Computer-Aided Design

Explain in your own words what computer-aided design (CAD) is and name two things you can do with a virtual model before manufacturing it (rotate it to see it from all sides, measure it, modify it). Why do you think almost no modern object is designed purely on paper anymore?

List three advantages of designing a part on the computer instead of directly building the prototype from wood or cardboard, considering: the material wasted when making a mistake, how easy it is to change a dimension, and the possibility of showing the design to someone who is far away.

In CAD, each shape is placed using exact dimensions: width, length, and height in millimetres. Compare this way of working with freehand drawing: what is gained in accuracy, what is lost in speed for an initial idea, and at which stage of the design process each is suitable.

CAD parts are built from primitives: basic ready-made shapes such as a cube, cylinder, sphere or cone. State which primitive you would choose as a starting point to model each object and why: (a) a mug, (b) a die, (c) an ice cream cone, and (d) a football.

When you open a 3D CAD program, you see the workspace with a grid, a view cube in one corner, and a toolbar with the available shapes. Explain the purpose of each of these three elements and why the grid helps to position parts accurately.

The figure shows, at the top, three CAD primitives and, at the bottom, two operations that combine shapes, but without naming them.

1. a) Name the three primitives at the top (primitive 1, 2, and 3).
2. b) Deduce what each operation at the bottom is (operation a and operation b) —the union of two shapes or the difference between them—, explaining what happened to the original shapes in each result.

Extrusion converts a flat 2D shape into a solid by giving it thickness, as if you were stretching the shape upwards. Explain the concept using an everyday example (a churro press shaping dough, or a biscuit cut with a cutter and lifted up) and state what solid is created by extruding: (a) a circle, (b) a square, and (c) a star.

A glass can be modelled by subtracting a small cylinder from a large cylinder: the small one hollows out the inside. Reason which two operations (union or subtraction) you would use to model these objects and with which primitives: (a) a ring, (b) an inkwell with its hole, and (c) a simple hammer (handle and head).

Write the step-by-step modelling instructions for a glass in Tinkercad: which cylinder you create first and with what dimensions, how you turn the second cylinder into a 'hole', what the grouping operation does, and how you check that the wall has the thickness you wanted.

You want to model a bone-shaped keyring for a friend's dog: a thick cylinder at each end and a bar in the middle, plus a hole for the ring. Describe the sequence of operations in the program (primitives, unions, and subtractions, in order) and estimate the dimensions in millimetres you would give it so that it fits in a pocket.

In the classroom, three applications are used: Tinkercad (3D parts by joining shapes, very simple and browser-based), SketchUp (3D modelling of objects and spaces) and LibreCAD (2D technical drawings). Choose which one you would use for each task and justify your answer: (a) the two-dimensional floor plan of the classroom with its measurements, (b) a 3D keyring to print, and (c) the 3D model of the school library.

A classmate wants to draw a floor plan of their bedroom in Tinkercad and takes twice as long as with a 2D CAD program like LibreCAD. Explain what type of task is suitable for 2D CAD and what for 3D CAD, and reason why using the wrong program makes the work slower even though both 'draw parts'.

To reorganise the Technology workshop classroom ($12 \times 8\ \text{m}$) and check that the $1,2\ \text{m}$ safety walkways between machines are respected, the team must model the space and its furniture. One student insists on using Tinkercad, while another proposes using software like SketchUp.

1. a) Analyse two serious technical difficulties you will encounter if you try to model the entire classroom in Tinkercad (consider the limits and typical scale of the workplane, and how accumulating dozens of complex workbenches and tools through Boolean unions of primitives will affect browser performance).
2. b) Justify which of the two programs is more suitable for this specific project, explaining what advantages it offers (such as working at real scale with architectural dimensions or inserting pre-designed components) compared to solid modeling using basic primitives.

The figure shows the screen of a CAD program with a finished model and the three views generated by the program from it, labelled as view a, view b, and view c.

Identify which view (a, b, or c) is the front elevation, which is the plan view, and which is the side elevation of the model, and explain the advantage of having the program generate them automatically instead of drawing them by hand, especially when the model is modified.

When the model is finished, it is exported to a file to share or print it. Explain what information you think that file saves about the part (its shape, not decorative colours) and why a specific format called STL, which describes the surface of the part using small triangles, is used for 3D printing.

