American National Standard Machine Screws and Machine Screw Nuts
This Standard ANSI B18.6.3 covers both slotted and recessed head machine screws. Dimensions of various types of slotted machine screws, machine screw nuts, and header points are given in Tables 1 through 12. The Standard also covers flat trim head, oval trim head and drilled fillister head machine screws and gives cross recess dimensions and gaging dimensions for all types of machine screw heads. Information on metric machine screws B18.6.7M is given beginning on page 1708.
Threads
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Except for sizes 0000, 000, and 00, machine screw threads may be either Unified Coarse (UNC) and Fine thread (UNF) Class 2A (see American Standard for Unified Screw Threads starting on page 1865) or UNRC and UNRF Series, at option of manufacturer. Thread dimensions for sizes 0000, 000, and 00 are given in Table 7 on page 1698.
Threads for hexagon machine screw nuts may be either UNC or UNF, Class 2B, and for square machine screw nuts are UNC Class 2B.
Length of Thread
Machine screws of sizes No. 5 and smaller with nominal lengths equal to 3 diameters and shorter have full form threads extending to within 1 pitch (thread) of the bearing surface of the head, or closer, if practicable. Nominal lengths greater than 3 diameters, up to and including 1 1⁄8 inch, have full form threads extending to within two pitches (threads) of the bearing surface of the head, or closer, if practicable. Unless otherwise specified, screws of longer nominal length have a minimum length of full form thread of 1.00 inch. Machine screws of sizes No. 6 and larger with nominal length equal to 3 diameters and shorter have full form threads extending to within 1 pitch (thread) of the bearing surface of the head, or closer, if practicable. Nominal lengths greater than 3 diameters, up to and including 2 inches, have full form threads extending to within 2 pitches (threads) of the bearing surface of the head, or closer, if practicable. Screws of longer nominal length, unless otherwise specified, have a minimum length of full form thread of 1.50 inches.
Table 1. Square and Hexagon Machine Screw Nuts ANSI B18.6.3-1972 (R1991)
All dimensions in inches. Hexagon machine screw nuts have tops flat and chamfered. Diameter of top circle should be the maximum width across flats within a tolerance of minus 15 percent. Bottoms are flat but may be chamfered if so specified. Square machine screw nuts have tops and bottoms flat without chamfer.
Diameter of Body
The diameter of machine screw bodies is not less than Class 2A thread minimum pitch diameter nor greater than the basic major diameter of the thread. Cross-recessed trim head machine screws not threaded to the head have an 0.062 in. minimum length shoulder under the head with diameter limits as specified in the dimensional tables in the standard.
Designation
Machine screws are designated by the following data in the sequence shown: Nominal size (number, fraction, or decimal equivalent); threads per inch; nominal length (fraction or decimal equivalent); product name, including head type and driving provision; header point, if desired; material; and protective finish, if required. For example:
- ¼ - 20 × 1 ¼ Slotted Pan Head Machine Screw, Steel, Zinc Plated
- 6 - 32 × ¾ Type IA Cross Recessed Fillister Head Machine Screw, Brass
Machine screw nuts are designated by the following data in the sequence shown: Nominal size (number, fraction, or decimal equivalent); threads per inch; product name; material; and protective finish, if required. For example:
- 10 - 24 Hexagon Machine Screw Nut, Steel, Zinc Plated
- 0.138 - 32 Square Machine Screw Nut, Brass
Table 2. American National Standard Slotted 100-Degree Flat Countersunk Head Machine Screws ANSI B18.6.3-1972 (R1977)
a When specifying nominal size in decimals, zeros preceding the decimal point and in the fourth decimal place are omitted.
All dimensions are in inches.
Table 3. American National Standard Slotted Flat Countersunk Head and Close Tolerance 100-Degree Flat Countersunk Head Machine Screws ANSI B18.6.3-1972 (R1991)
a When specifying nominal size in decimals, zeros preceding the decimal point and in the fourth decimal place are omitted.
b These lengths or shorter are undercut.
c May be rounded or flat.
All dimensions are in inches.
