Exercises: Industrial manufacturing techniques

Classify these components according to the most likely manufacturing process (casting/moulding, machining or forming) and justify your choice: a remote control housing, a precision splined shaft, a copper pipe, an engine block and a car bodywork sheet.

Explain what chip-removal machining is and describe three basic operations (turning, milling and drilling): in each one, indicate what moves (the workpiece or the tool) and what shape is obtained.

A steel sheet $3{,}0\ \text{mm}$ thick enters a rolling mill, as shown in the figure, whose rollers reduce the thickness by $20\,\%$ on each pass. Calculate the thickness at the exit of the first pass and explain why rolling is a forming process by plastic deformation.

Describe the sand casting process step by step (pattern, sand mould, pouring, demoulding, cleaning) and give two advantages and two limitations of the process compared with machining.

Explain what thermoplastic injection moulding consists of (plasticising the material, injecting it into the mould, cooling and ejecting) and name four everyday objects manufactured this way. Why is the mould so expensive and yet the process is profitable?

Explain what forging is and what difference there is between open-die forging (with a hammer, with the smith guiding the workpiece) and closed-die forging (hot drop forging). Why are forged components usually stronger than cast components of the same shape?

Describe the extrusion process using the analogy of a toothpaste tube and name three extruded products (aluminium profiles, pipes, sheathed cables). Why does extrusion only produce components of constant cross-section?

For each geometry, indicate the appropriate machine tool (lathe or milling machine) and explain your choice: a cylindrical shaft with a keyway, a prismatic housing with cavities, an external thread and a flat guide with slots.

A company needs to manufacture plastic housings for a portable speaker. Analyse which process you would choose if the planned production is $100\,000$ units per year and which if only five prototypes are needed to present to investors, explaining the fixed costs (moulds, tooling) and per-piece costs of each option.

Compare chip-removal machining with forming processes (forging, rolling, stamping) from three perspectives: material utilisation, energy consumed and state of the internal structure of the resulting component. Why are engine connecting rods usually forged rather than machined from a solid block?

Order the manufacturing stages of a cast aluminium housing for a water pump: (a) machining of mounting surfaces and threads, (b) sand moulding and pouring, (c) final inspection and dimensional control, (d) sand removal and deburring, (e) pattern and mould design with gating systems. Explain what is done at each stage.

Welding is the main permanent joining process in metal structures. Compare shielded metal arc welding (SMAW/stick welding), MIG/MAG and TIG: what provides the filler material in each, what gas is used and a typical application of each process.

A car bodywork is manufactured by sheet metal press stamping. Describe the sequence (blanking/cutting, deep drawing, bending, punching), what defects can appear (wrinkling, cracking, springback) and why this process is only cost-effective for mass production.

Match each process with the dimensional accuracy and surface finish it can achieve without subsequent operations: sand casting, machining, rolling and 3D printing. Explain why the mating surfaces of a cast component are almost always machined afterwards.

An injection mould costs $30\,000\ \text{€}$ and each injected component costs $0{,}50\ \text{€}$; manufacturing the same components by 3D printing costs $6\ \text{€}$ per unit without a mould. a) Calculate from how many components the mould pays for itself. b) Comment on what other factors, besides unit cost, influence the decision (lead time, design changes, surface finish).

Compare a conventional milling machine with a CNC machining centre: what numerical control offers (repeatability, complex geometries, automatic tool changing) and in which cases a manual machine is still reasonable (one-off component, quick repair, training).

It is necessary to manufacture $2\,000$ aluminium pulleys of diameter $120\ \text{mm}$ with a central hole of $20\ \text{mm}$ and a keyway. Design the complete manufacturing plan: process for obtaining the blank (casting or bar stock?), machining sequence with specific machine tools, necessary tooling and fixtures, and intermediate and final quality controls.

A company is deciding whether to manufacture a component with a very complex geometry containing internal cooling channels for a turbine prototype (three units) using CNC machining or metal 3D printing. Analyse the advantages and disadvantages of each technology for this specific case (achievable geometry, surface finish, material properties, cost and lead time) and make a justified decision.

In the assembly of a batch of shafts into bearings whose bore dimension is $\varnothing\,25^{+0{,}021}_{0}\ \text{mm}$, it is specified that the fit must be a clearance fit. The target machining dimension set on the drawing for the shaft was $\varnothing\,25_{-0{,}040}^{-0{,}020}\ \text{mm}$. However, during incoming inspection, it is detected that several shafts assemble with interference and jam rotation. When measuring five problematic shafts at standard temperature ($20\ ^\circ\text{C}$), the following are obtained: $25{,}024\ \text{mm}$, $25{,}020\ \text{mm}$, $25{,}018\ \text{mm}$, $25{,}015\ \text{mm}$, and $25{,}012\ \text{mm}$.

1. a) Calculate the theoretical maximum and minimum clearance that should have been obtained, and mathematically demonstrate what type of actual fit occurs when assembling these defective parts into the bearings.
2. b) The operator states: "I measured the parts right after taking them off the lathe and they were at $24{,}970\ \text{mm}$; the defect must be in the bearings." Technically refute this claim by calculating the temperature increase $\Delta T$ that would be sufficient for the steel part (thermal expansion coefficient $\alpha = 12 \cdot 10^{-6}\ \text{K}^{-1}$) to undergo during dry machining to distort the measurement at the machine.
3. c) Justify which other two systematic errors in the external turning process (related to tool wear or machine setup) cause the actual dimension to be larger than the programmed nominal dimension.

