Dimension and Extension line placement 1. Always place shorter dimensions nearest to the object lines. Dimension lines should never cross. However, extension lines may cross each Other. 2. Extension lines should never be shortened. 3. Dimension lines should never coincide with any object line or center line of the drawing. Therefore no line should be used as dimension line nor coincide with a dimension line. 4 Dimensions should line up in chain fashion or be grouped together as much as possible. S. Do not repeat a dimension.
Repeating a dimension will only confuse the reader and could cause errors in the manufacturing process. 6. Dimensions should be given in views here the shapes are presented in profile and/or contour. 7. Dimensions should always be placed tot or outside tot view where possible. 8. Hole features must be located and given size in the view where they appear as a circle not as a rectangle or hidden lines. Never dimension to hidden lines. 9. Avoid a complete chain of dimensions. Either omit one or use reference notation. Reference notation indicates that a dimension is used for information purposes only.
Indicate a reference dimension by placing a parenthesis around the dimension 10. Cylinder location and size constraint. Locate cylinders in the auricular view but give its diameter and length in the rectangular view. Dimensioning Symbols 1. Symbols represent specific information which could be difficult to duplicate in note form. They aid in clarity, ease Of CAD drawing presentation, and above all, save time. Seven such symbols are show below. 2. Local Notes: Local notes apply to specific features only and are connected to a feature by a leader.
Notes should always be lettered horizontally. Leader arrows for notes should always point toward the circular view of the hole feature and if extended, would pass through the center. Direction of Dimension Figures TWO systems Of placing dimensions are used: -k unidirectional(Mechanical): Dimensioned numeral values and text are placed so that they can be read only from the bottom Of the drawing. * Aligned(Architectural): Dimensioned numeral values and text are placed so that they can be read either from the bottom or right side of the drawing.
Ordinate Dimensioning Ordinate dimensioning is also known as Datum dimensioning or Baseline dimensioning. Dimensions may be applied either using dimension lines or arrowheads or without such. A much clearer method to presenting a part consisting of numerous holes is Tabular Ordinate dimensioning, This method involves labeling each hole feature with a letter and then providing a Hole Chart indicating the X location constraint and Y location constraint with a complete size description of each hole feature.
This eliminates the need for both dimension and extension lines to locate hole features, SECTIONING On many occasions, the interior of an object is complicated or the component parts of a machine are drawn assembled. The interior features are represented by hidden lines in usual orthographic views, which results in confusion and difficulty in understanding the drawing. In order to show such features clearly, en or more views are drawn as if a portion had been cut away to reveal the interior. This procedure is called sectioning and the view showing the cut away picture is called section view.
A section is an imaginary cut taken through an object to reveal the shape or interior construction. The imaginary cutting plane is projected on a standard view so that the sectional view with orthographic representation is obtained. A sectional view must show which portions of the object are solid material and which are spaces. This is done by section lining (cross-hatching) the solid parts with uniformly spaced thin lines generally at 45џ, Section Views The technique called section views is used to improve the visualization of new designs, clarity emulative drawings and decapitate the dimensioning to drawings.
For mechanical drawings section views are used to reveal interior features of an object that are not easily represented using hidden lines. Sectional drawings are emulative technical drawings that contain special views of a part or parts, a view that reveal interior features. In the figure a regular emulative drawing and a sectioned emulative drawing of the same part in the front view, the hidden features can be seen after sectioning. Traditional sections views are based on the use of an imaginary cutting plane that cuts through the object to reveal interior features.
What are the types of sections? A cutting plane does not necessarily need to cut the whole object. There are three major types of sections used in engineering drawing: * full section – The view obtained even the cutting plane is right across the object. K half section – The view Obtained When the cutting plane goes half way across the Object to the centre line. It is used for symmetrical objects (the same either side of the centre line). The cutting plane only removes one quarter Of the Object. Part section – A method of showing internal detail for one small section of a drawing only.
Each sectional view is linked to a cutting plane identified by a subtitle placed below the view egg SECTION A-A, SECTION B. A. Full sectional views When the cutting plane is right across the object it results in a full sectional view (commonly referred to as a full section). Half sectional views Halt sectional views are used when an object is symmetrical (the same either side of the centre line), One half is used as a sectional view to shove the inside and the other half shows the outside view. The cutting plane only removes a quarter of he object.
