Chapter 9: Solid Modelling – Machine Drawing with AutoCAD

Chapter 9

Solid Modelling

Chapter Outline

With the advancement of computer hardware and software, there has been a drastic change in the design procedure generally followed in recent times. Previously, the first stage was the conceptual designing of a component or group of components to meet certain requirements. Afterwards, based on the design calculation and functional requirements, orthographic drawings were prepared. Before going to production, frequently, a prototype of the component(s) was developed and subsequently tested. Once everything was found to be perfect, production drawings were released for manufacturing. If any defects were found, the entire process had to be repeated.

If you watch a product development cycle, you will realise that until an actual prototype (it may even be a scaled-down model) is made, nobody has a clear idea about the appearance of the product which is, in fact, a very important criterion from the aesthetic and marketing point of view.

In recent times, the availability of faster computers and sophisticated CAD softwares at affordable prices, has made it possible to develop the product using solid modelling facilities. This is the process of developing a product that has all the attributes of the actual solid object. Solid models make it easy to visualise the product. Even a non-technical person can percieve the product and any modification made to it. Once the development of the basic solid model is completed, design analysis is carried out using any standard FEM (Finite Element) package. This is also an easy task as most of the CAD packages are compatible with to FEM packages. For the analysis, you need information regarding the physical attributes of an object such as volume, mass, moment of inertia, material properties, and so on. All these properties can be imparted to the object developed through solid models. Thus, the object can be subjected to various tests to ensure its performance as per product specifications. This eliminates the need of building expensive prototypes and makes the product development cycle shorter.

The addvantage of solid modelling has gone beyond the design office. In fact, it has become the core of the manufacturing process. The recent trend is to produce the components accurately and economically through NC/CNC machines. Codes have to be fed into the machine for each component to be manufactured. AutoCAD and other advanced CAD software packages provide the facility to automatically generate these codes from the solid models of the component.

AutoCAD creates the solid structures in three different ways—

  1. Wireframe model,
  2. 3-D surface mesh model, and
  3. Solid model.

A wireframe model is a skeletal framework of a 3-D object. It provides the outline edges of the model using points, lines, and curves but it does not have any flesh. Wireframe models are created by placing 2-D objects in 3-D space and then introducing Z coordinate values. These models can also be created by using 3-D polylines and splines.

A surface model is essentially stretching skin to the skeletal. This is acheived by providing surfaces in addition to edges. The surfaces are made up of polygonal meshes that may be planer and curved. While the Shademode, Hide, and Render commands have no bearing on wireframe models, they are applicable on surface models adding nice visual effect.

When 3-D objects are created with the help of a wireframe model or a surface model, they may appear to be the real object, but they do not have the properties of a typical solid object. Since you will be working with engineering components, both these modules may not have much application. Therefore, we will limit our discussion to 3-D drawings, predominantly around AutoCAD solid models only. If you are interested in knowing more about wireframe or surface models, you may go through the AutoCAD users manual or any book dedicated to AutoCAD.

SOLID PRIMITIVES

In case of a 2-D drawing, line, arc and circles are treated as the basic drawing primitives as it is possible to generate any geometrically defined 2-D object by different combinations of these primitives. Similarly, AutoCAD provides predefined and user-defined solid shapes to build the solid model. These shapes are the building blocks for a complex solid object and hence they are generally referred to as solid primitives. The predefined shapes are box, wedge, cone, cylinder, sphere and torus. In addition, the user-defined shapes can be created by two commands—Revolve and Extrude.

Unlike a two-dimensional drawing, the visual aspect is very important for the presentation of solid objects. Since a three-dimensional object is displayed on a two-dimensional drawing screen (computer monitor), it is necessary to take the help of additional lines apart from the edges to represent each surface of the solid. The more the number of lines, better the visual effect. The number of lines in a solid model representation is controlled by the value assigned to Isolines system variable. The number of lines determines the number of iterations needed to create a solid. Naturally, if the value of the isolines variable is very high, it may take significantly more time to generate a complex solid on the screen. Therefore, it is advisable to use a realistic value for this variable (see notes on isolines).

To create solid primitives, information about the part geometry such as length, width, height, radius, rotation angle, and so on is required. The height of the primitives is always along the positive Z-axis, perpendicular to the construction plane. In the section to follow, you will learn how to use solid primitives to build up complex machine components.

CONSTRUCTION AND DISPLAY OF 3-D SOLID PRIMITIVES

Solid models are three-dimensional (3-D) models with specific database properties defining the models, though virtually, with mass and density. As a general practice, in solid modelling, initially the basic primitives are created and then transformed into more complex shapes (composite models) by executing Boolean operations such as Addition and Subtraction. Finally, some 3-D editing operations follow to give them the exact shape and form. We shall begin with the creation of primitives, which are the basic building blocks of solid models. They are

 

Box Cylinder Cone
Wedge Sphere Torus

Sphere

The Sphere command can be used to create a solid sphere. You can invoke this command either from the command line or from pull down menu (Draw > solids > Sphere). The command has only one option: Center of sphere. The center of the sphere automatically aligns with the Z axis of the current UCS. The command can be executed by the following prompt sequence.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Sphere ↵ Initiates the Sphere command.
Current wireframe density:  
Isolines = Current The current status of Isolines is given by this statement.
Specify center of sphere < 0, 0, 0 >: Specify Center point.

The object on the screen appears as shown in Fig. 9.1. This object does not resemble a solid model. Rather, it looks like a 2-D drawing of the sphere. In fact, Fig. 9.1 shows the top view of the sphere you have just created by the sphere command. For a 3-D view as shown in Fig. 9.2, you have to change the viewing direction with the help of the Viewpoint command. Before proceeding further with solid models, we shall, therefore, discuss the concept of viewpoint and the related command in the following section.

To get a 3-D view, you will have to change the viewpoint to (1, 1, 1). The sphere now appears as shown in Fig. 9.2.

 

FIG. 9.1   Sphere viewed from Z-axis (top view)

 

FIG. 9.2   Sphere viewed from (1, 1, 1) (isometric view)

VIEW DIRECTIONS IN AutoCAD–VIEWPOINT

By default, the AutoCAD drawing screen is the X-Y plane, with the origin situated at the lower left corner of the screen. Naturally, we view the objects created on the screen from the top (plan view). In other words, the observer is situated on the Z-axis and looking down.

