Part 3

Module 14: Trimming and Extending

Learning Outcomes

When you have completed this module, you will be able to:

  1. Describe the geometry of regular polygons.
  2. Apply the TRIM and EXTEND commands to shorten and lengthen drawing objects at selected cutting edges.

Geometry Lesson: Regular Polygons

A polygon is defined as any plane figure bounded by straight lines. A regular polygon is a polygon that has equal angles, equal sides, and can be inscribed or circumscribed around a circle. AutoCAD has a command to automatically construct polygons and it will be taught in a future module. For now, it is important for you to be able to construct polygons using geometry.

The first nine regular polygons are shown in Figure 14-1.

Figure 14-1
Regular Polygons

Any regular polygon can be inscribed in or circumscribed around a circle as shown, using a hexagon, in Figure 14-2.

Figure 14-2
Inscribed and Circumscribed Regular Polygon

It is important to understand and know the geometry involved to construct a triangle, square, hexagon, and octagon as shown in Figure 14-3. Study each one and try to understand how they are constructed and the angles used to construct them.

Figure 14-3
Geometry of Four Regular Polygons

The following are the steps used to draw a hexagon in an AutoCAD drawing. See Figure 14-4.

Step 1

Draw 6 lines on layer Construction from the center to each corner, 60 degrees apart.

Step 2

Draw six lines snapping from the end of one line to the end of the next line.

Step 3

Turn layer Construction off.

Figure 14-4
Drawing a Hexagon

Trimming and Extending

Not every object can be drawn using exact coordinates or locations as has been taught to this point in the book. There are times where it is essential that objects be drawn to approximate lengths and sizes and then trimmed or extended to their exact size and location. This does not mean that you can guess at the size and location of the objects. Objects must still be drawn with 100 percent accuracy. The length can be approximated and then adjusted using the TRIM or EXTEND commands. These two commands are a big part of you becoming more productive.

In AutoCAD 2021 there have been changes to the Trim and extend commands and the default settings will not produce the expected results in the following tutorials. The default MODE setting, QUICK, will give access to the [cutting edges/Crossing/mode/Project/erase] options, which should be set as explained later in this tutorial. Then, as a last step, the mode should be set to standard. This will produce the expected behavior of TRIM and EXTEND. If you do not see the above options, set the MODE to Quick, make the setting changes and then restore the MODE setting to Standard.

USER TIPS: With both the TRIM and EXTEND commands, a very common beginner’s error is picking the wrong objects first. Always select the cutting edge object(s) first, press the Enter or Space next, and then select the objects to be trimmed or extended.

AutoCAD Command: TRIM

The TRIM command is used to shorten objects to meet the edges of other objects. It is not required for the object being trimmed to intersect with the object forming the cutting edge.

Shortcut: TR

Modify Toolbar
Modify Pull-down
Home Ribbon

AutoCAD Command: EXTEND

The EXTEND command is used to lengthen objects to meet the edges of other objects. It is not required for the object being extended to intersect with the object forming the cutting edge.

Shortcut: EX

Modify Toolbar
Modify Pull-down
Home Ribbon

WORKALONG: Using the TRIM and EXTEND Commands

Step 1

Start a new drawing using the template: 2D English.

Step 2

Save and name the drawing: AutoCAD 2D Workalong 14-1.

Step 3

Create the layers: Object and Construction. Set layer: Construction as the current layer.

Step 4

Using the figures as a reference, enter the LINE command, as shown on the below, to draw the construction lines shown in Figure Step 4B. (Figure Step 4A and 4B)

Figure Step 4A

Command: L

Specify first point: 2.5,4.5

Specify next point or [Undo]: @1,0

Specify next point or [Undo]: @2,0

Specify next point or [Close/Undo]: @1,0

Specify next point or [Close/Undo]: @0,4

Specify next point or [Close/Undo]: @0,1

Specify next point or [Close/Undo]: @-3,0

Specify next point or [Close/Undo]: @2<220

Specify next point or [Close/Undo]:

Command: L

Specify first point: (end) P1

Specify next point or [Undo]: @0,5

Command: L

Specify first point: (end) P2

Specify next point or [Undo]: @1<145

(The length of this line can be any length but the direction must be accurate. I purposely drew it shorter to demonstrate the EXTEND command.)

