Exercises: Technical drawing and CAD

Explain the difference between a freehand sketch, a working sketch and a standard engineering drawing, and indicate at what point in a technical project each one is used.

Explain what a functional block diagram is and what it is used for. What does each block represent, and what do the arrows connecting them represent?

The figure shows the functional block diagram of a sound system, but one of the blocks is covered with a question mark, as shown in the figure.

Indicate which block is missing and explain what function it performs within the system, taking into account how the signal changes as it passes through it.

Explain what the front view, plan view and side view of a part are, and why a technical drawing usually includes all three views instead of just one.

A drawing is drawn to a scale of 1:10. Explain what this means and calculate how long a real edge of $60\ \text{mm}$ measures on the drawing.

What is dimensioning a drawing? Name the elements of a dimension (dimension line, projection lines, arrowheads/ends, and dimension value) and explain why the value must be the real measurement of the part and not that of the drawing.

Give two advantages of making an initial freehand sketch and two advantages of modelling afterwards with CAD software, and indicate at what point in the design process you would use each one.

Explain what a cut or cross-section is in a technical drawing and in which cases it is essential to understand a part (hollow interiors, cavities, components inside others).

Explain what the acronyms CAD, CAE and CAM stand for, what is done with each type of tool, and in what order they are used throughout product development, from idea to manufacturing.

You have designed a desk lamp and need two graphic documents: one to show the idea to a client and another so that the workshop can manufacture it without errors. State which type of representation you would choose in each case and justify your choice based on the information it must contain.

A washing machine receives clothes, water, detergent and the selected programme as inputs; as outputs, it returns clean clothes and wastewater to the drain. Draw its functional block diagram (input → process → output) in your exercise book, and write down here which blocks you drew and what flows of matter, energy or information travel along each arrow.

You can do it on paper and send a photo with your answer.

A real part measures $240\ \text{mm}$. Calculate how long it will measure when drawn to scale 1:20 and to scale 1:5, and state which of the two drawings will accommodate more detail and why.

STL and STEP files are used to exchange 3D models. Explain what each type contains (a triangle mesh versus the exact geometry of the solid) and reason which one you would use to send a part for printing and which one to keep editing it in another CAD program.

CAE tools allow simulating a part before manufacturing it. Give an example of a design decision that might change after viewing a stress simulation (reinforcing an area, thinning another) and explain what is saved through this virtual check (broken prototypes, material, time).

In CAM manufacturing, the computer generates the toolpaths for the machine tool. Explain what data these toolpaths must contain (tool selection, feed rate, depth of cut per pass) and what the consequences are if one of these parameters is incorrect.

A garden irrigation system has this functional diagram: timer → solenoid valve → spray heads; additionally, a rain sensor can override the irrigation. Today it did not water at its scheduled time. Order the checks you would carry out on each block (from most likely to least likely) and justify this diagnostic order.

A team has implemented an automatic irrigation system by connecting the output of a soil moisture sensor directly to the relay of an electrovalve, configured to open when moisture drops below 30 %. In field tests, they observe two serious problems: the valve undergoes violent and continuous switching (chattering effect) when moisture is around that 30 %, and the plants furthest from the sensor die from water stress even though the sensor indicates adequate moisture.

Redesign the functional block diagram of the system to solve both malfunctions. Identify which new signal conditioning, logic control, and feedback blocks you must incorporate; define the nature of the signals connecting each block (analog, digital, or modulated), and technically justify how your design eliminates the destructive cycling of the actuator.

A designer models an L-shaped bracket with four holes for M6 bolts equidistant from the edges using parametric CAD. To save time, they fix the position of the holes using absolute numerical distances to the coordinate origin, instead of defining geometric constraints of symmetry and offset relative to the faces of the solid. When the client requests extending the base length from 100 mm to 160 mm and switching to an M8 metric thread, the model breaks or generates a useless part.

1. a) Explain what happens geometrically to the holes when altering only the length dimension in that poorly parameterized model.
2. b) Design the feature tree and parametric relationships (geometric constraints and equations between dimensions) that ensure that any future modification in the total length or in the bolt metric automatically adapts the position, diameter, and countersinking of the holes without rebuild errors.

A machining workshop receives an isometric perspective sketch of a stepped shaft with a keyway and a blind hole. The operator states: 'Since the drawing looks well-proportioned, I can measure directly on the perspective with a calibrated ruler, deduce the depths, and manufacture the shaft without needing technical drawings'.

Technically refute the operator's statement, justifying at least three critical geometric or dimensional discrepancies that will arise between the resulting part and the intended one (consider isometric reduction factors, visual ambiguity in hidden features, and dimensional and geometric fit tolerances). Determine which minimum standardized views, cuts, or sections, and what dimensioning information are strictly essential to ensure the interchangeability of the shaft.