The concept of viewpoint is being introduced by AutoCAD by means of two components:

  1. Position of the object (also known as target), and
  2. Position of the observer in relation to the object.

The position of the object or the target location is fixed at the origin (0, 0, 0) regardless of the actual volume measurements and converted to a wholesome point object. This is set with the AutoCAD system variable Target, the default setting being (0, 0, 0). However, you can change the position of the observer. Let us now focus our attention on Fig. 9.3 in which the position of the object, and the observer and the viewing direction, for a particular case, is clearly shown. Now, it is clear that you observe the sphere as shown in Fig. 9.1 when the observer location is (0, 0, 1). Similarly, Fig. 9.2 is visible when the observer location is (1, 1, 1) which is, in fact, an isometric view of the sphere.

 

FIG. 9.3   Viewing in AutoCAD

 

From the previous discussion, it is clear that the default setting of the position of the observer is (0, 0, 1). You can change the observer's position with the help of the Viewpoint (Vpoint) command. The AutoCAD prompts are as follows.

Command: Vpoint ↵

Current view direction: VIEWDIR = Current

Specify a view or [Rotate] <display compass and tripod>: Specify

viewpoint location

We have used unitary units of coordinate measurement for easy understanding but if you use 2, 2, 2 instead of 1, 1, 1 the same result will be obtained.

Vpoint Coordinate Input – Orthographic, Isometric, Dimetric and Trimetric View

Fig. 9.4 shows how to obtain top, front, and right side view of a solid cube by changing the appropriate coordinate values at the Vpoint command prompt.

When you enter a positive or a negative number for only one axis, the other being zero, you are actually generating a standard orthographic view (that is, Top, Front or Side view). When you enter the same positive values for all three axes, you get an Isometric view.

The numerical value of the input, as we said earlier, has no effect on the size of the object because the view generated in all cases is based on parallel projection and the view generated is a Zoom Extent view.

 

FIG. 9.4   Generation of different views

 

We present here an exercise to create some standard Vpoint values that are very useful for obtaining different orthographic views. We shall first develop a solid box using the AutoCAD Box command and then explore the effect of the Vpoint command on it.

  1. Set the limits 0, 0 and 15, 15.
  2. Create a solid box using the following command sequences.
    Command: Box ↵
    Specify corner of box or [Center] < 0, 0, 0 >: 5, 5
    Specify Corner or [Cube/Length]: L ↵
    Specify Length: 5 ↵
    Specify Width: 4 ↵
    Specify Height: 3 ↵
  3. Your first view of the Box is as shown in Fig. 9.5a. Write ‘TOP VIEW’ on the square that you see on the screen (Fig. 9.5b).
  4. Revoke the Vpoint command. Observe the view (Fig. 9.6) by entering the coordinate entries as (–1, –1, 1).
  5. Change Vpoint to 0, –1, 0 and in the square of the resulting view (Fig. 9.7) write ‘FRONT VIEW’.
  6. Change Vpoint to 1, –1, 1 and see the resulting view (Fig. 9.8) as given here.

 

FIG. 9.5

 

FIG. 9.6

 

FIG. 9.7

 

FIG. 9.8

 

Do this exercise for the other surface as per Table 9.1 given below and view it from different viewing angles for a better understanding of the viewpoints in AutoCAD.

 

Table 9.1

Vpoint Value View Displayed
0, 0, 1 Top
0, 0, –1 Bottom
0, –1, 0 Front
0, 1, 0 Rear
1, 0, 0 Right side
–1, 0, 0 Left side
1, –1, –1 Bottom, front, right side
1, 1, 1 Top, rear, right side (isometric)

Standard Viewpoints

You can choose suitable viewpoints also from ten standard viewpoints offered by AutoCAD. They are very useful because they are the most commonly used viewpoints. As they show viewpoints relative only to WCS (not the current UCS) they are useful when you work using WCS.

To use these preset viewpoints click the View flyout of the Standard toolbar. You can also choose View → 3-D Views and choose the Viewpoint from the submenu.

To generate a Dimetric view, enter the same value for two of the three axes values. It means that the direction vector makes an equal angle with two of the axes. If you want to generate a Trimetric view where all three axes angles are different, enter three different values for each of the three axes. Fig. 9.9 illustrates the difference among the three views mentioned above.

 

FIG. 9.9

 

Now that you have gained a thorough idea of the concept of viewpoint, we shall explore the methods of creating other primitives now.

Cone

The Cone command creates a solid cone with a base that could be elliptical or circular as shown in Fig. 9.10. It has two options—Elliptical and Center point—the latter option being the default option. The base of the cone lies on the X-Y plane by default and the height of the cone is along the Z-axis. You can change the base plane (XY plane) by using the UCS command options. You can also define the apex point to define the cone height. The AutoCAD prompt sequence will ask you for different parameters for these two types of cones which have been shown in Fig. 9.10. Let us now see the prompt sequence.

 

FIG. 9.10   Cone with different bases

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Cone↵ Initiates the Cone command.
Current frame density: Isolines Current Provides the status of the current display by the number of isolines.
Specify center point for base of cone or [Elliptical] < 0, 0, 0 >: Specify the center point of the base circle of the cone or choose the elliptical option.
Specify radius for base of cone or [Diameter]: If you continue in the circular option then specify the radius or diameter of the base circle. Specify height of the cone as depicted in Fig. 9.10 (height of the Apex point from the center of the base circle).
Specify height of cone or [Apex]: You can dynamically pick the points with the cursor or enter a value.
If you choose the Elliptical option, provide the standard necessary inputs of an Ellipse.

 

FIG. 9.11   Solid cone with isolines = 4

Isolines

Fig. 9.11 shows the same cone as that of Fig. 9.10a but reducing the number of ruled lines defining its inclined surface. These lines are employed in order to give a three-dimensional visual effect to the solid object. These ruled lines are called Tessellation lines and their number can be controlled by the AutoCAD Isolines system variable. As the number of Isolines are increased, the object will look more realistic. However, it is not advisable to use a very high value of Isolines because with the increase in the numbers of Isolines, AutoCAD takes a longer time to generate the model. The command prompt is as follows.

