15. Lines on an engineering drawing signify more than just the geometry of the object and it is important that the appropriate line type is used. Line Thickness For most engineering drawings you will require two thickness', a thick and thin line. The general recommendation are that thick lines are twice as thick as thin lines. Line Styles Other line styles used to clarify important features on drawings are: A thin line is used for hatching, leader lines, short centre lines, dimensions and projections. A thick continuous line is used for visible edges and outlines. Dashed lines are used to show important hidden detail for example wall thickness and holes.. Thin chain lines are a common feature on engineering drawings used to indicate centre lines. Centre lines are used to identify the centre of a circle, cylindrical features, or a line of symmetry.
18. Dimensioning A dimensioned drawing should provide all the information necessary for a finished product or part to be manufactured. An example dimension is shown below. Dimensions are always drawn using continuous thin lines. Two projection lines indicate where the dimension starts and finishes. Projection lines do not touch the object and are drawn perpendicular to the element you are dimensioning. All dimensions less than 1 should have a leading zero. i.e. .35 should be written as 0.35
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Editor's Notes
By definition for each element of a orthographic projection drawing you only present 2 of the three dimensions. Think of it as an observer look at one face, what do they see. Any orthographic projection drawing normal has three views… Front view, Top view and side view (Right or left side view)
Although any face could be chosen to be the front, once front and two other face are selected all are determined. There are really SIX PRINICPAL VIEWS as defined in the diagram. Generally do not need all six to fully describe the object. A conventional Engineering Drawing will normally have 2 to 3 views unless it required more views to describe the geometry/ profile. We know which ones they are on the drawing, because we always present them in the same relationship to each other. I.e. Top above front, right to right of front, etc. This convention is called as the Third angle method.. The other method in which the views can be placed is the First angle method in which the Top view is below front view, Right side view is on left side of front view. For this class we will be following the Third angle convention. These are often called orthographic projections – because the line of sight is perpendicular to the principal view
Pick the views which will help in describing the object with highest clarity. Explain what is an auxiliary view. Explain that they are drawn to show specific features that are not clear in the Principal views.
The Idea is to have them take an object from the table. Declare front. FRONT View is the MOST DESCRIPTIVE VIEW OF THE OBJECT. The view that gives MORE INFORMATION ABOUT THE OBJECT. Rotate 90 degrees “up” to get top view. Rotate Back. Rotate 90 degrees clockwise to get right side. This give three principal views commonly used.
At this point, give an introduction to Glass-box approach for developing orthographic projection drawings. Student slides contain snapshots of the animation
The object, whose orthographic projection needs to be drawn, is enclosed in a glass-box
Project points on the front view of the glass-box
Project points on the the top view of the glass-box, just as done for front
Project points on the right view of the glass-box, just as done for front and top
Unfold the glass box, see how the views align
Unfold the glass box, see how the views align
Instructor: Third angle projection is normally used in the US while Europe uses the First Angle projection. Note the symbols at the bottom of each one which tell you which projection that you are viewing. These can be confusing to students. We are only highlighting the fact that there are different ways to represent projections. It is not expected for students to fully understand the differences. From Fundamentals of Graphic Communications by Bertoline, McGraw-Hill
Note that the views are placed and aligned in the manner shown in the diagram. Remind the students that they have to follow the above convention for all their home work problems and exam problems. It is very important to maintain the alignment and correct placement relative to each other. Means line for top (and bottom) is straight across for both front view and right side view for example. Same thing between front and top for sides. Note : The following can be seen from the slide: Top View and front view have the same width Front View and Right / Left side view have the same height. The depth of Top view is same as the width of right/ left side view.
From: Bertoline, Figure 2.40/ Pg 43 Note the thickness specified for given three types of lines The precedence of lines governs which lines are drawn when more than one line occupies the same position on a drawing. For example the figure above shows that while a visible line takes precedence over all other lines, hidden line and cutting plane line take precedence over center lines. Standard engineering drawing practices requires the use of standard linetypes, which are called the alphabet of lines. The sizes show the recommended line thicknesses.
From Bertoline: Figure 2.38 / Pg 42 In engineering and technical drawing, it is important that hidden features be represented, so that the reader of the drawing can clearly understand the object. Thus we need hidden lines to emphasize that those features exist and are hidden in that particular view. We also need center lines to understand how the features defined in the 2D views translate into 3D. NOTE: It must be emphasized that hidden lines and center lines are used only on Orthographic projection drawings, never on isometric drawings Q: Do we need a convention for what line to show if two lines fall on top of each other? A: Yes! Otherwise features which are more important (eg: visible lines) would be overridden by less important features (eg: hidden lines) and the resulting drawing would be interpreted inaccurately. The next slide shows the convention followed.
![Orthographic Projections ,[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)