Engineering Graphics (BT-105) Unit 8 Notes

How unit 8 is examined

This unit covers CAD annotation, layers, edit commands, plotting and 3D modelling; layers (7 and 14 marks), edit commands and the types of 3D models carry the marks.

Applying dimensions and annotations

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. Dimensioning in CAD adds measured sizes to a drawing, and annotation adds text, notes and labels that explain it.

Key points.

Dimension commands (linear, aligned, radius, diameter, angular) measure the object automatically, so the value always matches the geometry.
Dimensions and text should sit on their own layers, so they can be switched off or locked.
A dimension style fixes text height, arrow size and extension-line gap once for the whole drawing.
Text is added with single-line or multiline text commands, in a style with a chosen font and height.

Layers: setting up and using customised layers

Definition. <mark>A layer is a transparent overlay in CAD on which related objects are drawn, so that the drawing is organised and each group is controlled by colour, linetype, lineweight and visibility together.</mark>

Key points.

A drawing is a stack of transparent sheets, and the drawing seen on screen is all the visible layers laid over each other.
Every layer has a name, a colour, a linetype (continuous, hidden, centre), a lineweight and a plot on/off setting.
Objects drawn on a layer take that layer's properties (ByLayer), so changing the layer changes all its objects at once.
On/off hides a layer, freeze/thaw hides it and also skips it when regenerating, and lock/unlock keeps it visible but not editable.
Only one layer is current, and new objects go on it; an existing object is moved by choosing another layer in its properties.
Customised layers such as DIM, HIDDEN, CENTRE, TEXT and OUTLINE are made once and saved in a template file.
Uses: architecture (walls, doors, electrical, plumbing, text), mechanical drawings (outline, hidden, centre, dimensions, hatching) and PCB design (top copper, bottom copper, silkscreen, solder mask).

Implementation.

Step 1: Type LAYER (or click Layer Properties) to open the Layer Properties Manager.
Step 2: Click New Layer and type a name such as HIDDEN.
Step 3: Set its colour, linetype and lineweight.
Step 4: Set the layer current, then draw, or select objects and change their layer.
Step 5: Use on/off, freeze/thaw and lock to control what is shown or edited.

Viewports (for the short note). A viewport is a window showing a view of the model. In model space the screen is split into tiled viewports, so front, top and side views show together. In layout (paper) space floating viewports are placed on the sheet, each with its own scale, for printing.

Edit commands (for the short note). These are TRIM, EXTEND, OFFSET, FILLET, CHAMFER, MOVE, COPY, ROTATE, SCALE and MIRROR, explained in the next topic. Types of projections are in the last topic.

Answer frame. Open with the definition; draw a stack of three transparent layers over one drawing; develop points 1-6, then the examples; close with the Layer Properties Manager steps. For "examples where layering is useful", give architecture, mechanical and PCB, then the benefit of control.

Asked: [7 marks] (Jun 2022, Dec 2024) Explain the layering concept with examples. How is it implemented in CAD software? Asked: [7 marks] (Nov 2022) Give some examples where the layering concept is useful. Asked: [14 marks] (Jun 2023, Dec 2023) Write short notes on (any two): i) Viewports ii) Layering concept iii) Edit commands used in CAD / Types of projections. Pitfall: Freezing a layer is not the same as turning it off; a frozen layer is also left out of regeneration.

Changing line lengths: extend, lengthen and edit commands

Definition. <mark>Edit commands modify existing objects; EXTEND lengthens a line to meet a boundary edge, and LENGTHEN changes a line's length by a set amount.</mark>

Key points.

Edit commands save redrawing, since existing geometry is changed instead of new geometry being drawn.
Select the boundary edge first, then click the line to be extended, and it stretches to meet the edge.
Lengthen changes an open line by a delta, a percentage or a total length.
Transformation commands (move, copy, rotate, mirror) reposition objects and need a base point.

Command	Function
TRIM	Cuts objects at edges set by other cutting objects.
EXTEND	Lengthens objects to meet the edges of other objects.
FILLET	Rounds the corner of two lines with a given radius.
CHAMFER	Makes a bevelled edge between two non-parallel lines.
OFFSET	Makes parallel lines, curves or concentric circles at a set distance.
MOVE	Shifts objects from a base point to a second point, with no duplicate.
COPY	Duplicates objects from a base point to one or more points.
ROTATE	Turns objects about a base point by an angle.
MIRROR	Makes a reflected copy about a two-point mirror line.

Answer frame. Open with "Edit commands modify existing drawing objects"; pick five from the table with their function; close with an example, such as offset then trim to form a slot.

Asked: [7 marks] (Nov 2022) Name and explain five edit commands used in CAD; explain move, copy, rotate and mirror.

Printing documents with the print command

Definition. The PLOT/PRINT command sends a drawing to a printer or plotter, or to a PDF file.

Key points.

The dialog sets the printer, paper size, plot area (display, extents, window or layout) and orientation.
The plot scale must match the drawing units, for example 1:1 or 1:50.
A plot style table controls colour and lineweight on paper.
Preview before printing avoids wasted paper.

Sectional views of composite solids and true shape

Definition. A composite solid is made of two or more right solids joined together, and its section is cut by a plane to show the inside.

Key points.