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Isolines↵ Initiates the Isoline command.
Enter new value for Isolines < 4 >: Specify new value for Isolines by entering a number. The default or present value is shown inside the bracket.

After the execution of the command you will have to Regen to see the effect of this command.

For solid modelling there are two other very important system variables that are useful to generate different types of viewing options and grades of smoothness of the surface of the models. These options are: 1) Facetres and 2) Displslh. These variables are important for shading and rendering purposes.

Cylinder

You can use the Cylinder command to create a solid cylinder with two options for the base—circular and elliptical as shown in Fig. 9.12. The base of the cylinder lies on the X-Y plane, and the height of the cylinder is along the Z axis. You can change the base plane (XY construction plane) using the UCS command options. The following is the prompt sequence for the Cylinder command.

 

FIG. 9.12   Cylinder with different bases

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Cylinder↵
Current wireframe density:
Isolines = 8
Initiates the Cylinder command and displays the position of Isolines.
Specify center point for base of cylinder or: [Elliptical] < 0, 0, 0 > Specify the center point of the base circle or select the Elliptical option.
Specify radius for base of cylinder or
[Diameter]:
Enter the radius of the base circle.
Specify height of cylinder or:
[Center of other end]
Enter the height of the cylinder. You can indicate the center of the other end as well. If you choose the Elliptical option, feed the standard necessary inputs of an ellipse.

Hide

For any complicated object, the view in solid may look very confusing due to the presence of too many lines on the screen. In such a situation, it may be essential to remove the hidden lines temporarily to have a realistic view of the same. The user can use the Hide command for this purpose. In Fig. 9.13 the same Cylinder and Cone drawn previously, is displayed with Hide option. It is to be remembered that further editing operations cannot be performed with Hide object. In order to get back to the original drawing use Regen command.

 

FIG. 9.13   Cylinder and Cone with Hide option

Torus

A doughnut shape, as shown in Fig. 9.14, can be created using the Torus command. The user will have to enter the diameter or radius of the torus and the diameter or radius of the tube. The radius of the torus is the distance from the centre of the torus to the center line of the tube. The following is the prompt sequence for the Torus command.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Torus ↵
Current wireframe density, Isolines = Current
Initiates the Torus command and the display status is shown.
Specify center of torus or [Diameter] < 0, 0, 0 >:
Specify radius of torus or
[Diameter]:
Specify radius of tube or [Diameter]:
Specify the center of the torus and also the radii of the torus and the tube as depicted in in Fig. 9.14.

 

FIG. 9.14   Torus with all positive values

 

This radius can have a positive or negative value. If the value is negative, the torus has a football like shape (Fig. 9.15). In Fig. 9.16, a torus is shown with radius equal to –2.0 and radius of the tube being +2.4. If the radii of the tube and the torus are both positive and the radius of the tube is greater than the radius of the torus, the resulting solid looks like a sphere with depressions at both the ends (Fig. 9.16). The torus is centered on the construction plane (X-Y plane), the top half of the torus is above the construction plane, and the other half is below the construction plane.

 

FIG. 9.15   Radius = –2.0 and 2.4

 

FIG. 9.16   Tube radius > torus radius

Wedge

A wedge is a box cut along a diagonal plane as shown in Fig. 9.17 and the Wedge command is used to create it. As in the Box command, the user has to enter the first and the second corner of the wedge. These two points define the base of the wedge, which is always drawn parallel to the current XY plane. The wedge will taper towards the positive X axis. The following is the prompt sequence for the Wedge command.

 

FIG. 9.17   Solid wedge

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Wedge ↵
Specify first corner of wedge or
[Center] <0, 0, 0>:
Specify corner or [Cube/Length]:
Specify height:
Initiates the Wedge command. Give specific inputs. In case you choose the Cube option, the length of one side will create the wedge.
CREATING EXTRUDED SOLID

It is also possible to create user-defined solid objects with the help of the Extrude command. This is a very powerful command that can efficiently create a solid object of irregular shape with the base as shown in Fig. 9.18. In order to execute this command, you have to define the base first. As per the AutoCAD prompt sequence, select the object for extrusion, the height of extrusion, and the extrusion taper angle. A positive angle results in a tapering in the inward direction (Fig. 9.18a) whereas a negative angle indicates a tapering in the outward direction (Fig. 9.18b), both being expressed with respect to a positive Z axis.

 

 

FIG. 9.18   Creation of extruded solids

 

Given this information, AutoCAD evaluates the boundaries and does tessellation calculations for each element in the composite region. You can manipulate the Isolines system variable to set the number of tessellation lines. It is possible to extrude closed objects like polylines, polygons, rectangles, circles, ellipses, closed splines, donuts, and regions. Note that AutoCAD does not permit extrusion of 3-D objects within blocks and polylines with intersecting segments.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Extrude ↵
Current wireframe density Isolines = Current
Initiates the Extrude command and the position of Isolines is displayed.
Select objects: ↵
Select objects:↵
Select the base region.
Specify height of extrusion or [Path]: Enter a value for height or select the Path option. A positive value extrudes along the positive Z axis and a negative value extrudes along the negative Z axis.
Specify angle of taper for extrusion <0>: Enter the angle of taper as discussed above.

Using the Path option, one can generate complicated extruded objects (for example, a helical spring) along the selected path. The extrusion path should not lie on the same plane as the object (Fig. 9.19).

 

FIG. 9.19   Extruded command with Path option

CREATING REVOLVED SOLID

The Revolve command can create a solid by rotating a two-dimensional surface formed by a closed pline or circle about an axis. The degree of rotation can be controlled as per requirement. Fig. 9.20b shows a solid object created from the 2-D surface (Fig. 9.20a) with the angle of revolution being equal to 270°.

