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Engineering Graphics, Class 8 Orthographic Projection Mechanical Engineering Department

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### Transcript of class 8 - Orthographic Projection€¦ · Orthographic Projection Orthographic projection is a...

Engineering Graphics, Class 8Orthographic Projection

Mechanical Engineering Department

Multi view drawings Multi view drawings provide accurate shape descriptions of objects.

Using multi-view drawings, the three dimensional object information can be represented in paper, which is a flat, two-dimensional drawing space. This is done by drawing images of the object from multiple directions.

Technical drawings usually use lines to outline the object’s features. Visual qualities such as color and texture are specified in writing.

Orthographic Projection

Orthographic projection is a system of drawing views of an object using perpendicular projectors from the object to a plane of projection. It is the most popular method of shape description, wherein an undistorted image of the object appears in a flat, transparent, but imaginary projection plane.Between the observer and the object, imagine a plane or a pane of glass that represents a plane of projection. The outline on the plane of projection shows how the object appears to the observerTheoretically, the observer is at infinite distance from the object so that the lines of sight are parallel

Orthographic Projection

Start by imagining perpendicular lines, or projectors, from all points on the edges or contours of the object to the plane of projection.Each projector will pierce the plane of projections at a single point.Projector from point 1 on the object pierces the plane of projection at point 7, which is a view or projection of the pointThe same procedure applies to point 2, whose projection is point 9Since 1 & 2 are end points of a straight line on the object, the projections 7 & 9 are joined to give the projection of the line 1-2

Orthographic Projection

The same procedure can be applied to curved lines (see the top curved contour of the object)

Point 5 on the curve is projected to the plane at 6

The projection of an infinite number of such points on the plane of projection results in the projection of the curve

Projection Methods

Frontal plane: is the plane of projection on which the front view is projected

Horizontal plane: is the plane of projection on which the top view is projected

Profile plane: is the plane of projection on which the side view is projected

TOP VIEW R SIDE VIEWFRONT VIEW

The Glass Box

If planes of projection are placed parallel to the principle faces of the object, they form “glass box”

In this method, the observer is always on the outside looking in, the object is seen through the planes of projection

Since the glass box has six sides, six views of the object can be obtained

The Glass Box

To show the views of a solid object on a flat sheet of paper, it is necessary to unfold the planes so that they will all lie in the same plane

All the planes except the rear plane are hinged to the frontal plane,

The rear plane is hinged to the left-side plane

The Glass Box

Each plane is revolved outwardly from the original box position until it lies in the frontal plane, which remains stationary.

The hinge lines of the glass box are known as folding lines

Observe that lines extend around the glass box from one view to another on the planes of projection. These are the projections of the projectors from points on the object to the views

The Glass Box

The projector 1-2 is projected on the horizontal plane at 7-8 and on the profile plane at 16-17

When the top plane is folded up, lines 9-10 and 7-8 will become vertical and lines up with 10-6 and 8-2, respectively

Thus, 9-10 & 10-6 form a single straight line 9-6, and 7-8 & 8-2 form a single straight line 7-2

This is explains why the top view is the same width as the front view & why it is placed directly above the front view.

The Glass Box

The front, top, and bottom views line up vertically and are the same width

The front, left side, rear & right-side views line up horizontally and are the same height

The Glass Box

Note that lines OS & OW and lines ST & WX are respectively equal.

Thus, it seems that the top view must be the same distance from the folding line OZ, as the right side view is from the folding line OZ

The Glass Box

Since the back view is a mirror image of the front view, the bottom is a mirror of the top, and the L side is a mirror of the R side, there are three redundant views in the glass box.

The standard, or most common, views are the front, top, and right-side views.

Folding Lines

Folding line HF, between the front and the top views is the intersection of the horizontal and frontal planes

Folding line FP, between the front and the right-side views is the intersection of the frontal & the profile planes

The distances X & Y, from the front view to the respective folding line, are not necessarily equal, since they depend on the relative distances of the object from the horizontal and profile planes.

However, distance D1, from the top and side views to the respective folding lines, must always be equal

Folding Lines

The views may be any desired distance apart and the folding lines may be drawn anywhere between them, as long as distances D1 are kept equal and the folding lines are at right angles to the projection lines between the views.

The folding lines may be omitted from the drawing if desired.

Surfaces, Edges, and Corners

To analyze multiview projections, the component elements that make up most solids must be considered.

A surface may be frontal, horizontal, or profile, according to the plane of projection to which it is parallel

If the plane surface is perpendicular to a plane of projection, it appears as a line or edge view (EV), (a)

If it is parallel, it appears as a true size (TS), (b)

If it is situated at an angle, it appears as a foreshortened (FS) surface, (c)

Surfaces, Edges, and Corners

Intersection of two plane surfaces produces an edge, or a straight line, such a line is common for both surfaces and forms a boundary line for each.

