Type of threads: There are mainly two types are:-
External threads: The threads cut on the outside surface of a male member, such as a bolt, stud, or screw, are known as external threads.
Internal threads: The threads cut on the inside surface of a hollow cylindrical body (female member), such as a nut, ring or a socket, are called internal thread.
Elements of threads: The terms used in relation to the screw threads are called “elements of threads”.
Major diameter (D): It is the largest diameter of a screwed part measured normal to the axis of the threads. It is also known as outside diameter or normal threads.
Minor diameter (d): It is the smallest diameter of screw parts measured normal to the axis of the threads. It is also known as rod diameter or core diameter.
Pitch (P):- It is the distance between two corresponding points on two adjacent threads, measured parallel to the axis of the threads.
Pitch diameter (pd): For cylindrical screwed parts it is the diameter of that imaginary cylinder of which the surface will intersect the threads at such points where the widths of the threads will equal the adjacent widths of the spaces between them.
Crest: It is the top most point on the outer surface where the two flanks of a thread join.
Root: It is the inner most point in the thread gulley which represents one extremity of the core diameter or minor diameter.
Depth of thread (t): It is the distance between crest and root of a thread measured normal to the axis of the screwed parts and is given by {t=D-d/2}
Thread angle (Alfa): It is the total included angle between the flanks of a thread or between the flanks of two adjacent threads.
Lead: It is the distance a screwed part advances parallel to its axis in one rotation. In single start threads it is equal to the pitch of the threads.
Right and left-hand threads: When we looked at a screwed part in a direction normal to its threads will be found slopping downwards from top, from left to right or from right to left than ¼ inch diameter. This threads cures in include starts of threads angle of 47.6 degree or with crest and root rounded.
Start of threads: There are many starts of threads:-
1. Single starts threads: Lead and pitch are same or equal.
2. Double starts threads: Pitch has no change but lead has double of pitch. [p= lead/2]
3. Triple starts threads: Pitch has no change but lead has triple of pitch. [p= lead/3]
The pitch will be equal to the lead in case of single start threads.
Forms or types of screw threads: The main thread forms are the following:
British standard with worth (b.s.w.) threads: This threads from ,which carries an included angle of 55 degree ,was hitherto the most widely used threads from on most types of bolts, nuts, studs etc., but I is now being largely replaced by metric threads.
British association (b.a.) threads: They are preferred over B.S.W. threads for all screwed parts having less than 1/4 “diameter. There threads carry an included angle of 47 *1/2 degree with both crests and roots rounded. It is generally used small instrument screws.
Buttress threads: It is a combination of V and square threads with one flank normal and the other included at 45 degree to the axis and roots and crests all flat. But, it has a disadvantage that for strength it can be effectively used in only one direction. Some applications of three threads can be seen in carpenters vices, airplane- jacks, and screw of bench vices.
Seller’s threads or American national threads: These threads, adopted by U.S.A. carry an included angle of 60 degree. They are of ‘V’ shape and both crests and roots are flat, in U.S.A.
Square threads: Square threads carry their roots and crests flat and both flanks normal to the axis of the threads. They are used for power transmission and quicker movements of screwed parts.
Acme threads: It is a modified form of square threads, combining the advantage of strength of the former with specific feature of quick engagement and disengagement. Cutting of these threads is relatively easier. Their main application is in power transmission.
A common use of these can be seen on the lead screw and slit half nut of a lathe machine. The angle in ACME threads is 29 degree.
Knuckle threads: It is another modified form of square threads with both crests and root made semi –circular. They enable a quick power transmission and smooth operation and can successfully withstand heavy wear and rough handling. Their production through machining is relatively difficult.
They are best produced through rolling and casting. They are used in coupler of railway carriages and electrical bulbs.
Unified threads: These threads, mostly adopted by western countries, carry an included angle of 60 degree. It will be noticed that the shapes of roots and crests vary in internal and external threads.
U.N.C. = Unified course threads.
U.N.F. = Unified fine threads.
U.N.S. = Unified special threads.
I.S.O. threads: International organization for standardization has adopted the basic profile of unified the ends and has developed two main series of threads, one in inch system and the other in metric system. The B.I.S. formerly known as “INDIAN STANDARDS INSTITUTION” (I.S.I.) has adopted the basic profile of I.S.O. metric threads.
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