourth decimal place shall be omitted.
  4. Point diameters specified apply to screw blanks before roll threading.
  5. For determining the effective grip length of Type BP screws, see Appdx. V11, page H-58.
  6. For additional requirements refer to introductory Notes and General Data, page H-11.

Dimensions For Type C Tapping Screws

Table V1 Dimensions of Threads and Points for Types C Thread Forming Tapping Screws

Nominal Size or Basic Screw Diameter Threads per inch D P S L
Major Dia Point Dia Point Taper Length Determinant Lengths for Point Taper Minimum Practical Nominal Screw Lengths
For Short Screws For Long Screws
Max Min Ref Max Min Max Min 90˘XHead Csk Heads 90˘XHead Csk Heads
2 0.0860 56 0.0860 0.0813 0.068 0.062 0.045 0.080 0.062 5/32 3/16 5/32 3/16
2 0.0860 64 0.0860 0.0816 0.070 0.055 0.039 0.070 0.055 1/8 3/16 1/8 5/32
3 0.0990 48 0.0990 0.0938 0.078 0.073 0.052 0.074 0.073 3/16 7/32 5/32 7/32
3 0.0990 56 0.0990 0.0942 0.081 0.062 0.045 0.080 0.062 5/32 3/16 5/32 3/16
4 0.1120 40 0.1120 0.1061 0.087 0.088 0.062 0.112 0.088 7/32 1/4 3/16 1/4
4 0.1120 43 0.1120 0.1068 0.091 0.073 0.052 0.094 0.073 3/16 7/32 5/32 7/32
5 0.1250 40 0.1250 0.1191 0.100 0.088 0.062 0.112 0.088 7/32 9/32 3/16 1/4
5 0.1250 44 0.1250 0.1195 0.102 0.080 0.057 0.102 0.080 3/16 1/4 3/16 1/4
6 0.1380 32 0.1380 0.1312 0.107 0.109 0.078 0.141 0.109 1/4 5/16 1/4 5/16
6 0.1380 40 0.1380 0.1321 0.113 0.088 0.062 0.112 0.088 7/32 9/32 3/16 1/4
8 0.1640 32 0.1640 0.1571 0.132 0.109 0.078 0.141 0.109 1/4 11/32 1/4 5/16
8 0.1640 36 0.1640 0.1577 0.136 0.097 0.069 0.125 0.097 7/32 5/16 7/32 9/32
10 0.1900 24 0.1900 0.1818 0.148 0.146 0.104 0.188 0.146 11/32 7/16 5/16 13/32
10 0.1900 32 0.1900 0.1831 0.158 0.109 0.078 0.141 0.109 1/4 11/32 1/4 5/16
12 0.2160 24 0.2160 0.2078 0.174 0.146 0.104 0.188 0.146 11/32 7/16 5/16 13/32
12 0.2160 28 0.2160 0.2085 0.180 0.125 0.089 0.161 0.125 5/16 13/32 9/32 3/8
1/4 0.2500 20 0.2500 0.2408 0.200 0.175 0.125 0.225 0.175 13/32 17/32 3/8 1/2
1/4 0.2500 28 0.2500 0.2425 0.214 0.125 0.089 0.161 0.125 5/16 13/32 9/32 3/8
5/16 0.3125 15 0.3125 0.3026 0.257 0.194 0.139 0.250 0.194 15/32 19/32 7/16 9/16
5/16 0.3125 24 0.3125 0.3042 0.271 0.146 0.104 0.188 0.146 11/32 15/32 5/16 15/32
3/8 0.3750 16 0.3750 0.3643 0.312 0.219 0.156 0.281 0.219 1/2 11/16 15/32 5/8
3/8 0.3750 24 0.3750 0.3667 0.333 0.146 0.104 0.188 0.146 11/32 1/2 5/16 1/2
7/16 0.4375 14 0.4375 0.4258 0.366 0.250 0.179 0.321 0.250 19/32 3/4 9/16 23/32
7/16 0.4375 20 0.4375 0.4281 0.387 0.175 0.125 0.225 0.175 13/32 9/16 3/8 17/32
1/2 0.5000 13 0.5000 0.4876 0.423 0.269 0.192 0.346 0.269 5/8 25/32 19/32 3/4
1/2 0.5000 20 0.5000 0.4906 0.450 0.175 0.125 0.225 0.175 13/32 9/16 3/8 17/32
See Notes 1         3        

Note:

  1. Where specifying ncminal size in decimals,zeros preceding decimal and in the fourth decimal place shall be omitted.
  2. Threads within point taper length shall have unfinished crests.
  3. Screws of these nominal lengths and shorter shall have point taper lengths specified for short screws. Longer lengths shall have point taper lengths specified for long screws.
  4. For additional requirements. Refer to Para. 2, page H-18.

II. Thread cutting screws are Types BF, BT, D, F, G, and T. types BF and BT have spaced threads, the other machine threads. Other differences related to the desgn of their cutting point. Before selecting a thread cutting screw, some thought should be given to the possible effect, if any, of the presence of small chips in the system.

Table 1 Type Designations for Tapping Screws and Metallic Drive Screws