If an edge is perpendicular to a plane of projection, it appears as a point (a)

If it is parallel to the plane of projection, it shows true length (b)

If not parallel, it appears foreshortened (c)

Thus, a straight line, always projects as a straight line or as a point

A corner, or a point, is the common intersection of three or more surfaces or edges

A point appears as a point in every view

In the figure lines divide the view into three areas. Each area must represent a surface at a different level

Surface A may be high, and surfaces B & C lower (b), or B may be lower than C (c)

Or B may be highest, with C and A each lower (d), or one or more surfaces may be inclined (e), or one or more surfaces may be cylindrical (f)…and so on

No two adjacent areas can lie on the same plane

Since an area (surface) in a view may be interrupted in several different ways, other views must be observed to determine which interpretation is correct

Similar shapes of surfaces

If a surface is viewed from several different positions, it will in each case be seen to have a certain number of sides and to have a certain characteristic shape

An L-shape surface (a) will appear as an L-shaped figure in every view in which it does not appear as a line

A T-Shaped surface (b), a U-shaped surface (c), or a hexagonal surface (d) will in each case have the same number of sides & the same characteristic shape in every view in which it appears as a surface

Using a Miter Line to Transfer Depth

Draw a miter line at 45º at a convenient distance to produce the view.

Sketch light lines projecting depth locations from points to miter line and then down into the side view as shown.

Using a Miter Line to Transfer Depth

Project additional points surface by surface.

Draw the view locating each vertex on the surface on the projection and miter line

More Inclined Surfaces

Cylindrical Surfaces

The single cylindrical surface is intersected by two plane (normal) surfaces forming two curved lines of intersection or circular edges (the bases of the cylinder). These circular edges are the only actual edges on the cylinder.

The cylinder (body or hole is represented on a drawing by its circular edges and the contour elements. An element is a straight line on the cylindrical surface, parallel to the axis of the cylinder.

Three views of a right-circular cylinder are shown. The right circular edges appear in the top views as circles A, and in the front view as horizontal lines 5-7 and 8-10, and in the side views as horizontal lines 11-13 and 14-16.

The contour elements 5-8 and 7-10 in the front view appear as points 3 and 1 in the top views. The contour elements 11-14 and 13-16 in the side views appear as points 2 and 4 in the top views.

Cylindrical Surfaces Example - Machining a Cap (1)

A first stage of machining a cap is shown. In stage 1, the removal of the two upper corners forms cylindrical surface A which appears in the top view as surface 1-2-3-4, in the front view as arc 5, and in the side view as surface 8-9-Y-X.

In the second stage, a large reamed hole shows in the front view as circle 16, in the top view as cylindrical surface 12-13-15-14, and in the side view as cylindrical surface 17-18-20-19.

Cylindrical Surfaces Example - Machining a Cap (2)

In the third stage, two drilled and counterbored holes are added producing four more cylindrical surfaces and two normal surfaces. The two normal surfaces are those at the bottoms of the counterbores.

Cylindrical Surfaces Example - Machining a Cap (3)

In the 4th stage, a cylindrical cut is added, producing two cylindrical surfaces that appear edgewise in the front view as arcs 30 and 33. In the top views, the cylindrical surfaces appear as surfaces 21-22-26-25, and 23-24-28-27 and in the side view as surfaces 36-37-40-38 and 41-42-44-43.

Cylindrical Surfaces Example - Machining a Cap (4)

Representation of Holes – Blind Drilled Holes

A drilled hole is a through hole if it goes through the member.

If the hole has a specified depth, the hole is called a blind hole. The depth includes the cylindrical portion of the hole only.

The point of the drill leaves a conical bottom in the hole drawn with the 30º-60ºtriangle.

Representation of Holes – Through Drilled or Reamed Holes

A through drilled or reamed hole is drawn as shown. The note tells how the hole is to be produced.

The tolerance is ignored when actually laying out the hole.

Representation of Holes – Drilled and Counterbored holes

A drilled and counterbored hole is a hole that is drilled then the upper part is enlarged cylindrically to a specified diameter and depth.

Wrong!

Representation of Holes – Drilled and Countersunk holes

A drilled and countersunk hole is a hole that is drilled then the upper part is enlarged conically to a specified angle and diameter.

Wrong!

Visualization

The ability to visualize or think in three dimensions is one of the most important abilities of successful engineers, and scientists.

In practice, this means the ability to study the views of an object and to form a mental picture of it, i.e. to visualize its three dimensional shape

To the designer it means the ability to form, a mental picture before the object even exists and the ability to express this image in terms of views.