In the program, you have already modelled one leg of a stool. Explain which operations you would use to finish the stool without remodelling the leg three more times (copying, rotating, and moving the existing leg), and what precaution you must take when placing the copies so that the stool does not end up crooked.

When 3D printing a model, two separate parts come out that should have been a single piece: in the program they were 'together' but not grouped, merely touching. Explain the difference between grouping two shapes (so they become a single part) and simply leaving them resting against each other, and how you would check this in the program before printing.

Plan the complete modelling of a phone stand for a desk: a base, an inclined backrest, and a front stop. Detail the primitives you would use for each part, the union or subtraction operations in order, the approximate dimensions in millimetres, and a final check before approving the model.

From the orthographic views of a technical part, the following is observed:

• Front view: symmetrical "T"-shaped silhouette with a total width of $60\ \text{mm}$, a height of $40\ \text{mm}$, an upper crossbeam with a height of $20\ \text{mm}$, and a central pillar with a thickness of $20\ \text{mm}$.

• Top view: $60 \times 25\ \text{mm}$ rectangle with the hidden edges of the central pillar.

• Left side view: $25 \times 40\ \text{mm}$ rectangle with a visible horizontal edge separating the crossbeam from the pillar.

1. a) Deduce the 3D shape of the part and detail two different strategies to model it in CAD: one using union of primitives and another using a large block from which hollow parts are subtracted. Specify the dimensions of each prism in both methods.
2. b) A classmate claims that to check if the CAD model matches the drawing, it is enough to rotate the free view with the mouse. Justify why this method leads to errors and explain how you should use the view cube and the program's orthogonal projection to verify each standardized view.

A part must have: a base block, two side through-holes, and a recess at the top. Reason whether the order of subtraction operations matters, what can happen if you create the recess before the holes (or vice versa), and write the sequence you would follow, justifying each step.

In a project design team, two conflicting views arise regarding which tool to use at each stage:

1. Sofía argues that you should turn on the computer and model directly in CAD from minute one, without making any prior pencil sketches.

2. Marcos contends that it is a waste of time to transfer a pulley bracket to CAD if they already have a perfect dimensioned drawing done by hand with set squares, ready to be manufactured in the workshop.

Rebut both positions with reasoned arguments:

1. a) Justify two disadvantages of starting to design directly in CAD during the brainstorming stage (consider the speed of exploring alternatives and the risk of blocking creativity by focusing on premature dimensions).
2. b) Justify two decisive technical advantages provided by modeling the part in CAD before manufacturing it (for example, regarding the assembly simulation with other parts, interference detection, or exporting to digital manufacturing) compared to only having the paper drawing.

Your model has very fine details: raised lettering half a millimetre high and small legs $1\ \text{mm}$ thick. Anticipate what problems such a part will have when 3D printed and redesign those details on paper so they survive printing, explaining the criteria you followed (minimum thicknesses, unnecessary details).

Three students are working simultaneously in Tinkercad on the collaborative design of a bridge using a shared link. At the end of the session, they discover three mistakes:

1. One teammate accidentally deleted the beam structure that another had finished because they selected it when making a drag-selection box to move their own piece.

2. Two modules designed separately do not fit together in the final assembly because one was designed with a $1\ \text{mm}$ snap grid and the other with a $0,1\ \text{mm}$ snap grid without agreeing on dimensions or connection points, resulting in misalignment.

3. The last $20$ minutes of work from one of them were lost because the workshop's Wi-Fi connection suffered micro-outages and the browser did not sync the changes.

1. a) Explain which technical feature or limitation of cloud-based work caused or worsened each of these three incidents.
2. b) Design a teamwork protocol (with three specific operational rules) to prevent these problems from happening again when modeling collaboratively.

In a manufacturing workshop, an intern claims: "Traditional technical drawing and standard views are dead; simply designing the part in 3D on the computer and sending the file to the machine is enough."

However, the workshop manager rejects the part and demands the 2D dimensioned technical drawing before starting machining.

1. a) Refute the student's statement by explaining why a 3D model alone is not sufficient to manufacture a real part accurately (mention at least two critical pieces of data that a 3D model without annotations does not specify but the technical drawing does, such as fit tolerances, surface finishes, or verification dimensions for quality control).
2. b) Imagine that the part is manufactured directly from the 3D file without a prior control drawing. Predict what will happen when this component has to be assembled with other parts and how knowledge of multiview projections (front, top, and side views) helps detect shape errors prior to manufacturing.