Domande Optimization and innovation of production processes (in inglese)

Introduction to DFMDFM Cycle and steps to follow

The goal of DFM (Design for Manufacturing) is to reduce production costs through the optimization of manufacturing processes and the reengineering of components.

In DFM, there is an iterative process (which follows this order: design, production, marketing) that begins only after a design proposal has been made.

The steps to follow are:

• Estimation of production costs
• Reduction of component costs
• Reduction of assembly costs
• Reduction of production support costs
• Evaluation of DFM effects and actions on other factors

The process of reducing total costs is iterative: it starts from the most expensive components and leads to the development of new technical solutions.

What principles is the black-box method based on, and how does it work?

In the DFM optimization process, it is often advantageous to purchase non-standard components from specialized suppliers, leveraging their high production volumes to reduce costs.

The black-box approach consists of imposing only the essential technical constraints on the supplier, without interfering with their production process. This leaves them free to choose the most efficient technologies, thereby achieving the lowest possible cost for the component.

What does the DFA index represent? How is it calculated?

The DFA (Design for Assembly) index, introduced by Boothroyd, is used to evaluate the efficiency of a product’s assembly process.

It is calculated as the ratio between the theoretical minimum number of parts multiplied by 3 seconds and the actual assembly time.

In practice, it measures the gap between the theoretical minimum assembly time and the real time required, making it possible to identify whether and where improvements can be made in the assembly process.

If the DFA index is high, the process is already very efficient and further intervention is not worthwhile. If the DFA index is low, it indicates that the assembly can be improved and that there are critical parts in terms of handling and insertion.

Explain the rules for simplifying assembly

• The component to be assembled should be inserted from above, ensuring good visibility.
• The component should be self-aligning.
• The component should not require specific orientations with respect to one or more axes.
• The component should be mountable with one hand, meaning it should not require lifting equipment.
• The component should not require tools for assembly.
• Linear motion is preferable to rotational motion.
• The component should lock automatically after insertion.

DFM and concurrent engineering

What is dual optimization? What are the steps?

Dual optimization is an optimization technique related to parameter design, in which the goal is to reduce output variability by adjusting the parameters involved through two methods performed consecutively:

• Scaling: reduces both the standard deviation and the mean value.
• Levelling: adjusts the output value so that it meets the specified values and ranges. It modifies only the mean value.

What are the phases of robust design and how is it applied?

Robust design aims to identify the best combination of design parameters that ensures a system response with minimal variation around the desired value, despite the presence of noise factors.

The approach consists of making both the product and the process insensitive to variations by selecting and optimizing the control factors that first influence the variance and then the mean of the response.

After the initial optimization, a refinement phase follows, focusing efforts on the most critical parameters for quality, while allowing greater flexibility for less influential ones, resulting in cost reduction.

Robust design also contributes to improving the reliability and safety of the designed system.

The three phases are:

• Concept design: understanding the objectives and mission of the product.
• Parameter design: determining the optimal values of the control factors to achieve minimal sensitivity of the system to noise factors.
• Tolerance design: determining the tolerance values for the control factors as a cost/quality trade-off.

Define the robustness of a product/process and explain how to find an optimal configuration using parameter design. Provide an example.

The robustness of a product or process refers to its ability to maintain consistent performance with minimal variation around the desired output, despite the presence of noise factors (disturbances or uncertainties).

Design parameters (DPs) are the physical variables of the design that allow the Functional Requirements (FRs) to be satisfied and, in mechanical systems, they correspond to components or their characteristics.

According to Axiomatic Design (AD), the design process must be structured: it starts from the analysis of customer needs, which are translated into FRs and constraints. Then, the DPs that satisfy them are selected, and the design matrix is built, favoring solutions with low coupling.

The design is then decomposed through a zigzagging process, also including the process domain (PVs) if production is considered. This is followed by detailed development, estimation of costs and resources, comparison of alternatives using information content, optimization of robustness, and verification of consistency with customer needs.

To achieve an optimal configuration using parameter design, control factors are selected and adjusted to minimize output variability (variance) first, and then to align the mean value with the desired target.

Example:

A classic example of a coupled design is the traditional two-handle faucet: the FRs (flow rate and temperature control) are not independent, since adjusting one handle affects both.

A decoupled solution is the single-handle mixer tap, which allows independent control of flow rate (vertical movement) and temperature (horizontal movement), thus improving robustness and usability.

Concurrent engineering rules

Concurrent Engineering (CE) is a business management approach based on the idea that designers must thoroughly understand customer needs, and it involves the parallelization of activities in product development.

