Draw10w - Pictorial Drawing (isometric Drawing) 68131n

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Pictorial Drawing (Isometric Drawing)

In axonometric projection, an observer is considered to be at infinity and the visual rays are parallel to each other and perpendicular to the plane of projection. An isometric projection is one in which all angles between the axonometric axes are equal. To produce an isometric projection (isometric means “equal measure), it is necessary to place the object so that its principal edges or axes make equal angles with the plane of projection and therefore foreshortened equally. In this position, the edges of a cube would be projected equally and would make equal angles with each other. The projections of the axes make angles of 120 with each other and are called isometric axes. Any line parallel to one of these is a nonisometric line. The angles in the isometric projection of the cube are either 120 or 60, and

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all are projections of 90 angles. In an isometric projection of a cube, the faces of the cube and any planes parallel to them are called isometric planes. All distances in the isometric scale are 2/3 times the true size. When a drawing is prepared with an isometric scale or when the object is actually projected on a plane of projection, it is called an isometric projection. When a drawing is prepared with an ordinary scale, it is an isometric projection. The isometric drawing is about 25 / larger than the isometric projection, but the pictorial value is obviously the same in both. It is usually advantageous to make an isometric drawing than projection. In constructing an object composed only of normal surfaces, all measurements are made parallel to the main edges of the main enclosing box. Inclined surfaces are located by offset or coordinate measurements along the

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isometric lines. Oblique surfaces may be drawn by establishing the intersections of the oblique surface with the isometric planes. The choice of the directions of the isometric axes should be determined by the position by which the object is usually viewed or by the position that best describes the shape of the object. Nonetheless, the angles between the axes must remain 120. Offset measurements are drawn parallel to certain edges of the main block in the multiview drawings. Hence, they will also be parallel to the same edges in the isometric drawings. Hidden lines are omitted unless they are needed to make the drawing clear. Centers lines are drawn if they are needed to indicate symmetry or if they are needed for dimensioning.

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Objects of rectangular shape may be easily drawn by means of box

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construction, which consists simply in imagining the object to be enclosed in a rectangular box whose sides coincide with the main faces of the object. A box is drawn lightly with construction lines, the features are drawn and finally, required lines are made heavy. Since the only lines of an object that are drawn true length in an isometric drawing are the isometric axes or lines parallel to them, non-isometric lines cannot be set off directly with the scale. Non-isometric lines will not be true length in isometric drawing. Angles project true size only when the plane of the angle is parallel to the plane of the projection. An angle may project larger or smaller depending on its position. Since in isometric, the various surfaces of the object are usually inclined to the plane of projection, it follows that angles generally will not be projected true size. Angular measurements

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must be converted to linear measurements along isometric lines. A regular protractor cannot be used to set off angles in isometric. Isometric protractors for setting off angles on isometric surfaces are available from drafting supply dealers. If the general shape of an object does not conform somewhat to a rectangular pattern, it may be drawn using the box construction. An irregular object may also be drawn by means of a series of sections. Curves may be drawn in isometric by means of a series of offset measurements similar to what was previously discussed. Enough points should be chosen to fix accurately the path of the curve; the more points used, the greater the accuracy. Offset gridlines are then drawn from each point parallel to the isometric axes. A light freehand curve is then drawn through the points. The final curve is heavied with the aid of the

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irregular curve, and all straight lines are darkened to complete the isometric. If a circle lies in a plane that is not parallel to the plane of projection, the circle will be projected as a true ellipse. The ellipse can be constructed by the method of offsets. Draw parallel line spaced at random across the circle; then transfer these lines to the isometric with the aid of dividers. Draw the final ellipse with the aid of the irregular curve. A variation of the method of offsets provides eight points on the ellipse. Circumscribe a square around a given circle, and draw diagonals. Through the points of intersection of the diagonals and the circle, draw another square. Draw this construction in the isometric, transferring distances with the dividers. Another alternate is to divide the circle into 12 parts using a 30 X 60 triangle. Lines parallel to the sides of the square are drawn to these points and the entire construction is done on the isometric. Ellipse guides and ellipsographs are available as aids.

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In drawing objects characterized by open or irregular interior shapes, isometric sectioning is as appropriate as in multiview drawing. In an isometric full section, draw the cut surface first and then draw the portion of the object that lies behind the cutting plane. In an isometric half section, make an isometric drawing of the entire object and then cut the surfaces. At times, isometric broken-out sections are used. Section lining in isometric drawing is similar to that in multiview drawing. Section lining at an angle of 60 with the horizontal is recommended. The direction can be changed if at this angle, the lines would be parallel to a prominent visible line bounding the cut surface or to other adjacent lines of the drawing.

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Two methods of isometric dimensioning are approved by ANSI- namely, the pictorial plane (aligned) system and the unidirectional system. Vertical and not inclined lettering is used for either method. In the aligned system, the extension lines, dimension lines and lettering are all drawn in the isometric plane of one face of the object. The horizontal guidelines are drawn parallel to the dimension lines. The vertical guidelines and barbs of the arrowhead are drawn parallel to the extension lines. In the unidirectional system, the extension and dimension lines are all drawn in the isometric plane of one face of the object. The barbs of the arrowhead should line up parallel to the extension lines. However, the lettering for the dimensions is vertical and reads from the bottom of the drawing. Reference: Giesecke, F. (2001). Technical Drawing. Singapore: Pearson Education Asia.