The cutting plane (usually a section plane) is shown in the view where it appears as a line.
The cut points are projected to the other view to get the sectional view.
The true shape is found by an auxiliary view drawn parallel to the cutting plane.
The cut surface is hatched at 45 degrees.

Computer-aided design (CAD) software modeling of parts and assemblies

Definition. Part modelling builds each component as a 3D solid, and assembly modelling joins the parts using constraints.

Key points.

A part is made from sketches, extrudes, revolves and cuts, and it stores its dimensions.
An assembly places parts with mate constraints such as coincident, concentric and distance.
Interference checking finds parts that collide.
Drawings, bill of materials and exploded views come directly from the model.

Parametric and non-parametric models: solid, surface and wireframe

Definition. <mark>3D modelling is the creation of a three-dimensional computer model of an object; its main types are wireframe, surface and solid.</mark>

Key points.

Wireframe shows only edges and vertices, so it is light and fast but has no faces, volume or hidden-line removal.
Surface modelling adds skin faces over the wireframe, so it suits complex contours like car bodies, but it has no volume.
Solid modelling stores the volume, so mass, centre of gravity and collisions can be found; it uses CSG (union, subtract, intersect) or B-Rep (faces, edges, vertices).
Parametric models are driven by dimensions and constraints, so changing a value updates the model; non-parametric models are edited directly with no stored history.
Compound solids are built with Extrude, Revolve, Sweep and Loft, then joined by Union, Subtract and Intersect.
Visualisation uses 3D Orbit, visual styles (wireframe, shaded, realistic), perspective and multiple viewports.

Wireframe method.

Cone: draw the circular base, mark equal divisions, fix the apex on the axis, and join the apex to each division by generator lines.
Pyramid: draw the polygon base, fix the apex above its centre, and join the apex to each base vertex.
Prism: draw the bottom polygon, copy it along the axis to make the top, and join matching vertices with vertical edges.

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Answer frame. For wireframe, open with the definition, draw the three figures with vertices, and give the method of each. For 3D modelling, define it, then wireframe, surface and solid with pros and cons; close with parametric. For compound solids, give tools, Boolean operations, then visualisation.

Asked: [7 marks] (Jun 2022) Explain the method of drawing wireframe models of a cone, pyramid and prism. Asked: [7 marks] (Jun 2024) What is 3D modeling? Explain types of modeling. Asked: [7 marks] (Jun 2025) Discuss how CAD software supports the visualisation and creation of complex 3D objects such as compound solids, with specific tools.

Planar projection theory, including perspective, isometric, multiview, auxiliary and section views

Definition. Projection draws a 3D object on a plane by lines from its points; planar projection uses straight projectors.

Key points.

Parallel projection has parallel projectors and includes orthographic (multiview and auxiliary), oblique and axonometric (isometric).
Perspective projection has projectors meeting at the eye, so distant objects look smaller and it looks natural.
Multiview gives true shapes on separate planes; isometric shows all three faces at 120 degrees with foreshortened axes.
A section view shows the inside by an imagined cut.

Last-minute revision

A layer is a transparent overlay that holds related objects and shares colour, linetype and lineweight.
Layer states: on/off, freeze/thaw, lock/unlock.
Layer command: LAYER opens the Layer Properties Manager.
EXTEND lengthens to a boundary; TRIM cuts to a boundary.
FILLET rounds a corner with a radius; CHAMFER bevels it.
OFFSET makes parallel copies at a distance; MIRROR reflects about a line.
Model-space viewports are tiled; layout viewports float and carry the print scale.
Wireframe = edges only; surface = skin; solid = volume.
Solid methods: CSG and B-Rep.
Parametric models update from dimensions and constraints.

Memory hooks

Layers are tracing sheets stacked on a drawing.
Wire, Skin, Solid: the three models from thin to full.
TEF-CO: Trim, Extend, Fillet, Chamfer, Offset.
UnSub-Int: Union, Subtract, Intersect.

Coverage checklist

applying dimensions to objects, applying annotations to drawings: no past questions.
Setting up and use of Layers, layers to create drawings, Create, edit and use customized layers: layering (Jun 2022, Dec 2024), layering examples (Nov 2022), short notes (Jun 2023, Dec 2023).
Changing line lengths through modifying existing lines (extend/lengthen): five edit commands (Nov 2022).
Printing documents to paper using the print command: no past questions.
Drawing sectional views of composite right regular geometric solids and project the true shape of the sectioned surface: no past questions.
Computer-aided design (CAD) software modeling of parts and assemblies: no past questions.
Parametric and non-parametric solid, surface, and wireframe models: wireframe (Jun 2022), 3D modeling types (Jun 2024), compound solids (Jun 2025).
Planar projection theory, including sketching of perspective, isometric, multiview, auxiliary, and section views: no past questions; types of projections in the Jun 2023 short note.

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Engineering Graphics (BT-105) - Unit 8 Short Notes

How unit 8 is examined

Applying dimensions and annotations

Layers: setting up and using customised layers

Changing line lengths: extend, lengthen and edit commands

Printing documents with the print command

Sectional views of composite solids and true shape

Computer-aided design (CAD) software modeling of parts and assemblies

Parametric and non-parametric models: solid, surface and wireframe

Planar projection theory, including perspective, isometric, multiview, auxiliary and section views

Last-minute revision

Memory hooks

Coverage checklist