Threads and threading
A screw thread is a ridge of uniform section in the form of a helix in the internal or external surface of a cylinder. The subject of thread and threading is of vital importance to a marine engineer because every machine or component in an engine room is held together by fasteners having some kind of threads. The threads are used as fasteners, for adjustment, and to transmit power or motion. Junior engineers and the marine engineers in general must be able to recognize a nut or a bolt just by viewing it. At the start of an overhauling operation of a machine the engineer in charge must have a look and tell the duty motorman or the fitter what all spanners to bring. This skill by which an engineer can recognize a nut or bolt is viewed very highly in merchant marine and generally defines the skill and the efficiency of the engineer.
Method of making threads:
The different methods by which thread are manufactured are as follows:
Lathe: Thread cutting is done by chasing tools
Rolling: Or more accurately, cold rolling is the most economical and fastest method of manufacturing.
Screw-broaching: Used in the automotive field.
Sand casting and other casting methods: These methods are rarely used nowadays except in very large applications.
Milling: By using a milling machine and a milling cutter.
Die and tap cutting: It is what is used in the engine room as the other methods are used on shore. It is discussed later in the article.
Grinding: By single or multiple edge grinding wheel.
Right hand and left hand threads:
There are two types of threads in use, right handed and left handed threads. Right hand threads are used extensively in engineering practice for all nuts, bolts, and fasteners; however in some specialized applications left hand threads are used. For example in purifier bowl locking nuts, impeller locking nuts of centrifugal pumps, etc. If a nut advances axially by rotating in a clockwise direction it is a right hand thread, but if it advances axially by rotating anticlockwise, then it is left handed, which is also defined as a screw thread that is screwed in by rotating counterclockwise.
Start of threads and Pitch:
A thread can be a single start, double start, or multiple starts. In the case of a single thread a single thread groove would be all over, but in the case of multiple start threads, multiple thread grooves, equidistant from each other would be all over. The purpose of a multiple start threading is to give a faster axial movement, for same rotation of the nut. The axial movement of the nut in one rotation is called the lead of the thread. In a single start thread the pitch of the thread is the lead. In the case of multiple start threads the pitch is equal to the lead divided by the number of starts.
Pitch = Lead/no. of start
The pitch of a thread can be measured very easily by the help of a vernier caliper. Simply measure the distance of ten thread ridges or crest. The measured value divided by ten would be the pitch of the thread in a single start thread. Of course, because of some human error and the error of the vernier caliper, you would have to round the value. Generally pitch figures are like 1.0 mm, 1.5 mm, 1.75 mm, and 2.0 mm etc.