AutoCAD 3D Tutorial(1).pdf

UNIVERSITY OF NEW SOUTH WALES

FACULTY OF THE BUILT ENVIRONMENT

TUTORIAL INTRODUCTIONS TO CAD

AutoCAD 2000 in 3D - A Monkish Shot Tower

Jim Plume

As you are probably aware, the

traditional technology for

manufacturing lead shot involves

dropping blobs of lead from

significant heights such that, as the

lead cools during its descent, it

forms perfect spherical balls ….

This tutorial introduces the concepts and techniques used to build 3D models in

AutoCAD 2000. As a vehicle for learning these techniques, we develop a model

of a whimsical structure that I have called the Monkish Shot Tower after a group

of imaginary Monks who have refined the technology of manufacturing lead shot

for firearms, and the like. The tower is designed around the perfect number 7,

features an external spiral stair, an elaborate canopy on top, and a hole down

the centre. The final structure is illustrated in Figure 1 below.

This tutorial will introduce the following concepts and techniques:

simple solid modelling with solid primitives & CSG operations (the tower);

construction of complex solid objects (stair and landing);

user coordinate systems and positioning of objects (external stairs);

advanced use of the CSG operations (tapering the stairs);

simple surface modelling (the roof canopy and handrails);

surface modelling versus solid modelling (the handrails);

meshed surfaces (the landing “sails” for shading).

Figure 1 Four Perspective Views of the Monkish Shot Tower

Updated: May 2000

AutoCAD 2000 in 3D - A Monkish Shot Tower

GETTING STARTED

As with every drawing that you produce with AutoCAD you should begin by

establishing your drawing area by setting the Drawing Limits , Units and using

Zoom All to expand out to a full view of your drawing area. Do that now, setting

the limits to 20000 x 15000 .

Layer Control button

For this model, we will make use of several different layers for handling different

parts of the model. Begin by calling up the Layer Control dialogue box (see

button at left) and create two new layers called “Construction” and “Tower”,

setting each to a different colour (I used green and white respectively). The

Construction layer will be used for construction lines that do not form part of the

final drawing. The Tower layer will contain the central tower.

Hint :

To create a new layer in this

dialogue box, simply click the New

button and type a name for the

layer in the list. To set the colour,

click on the colour box adjacent to

the corresponding layer.

The last thing to do before beginning the tower is to create a single reference

point for this model … we will do this by placing a Point entity on the

Construction layer at the centre of the drawing area (which will become the

centre of the base of the Shot Tower). We will be using Points a few times in

this drawing as reference markers, so we need to set a Point Style that will be

easily visible (the normal way AutoCAD marks a point is with a single dot … we

will use an X-shaped cross instead). To do all that, proceed as follows …

Begin by setting the current layer to Construction — pull-down the list of layers

(illustrated at left) and click on Construction … then proceed as follows …

Pull-down layer list …

F o rmat >

P oint Style

Select from the pull-down menus — in the resulting

dialogue box select the X-shaped cross and then click OK .

[Draw.Point]

Issue the Point command (illustrated) and click a position

at about the centre of the drawing area …

Draw point tool …

< Esc >

To cancel the command.

CONSTRUCTING THE CENTRAL TOWER

In keeping with the principle that everything in this Shot Tower is designed

around the perfect number 7, we will construct a 7-sided polygonal tower, 19.6

metres high (7 levels at 2700), with an outside diameter of 7 metres, and

tapering inwards at a gradient of 1:70 (which converts to an angle of 1.3 o off the

vertical). Proceed as follows:

Set the current layer to Tower …

[Draw.Polygon] Select from the Draw Toolbar .

Polygon tool

Number of sides — then use the |NODe| object snap mode

to fix the centre at the point entity just created.

Specify an inscribed polygon.

3500

↵↵

Radius of the polygon.

[Modify.Offset]

Issue the Offset command (as illustrated).

Offset tool

300

Offset distance — select the polygon just drawn and then

point to the inside to show the side to offset — press

complete the command.

We now have to extrude these two polygons in the Z-direction to the height

required, specifying the 1 in 7 taper as we do so. We will then subtract the inner

Updated: May 2000

AutoCAD 2000 in 3D - A Monkish Shot Tower

one away from the outer one to create a polygonal shell with an empty interior.