 

FIG. 9.20   Solid created with 270° angle of revolution

 

The command prompt is as follows.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Revolve ↵
Current wireframe density: Isolines = current
Select objects:↵
Initiates the Revolve command.
Select objects:↵
Specify start point for axis of revolution or define axis by [Object/X(axis)/Y (axis)]:
Specify endpoint of axis:
Specify angle of revolution < 360 >:
Select the region.
CREATING COMPOSITE SOLIDS

It is possible to create a composite solid by combining two or more solid primitives. This is achieved by using Boolean functions provided by AutoCAD. A user can add, subtract, and find the intersection of existing solids to create complex solid models of real objects. The tools to invoke the three commands are located on the solids editing Tool bar or under the solid editing command located in the Modify pull-down menu.

Union

The Union command is used to create complex solids (or regions for 2-D objects). Several solids or regions can be combined by the same command. Once this command is executed, both objects lose their separate identity. Although they may not look different, their properties (for example, volume, moment of inertia, radius of gyration, and so on) will be upgraded accordingly so that they will be treated as a single object in future operations. However, the solids cannot be combined with the regions. If the selection set contains both regions and solids, AME will automatically combine the solids and the regions separately.

 

 

The prompt for Union command is as follows.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Union↵ Initiates the command.
Select objects:↵ Select the first object.
Select objects:↵ Select the second object.
Select objects:↵ Confirm the selection by pressing Enter.

A cylinder and a box are placed side by side, with the axis of the cylinder coinciding with the axis of the side wall of the box as shown in Fig. 9.21a. Following the Union command, their appearance is slightly modified as illustrated in Fig. 9.21b.

 

FIG. 9.21   Union of two solids

Subtract

With the help of the Subtract command, you can remove the common area of one of solids from another. For example, a cylinder is placed on the object shown in Fig. 9.22a. The resulting objects is shown in Fig. 9.22b. Now, it is very easy to create a hole in the object by subtracting the cylinder from the former.

 

FIG. 9.22   Subtraction of a cylinder from a solid

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Subtract ↵
Select solids and regions to subtract from …
Initiates the Subtract command.
Select objects:↵ Select the composite solid (Fig. 9.22a).
Select objects:↵ Confirm the selection by pressing Enter.
Select solids and regions to subtract … Select objects: Select the inner cylinder.

You may not see any change in the shape of the solid. However, if you invoke the Hide option, you can observe the difference. Under the List command, when you select the solid, the entire solid will be highlighted, indicating that it is now a single solid with a hole.

Intersect

When two solids are placed such that they intersect one another with some portion common to both the objects, it is possible to keep the intersecting portion of both the solids on the screen, removing the rest. This is possible with the Intersect command. This command can be invoked from the Solids Editing toolbar, from the Modify menu (choose Solids Editing > Intersection), or by entering Intersect at the Command prompt. If the objects do not intersect, they do not have any overlapping area or volume, and hence AutoCAD will display a “Null solid created-deleted” prompt.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Intersect↵ Initiates the Intersect command.
Select objects:↵ Select the first object.
Select objects:↵ Select the second object.
Select objects:↵ Confirm the selection by pressing Enter.

Fig. 9.23 illustrates the Intersect command being executed over two solid objects—a cylinder and a box.

 

FIG. 9.23   Intersect command

Interfere

The Interfere command can be used to find the interference of two or more solids. This command is similar to Intersect but for one difference—here the original solids remain. AutoCAD creates a new, third solid from the common volume of the two solids. It can also be used to show or highlight the common volume of several pairs of solids. In the figure below we have shown how the newly formed solid can be removed keeping the original solids intact in their position and also the highlighted pairs in dotted lines. If the objects do not intersect, they have no common volume, therefore, Interfere cannot be used with them. This command can be invoked from the solids Tool bar (select the Interfere button), from the Draw menu choose Solids > Interference, or by entering Interfere at the command prompt. This command is used mostly to show the interference of pipelines, gear-pairs teeth, and so on.

 

FIG. 9.24   Interfere command

CREATING FILLETS AND ROUNDS (FILLET)

We are well aware of the Fillet command from our knowledge of 2-D drawing in AutoCAD. The same command can be employed to create fillets and rounds for solid objects as well. AutoCAD is intelligent enough to determine, depending on the geometry of the solid, whether to create a fillet or a round at the selected edges. The visual appearance of fillet on solid objects are given with the help of tessellation lines. If they are found to be too dense, use the Isolines command to change the number of lines. Fig. 9.25 shows the effect of Fillet command executed on one edge of a box. For complicated objects, the value of the fillet radius should be such as to achieve it physically while rounding the edge.

During the execution of the Fillet command on solids, AutoCAD comes up with a different set of prompts than would appear in a 2-D operation. The first change you will notice is that here you can change the fillet radius without restarting the Fillet command (see the prompt sequence). After you specify the radius or accept the default value, you will be prompted to select the edges you want to select. If you want to fillet the same edge that you initially selected, press Enter. To fillet additional edges (by using the same fillet radius), select each of them. You can also fillet a tangential sequence of edges by typing C, denoting the Chain mode. When you are in the Chain mode, selecting any edge will lead AutoCAD to operate filleting on all other edges tangential to that surface. Fig. 9.25 illustrates the difference between Edge mode and Chain mode.

 

FIG. 9.25   Edge mode and Chain mode

 

 

To round the top and corner of the edges of a base plate shown in Fig. 9.26, use the following instructions. You should begin by filleting the four corner edges and consult the figure for selecting edges.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Fillet ↵ Initiates the command.
Current setting: Mode = Trim, radius = 0.5000 Show the status.
Select first object or [Polyline/Radius/Trim]: Select one of the vertical edges.
Enter fillet radius < 0.5000 >: ↵ Accept the fillet radius or supply your value.
Select an edge or [Chain/Radius]: ↵ Select a second vertical corner edge.
Select an edge or [Chain/Radius]: ↵ Select a third vertical corner edge.
Select an edge or [Chain/Radius]: ↵
Select an edge or [Chain/Radius]: ↵
Select a fourth vertical corner edge (Fig. 9.26a).

Four edges have been selected for fillet and your drawing will now look like Fig. 9.26b. Now complete the exercise by using the Chain option to fillet the entire top edge in one step. The command sequence is given below.

