17 CASE STUDY 125THICKNESS. AN OPTION IN THE SHELL COMMAND ALLOWS SP...

3.17 Case Study

125

thickness. An option in the shell command allows specified surfaces to be removed

from the shell operation, and in this case the top and front surfaces have been

removed to create the scoop shape of the shovel. The shell command is very useful

in design for the injection molding process where a uniform wall thickness is desir-

able to achieve uniform cooling and shrinkage. It can also be useful for processes

such as thermoforming and sheet metal forming where a sheet of uniform thickness

is used as the raw material.

The ribs were added to increase the moment of inertia of the shovel in the

direction of the expected snow load and thereby to stiffen the shovel head. Stiffening

the shovel through geometric changes allows a reduction in overall wall thickness,

which relates to a reduction in cost and weight. Notice how the use of the shell com-

mand greatly simplifies the design of these ribs, which would have been quite diffi-

cult to model without use of this command.

The next step in the design of the shovel head is to add the interface

between the shovel head and the shaft. The wire frame sketch of this interface

is shown in Figure 3.40. To make this sketch and locate it properly, a reference

plane has been added to the shovel. This plane is placed to be perpendicular

to

the rear face of the shovel so that wire frames that will be sketched and extruded

••••••

,.40

126

Product

Design, Computer

Aided Design (CAD), and Solid Modeling

Chap. 3

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on it will be parallel to the rear of the shovel. Onto this plane, the outermost

lines of the shovel head have been

focused

to give a reference for centering the

interface. The interface is sketched as a tubelike structure to create a slip-fit with

the shaft.

Figure 3.41 shows the shovel head after the interface section is extruded. This

interface is extruded to a distance so that its full length extends past the shovel head.

The interface is then cut off at an angle to match the bottom of the shovel. It is also

lengthened, and the remaining wall of the shovel head inside the tube is removed.

The result of these operations is shown in Figures 3.42 and 3.43.

Next, as shown in Figure 3.44, the resulting hole on the bottom of the

shovel is sealed. and a hole is added for the fastener that attaches the shovel

head to the shaft. This step completes the design of the shovel scoop itself. The

next step is to add the molded-in aluminum cutting blade. Figure 3.45 shows a

bottom view of the shovel with a wire frame sketch of the plastic section that

will encase the cutting blade. Once this section is extruded, the wire frame for

the blade itself is generated as in the bottom front isometric view in Figure 3.46.

Notice that in this case the blade is drawn as an integral component of the shovel

head. Later, a second model of the blade will have to be generated that includes

the section that is encased in the plastic. This section will require several slots so

that during injection molding, the plastic will flow through them and mechani-

cally entrap the blade.

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