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# Activity 213 Making Sketches in CAD

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Purpose

It would be great if computer systems were advanced enough to take a mental image of an object, such as the thought of a sports car, and instantly generate it as a three-dimensional computer aided design (CAD) model on a computer screen. Unfortunately, computer systems have not advanced to this point. For now, engineers must continue to express ideas as sketches.

A CAD model can quickly display an engineer’s ideas in a “real” setting. That is, once an engineer has developed a model in CAD, the idea can be shared much more easily.

As is the case with technical sketching, CAD models must begin as sketches of points, lines, or shapes. The major difference between a freehand sketch and a CAD sketch is accuracy. The lines of a CAD sketch can be drawn perfectly straight, with start and end points that occur in exact locations in space. A line may also be given precise length through the use of dimensions. If more than one line is being sketched, they can be made perfectly parallel or perpendicular, or given a specific angle. CAD programs give designers the ability to sketch any kind of geometry, along with the ability to dimension, extend, rotate, mirror, copy, pattern, move, trim, or erase it.

The ability to realize CAD models through sequentially developing geometric sketches is a critical skill that designers in multiple engineering disciplines use to make mental images into money-making products.

Equipment

; Computer with 3D CAD solid modeling program

; PLTW. bmp graphic image

o Dimensioning Activity

o Trim Practice

o Extend Practice

o Move Practice

o Rotate Practice

o Geometric Constraints

o Project Geometry

o Wristband

o Water Bottle

IED Unit 2 Lesson 2.1 Activity 2.1.3 Making Sketches in CAD Page 1

Procedure

In order to effectively use a CAD program as a design tool, a designer must know what sketching tools are available and how they work. This activity will help you to understand and utilize the sketching tools that are common to most CAD programs.

There are 19 exercises in this activity. Many of the exercises require the creation of a new CAD file, and the replication of the images pictured. Other exercises require the manipulation of an already existing file(s). As you finish each exercise, initial the graphic, save the CAD file, document the file name and location on the line provided, and submit this activity to your instructor for evaluation.

1. The spline tool is used to create irregular curves, such as the involute curve on a

gear tooth, or the contour of a car body surface. Create a new CAD file, and use

the spline sketch tool to draw two irregular curves and two closed shapes that

approximate the figures pictured above. Note the locations and number of points

in each spline.

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2. Create a new CAD file and use the circle and ellipse sketch tools to replicate the figures shown above. Label the images as shown using the text tool.

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3. Many CAD programs give the user the ability to define arcs through several methods. These methods may include: defining the size of an arc by establishing three points of tangency, referencing two points of tangency, or identifying a center point and two points of tangency. Create a new CAD file and use the line

and arc sketch tools to replicate the figures shown above. Label the images as shown using the text tool.

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4. The fillet tool creates a round where two lines meet at a corner. The size of the round is identified as a radius value. Create a new CAD file and draw a rectangle that is approximately 2 inches wide by 1.25 inches tall. Use the fillet sketch tool

to round off the top right to .25 inch radius. Then, round off the bottom right corner with a .75 inch radius. Lastly, round off the bottom left hand corner with a .5 inch radius.

5. The chamfer tool is often used to break a corner, which results in a softer edge

on a part. This process is typically done in one of three ways: by identifying the distance of one side of a 45? angle, by identifying the individual lengths of each side of the chamfer, or by identifying length of one side and the angle of the chamfer. Create a new CAD file and draw a rectangle that is approximately 2 inches wide by 1.5 inches tall. Use the chamfer sketch tool and all three

methods explained to create angles at the top right and bottom corners of the rectangle according to the dimensions given in the figure shown above.

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6. Regular polygons are multisided shapes that have sides of equal length. They may be inscribed or circumscribed within a given radius. The raw materials that are used to produce engineered objects are often manufactured in the shape of regular polygons. Create a new CAD file and use the polygon sketch tool to

draw the series of shapes pictured above. Use the text tool to label the names of each of the regular polygons.

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Before

After

7. CAD programs allow the designer to mirror images across lines, which is a useful tool when designing parts that have high degrees of symmetry. Create a new CAD file and use the line and circle sketch tools to create a similar figure to the one shown in the Before image. A regular vertical line may be used as the mirror line. The top and bottom horizontal edges must terminate at the vertical mirror line. Use the mirror sketch tool to mirror the figure across the mirror line.

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Before

After

8. The ability to pattern a shape or element allows the designer to save time and maintain accuracy. Create a new CAD file and draw a circle with a diameter of approximately 2.25 inches. Use the polygon sketch tool to create an isosceles triangle that would fit within a .25 inch diameter circle. Orient the triangle so that it is pointing toward the top quadrant of the circle. The center of the triangle should be approximately 7/8 inch from the center of the circle. Use the pattern sketch

tool to create a copy of the triangle 12 times (the number of instances includes the object being patterned) around the center of the circle.

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Before

After

9. The Pattern sketch tool allows the designer to create a pattern from one or several objects. The direction or orientation of the pattern is derived from existing lines on the sketch. Create a new CAD file and draw a rectangle that is approximately 4 inches wide by 3.25 inches tall. Create a 3/8 inch diameter circle in the lower left hand corner. Locate the center of the circle approximately 3/8 inch from the bottom and left edges. Use the pattern sketch tool to create

multiple copies of the circle. The circle pattern must have seven columns and six rows, and fit within the boundaries of the rectangle.

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Before

After

10. The need to create geometry that is identical in shape and parallel is very common in engineering design. The Offset sketch tool is used to make this

process quick and accurate. Create a new CAD file and draw the figures pictured in the before image. Use the Offset sketch tool to offset the geometry of each figure outward two times.

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