At an avenue intersection, drivers complain that, if a pedestrian presses the button immediately after the vehicular traffic light turns green, it changes to the red phase within a few seconds, causing dangerous sudden braking. Furthermore, if several pedestrians repeatedly press the push button while crossing, the system chains pedestrian cycles without allowing vehicular traffic to clear.

Design the functional block diagram of a traffic light controller that corrects these faults. Identify the inputs, timing blocks, control logic, and outputs to the actuators; specify the signal type of each flow (logic pulses, analog clock signals, status buses), and develop the state diagram or logic sequence that ensures a non-interruptible minimum vehicular green time and a balanced management of requests.

A development team must urgently redesign the ergonomic housing of a portable power tool to reduce weight and improve motor ventilation. The team leader proposes skipping the freehand sketching phase and starting directly by modeling 3D solids in CAD to 'save time and send the files to be machined as soon as possible'.

Critically evaluate this proposal:

1. a) Justify why forcing the immediate jump to CAD modeling usually makes the project more expensive and restricts the search for optimal solutions in products with organic geometries or anatomical interaction.
2. b) Identify at what exact technical milestone in the design process the dimensioned sketch ceases to be sufficient and becomes an unacceptable source of risk for manufacturing (consider finite element thermal analysis, housing fit tolerances, and injection mold manufacturing).

A workshop receives the manufacturing drawing of a steel anchor plate. When checking the documentation, the quality control manager finds three technical anomalies:

1) A center-to-center distance between holes appears dimensioned as 120 ± 0,2 mm in the front view, but in the top view the same dimension appears as nominal 122 mm without tolerance.

2) The title block indicates 'Scale 1:1', but when checking the printed drawing with a scale ruler, the dimension labeled as 200 mm measures exactly 100 mm on paper.

3) In the front view, a hidden edge is drawn with a dashed line that does not correspond to any change of plane or hole in either the top view or the side view.

Analyze the impact that processing the plate while ignoring each of these errors would have on machining and final assembly. Then, detail the technical correction protocol that must be applied according to UNE-EN ISO technical drawing standards (criterion of precedence of dimensions over drawing, elimination of redundant dimensions, and the principle of projection alignment of views).

A subcontracted company must manufacture and assemble 100 units of a gear reducer designed by your team, under the constraint that they will not have technical support to resolve questions during the process. If you only provide them with the part drawings containing the nominal geometric dimensions of each part, the probability that the reducers will seize up or break in service is almost total.

1. a) Justify what critical technical information missing from those nominal part drawings will cause mechanical failure (analyse the omission of dimensional and geometric tolerances, standardised ISO fits for bearings, surface finishes, and tightening torques).
2. b) Structure the complete technical manufacturing dossier that you must supply. For each document in the dossier, justify which specific assembly or machining problem it prevents and how the item callouts on the assembly drawing are linked to the bill of materials (BOM).

In an industrial plant, a bronze bushing serving as a plain bearing for a rotating shaft breaks down. Given the urgency, a technician proposes measuring the worn bushing with a vernier caliper and ordering an identical replica from the lathe with those exact measurements.

1. a) Technically justify why this direct copying procedure will, with high probability, cause seizure or unacceptable play when installing the new component (analyse non-uniform wear, the discrepancy between the actual measurement and the design nominal dimension, and the impossibility of deducing the shaft-hole fit system by measuring a single used part).
2. b) Design the correct reverse engineering protocol that should be followed: determine which auxiliary measurements on undamaged elements of the assembly (shaft and housing) must be performed and which ISO tolerance and fit tables (IT) must be applied to produce the final shop drawing.

Unit challenge. Quality control and ecodesign of household packaging.

Imagine you work in the technical office of a packaging company. You must audit an everyday product from your home (soda cans, milk cartons, or their caps) and document a technical improvement proposal.

1. Metrology and quality control: Measure $5$ units of the same packaging or product using a ruler or calliper. Record their height and diameter in LibreOffice Calc, calculate the mean and standard deviation, and determine whether they meet an allowable tolerance of $\pm 0,5\text{ mm}$.

2. Graphic expression and CAD: Model the part at full scale using Tinkercad or FreeCAD. Generate a dimensioned drawing with its main standard views (front, top, and side view) applying basic dimensioning standards.

3. Life cycle assessment (LCA): Identify the primary material and its environmental impact. Propose and justify a design modification that reduces the material mass by at least $5\,\%$ or facilitates its separation and final recyclability.

Submit a brief technical report (two pages) with the statistical table, screenshots of the dimensioned CAD model, and your ecodesign proposal.

Assessment criteria:

• Rigour in empirical measurement and correct application of statistical quality control.

• Precision in 3D CAD modelling and compliance with dimensioning standards.

• Technical coherence and environmental justification of the proposal regarding the product's life cycle.