Once we have done that, we will position a box to serve as the doorway at

ground level and subtract that away from the shell to create the opening. That

will complete the basic tower! To do all that, proceed as follows …

V iew >

T o olbars

Open the Toolbars dialogue box, scroll down the list to find

the Solids toolbar, and then click on the check-box next to

it. Position it anywhere convenient on the screen, taking a

moment to examine each tool, and noting the brief

description of each on the status line as you move the

mouse across them..

Solids Toolbar with the Extrude

command highlighted ..

[Solids.Extrude] Pick the Extrude command (as illustrated) — don’t forget to

watch the prompts on the command line so that you

understand the process! — select both polygons and then

press

Solid Modelling :

With solid modelling we sculpture

objects by assembling the

primitives shown on this toolbar

using the three CSG operations of

Union, Difference (subtract) and

Interesection. As you will discover

in this tutorial, there is almost

nothing that cannot be formed

using these tools.

to complete the selection.

19600

Height of each extrusion.

1.3

Taper angle .

To see the effect of this in 3D, select V iew > 3 D Views > S E Isometric from the

pull-down menus … this will create a parallel projection ( not a perspective ) of

the 3D object looking from the +X / -Y quadrant. You can then use the normal

Pan and Zoom commands to alter the screen display to suit you own purposes,

while maintaining that parallel projection. Experiment with this until you have a

3D view of the whole tower near the centre of your drawing area.

The CSG tools are located on the Solids Editing Toolbar (illustrated). You can

keep this on the screen or you may feel it is easier to execute these commands

through the pull-down menus …

M odify > Solids

Editi n g >

S ubtract

Issue the Subtract command — note that this command

allows you to select two sets of objects: the second set are

then subtracted from the first — we will have only one

object in each set in our case — select the outer shape

and press

Solids Editing Toolbar

(the “primary” object), and then select the

inner object and press

Real-Time Pan & Zoom

again (the object to be subtracted

from the “primary” one) — don’t forget to watch the

prompts to understand how this works .

These new tools are great for

moving around your drawing …

hold down the left button while

dragging the mouse up and down.

hide

Issue the Hide command — this draws the current view

with all hidden lines removed or truncated as necessary —

notice that your tower is now one single solid object which

is hollow down the centre .

User Coordinate System (UCS) :

This is a 3D cartesian coordinate

system that operates independently

of AutoCAD’s fixed World

Coordinate System (WCS). It can

be positioned and orientated

anywhere in space (at any angle).

All 2D drawing is always done in

the UCS X-Y plane: normally, the

UCS is coincident with the WCS!

The next step is to punch a doorway through the base of the tower. Begin by

zooming in to the base of the tower with a view similar to that shown in Figure 2,

over the page ( Hint : use the new real-time pan and zoom tools in the standard

toolbar) . To make this process simpler, we will position the UCS (see

explanation at left) as illustrated (Figure 2), with the origin at the outside corner

of the tower and the X-axis in line with its base. To do that, follow these steps:

ucs

↵↵

Issue the UCS command to alter the position of the current

UCS — notice the command prompt, showing options for

saving and restoring UCS settings .

This means that you can use all the

2D drawing commands that you are

used to, only place them anywhere

in space by first positioning the

UCS. This would allow you, for

example, to draw a cross-hatch

pattern on a sloping roof plane!

↵↵

Select the New option … notice the variety of ways that

you can define the UCS … we will use many of these in the

course of this tutorial …

Updated: May 2000

AutoCAD 2000 in 3D - A Monkish Shot Tower

Select the 3point option keyword — this is the most

versatile option, allowing us to define the new UCS

precisely — watching the prompts, proceed as follows …

|ENDpoint|

Use the object snap to put the origin at 1 .

|ENDpoint|

U se object snap again to identify a “ point on the X-axis ” at

2 .

|ENDpoint|

And again, to no m inate a point on the “ positive Y portion of

the XY plane ” at 3

Re-oriented UCS icon …

Notice that the UCS Icon has

now dropped the “W” (no

longer World Coordinates) and

has been positioned at the new

origin, oriented to suit the new

UCS. See note at left to move

the icon back to the lower left

corner of the screen . We now

use the Box primitive to

construct the doorway … be

careful that you use the “corner

box” rather than the “centre

box” …

NOTE : if the UCS icon ha s been

placed at the origin (point 1 in

Figure 2), then type the command

ucsicon

for

“No Origin” .. that will force the icon

back to the lower left corner of the

screen

and then type n

Figure 2 Constructing the Door Opening at

the base of the Tower

[Solids.Box]

Select from the Solids toolbar — AutoCAD will prompt for

the corner position of the box …

Box tool in the Solids Toolbar.

|ENDpoint|

U se the object snap mode to place the corner of the box at

1 in Figure 2.

