Welding Engineer Caricature

Fig 18/7 Changes in direction of a section line

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1ST ANCLE WWJECTIO*

HALF StCnON ON T-Y

HALF StCnON ON T-Y

SECTION Z-Z ONE MOL Ol NC SHOW*, roe CLAWTYI

SCREW THREADS The screw thread is probably the most important single component in engineering. The application of the screw thread to nuts, bolts, studs, screws, etc. provides us with the ability to join two or more pieces of material together securely, easily and. most important of all, not permanently. There are other methods of joining materials together but the most widely used ones—riveting, welding and (very common these days) using adhesives. are all permanent. It is true that these methods are cheaper, but when we know that we might have to take the thing apart again we use the screw thread. Since the screw thread is so important it is well worth while looking at the whole subject more closely.

The standard thread, in this country, for many years was the Whrtworth; this thread was introduced by Sir Joseph

Whitworth in the 1840's. It was the first standard thread, previously a nut and bolt were made together and would fit another nut or bolt only by coincidence At the time, it was a revolutionary step forward.

The 8SW thread and its counterpart the BS Fine thread were the standard threeds in this country until metrication and will probably be in use for many years.

However, the U.S.A. developed and adopted the Unified threed as their standard end countries using the Metric System of measurement had their own Metric thread forms. It became increasingly obvious that an international screw thread was needed

As far as this country was concerned, the breekthrough came when it was decided that British Industry should adopt the Metric System of Weights and Measures The International Organisation for Standardization (ISO) has formulatod a complex set of standards to cover the whole range of engineering components.

Their thread, the ISO Is now the international standard thread. The ISO and Unified thread profiles are identicel. The Unified thread is the Standard International thread for countries which are still using Imperial units. The ISO basic thread form is shown in Fig. 18/9.

Drawing Template Basic Geometric Forms

Fig 18/9 Basic form of ISO thread

Drawing Template Basic Geometric Forms

Fig. 18/14 Buttress thread profile

The Buttress threed. Fig 18/14. combines the vn threed and the square thread without retaining any of their disadvantages. It is a strong thread and has less friction then a vee thread. Its main application is on the engineer's vice although it is sometimes seen transmitting power on machines.

Fig. 18/14 Buttress thread profile

Drawing screw threede

Drawing a screw thread properly is a long and tedious business. A square thread has been drawn in full in Fig. 15/12 and you can see that this type of construction would take much too long a time on a drawing that has

EXTERNAL

several threads on it and would be physically impossible on a small thread.

There are conventions for drawing threads which make life very much easier Three conventional methods of representing screw threads are shown in Fig. 18/15. The

INTERNAL

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(Used only in technical illustrations)

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