Shells

Creating Shells

In This Chapter

14

This tutorial teaches you how to create and edit a shelled part in Autodesk® Mechanical Desktop®. With the shell feature, you can create complex parts with walls of varying thickness. In the tutorial, you add a shell feature to an existing die cast engine part, and then edit the shell. The procedures you learn here can be applied to a variety of shelledparts.

s Adding a shell feature to an

existing part
s Modifying wall thickness s Adding multiple thickness

overrides
s Managing thickness overrides

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Key Terms
Term
converging radial shapes

Definition
A sharp corner where cylindrical faces, such as fillets, are offset and converge to form a zero radius. Parts with more complex shapes, such as variable radius fillets andsurfcuts, need a shell thickness large enough so that the offset face does not converge. The offset value initially applied to all faces of your part. Face on a shelled part you select that will not be offset. Face on a shelled part that was previously excluded can be selected, or reclaimed. Reclaimed faces are offset by the default thickness. A Mechanical Desktop® feature that cuts portions ofthe active part by offsetting its faces. An offset value that takes precedence over the default thickness of a shell feature. You can define a list of thickness override values and apply them to any face on the part.

default thickness excluded face reclaimed face

shell

thickness override

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Chapter 14 Creating Shells

Basic Concepts of Creating Shells
Unlike other sketchedor placed features, a shell feature is initially applied to all the faces of your active part, instead of only those you select. It doesn’t need parametric dimensions to control placement. A part can have only one shell feature. When you add a shell feature to a part, Mechanical Desktop creates new faces by offsetting existing ones inside or outside of their original positions. You can also choosethe midplane option, which offsets faces by half the entered value to one side and half to the other side. Mechanical Desktop treats continuously tangent faces as a single face when offsetting. This illustration shows the progression for adding a shell feature to a part and then a thickness override to the cylindrical face. The faces tangent to the cylindrical face are also offset in theoperation.

You can change a shell feature in different ways. You can change the offset type and offset values in the Shell Feature dialog box. If you choose to exclude or reclaim faces, or change thickness overrides, you select faces on your part. In both cases, to apply your modifications you must update the part.

Adding Shell Features to Models
In this tutorial, you add a shell feature to theexisting model of a clutch housing for a 250cc two-stroke engine. The shell feature requires an excluded face and multiple thickness overrides. You apply what you’ve learned in previous chapters about constrained sketches and features to examine an existing part. You then add a shell feature to the part, later modifying it to suit your design requirements.

Basic Concepts of Creating Shells

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Using Replay to Examine Designs
First, review the clutch housing design. Open the file clutch.dwg in the desktop\tutorial folder. The clutch housing has been modeled to a point in the design where it is ready for you to add the shell feature. It contains six extruded features.

NOTE Back up the tutorial drawing files so you still have the original files if you
make a mistake. See“Backing up Tutorial Drawing Files” on page 40.

For the constraint system in this model, construction lines were used to align its cylindrical features. You can change the angle of the features with respect to one another to alter the design to fit engines of different sizes. To replay the design of a part 1 Use AMREPLAY to review the process used to build this model, responding to the prompts....