Oblique projection

In oblique projection, the object is aligned such that one face (the front face) is parallel to the picture plane. The projection lines are still parallel but they are not perpendicular to the picture plane.

This produces a view of the object that is 3D. The front face is a true view (see Figure 2.7). It has the advantage that features of the front face can be drawn exactly as they are, with no distortion. The receding faces can be drawn at any angle that is convenient for illustrating the shape of the object and its features. The front face will be a true view, and it is best to make this one the most complicated of the faces. This makes life easier! Most oblique projections are drawn at an angle of 45 ~ and at this angle the foreshortening is 50%. This is called a Cabinet projection. This is because of its use in the furniture industry. If the 45 ~ angle is used and there is no foreshortening it is called a Cavalier projection. The problem with Cavalier projection is that, because there is no foreshortening, it looks peculiar and distorted. Thus, Cabinet projection is the preferred method for constructing an oblique projection.

An oblique drawing of the bearing bracket in Cabinet projection is shown in Figure 2.8. For convenience, the front view with circles was chosen as the true front view. This means that the circles are true circles and therefore easy to draw. The method of construction for oblique projection is similar to the method described above for isometric projection except that the angles are not 30 ~ but 45 ~ .

Enclosing rectangles are again used and transposed onto the 45 ~ oblique planes using 50% foreshortening.

Engineering Drawing for Manufacture

by Brian Griffiths

Publisher: Elsevier Science & Technology Books

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Critical Role of Computers in Modern Manufacturing

A number of steps are involved in manufacturing a part from its conceptualization to production. They include product design, process planning, production system design, and process control. Computers are used extensively in all these stages to make the entire process easier and faster. Potential benefits of using computers in manufacturing include reduced costs and lead times in all engineering design stages, improved quality and accuracy, minimization of errors and their duplication, more efficient analysis tool, and accurate control and monitoring of the machines/processes, etc. Some of the applications of computers in manufacturing are shown in Figure 1.5. In computeraided design (CAD), computers are used in the design and analysis of the products and processes. They play a critical role in reducing lead time and cost at the design stages of the products/process. Also, computers may be utilized to plan, manage, and control the operations of a manufacturing system: computer-aided manufacturing (CAM) (Bedworth, Handerson, and Wolfe, 1991). In CAM, computers are either used directly to control and monitor the machines/processes (in real-time) or used off-line to support manufacturing operations such as computer-aided process planning (CAPP) or planning of required materials. At higher levels, computers are utilized in support of management. They play a critical role in all stages of decision making and control of financial operations by processing and analyzing data and reporting the results (management information systems, MIS) (Hollingam, 1987). Computers facilitate integration of CAD, CAM, and MIS (computer-integrated manufacturing, CIM) (Vajpayee, 1995) (see Figure 1.5). They provide an effective communication interface among engineers, design, management, production workers, and project groups to improve efficiency and productivity of the entire system. THE MECHANICAL SYSTEMS DESIGN HANDBOOK Modeling, Measurement, and Control OSITA D. I. NWOKAH YILDIRIM HURMUZLU Southern Methodist University Dallas, Texas CRC PRESS Boca Raton London New York Washington, D.C.

FORMING - DRILLING - REAMING - BORING

FORMING

Forming is the process of turning convex, or concave or of any irregular shape. Form turning may be accomplished by the following methods:

1.using a forming tool

2.combining cross-land longitudinal feed.

3.tracing or coping a template.

DRILLING

Drilling is the operation of producing a cylindrical hole in a work piece by the rotating cutting edge of a cutter known as drill.

REAMING

Reaming is a process of finishing and sizing a hole, which has been drilled or bored. The tool used is called reamer, which has multiple cutting edges. The reamer is held on tailstock spindle, either direct or through a drill chucks and is held stationary while the work is revolved at a very low speed. The feed varies from 0.5 to 2mm per revolution.

BORING

Boring is the operation of enlarging and truing a hole produced by drilling, punching, casting or forging.

1.The work is revolved in a chuck or a faceplate and the tool, which is fitted to the tool post, is fed in to the work.

2.The work is clamped on the carriage and a boring bar holding the tool is supported between the centers and made to revolve.

fr. NTTF ( NETTUR TECHNICAL TRAINING FOUNDATION)

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Torsion

A bar is under torsional stress when it is held fast at one end, and a force acts at the other end to twist the bar. In a round bar (Fig. 4.9) with a constant force acting, the straight-line ab becomes the helix ad, and a radial line in the cross-section, ob, moves to the position ad. The angle bad remains constant while the angle bod increases with the length of the bar. Each cross section of the bar tends to shear off the one adjacent to it, and in any cross section the shearing stress at any point is normal to a radial line drawn through the point. Within the shearing proportional limit, a radial line of the cross section remains straight after the twisting force has been applied, and the unit shearing stress at any point is proportional to its distance from the axis.

The twisting moment, T, is equal to the product of the resultant, P, of the twisting forces, multiplied by its distance from the axis, p. Resisting moment, T,, in torsion, is equal to the sum of the moments of the unit

shearing stresses acting along a cross section with respect to the axis of the bar. If d A is an elementary area of the section at a distance of z units from the axis of a circular shaft [Fig. 4.9 (b)], and c is the distance from

the axis to the outside of the cross section where the unit shearing stress is Z, then the unit shearing stress acting on dA is (ZZ/C) dA, its moment with respect to the axis is (zz2/c) dA, and the sum of all the moments

of the unit shearing stresses on the cross section is f (rz2/c) dA. In this expression the factor fz2 dA is the polar moment of inertia of the section with respect to the axis. Denoting this by J the resisting moment may be written zJ/c.

The polar moment of inertia of a surface about an axis through its center of gravity and perpendicular to the surface is the sum of the products obtained by multiplying each elementary area by the square of its distance from the center of gravity of its surface; it is equal to the sum of the moments of inertia taken with respect to two axes in the plane of the surface at right angles to each other passing through the center of gravity section of a round shaft.

The analysis of torsional shearing stress distribution along noncircular cross sections of bars under torsion is complex. By drawing two lines at right angles through the center of gravity of a section before twisting,

and observing the angular distortion after twisting, it has been found from many experiments that in noncircular sections the shearing unit stresses are not proportional to their distances from the axis. Thus in a rectangular bar there is no shearing stress at the comers of the sections, and the stress at the middle of the wide side is greater than at the middle of the narrow side. In an elliptical bar the shearing stress is greater along the flat side than at the round side.

Plastics

Engineered

Product

Design

Dominick Rosato and

Donald Rosato

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