A Text Book on Engineering Graphics - Central Board of Secondary ...

CHAPTER 2
2.1 INTRODUCTION
2.2 SCREW THREAD
MACHINE DRAWING
A. DRAWING OF MACHINE PARTS
In our day to day life, we come across many objects where bolts and nuts are used to join two
pieces together. For example we use wooden furnitures like desks, stools, tables etc. in school,
showing bolts, nuts and screws. Such machine parts which are used to connect two pieces
together are called as fasteners. There are two types of fasteners, viz, temporary fasteners and
permanent fasteners. Threaded fasteners like bolt and nut are temporary fasteners. The process
of joining different machine parts of machine or engineering products is called as fastening.
Permanent fastening such as welding, riveting etc. join two parts together permanently and they
cannot be separated without breaking the fastening, but in the case of temporary fastening, the
parts are joined together temporarily and can be separated easily without breaking the
fastening.
HELICAL SCREW THREAD
Fig 2.1 a
Recall that we have studied helix in class XI. A continuous helical groove cut along the outer
circumference of a cylindrical surface is called a screw thread. A screw thread is an operating
element of temporary fastening. Screw thread occurs on practically all engineering products.
FIG.2.1 shows a screw thread/helical groove on a cylindrical rod.
40 ENGINEERING GRAPHICS