MACHINE SCREWS GENERAL DATA

- Machine screws shall be manufactured from carbon steel and shall have a minimum tensile strength of 60000 psi unless otherwise specified, . Machine screws may also be made from higher strength steels, stainless steel, brass, monel, aluminum alloys or other materials as agreed between the manufacturer and purchaser.

- The threads of machine screws except for the No.0000, No.000 and No.00 sizes shall be Unified standard, Class 2A, UNC and UNF series (or UNRC and UNRF series at option of manufacturer) For threads with additive finish, the maximum diameters of class 2A may be exceeded by the amount of allowance.

- Machine screws with sizes No. 5 and smaller with nominal lengths equal to three times diameter and shorter shall have full thread extending to within one pitch (thread) of the bearing surface of the head, or closer, if practicable. Nominal lengths greater than three times diameters up to and including 11⁄8 in., shall have full thread extending to within two pitches (threads) of the bearing surface the head, or closer, if practicable. Screws of longer nominal lengths shall, unless otherwise specified, have a minimum length of full thread of 1 in.

- Machine screws with sizes No. 6 and larger with nominal lengths equal to three times diameter and shorter shall have full thread extending to within one pitch (thread) of the bearing surface of the head, or closer, if practicable. Nominal lengths greater than three times diameter, up to and including 2 in., shall have full thread extending to within two pitches (threads) of the bearing surface of the head, or closer, if practicable. Screws of longer nominal lengths shall, unless otherwise specified, have a minimum length of full thread of 1.50 in.

- The diameter of body on machine screws shall not be less than the Class 2A thread minimum pitch diameter nor greater than the basic major diameter of the thread.

- Unless otherwise specified, machine screws shall have plain sheared ends.

- The tolerance on the length of machine screws shall conform to the following table for the respective screw types.

  Tolerance on Length for Nominal Screw Size
Nominal Screw Length 0000 Through 00 0 Through 12 1/4 Through 3/4
Up to 1⁄2 in., incl. −0.01 −0.02 −0.03
Over 1⁄2 to 1 in., incl. −0.02 −0.03 −0.03
Over 1 to 2 in., incl. --- −0.06 −0.06
Over 2 in. --- −0.09 −0.09

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. Ex: 1⁄4− 20 × 11⁄4 Slotted Truss Head Machine Screw, Steel, Zinc Plated

Reference:
  • ASME B18.6.3 - 2003, Machine Screws and Machine Screw Nuts