 

FIG. 9.26

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Fillet ↵ Initiates the command.
Current setting: Mode = Trim, radius = 0.5000 Showing the status.
Select first object or [Polyline/Radius/Trim]: Select one of the vertical edges.
Enter fillet radius < 0.5000 >: ↵ Accept the fillet radius or supply your value.
Select an edge or [Chain/Radius]: C↵ Select the chain mode.
Select an edge or [Chain/Radius]: C↵ Select a second top edge.
Select an edge or [Chain/Radius]: C↵ Select a third top edge.
Select an edge or [Chain/Radius]: C↵ Select a fourth top edge.

All the top four edges are filled and your drawing will resemble Fig. 9.26c.

CHAMFER

In case of a 2-D drawing, the Chamfer command bevels the edge(s) of a closed area. The same command when applied for solids, bevels the edges of a solid through a specified distance. This command automatically subtracts the chamfered volume from the solid.

The Chamfer command requires you to select the surface you want to chamfer; select the edges where chamfer is needed, and indicate the chamfer distances of the base and the adjacent surfaces. If both the edges to be chamfered are on the same layer, the resulting chamfer assumes the same layer. In case the two edges are on different layers, the resulting chamfer assumes the current layer.

Fig. 9.27a shows a box with two holes. In order to chamfer the top edge of the hole, chamfer distances have to be specified first. Again, invoke the chamfer command and notice the current chamfer distances. Once the command is properly executed, the object will be modified as shown in Fig. 9.27b.

 

FIG. 9.27   Chamfer command

 

The command prompt is as follows.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Chamfer ↵ Initiates the command.
(Trim mode) Current chamfer Dist 1 = 0.75, Dist 2 = 0.75: The chamfer distances are already set.
Select first line or [Polyline/Distance/
Angle/Trim/Method] : Base surface selection …..
Select edge.
Enter surface selection option [Next/OK (current)]: Enter Next, if needed, until the cylinder edges are highlighted; then, press Enter.
Specify base surface chamfer distance <0.75>: ↵  
Specify other surface chamfer distance < 0.75 >: ↵  
Select an edge or [Loop]: Select edge.

FIG. 9.28   Loop mode

 

If you do not want to chamfer all the edges of the base surface, you can select the edges individually. But, if you want to bevel all the edges around the base surface, type L to switch to the Loop mode. Fig. 9.28 illustrates the difference between the operations of Edge mode and Loop mode.

SECTION (CREATING A CROSS-SECTION)

Chapter 5 discussed in detail ‘section’ and how to obtain sectional views for orthographic projection drawings. The trend now is to develop the model in 3-D. Therefore, it is essential to have a sectional view of a solid along any desired plane and passing through any location for better understanding of the object. This command does not directly produce a sectional view. It just cuts the solid into two halves along the sectioning plane specified through the command and creates a region on the current layer and at the location of the cross-section.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Section ↵ Initiates the command.
Select objects: Select the object to be sectioned.
Specify first point on Section plane by [Object/Z axis/View/XY/YZ/ZX] < 3 points >: Specify the cutting plane by choosing any one option (see note and figure below).

The cutting plane line can be specified in different fashions as mentioned below.

3Points option   This option defines the sectioning plane by three points (Fig. 9.29a). After specifying the first point, AutoCAD prompts you to select the other two points on the sectioning plane.

Object option   This option aligns the section plane with a specific object (Fig. 9.29b). This object can be a circle, ellipse, circular or elliptical arc, 2-D polyline segment, or spline.

Z axis option   This option defines the section plane by one point on the sectioning plane and one on the Z axis of the plane (Fig. 9.29c). The command prompt is

 

Specify a point on the section plane:

Specify a point on the Z axis (normal) of the plane:

 

FIG. 9.29   Options for selecting sectional plane

 

View option   This option aligns the section plane with the current viewing plane (Fig. 9.29g) and perpendicular to the viewing direction. The command prompt is

Specify a point on the XY plane < 0, 0, 0 >:

XY option   This option aligns the section plane parallel to the XY plane (Fig. 9.29d). The command prompt is

Specify a point on the XY plane < 0, 0, 0 >: Select a point through which the plane passes.

Similarly, the YZ and ZX options align the sectioning plane with the respective planes (Fig. 9.29e and f).

Example:   Let us take the illustration of the solid object shown in Fig. 9.30 the sectional view of which has to be obtained. The sectional plane is assumed to be passing through the line of symmetry. To obtain the cross-section created by the Section command, first cut the object (Fig. 9.30a) into half. The sectional region will be still positioned inside your solid. It is advisable to move (a copy of) the region to another position (Fig. 9.30b). Change the Vpoint appropriately to obtain the required orthographic view (Fig. 9.30c) and add hatching according to drawing convention (Fig. 9.30d). You may have to add some lines to the AutoCAD view of the section (Fig. 9.30c), as marked with arrows in Fig. 9.30d, to comply with the engineering drafting convention.

 

FIG. 9.30   An example showing the Section command

CONCEPT OF WORKING PLANE

When we create three-dimensional models, we always begin to construct their geometry on a flat surface, usually referred to as the working plane. In AutoCAD, this default working plane is the XY plane. Let us consider the solid model of an object shown in Fig. 9.31a. In the figure, AFHL represents the XY plane. Now, if we want to create the two different circles, lying on two different planes—DEGI and EFGH—you have to redefine each of these planes as the working plane (XY plane) before drawing the circles because AutoCAD can only draw on the working plane. So, there must be a system for shifting and changing the working plane as and when we desire. In AutoCAD this is achieved by means of two systems of coordinates, namely, the World Coordinate System (WCS) and the User Coordinate System (UCS).

Let us explore them in some detail.

 

FIG. 9.31

World Coordinate System (WCS)

This system is what may be called the Master coordinate system in the AutoCAD drafting environment. In this system, the X, Y and Z coordinates of any point are set at (0, 0, 0) and evaluated. The origin is located at the lower left corner of the screen. The orientation of these axes cannot be changed. All dimensions, be it in 2-D or 3-D, are referred to the WCS and AutoCAD maintains its database relating only to WCS. WCS can always be used as a frame of reference and it guarantees that you are never “lost” in 3-D space.