Visualization

Visualization

The engineer is the master planner in the construction of new equipment, structures, or processes

The ability to visualize and to use the graphics language as a means of communication or recording of mental images is indispensable

Even experienced engineers may not be able to look at a multiview drawing and instantly visualize the object

It is necessary to study the drawing, to read the lines in a logical way, and to piece together the little things until a clear idea of the whole emerges .

Alternate Positioning of Views

If three views of a wide, flat object are drawn using the conventional arrangement of views, a large wasted space is left on the paper

In such cases, the profile plane may be considered hinged to the horizontal plane instead of the frontal plane

This places the side view beside the top view, this results in better spacing & sometimes makes the use of a reduced scale unnecessary

Alternate Positioning of Views

If three views of a wide, flat object are drawn using the conventional arrangement of views, a large wasted space is left on the paper

In such cases, the profile plane may be considered hinged to the horizontal plane instead of the frontal plane

This places the side view beside the top view, this results in better spacing & sometimes makes the use of a reduced scale unnecessary

Partial Views

A view may not need to be complete but may show only what is necessary for the clear description of the object. Such a view is a partial view

A break line may be used to limit the view, as shown in (a) and (b).

If symmetrical, a half-view may be drawn on one side of the center line (c), or a partial view, “broken out”may be drawn (d).

Do not place a break line where it will coincide with a visible or hidden line

Partial Views

Occasionally the distinctive features of an object are on opposite sides.

In either complete side view there will be a considerable overlapping of shapes.

In such cases two side views are often the best solution. The views are partial views and in both, certain visible and invisible lines have been omitted for clarity.

Revolution Conventions

Regular multiview projection may sometimes become awkward, confusing, or actually misleading

In part (a) there are three triangular ribs, three holes equally spaced in the base, and a keyway.

Regular projection causes the lower rib to appear in a foreshortened position and the holes do not appear in true relation to the rim. Additionally, the keyway is projected as a confusion of a hidden line.

In this case, the revolution convention shown in part (c) is preferable.

Revolution Conventions

Features are revolved in the front view to lie along the vertical center line, from where it is projected to the correct side view.

Removed Views

Features are revolved in the front view to lie along the vertical center line, from where it is projected to the correct side view.

Removed Views

A removed view is a complete or partial view removed to another place on the sheet so that it is no longer in direct projection with any other view

A removed view may be used to show some feature of the object more clearly, possibly to a large scale, or to save drawing a complete regular view.

Removed Views

A viewing-plane line is used to indicate the part being viewed; the arrows at the corners show the direction of sight.

The removed view should be labeled VIEW A-A or VIEW B-B and so on, the letters refer to those placed at the corners of the viewing-plane line.

Intersections and tangencies

Fillets and Rounds

A rounded interior corner is called a fillet, and a rounded exterior corner is called a round.

In cast or forged objects, two intersecting rough surfaces produce a rounded corner. If one or both of these surfaces is machined, the corner becomes sharp. On a drawing, a rounded corner means that both intersecting corners are rough, and a sharp corner means that one or both surfaces has been machined.

On working drawings, fillets and rounds are never shaded.

Conventional Fillets, Rounds and Runouts

A runout is produced when a filleted or rounded corner between two plane surfaces intersects a cylindrical surface. The small curve of intersection is called a runout, and is shown in the drawing to represent this intersection.

In the figures below, the radiuses of the runouts differ because of the different shapes of the horizontal intersecting members.

Conventional Fillets, Rounds and Runouts

A runout results when a rounded web intersects a rounded corner.

At (e), the top surface of the web is flat with only slight rounds along the edge, while at (f), the top surface of the web is considerably rounded.

When two different sizes of fillets intersect, (g), the direction of the fillet is dictated by the larger fillet.

First Angle Projection

If the vertical and horizontal planes of projection are considered indefinite in extent and intersecting at 90º with each other, the four dihedral angles produced are the first, second, third and fourth angles as shown. The profile plane intersects these two planes, and may extend into all angles.

If the object is placed below the horizontal plane and behind the vertical plane, as in the glass box, the object is said to be in third angle projection. In this case, the observer is always outside, and is looking in, so that for all views, the lines of sight proceed from the eye through the planes of projection, and to the object.

First Angle Projection

If the object is placed above the horizontal plane, and in front of the vertical plane, the object is in first angle. In this case, the observer always looks through the object and to the planes of projection.

In this case, the views are projected from the object into a plane. When the planes are unfolded, the right-side view falls at the left of the front view, and the top view falls below the front view.

The only ultimate difference between third-angle and first-angle projection is in the arrangement of the views. The views themselves are the same in both systems.

First Angle Projection

The symbols used to specify first angle or third angle projections are those of a truncated cone. The circles are visible when viewed from the smaller diameter side.

In the first angle projection, these visible circles are projected towards the large diameter side of the cone, but in the third angle projection they are projected towards the small diameter side, from where they are viewed.

In the US and Canada, the first angle projection is the current standard