Table 4. American National Standard Slotted Undercut Flat Countersunk Head and Plain and Slotted Hex Washer Head Machine Screws ANSI B18.6.3-1972 (R1991)
a When specifying nominal size in decimals, zeros preceding the decimal point and in the fourth decimal place are omitted.
b These lengths or shorter are undercut.
a Unless otherwise specified, hexagon washer head machine screws are not slotted.
All dimensions are in inches.
Table 5. American National Standard Slotted Truss Head and Plain and Slotted Hexagon Head Machine Screws ANSI B18.6.3-1972 (R1991)
a Where specifying nominal size in decimals, zeros preceding decimal points and in the fourth decimal place are omitted.
a Unless otherwise specified, hexagon head machine screws are not slotted.
All dimensions are in inches.
Table 6. American National Standard Slotted Pan Head Machine Screws ANSI B18.6.3-1972 (R1991)
a Where specifying nominal size in decimals, zeros preceding decimal and in the fourth decimal place are omitted.
All dimensions are in inches.
Table 7. Nos. 0000, 000 and 00 Threads ANSI B18.6.3-1972 (R1991) Appendix
a Where specifying nominal size in decimals, zeros preceding decimal and in the fourth decimal place are omitted.
b There is no allowance provided on the external threads.
c The minor diameter limits for internal threads are not specified, they being determined by the amount of thread engagement necessary to satisfy the strength requirements and tapping performance in the intended application.
Screw Slot Sizes
All dimensions are in inches.
Table 8. American National Standard Slotted Fillister and Slotted Drilled Fillister Head Machine Screws ANSI B18.6.3-1972 (R1991)
All dimensions are in inches.
1 Where specifying nominal size in decimals, zeros preceding decimal points and in the fourth decimal place are omitted.
2 Drilled hole shall be approximately perpendicular to the axis of slot and may be permitted to break through bottom of the slot. Edges of the hole shall be free from burrs.
3 A slight rounding of the edges at periphery of head is permissible provided the diameter of the bearing circle is equal to no less than 90 percent of the specified minimum head diameter.
Table 9. American National Standard Slotted Oval Countersunk Head Machine Screws ANSI B18.6.3-1972 (R1991)
a When specifying nominal size in decimals, zeros preceding decimal points and in the fourth decimal place are omitted.
b These lengths or shorter are undercut.
All dimensions are in inches.
Table 10. American National Standard Header Points for Machine Screws before Threading ANSI B18.6.3-1972 (R1991)
All dimensions in inches. Edges of point may be rounded and end of point need not be flat nor perpendicular to shank. Machine screws normally have plain sheared ends but when specified may have header points, as shown above.
Table 11. American National Standard Slotted Binding Head and Slotted Undercut Oval Countersunk Head Machine Screws ANSI B18.6.3-1972 (R1991)
a Where specifying nominal size in decimals, zeros preceding decimal points and in the fourth decimal place are omitted.
b Unless otherwise specified, slotted binding head machine screws are not undercut.
a These lengths or shorter are undercut.
All dimensions are in inches.
Table 12. Slotted Round Head Machine Screws ANSI B18.6.3-1972 (R1991) Appendix
a When specifying nominal size in decimals, zeros preceding decimal point and in the fourth decimal place are omitted.
All dimensions are in inches.
Not recommended, use Pan Head machine screws.
Machine Screw Cross Recesses
Screw Head Slot Files Extractor
Four cross recesses, Types I, IA, II, and III, may be used in lieu of slots in machine screw heads. Dimensions for recess diameter M, width N, and depth T (not shown above) together with recess penetration gaging depths are given in American National Standard ANSI B18.6.3-1972 (R1991) for machine screws, and in ANSI/ASME B18.6.7M-1985 for metric machine screws.
ANSI Cross Recesses for Machine Screws and Metric Machine Screw
Summary
This article presents information about standard machine screws and nuts, including the different types and tables of sizes with key dimensions. Learn more about fasteners from the Machinery's Handbook, 30th Edition, which is published and available from Industrial Press on Amazon.
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