User Coordinate System (UCS)

The User Coordinate System enables you to establish your own coordinate system. Thus, by changing the position and orientation of the UCS, you can make any surface of a solid object as your working plane for the subsequent development of the model. Moving the UCS can make it easier to work on particular sections of a drawing. Rotating the UCS helps specify points in 3-D or rotated views. The Snap, Grid, and Ortho modes all rotate in line with the new UCS. UCS can be relocated using several methods.

  1. Specify a new X Y plane.
  2. Specify a new origin.
  3. Align the UCS with an existing object.
  4. Align the UCS with the current viewing direction.
  5. Rotate the current UCS around any of its axes.
  6. Select a previously saved UCS.

The different options of the UCS command are given in detail in Appendix 6.

UCS Icon

When you work with AutoCAD, you must have noticed the UCS icon usually at the lower left corner of your drawing screen. The UCS icon is displayed mainly in one of the two ways, depending upon the drawing mode you have implemented. For 2-D drawings, the icon will be displayed as a flat ‘L’ shaped icon, as in Fig. 9.32a, whereas for a 3-D drawing, it will be displayed as a shaded 3-D axis icon (Fig. 9.32b).

Notice that the icon is formed in the shape of an ‘L’, with arrowheads pointing in the positive directions of the X and Y axes. Usually, you may observe a letter ‘W’ below the letter Y. It indicates that, at present, UCS is aligned to WCS. This may not be the case always, causing the ‘W’ to disappear as shown in Fig. 9.32c. The ‘+’ sign displayed at the center of the X and Y axes signifies that UCS is now positioned at the origin of the X, Y, Z axes of UCS or the current working plane.

 

FIG. 9.32

CREATING A SOLID MODEL OF A BLOCK WITH STEPS

Now that we have acquired enough knowledge about the solid modelling features of AutoCAD, let us now apply them to create a solid model of an object shown in Fig. 9.43. The essential dimensions are given in Fig. 9.33. A semicircular cavity is cut on the top surface of the object. The depth of the semicircular cavity varies along the length. The detailed dimensions of the cavity can be understood from Fig. 9.40. The 3-D drawing is developed according to the following steps.

  1. Open a new drawing and set the limits as (0, 0) to (12, 9). Also create separate layers for i) Object ii) Geometry iii) Text iv) Dimension v) Viewport vi) Border, and vii) Title block. You can choose colours for each of these layers separately. Keep Object Snap on, choosing Endpoint, Midpoint, Center, Perpendicular, and other options.
  2. Turn on Grid and Snap if you prefer to work with them and give a space of one unit for both.
  3. View the whole drawing area by Zoom > All command and draw a Box giving the coordinate value for the corner as (1, 1, 0). Choose the Length option and enter 7 for length, 4 for width, and 1 for height.
  4. Now invoke the Vpoint command and enter (–1, –1, 1) as the input value. Use Real Time Zoom to move the model a bit so that the UCS symbol moves to the origin point (0, 0, 0). The resulting view looks like Fig. 9.34.
  5. You may turn off the Grid and Snap now. Next, we shall create four cylinders starting from the lower surface of the base block. For this purpose, it is better to set the lower surface of the box as your working plane. Choose the Origin option from the UCS toolbar and put it at the bottom left corner of the box. In this option, the origin of the UCS is merely changed, the direction of the X, Y, and Z axes remains unaltered as is evident from Fig. 9.35. This will facilitate drawing any geometry on the bottom surface of the box, taking that corner point as origin (0, 0, 0).
  6. Now draw four cylinders at coordinates (with respect to the new position of the UCS) (i) (0.75, 0.75, 0), (ii) (6.25, 0.75, 0.0), (iii) (6.25, 3.25, 0), and (iv) (0.75, 3.25, 0). All of them have the same radius and height of 0.25 and 1.0 respectively. Your figure will look like Fig. 9.35.
  7. Subtract the cylinders from the base block to make the holes. Use the Subtract option from the Solid Edit toolbar and select objects according to the prompts.
  8. Now chamfer the edges of the holes. Give both the chamfer distances as 0.25. The resulting view after the chamfering will be like in Fig. 9.36.
  9. Our next attempt will be to develop a truncated pyramid on the top surface of the base block. It will be helpful to place the UCS on this surface. Again, use the origin option of UCS and shift its origin to the lower left corner of the upper surface as stated earlier. In order to pick the origin properly, you may use Endpoint Object Snap. The orientation of the new UCS is as shown in Fig. 9.37.
  10. Create the pyramidal block above the base block by first creating a rectangle of size 4 × 3 units. Then extrude it to a height of 4 units and a tapering angle of 4°. For creating the base geometry of the pyramid, you have two options. Either draw a closed polyline or draw a rectangle (which also behaves as a polyline). Place the lower left corner of the rectangle at the coordinate point (1.5, 0.5) considering the lower left corner point of the upper surface of the base block as the origin point (0, 0, 0) because the UCS has been shifted to this point now.
  11. Now fillet the four corners of the rectangle by choosing the Polyline option and giving the filleting radii as 0.5. Then extrude the resulting figure by giving the values of the height and tapering angle as stated in Step 10. Next, convert the upper and the lower part of the model into one single object by the Union command (Fig. 9.37).
  12. Fillet the bottom edges of the vertical pyramidal block where it meets the horizontal plane of the top surface of the base block. Use filleting radius as 0.3. Opt for the Chain option for selecting the edges. The resulting figure after these two filleting and extrusion will be as shown in Fig. 9.38.
  13. Create a Polyline geometry on the top surface of the pyramidal block. This will be used as the outline of a revolved surface required for the creation of the cavity using the Revolve command. For drawing the outline, shift the UCS to the midpoint of the left hand side of the top surface of the pyramidal block as shown in Fig. 9.38 using Object Snap and the Origin option of the UCS as before. Here, convert your 2-D UCS symbol into a 3-D UCS symbol by invoking the Shade toolbars (Fig. 9.39) and clicking the 3-D Wireframe icon. The 3-D UCS symbol is shown in Fig. 9.41.
     In order to create the cavity at the top of the pyramidal block, a closed area is formed on the top surface (Fig. 9.41). The relevant dim-ensions are adop-ted from the detailed dimension of the sectional out-line given in Fig. 9.40. You should be careful here to end the polyline with the Close command.
  14. Now create a solid revolved object taking the X axis of the newly placed UCS as the axis of rotation. Use the Revolve command and select the object — the Pline outline you have just created. Opt for the X axis at the AutoCAD prompt for selecting the axis of rotation and give it a complete rotation of 360°. Your figure will now appear as in Fig. 9.42.
  15. Finally, Subtract the revolved section from the pyramidal block. The complete solid object is now ready, as shown in Fig. 9.43. You may shift the UCS to get a clear view.
  16. For a better presentation, you can shade the model choosing different options from the Shade toolbar. Just click the different icons in the toolbar to change the view of your model. Given below is an example of a shaded model.

