A screw thread is the functional element used on a bolt, stud , set-screw or any other threaded piece or component. A screwed piece consist essentially of a cylinder of round core along along which a projection, of thread,winds its way in the form of a helix or, it can be said that a screw thread is formed by cutting helical groove on a cylindrical surface (outside or inside). All screwed pieces fall under the category of temporary fasteners .
screw thread and fastening by screwed pairs have frequent occurrence in the machine parts. The usual form, method of representation and proportions of screw thread of prime important in machine drawing and hence should be clearly understood .
HELIX
It is the curve generated by a point moving the surface of a cylinder or cone in circumferential direction at a constant angular speed and with a simultaneous uniform rate of advance in an helix direction. The amount of this advance for one revolution is called lead of the helix.
Thread Term And Nomenclature
1. Screw Thread - It is ridge of uniform cross section produced by forming on the outside or inside surface of a cylinder or a cone a continuous helical groove , such that the distance measured parallel to the axis between to corresponding point on this contour is proportional to their relative angular displacement abut the axis.
2. EXTERNAL THREAD - A thread on the outside surface of a member such as bolt stud or screw.
3. Internal Thread - A thread on the inside surface member such as a threaded hole or a nut.
4. Crest- The top surface joining the two sides of a thread , or that part connecting adjacent flanks at the top of the groove.
5.Root- The bottom surface joining the sides of the two adjacent threads, or the part which connects adjacent flanks at the bottom of the groove.
6. Angle of Thread - It is the angle between the flanks, measured in an axial plane.
7. Flank - It is the surface between the crest and the root of a thread.
8. Depth of Thread - It is the distance measured perpendicular to the axis, between crest and root.
9. Pitch - It is the distance measured axially between corresponding point on the consecutive thread forms in the same axial plane and on the same side of the axis.
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