Command:

Figure Step 4B

AUTHOR’S COMMENTS: Notice how the object is drawn with a series of construction lines that go from point to point on the object rather than from corner to corner. This is done so that the endpoints of the lines can be used as snap point later in the construction.

Step 5

Enter the TRIM command as shown below. (Figure Step 5).

Command: TRIM

Current settings: Projection=UCS, Edge=None

Select cutting edges …

Select objects or <select all>:

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: E

Enter an implied edge extension mode [Extend/No extend] <No extend>: E

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

AUTHOR’S COMMENTS: The Edge option in the TRIM command should be set to Extend most of the time. Once it is set on your computer, it will stay that way until you change it.

Command: TRIM

Current settings: Projection=UCS, Edge=Extend

Select cutting edges …

Select objects or <select all>: P3 1 found

Select objects: P4 1 found, 2 total

(These lines are the cutting edges.)

Select objects:

(Press enter or space.)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P5

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P6

(These are the lines to trim. Select the line close to the end to be trimmed.)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

Figure Step 5

Step 6

Enter the EXTEND command, as shown below, to extend the line to top line in object. (Figure Step 6A and 6B)

Command: EXTEND

Current settings: Projection=UCS, Edge=Extend

Select boundary edges …

Select objects or <select all>: P7 1 found

(The cutting edge.)

Select objects:

(Press enter or space.)

Select object to extend or shift-select to trim or [Fence/Crossing/Project/Edge/Undo]: P8

(The line to extend. Select the line close to the end that is to be extended.)

Select object to extend or shift-select to trim or [Fence/Crossing/Project/Edge/Undo]:

Command:

Figure Step 6A
Figure Step 6B

Step 7

Draw three construction lines starting at the midpoint of the bottom line. (Figures Step 7)

Command: L

Specify first point: (mid) P9

Specify next point or [Undo]: @0,1

Specify next point or [Undo]: @0,1.5

Specify next point or [Close/Undo]: @0,1.5

Specify next point or [Close/Undo]:

Command:

Figures Step 7

AUTHOR’S COMMENTS: Notice how the three lines are drawn end to end even though they look like one line. This is done to establish the endpoints so that they can be snapped onto later to construct the object.

Step 8

Enter the LINE commands, as shown below, to insert the construction lines for the hexagon. Each line starts at the end of the construction line drawn in Step 7. (Figure Step 8)

Command: L

Specify first point: (end) P10

Specify next point or [Undo]: @.5<30

Specify next point or [Close/Undo]:

Command: L

Specify first point: (end) P10

Specify next point or [Undo]: @.5<90

Specify next point or [Close/Undo]:

Command: L

Specify first point: (end) P10

Specify next point or [Undo]: @.5<150

Specify next point or [Close/Undo]:

Command: L

Specify first point: (end) P10

Specify next point or [Close/Undo]: @.5<210

Specify next point or [Close/Undo]:

Command: L

Specify first point: (end) P10

Specify next point or [Undo]: @.5<270

Specify next point or [Close/Undo]:

Command: LINE Specify first point:

Command: L

Specify first point: (end) P10

Specify next point or [Undo]: @.5<330

Specify next point or [Close/Undo]:

Command:

Figure Step 8

Step 9

Enter the LINE command, as shown below, to construct the lines for the hexagon. Ensure that you have Autosnap enabled and you snap to the endpoints of the lines. (Figure Step 9)

Command: L

Specify first point: (end) P11

Specify next point or [Undo]: (end) P12

Specify next point or [Undo]: (end) P13

Specify next point or [Close/Undo]: (end) P14

Specify next point or [Close/Undo]: (end) P15

Specify next point or [Close/Undo]: (end) P16

Specify next point or [Close/Undo]: (end) P11

Specify next point or [Close/Undo]:

Command:

Figure Step 9

Step 10

Enter the LINE and CIRCLE commands, as shown below, to draw the slot. (Figure Step 10A and 10B)

Command: L

Specify first point: (end) P17

Specify next point or [Undo]: @1,0

Specify next point or [Undo]:

Command: L

Specify first point: (end) P17

Specify next point or [Undo]: @-1,0

Specify next point or [Undo]:

Command: C

Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: (end) P18

Specify radius of circle or [Diameter]: D

Specify diameter of circle: .5

Command: C

Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: (end) P19

Specify radius of circle or [Diameter] <0.2500>:

Command: L

Specify first point: (quad) P20

Specify next point or [Undo]: (quad) P21

Specify next point or [Undo]: Command: L

Specify first point: (quad) P22

Specify next point or [Undo]: (quad) P23

Specify next point or [Undo]:

Command:

Figure Step 10A
Figure Step 10B
AUTHOR’S COMMENTS: Ensure that you snap to the object snap mode shown before each pick in the command.