 

FIG. 9.33   A solid model of a block

 

FIG. 9.34   Box with Vpoint set to (–1, –1, 1)

 

FIG. 9.35   Box with four cylinders

 

FIG. 9.36   Four holes after chamfering operation

 

FIG. 9.37   Box with pyramid on top

 

FIG. 9.38   Object after filleting the edges of the pyramid

 

FIG. 9.39   The Shade toolbar

 

FIG. 9.40   The outline of the Pline section

 

FIG. 9.41   Closed area formed on top of the block

 

FIG. 9.42   The revolved Pline section

 

FIG. 9.43   The final model after Subtraction

 

FIG. 9.44   The shaded solid model

ORTHOGRAPHIC PROJECTION VIEWS

In the previous section, we have described the procedure of creating a solid model of a fairly complicated object. In spite of a solid model having several advantages, it is not very useful from the manufacturing point of view. In order to manufacture an object, all dimensions are required to be put in the drawing. This is not possible in a solid model drawing since the inside of the object is not always clearly visible. In fact, an orthographic drawing is very handy in this respect; it is easy to obtain orthographic views of an object from its solid model.

The following commands are executed to obtain an orthographic project view of the object shown in Fig. 9.44.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Mvsetup ↵ Initiates the Mvsetup command.
Enable paper space Y/N: ↵ Accept the default option to enter paperspace by pressing Enter.
Enter an option [Align/Create/Scale
viewports/Options/Titleblock
/Undo]: C ↵
Enter ‘C’ to create viewports.
Delete objects/Create Viewports/undo/
<Create Viewports>: ↵
Just press Enter to opt for default option.
Enter layout number to load or [Redisplay]:<number of entry to load>: 2 ↵ From the displayed list, select ‘2’ for the standard engineering option in which four viewports will be generated.
Specify first corner of bounding area for viewport (s): Specify opposite comer: Pick two corner points to show the portion of the screen which the entire drawing should cover.
Specify distance between viewports in X direction <0>: 0.5 ↵
Specify distance between viewports in Y direction <0>: 0.5 ↵
These values indicate how much space to leave between the viewports.
Enter an option [Align/Create/Scale
viewports/Options/Titleblock
/Undo]: ↵
Press Enter once again and after a few seconds (depen ding upon the type of objects), orthographic projection views (front, top, and oneside view) along with an isometric view will be displayed, as shown in Fig. 9.45.
NOTE

It is safer to erase the default paper space view generated in single viewport of the object before executing this command or the views generated by the above command may overlap. In Fig. 9.45, the views in each viewport have been slightly modified using the Zoom and Pan commands to get better results. Hidden lines have to be incorporated separately at the appropriate places.

 

FIG. 9.45   Orthographic views generated by Mvsetup command

 

ANALYSING SOLIDS

The Massprop command can be used to analyse a solid model useful for engineering applications. This command will automatically calculate the mass properties of the solid. The density of the solid is assumed to be 1. When you enter this command, AutoCAD will list the properties of the solid.

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Massprop: ↵
Select objects:
Select the solid shown in Fig. 9.43.

 

The following information of the solid is obtained:

– – – – – – – –SOLIDS – – – – – – – –
     Mass 63.7670
     Volume 63.7670
     Bounding box X: 0.0000 – 7.0000
Y: 0.0000 — 4.0000
Z: 0.0000 — 5.0000
     Centroid X: 3.5061
Y: 2.0000
Z: 1.7675
     Moments of inertia X: 633.8235
Y: 1251.0000
Z: 1247.5181
     Products of inertia XY: 447.1480
YZ: 225.4116
ZX: 396.2137
     Radii of gyration X: 3.1527
Y: 4.4293
Z: 4.4231
     Principal moments and X-Y-Z directions about centroid
  I: 179.5138 along [0.9993 0.0000 0.0363]
J: 267.9224 along [0.0000 1.0000 0.0000]
K: 208.6140 along [−0.0363 0.0000 0.9993]
     Write analysis to a File ? < NO> : Y

If you enter Y (Yes) at this last prompt, the Create Mass and Area Properties options will be provided to store the data in a file with extension ‘mpr’.

ADVANCED EDITING COMMANDS

Mirror in 3-D (Mirror3D)

You can mirror any 3-D object using the Mirror command when the mirror plane (for a 3-D object, instead of mirror line, define a plane) is XY. However, if the mirror plane is other than the XY plane, then the Mirror 3D command has to be used.

The command sequence is as follows.

 

 

FIG. 9.46

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Mirror3D  Initiates the Mirror command.
Select objects: Pick the objects to be mirrored.
Select objects: Confirm the selection.
Specify first point of mirror plane (3point) or [Objects/Last/Zaxis/
View/XY/YZ/ZX/3points]
Pick one end of the mirror plane (first point in Fig.9.46a).
Specify second point on mirror plane; Pick the second point on the mirror plane.
Specify third point on mirror plane: Pick the third point on the mirror plane.
Delete source objects? [Yes/No] <N>: Enter Y to delete them or press Enter to keep them.

In Fig. 9.46b, the mirror object created is shown in a new position to explain the figure properly.

 

Align(in 3-D)

This command is often used to align objects while developing a solid model. The command actually gives the combined effect of two separate commands—Move and Rotate3D—in a single step. In its first prompt this command will ask you to select the objects to be moved. In the following prompts, it will ask you for three pairs of points. These are referred to as Source points and Destination points because the final alignment of the objects is totally dependent on their correlations.

