Chapter9

Page No - 1NETTUR TECHNICAL TRAINING FOUNDATION

CHAPTER 9

SECTIONAL VIEWS

Objectives:

At the end of this chapter trainee will be able to:-

Visualize sectional view. Draw sectional views.

Guidelines to instructor:

Explain the types of cams and followers.

Explain the steps for drawing displacement diagram and cam profile.

Need for drawing sectional views:


The orthographic views of an object may not be always give all the information clearly: they give only the external information of the object.The invisible features like holes, slots and internal contour are shown by means of hidden lines. But, when such features are too many or of complex nature, then the use of hidden lines makes the views more complicated and difficult to understand. In order to avoid too many hidden lines and to make the views more understandable, the object is assumed to be cut by one or more planes(imaginary) called section plane (also called cutting plane) in such a way that one portion of the part is assumed to be removed as to expose the required internal details of the object, to the direct view of the observer.

The cutting plane is generally assumed to be parallel to the plane on which the view of the object is projected. The part of the object between the section plane and the observer is assumed to be removed away and the remaining sectioned part is projected to get the sectional view. The sectional view thus, shows not only the shape of the section but also all the visible edges and contours of the object behind the section plane. The surfaces cut by section plane are specially marked by means of hatching lines.

When a section is assumed in one view, it does not affect the other views in anyway. The other views are drawn for the entire object.

The cutting plane line(section line) is included to show the location of the section and the direction of viewing. Section line is a chain thin line thickened at the ends. The direction of viewing the section is shown by arrows resting on the thickened ends and named by upper case letters like A-A,B-B.

The following are the different ways of sectioning a object.a. Full sectionb. Half section c. Local sectiond. Removed section/Revolved sectione. Offset sectionf. Thin section.


Full section:When an object is assumed to be cut right through and the front half( the portion in between the section plane and the observer) is removed, it is said to be in full section. The projection of the remaining portion is known as Full sectional view. In the sectional front view the complex internal details are clearly defined and the hatching lines denote the sectioned area. In the top view the location of the section plane is shown by section line with the direction of view.

Full section views may be classified as 1. Full sectional front view(sectional elevation)2. full sectional top view( sectional plan)3. full sectional right side view( sectional right end view)4. full sectional left side view( sectional left end view)

For obtaining full sectional front view of an object, it is assumed to cut by a section plane parallel to the vertical plane.For obtaining full sectional side view of an object, it is assumed to cut by a section plane parallel to profile plane.Similarly, for obtaining full sectional top view of an object, it is assumed to cut by a section plane parallel to the horizontal plane.

Half section:If the object is symmetrical, then a full section is not essential, since the sectional view will also be symmetrical. For such objects , a half section is enough to show the internal features. In half section, two cutting planes at right angles to each other are assumed to cut the object. That quarter of the object which is between the two cutting planes is removed and the remaining three by four portion of the object is projected to get the half-sectional view. The projection will be drawn half in full view and half in section. The sectioned half and the other half are separated by a centre line.

The various half-sectional views are1. top half in section2. bottom half in section3. left half in section4. right half in section.


Local section (Partial section or Broken section):

When only a small portion of an object is required to be shown in section to view the internal features, sectioning is considered to be done locally. A free hand irregular line of continuous thin type shows the boundary of the section.

Revolved section/Removed section:When the cutting plane is passed at right angles to the axis of the object, a cross section is obtained. The cross sectional view thus obtained is revolved through 900 and drawn on the front view itself, to give a revolved section.A removed section is obtained in the same manner, but the sectional view is drawn outside the main view. Generally the sectional view is drawn with reference to the extension of the cutting plane line.( if it is not convenient, it may be drawn away from its location.)

Offset section:In some objects, the internal features are placed at different positions. To show these internal features a single straight cutting plane is not enough. An offset cutting plane is assumed to cut the object, so that the cutting plane passes through the internal features which are required to be shown in sectional view. The section thus obtained is called as Offset section.

Thin section:Objects of small thickness like sheet metals, steel sections, gaskets, packing materials, shims and washers are shown entirely thickened in sectional view.


General rules for sectioning:1. Hatching is generally used to show areas of sections.

2. The simplest form of hatching is usually drawn by continuous thin line at a convenient angle , preferably 450 to the principal outlines or lines of symmetry of the sections.

3. Separate areas of the section of the same component shall be hatched in an identical manner.The hatching of adjacent components shall be carried out with different directions or spacing.

4. Spacing between the hatching lines should be chosen in proportion to the size of the hatched areas.

5. In the case of large areas, the hatching may be limited to a zone following the contour of the hatched area.

6. where sections of the same part in parallel planes are shown side by side, the hatching shall be identical, but offset along the dividing line between the sections if greater clarity is considered necessary.

7. Hatching shall be interrupted when it is not possible to place inscriptions outside the hatched area. Eg: Dimensioning in hatched area:

Dimension lines are normally shown outside the drawing, but in some unavoidable cases they can be shown inside the drawing itself. However dimension is to be drawn on the hatched area, the hatching lines are broken at the place where dimension value is to be written.

Chapter9

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