An industrial plant produces $1\ \text{L}$ high-density polyethylene (HDPE) bottles ($500\,000\ \text{units/year}$) by extrusion blow molding. In quality control, two recurrent defects are detected: crush failure at the bottom lower corners when stacking the containers, where the thickness drops below $0{,}35\ \text{mm}$, and microleaks at the bottom pinch-off weld line of the parison.

1. a) Physically justify why in conventional extrusion blow molding the wall thickness decreases critically at the corners and areas furthest from the bottle axis, and explain how this problem is corrected through dynamic control of the parison thickness along its length.
2. b) Analyze the possible causes of leaks in the bottom weld line and propose which process parameters (parison extrusion temperature, mold clamping pressure/speed, or cooling time) you should adjust to achieve a leak-tight and strong heat seal.
3. c) Design an at-the-line quality control plan that combines a destructive test and a non-destructive test to verify the leak tightness and mechanical strength of the produced containers.

For a competition single-cylinder engine, a series of $5\,000$ connecting rods made of chromium-molybdenum alloy steel is planned to be manufactured. A first test batch, produced by direct machining from a solid rolled flat bar, failed prematurely due to fatigue fracture at the transition between the shank and the big end.

1. a) Justify why hot closed-die forging provides a radically superior fatigue life compared to direct machining from a solid block or casting, analyzing the continuity and orientation of the material's flow lines (grain flow).
2. b) Design the complete manufacturing sequence, justifying each stage: state of the material after forging, heat treatment required to achieve an optimal balance between yield strength and toughness, machining operations needed for the critical areas (big-end and small-end bores), and final mechanical treatment to delay the propagation of surface microcracks.
3. c) Determine which destructive and non-destructive tests (at least two of each type) you must implement in quality control to certify both the mechanical properties and the absence of internal defects in the forged connecting rods.

In a turning cell, shafts are machined with a critical dimension $\varnothing\,30 \pm 0{,}05\ \text{mm}$. For statistical process control (SPC), subgroups of $n = 5$ parts are taken every hour. During the first four hours of a shift, the following average diameter values ($\bar{X}$) and ranges ($R$) are recorded:

• Hour 1: $\bar{X}_1 = 29{,}998\ \text{mm}$; $R_1 = 0{,}018\ \text{mm}$

• Hour 2: $\bar{X}_2 = 30{,}012\ \text{mm}$; $R_2 = 0{,}020\ \text{mm}$

• Hour 3: $\bar{X}_3 = 30{,}025\ \text{mm}$; $R_3 = 0{,}019\ \text{mm}$

• Hour 4: $\bar{X}_4 = 30{,}039\ \text{mm}$; $R_4 = 0{,}021\ \text{mm}$

(Constant for control limits with $n = 5$: $A_2 = 0{,}577$).

1. a) From the average range $\bar{R} = 0{,}020\ \text{mm}$ and taking the nominal value ($30{,}000\ \text{mm}$) as the center line, calculate the Upper and Lower Control Limits ($\text{LSC}$ and $\text{LIC}$) for the $\bar{X}$ mean chart.
2. b) Interpret the time evolution of $\bar{X}$. Although at hour 4 no individual shaft has yet exceeded the drawing tolerance ($30{,}05\ \text{mm}$), predict what will happen at hours 5 and 6 if no action is taken, and justify what physical cause of machining originates this pattern.
3. c) Decide and justify what immediate corrective action the operator must take before continuing production.

A workshop must machine $40$ identical flanges each with four drilled holes. Compare machining them one by one on a pillar drill using a template versus preparing a CNC program with a multi-part clamping fixture: analyse setup and cycle times, cost of errors and break-even point for time investment, and decide based on reasonable assumptions.

Evaluate the sustainability of two production routes for the same aluminium component: machining from a solid block (high swarf volume) versus pre-forging with minimal subsequent machining. Consider manufacturing energy, wasted material and its recycling (aluminium swarf is recycled, but with losses), tool life and final properties of the component. Conclude with a reasoned recommendation.

Unit challenge. Imagine you work in a technical office and must audit the manufacturing process of a metal component subjected to wear in your home (for example, an open-ended spanner, a bicycle sprocket, or pruning shears).

1. Data collection: Measure its main dimensions with a ruler or caliper (with a thickness or dimension of at least $0,5\text{ mm}$) and deduce the predominant mechanical stresses it undergoes in service.

2. Forming selection: Consult material datasheets in open databases (such as MatWeb) and select the optimal commercial alloy. Justify the appropriate manufacturing technique (forging, casting, or sintering) over machining by chip removal, taking into account energy cost and grain flow orientation.

3. Improvement treatment: Design the necessary thermal or surface cycle (e.g., quenching at $850\text{ }^\circ\text{C}$ followed by tempering, or case hardening), explaining what microstructural change you are aiming for to combine surface hardness and toughness.

4. Process sheet: Using LibreOffice or FreeCAD, create a one-page technical sheet that includes the dimensioned sketch and the orderly manufacturing sequence from raw material to final finish.

Evaluation criteria:

• Rigor in the metallurgical justification of the proposed thermal or surface treatment.

• Technical adequacy of the selected industrial manufacturing sequence.

• Precision in taking real measurements and formal quality of the technical sheet.