You can also use the Align command to scale objects as the moved objects are guided to match the alignment points. AutoCAD will calculate the distance between the first and second destination points and use it to determine the scale factor.

When you specify a third pair of points, you actually direct the object to rotate about a second axis. Here the objects are aligned so that the plane defined by the three source points aligns with the plane defined by the three destination points.

We give here two examples, with explanation for each step for the first. The second example (where we have marked the source and destination points) is an exercise for the reader.

Exercise 1   We shall realign a 3-D ring so that it fits around an axle port.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Align ↵ Initiates the Align command.
Select objects: Select the ring (Fig. 9.47a).
Select objects: ↵ Confirm the selection.
Specify first source point: Select point S1 (use OSNAP).
Specify first destination point: Select point D1 (Fig. 9.47b).
Specify second source point: Select point S2.
Specify second destination point: ↵ Select point D2 (use OSNAP).
Specify third source point or <Continue>: Ends the point selection.
Scale objects based on alignment points? [Yes/No] <No>: Allows the scaling of objects (Fig. 9.47c). You notice that the object is being resized andaligned.

 

FIG. 9.47   Application of Align command

 

Exercise 2   Fig. 9.48 shows an object before and after the align command. Readers are advised to do the exercise on their own for a better understanding of the command.

 

FIG. 9.48   Aligned object

 

Rotating in 3-D (Rotate3D)

When you want to simply rotate 3-D objects in the XY plane, use the Rotate command. However, if you need to rotate an object in any other plane, use the Rotate 3-D command. Here you can define the reference axis of the revolution in seven different ways. These seven options are given below in detail.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Rotate3D ↵ Initiates the Rotate 3-D command.
Select object: Select the object or objects you want to rotate by any selection method.
Specify first point on axis or define axis by [Object/Last/View/X axis/Y axis/Z axis/2 points]: Axis can be defined by the default 2Points option. You may use other options for defining the axis as discussed below.
Other options:  
1. Object: This option lets you define your axis of rotation by selecting any line, arc, circle, spline, or 2-D pline. If you choose curved object such as a circle or an arc, AutoCAD only allows the object to rotate around a line that starts at the object's center and extends perpendicular to the object.
2. Last: This option uses the last-defined axis as the new axis of rotation.
3. View: This option will fix the axis of rotation as parallel to the current view and passing through a selected point at the “Point on view direction axis < 0, 0, 0 >:” prompt.
4. X axis The axis of revolution will be parallel to the X axis and will pass through a point that you specify.
5. Y and Z axes Same as above, the axes being Y and Z.
6. 2 Points This is the default option. Specify two points, preferably by Osnap, which will define your axis of revolution.
After the selection of the axis is complete, the next prompt is as follows.  
Specify rotation angle or [Rotation]: Enter the angle of rotation or type R to specify a reference angle

Fig. 9.49 illustrates the application of the Rotate3D command to position the handle as required.

 

FIG. 9.49   Query Application of Rotate3D command

EXERCISE
  1. What is solid modelling? Name the solid primitives. How can you control the number of tessellation lines in a solid model? What do you obtain by the Massprop command?
  2. Pictorial views of different objects are shown in Figs. 2.29 and 2.30. Develop the solid models of the objects.
  3. Obtain the orthographic projection views from the respective solid models created in Question 1. Also incorporate the necessary changes wherever continuous lines will be replaced by hidden lines.
  4. Use the Section command to generate a sectional view of the solid objects developed in Question 1. Assume the section to be through the center of the object.
  5. Find out and compare the volume and weight of each object assuming they are made of steel and wood.
  6. Develop the solid models of the objects shown in Figs. 5.30 to 5.61 and hence obtain the sectional view of the objects.
  7. Develop the solid models of different components of a knuckle joint shown in Fig. 7.15. Take the necessary dimensions from Fig. 7.14.
  8. Using solid model editing commands, generate the assembled view of the knuckle joint. Generate the orthographic views from the solid model.
  9. Which prompt of the Wedge command identifies the location of the right angle?
  10. When using the Z-axis option of the Slice command, where will the point on the Z-axis (normal) of the plane be?
  11. Which command helps the draftsman find design interferences before construction?
  12. Fill in the blanks.
    1. You can use the _______ command to subtract solids.
    2. The _______ command can be used to create fillets and rounds.
    3. You can use the _______ command to list the properties of a solid.
    4. You can use the _______ command to edit the faces and body of the existing solids.
    5. The _______ command creates 3-D solids from two-dimensional regions and solids.
    6. _______ are solid shapes — predefined and user defined — with which you build your model.
    7. To draw a solid pyramid, use the _______ option of the Extrude command.
    8. Use the________command to draw a solid cylinder.
    9. The _______ command forms the basis for most 2-D to 3-D conversion packages.
    10. The _______ command behaves like a computer-controlled welding rod.
    11. Use the _______ command to remove one solid shape from another.
    12. Use the _______ command to cut through a solid object as though you are using a knife.
    13. The _______ command will compute the volume of a selected solid object.
  13. State whether the following are true or false.
    1. A composite solid consists of two or more solid primitives.
    2. The cone command creates a solid cone with an elliptical or a circular base.
    3. AutoCAD solids is based on PADL technology.
    4. The cross-section obtained by the Section command is automatically cross-hatched.
    5. The value of Isolines variable can be from 1 to 30.
    6. The base of the wedge is always drawn perpendicular to the current construction plane.
    7. It is necessary to regenerate an object to view changes in the Isolines display.
    8. There is no difference between filleting solids and filleting two-dimensional objects.
    9. There is no Angle option when chamfering a solid.
    10. There is no difference between chamfering solids and chamfering two-dimensional objects.
  14. Choose the correct option.
    1. The Center option of the Box command identifies the center of the box along (the X and Y axes, the X, Y and Z axes).
    2. When using the Object option to define the axis of revolution of the Revolve command, the object selected (must, does not have to) exist in the current XY plane.
    3. The Intersect command (removes everything that intersects between solids, removes everything that does not intersect between solids).
    4. The section that AutoCAD creates is a (solid, region, polyline).