Machinery's Handbook, 31st Edition
BRITISH Machine Screws 1803 Ends of Screws: When screws are made with rolled threads, the “lead” formed by the thread rolling operation is normally regarded as providing the necessary chamfer and no other machining is necessary. The ends of screws with cut threads are normally finished with a chamfer conforming to the dimension in Fig. 1a through Fig. 1d . At the option of the manufacturer, the ends of screws smaller than M6 (6-mm diameter) may be finished with a radius approximately equal to 1 1 ∕ 4 times the nominal diameter of the shank. When cone point ends are required, they should have the dimensions given in Fig. 1a through Fig. 1d .
Nominal Length
Nominal Length
90
d
d
Cut Thread Chamfered End Fig. 1b. Chamfer to Extend to Slightly Below the Minor Diameter
Fig. 1a. Rolled Thread End (Approximate Form as Rolled)
90
d
Radius Approx. 1 1 / 4 d
d
Nominal Length
Nominal Length
Fig. 1c. Cut Thread Radiused End (Permissible on Sizes Below M6 Diameter)
Fig. 1d. Cone Pointed End (Permissible on Cut or Rolled Thread Screws, but Regarded as “Special”)
Dimensions of 90-Degree Countersunk Head Screws: One of the appendices to this Brit ish Standard states that countersunk head screws should fit into the countersunk hole with as great a degree of flushness as possible. To achieve this condition, it is necessary for the dimensions of both the head of the screw and the countersunk hole to be controlled within prescribed limits. The maximum or design size of the head is controlled by a theoretical diameter to a sharp corner and the minimum head angle of 90 degrees. The minimum head size is controlled by a minimum head diameter, the maximum head angle of 92 degrees and a flushness tolerance (see Fig. 2). The edge of the head may be flat or rounded, as shown in Fig. 3 .
90°
Theoretical Sharp
Minimum Head Dia.
Sharp Corners Removed
Maxiumum Envelope Minimum Envelope Flushness Tolerance (Max.)
Flat Edge
92°
Fig. 2. Head Configuration
Fig. 3. Edge Configuration
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