Step 11

Enter the commands, as shown below, to draw the construction objects for the bottom arc. (Figure Step 11A and 11B)

Command: C

Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: (end) P24

Specify radius of circle or [Diameter]: D

Specify diameter of circle: 1

Command: L

Specify first point: (cen) P25

Specify next point or [Undo]: (end) P26

Specify next point or [Undo]:

Command: L

Specify first point: (cen) P25

Specify next point or [Undo]: (end) P27

Specify next point or [Undo]:

Command:

Figure Step 11A
Figure Step 11B
AUTHOR’S COMMENTS: I find it easier to draw circles rather than arcs. Then I trim them to construct the required arc.

Step 12

Enter the TRIM command, as shown below, to create the slot. (Figure Step 12A and 12B)

Command: TRIM

Current settings: Projection=UCS, Edge=Extend

Select cutting edges …

Select objects or <select all>: P28 1 found

Select objects: P29 1 found, 2 total

Select objects:

(Press enter or space.)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P30

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P31

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

Figure Step 12A
Figure Step 12B

Step 13

Enter the TRIM command, as shown below, to create the bottom arc. (Figure Step 13A and 13B)

Command: TRIM

Current settings: Projection=UCS, Edge=Extend

Select cutting edges …

Select objects or <select all>: P32 1 found

Select objects: P33 1 found, 2 total

Select objects: P34 1 found, 3 total

Select objects:

(Press enter or space)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P35

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:P36

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P37

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

Figure Step 13A
Figure Step 13B

Step 14

Enter the TRIM command, as shown below, to trim the line on the top right. (Figure Step 14A and 14B)

Command: TRIM

Current settings: Projection=UCS, Edge=Extend

Select cutting edges …

Select objects or <select all>: P38 1 found

Select objects:

(Press enter or space)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P39

Select object to trim or shift-select to extend or

[Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

Figure Step 14A
Figure Step 14B

Step 15

Change the layer of the objects that you want to reside on layer: Object. (Figure Step 15)

Figure Step 15

Step 16

Freeze layer: Construction. Your completed drawing should appear as shown in the figure. (Figure Step 16)

Figure Step 16

Step 17

Save and close the drawing.

MUST KNOW: The Edge option in the TRIM and EXTEND commands should always be set to Extend as shown below. Step 5 will show you how to set it. See also the 2nd paragraph in the notes on Trim an Extend.

Command: TRIM

Current settings: Projection=UCS, Edge=Extend

USER TIP: The TRIM and EXTEND commands areinterchangeable. If you enter either command, you can change over to the other command by holding down the Shift key. i.e.
Command: TRIM (See Figure Step 1.)

Current settings: Projection=UCS, Edge=None
Select cutting edges …Select objects or <select all>: P1 1 found
Select objects: P2 1 found, 2 total
Select objects: P3 1 found, 3 total
Select objects: P4 1 found, 4 total

Select objects:

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: E

(Enter the Edge option.)

Enter an implied edge extension mode [Extend/No extend] <No extend>: E

(Set it to Extend. This allows you to trim or extend when the objects do not physically touch.)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P5

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P6

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P7

(Press and hold down the Shift key to change to the EXTEND command.)

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P8

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P9

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: P10

Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]:

Command:

Figure Step 2                                                                                                      Completed Drawing

Key Principles

Key Principles in Module 14

  1. To improve your drawing productivity, draw the location and direction of objects with 100 percent accuracy but at an arbitrary length. Adjust the length with the TRIM or EXTEND commands.
  2. With both the TRIM and EXTEND commands, select the cutting edge objects first, press the Enter or Space key, and then select the object(s) to trim or extend.
  3. The TRIM and EXTEND commands are interchangeable by holding down the Shift key to change from one command to the other.