@1400,500,

2100

Specify the opposite corner of the box as a relative 3D

coordinate — alternatively, this could have been done by

entering the length, width and height of the box separately.

[Modify.Move]

Issue the normal Move command — select the box just

created and pres s

Move tool

— set the Base Point at the |MIDpoint|

position marked 4 (on the box just cre a te d ) and the

Second Point at the |MIDpoint| of line 1 - 2

[Modify.Move]

Re-issue the Move command — select the same object

and press

and pick any point as the Base Point …

@0,-100

Specify the Second Point of the displacement to move the

box so that it is centred on the thickness of the wall.

M odify > Solids

Editi n g >

S ubtract

again —

you should now have a door opening in the base of the

tower !

, then pick the box and press

Notice that we deliberately made the box wider than the wall’s thickness (500

instead of 300) so that when it was positioned, it protruded from both sides of

the wall … this is especially necessary in this case since the tower is tapered

and the wall leans in slightly. Generally , however, when performing this

type of operation, it is prudent to always oversize the subtracted object

to ensure that no thin slivers of material are left behind ! If you try to be

too precise, you can encounter precision errors.

Updated: May 2000

AutoCAD 2000 in 3D - A Monkish Shot Tower

Issue the Solids Subtract command – pick the tower first

and then press

CONSTRUCTING THE EXTERNAL WINDING STAIR

Discussion …

The decision to taper the tower inwards was deliberate because it leads to some

interesting complications in this otherwise simple model. The first of those

complications occurs with the stairs since they too have to taper inwards as they

climb the tower. We get over this complication very simply. We begin by

building the stairs straight up with no taper, each flight hugging the face of an

imaginary extrusion (downwards) of the 7-sided polygon that forms the inside

edge of the (tapered) top of the tower: this means that at the base of the tower

the stairs actually lie partially inside the tower. Once the stairs are complete, we

will create another tapered tower in the middle of the stairs and subtract that

away from them to create the internal taper — the stairs will then very neatly

follow the face of the real tower. We then do the same with the outside taper:

we build the flights very wide to begin with, and then at the end of the process

we construct a large tapered tower with very thick walls which we can also

subtract away from the stairs, thus creating the outer taper. This process is

illustrated in Figure 12. Producing the taper is easy: the harder part is

constructing the stairs to begin with …

First, we need a few construction lines —

makethe Construction layer the current layer again,

resetthe UCS back to “World” (issue the UCS command and then press

accept the default “World”),

and Zoom out to a view that allows you to see the top of the Tower.

We begin by tracing the inner edge of the top of the tower … to do that, we

switch on running object snap …

T ools >

Dra f ting

Settings

Call up the Drafting Settings Dialogue box with the Object

Snap tab (as illustrated) … I suggest that you select each

of the ones shown in the illustration at left … note the brief

explanation of how to use these if you have not done so

before …

Drafting Settings dialogue …

[Draw.

Polyline]

Issue the Polyline command — pick each of the 7 vertices

around the inside edge of the top of the tower …

With Running Object Snap turned

on, whenever you are using the

mouse to select a point, AutoCAD

will highlight and identify the

nearest snap point within the snap

aperture … to use that snap point

simply click the mouse button … if

the snap is ambiguous, pressing

the Tab key will cycle through the

available snaps at that point.

Close the polyline.

[Modify.Move]

Use a Window selection rectangle to select the polyline just

drawn (you work out why!) and then press

— set the

Base Point anywhere you wish (well away from everything

to avoid the running object snap) …

@0,0,-19600

↵↵

Second point of the displacement vector.

Re-position your view to the bottom of the Tower, then, with ORTHO mode

turned on (click the ORTHO button on the status line), construct two parallel

lines of roughly the same length (as illustrated in Figure 3) projecting out from

adjacent corners of the polygon just constructed.

Figure 3

Now choose V iew > 3 D Views > P lan View > W orld UCS and then zoom in on

the ends of those two lines — we will use these lines to set out the stairs in

profile as illustrated in Figure 4 (over the page).

[Draw.Line]

Issue the Line command — pick the start point at the

bottom end of the rightmost line.

@0,140

Height of riser ( multiple of 7!!! ).

@-280,0

Length of going ( another multiple of 7!!! ).

Updated: May 2000

AutoCAD 2000 in 3D - A Monkish Shot Tower

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