Lab Exercise 14-1

Time allowed: 40 minutes.

Drawing Name Template Units
AutoCAD 2D Lab 14-1 2D English Inches
Layering Scheme
Layer Name Objects on Layer Color
Construction Construction objects 253
Object 1 All objects Red

Step 1

Draw the object shown in the figure using the layering scheme. (Figure Step 1A and 1B)

Figure Step 1A
AUTHOR’S COMMENTS: Do not delete the construction lines.
Figure Step 1B

Step 2

Set the insertion units and check your drawing with the key.

Step 3

Turn layer: Key off and freeze layer: Construction.

Step 4

Save and close the drawing.

AUTHOR’S CONSTRUCTION TECHNIQUES: The following steps are the construction technique suggested by the author to help you learn how to construct objects using AutoCAD. It is only the suggested method and if you can complete the drawing accurately using a different construction technique, that is what is important. You may want to compare your construction technique with the authors.

AUTHOR’S CONSTRUCTION HINTS: Do your best to complete the lab exercise drawing without using the following hints. If you get stuck and cannot complete it on your own, use the following hints to help you.

Hint 1

Draw the lines in the correct location and direction but to any length and then trim or extend them. (Figure Hint 1)

Figure Hint 1

Hint 2

The lines in Figure Hint 2A must be drawn tangent to tangent (see Module 12) while the lines in Figure Hint 2B can be draw either tangent to tangent or quad to quad. (Figure Hint 2A and 2B)

Figure Hint 2A                                                                                    Figure Hint 2

Hint 3

The figure shows the construction lines. (Figure Hint 3)

Figure Hint 3

Lab Exercise 14-2

Time allowed: 30 minutes.

Drawing Name Template Units
AutoCAD 2D Lab 14-2 2D Metric Millimeters
Layering Scheme
Layer Name Objects on Layer Linetype Color
Construction Construction objects Continuous 253
Circle 1 Outside circle Center2 Red
Circle 2 Inside circles Continuous Red
Slots Slots Dashdot2 Blue

Step 1

Draw the object shown in the figure using the layering scheme. (Figure Step 1A and 1B)

Figure Step 1A

AUTHOR’S COMMENTS: Do not delete the construction lines.

Figure Step 1B
Completed Drawing

Step 2

Set the insertion units and check your drawing with the key.

Step 3

Turn layer: Key off and freeze layer: Construction.

Step 4

Save and close the drawing.

AUTHOR’S CONSTRUCTION TECHNIQUES: The following steps are the construction technique suggested by the author to help you learn how to construct objects using AutoCAD. It is only the suggested method and if you can complete the drawing accurately using a different construction technique, that is what is important. You may want to compare your construction technique with the authors.

AUTHOR’S CONSTRUCTION HINTS: Do your best to complete the lab exercise drawing without using the following hints. If you get stuck and cannot complete it on your own, use the following hints to help you.

Hint 1

Step 1

Draw only four construction lines at the correct angle as shown in the figure. Extend them to other side of the large circle. That will save you from drawing all 8 lines. (Figure Step 1A and 1B)

Figure Step 1A
Figure Step 1B

Step 2

Draw 8 circles locating their centers at the intersection of the lines and circle. (Figure Step 2)

Figure Step 2

Step 3

Draw 2 large circles by showing AutoCAD the radius by snapping to the intersection of the small circle and line. (Figure Step 3)

Figure Step 3

Step 4

Trim the large circles. (Figure Step 4)

Figure Step 4

Step 5

Trim the slots. (Figure Step 5)

Figure Step 5

Lab Exercise 14-3

Time allowed: 40 minutes.

Drawing Name Template Units
AutoCAD 2D Lab 14-3 2D Metric Millimeters
Layering Scheme
Layer Name Objects on Layer Color
Concrete Block All objects Blue

Step 1

Draw the top view of the concrete block shown in the figure using the layering scheme. (Figure Step 1A, 1B, and 1C)

Figure Step 1A
Figure Step 1B Concrete Block
Figure Step 1C Completed Drawing

Step 2

Set the insertion units and check your drawing with the key. Turn layer: Key off and freeze layer: Construction.

Step 3

Save and close the drawing.

License

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Introduction to Drafting and AutoCAD 2D Copyright © 2021 by Wally Baumback is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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