The 10 rules are:

• Understand the customer
• Use multidisciplinary product development teams
• Integrate the product development process (DFM/DFA)
• Involve external stakeholders (co-design)
• Use digital models (to facilitate information sharing)
• Integrate CAE/CAD/CAM systems
• Use simulation (to reduce costs compared to experimental testing)
• Ensure quality and reliability (apply robust design)
• Adopt an efficient development approach
• Continuous improvement

DFA – Design for assembly

What are the main assembly strategies in companies? Also explain the areas where each strategy is most suitable (optionally with a graph).

The main assembly strategies are:

• Manual assembly: typically used for products with low to high complexity in batch production.
  • Fixed-position manual assembly: the product remains stationary while components are brought to it.
  • Line manual assembly: multiple operators are arranged in assembly stations along a transport system that moves the product-in-progress from one station to another.
  • Assembly shop: consists of multiple assembly areas, each capable of assembling a certain number of components.
• Automated assembly: can be either rigid or flexible.
  • Rigid automated assembly: each assembly machine performs only one part of the product’s assembly cycle.
  • Flexible automated assembly: uses robots equipped with appropriate gripping systems to handle different types of objects.

Explain how selective assembly works and the organization required to implement it

The selective assembly process consists of measuring components and creating optimal pairings. This involves higher costs for inspection and classification, but eliminates costs related to reducing product variability.

This strategy is used when it is not economically convenient to reduce component variability, especially when components already have very tight tolerances.

In selective assembly, components are grouped into classes to ensure proper assembly compatibility. The components are classified based on their dimensions, and therefore all components must be measured.

When is a selective assembly strategy economically convenient?

A selective assembly strategy is convenient when the cost of measuring and managing the parts is lower than the additional cost required to increase production precision and, consequently, interchangeability.

What are the main differences between tolerance analysis, tolerance synthesis, and synthesis including quality costs? Also briefly explain the tolerance synthesis approach without quality costs.

• Tolerance analysis: the resulting tolerance of the assembly is calculated starting from the tolerances assigned to individual components.

  All component tolerances are defined and summed, then it is verified whether the functional tolerance is acceptable. This process is used in the verification phase to check that the chosen component tolerances ensure the required performance.

• Tolerance synthesis: starting from the required overall (functional) tolerance, the tolerances to be assigned to individual components are calculated (allocated).

  The functional tolerance is known, and the other tolerances are assigned accordingly to satisfy it. The objective is economic: beginning from the functional tolerance, component tolerances are distributed to minimize production costs while still meeting the required performance.

• Synthesis including production and quality costs: this approach also considers the quality costs associated with different functional tolerances.

  The functional tolerance is not chosen only based on design requirements, but as a compromise between production costs and quality costs. These costs have an inverse relationship: as one increases, the other decreases.

Tolerance synthesis without quality costs (brief explanation):

In this approach, the functional tolerance is fixed based on design requirements, and component tolerances are allocated solely to minimize production costs, without considering quality-related costs.

Briefly explain the tolerance synthesis approach without quality costs

Tolerance synthesis aims to find the optimal combination of component tolerances that ensures the required functional tolerance while minimizing manufacturing costs.

To achieve this, it is necessary to know the cost–tolerance relationship, often described by the Chase model:

Where A represents fixed costs, B the production cost related to tolerance, and k the sensitivity of the process to tighter tolerances.

Once the cost curves of the available processes are known, the combination of tolerances that meets the functional requirements at the minimum cost is selected.

Explain the purpose and functioning of the Boothroyd method

The Boothroyd method is used to estimate the potential assembly times of components.

In this method, a code ranging from 00 to 42 is assigned, which identifies the time required to assemble a component. It is based on standard tables that take into account the type of assembly (screwing, welding, gluing, etc.) and the geometry of the object to be assembled.

In particular, symmetry is defined through two symmetry angles:

• α (alpha): the angle of rotation the component must undergo, around an axis parallel to the insertion direction, to present the same geometry.
• β (beta): the angle of rotation the component must undergo, around an axis perpendicular to the insertion direction, to present the same geometry.

Assembly time calculation

The calculation of assembly times is complex because it depends on factors related to both the operator and the characteristics of the components. For this reason, standardized methods based on average time tables are used instead of arbitrary estimates.

The total assembly time is divided into handling time and insertion time.

Theoretical times can be estimated using dedicated tables, while actual times are defined in agreement with labor unions.

Among these methods are:

• Boothroyd Method
• Hitachi Method: this method assigns a score from 0 to 100 to each operation; the more complex the operation, the higher the score. The sum of the scores represents the overall complexity of assembling the product. A total of 20 operation classes have been defined.
• Westinghouse Method: a multi-factor approach that considers operation type, assembly method, and component size. Westinghouse used these factors to develop a unified tool for predicting assembly times.

For which operations are robots used? Sizes, batch volumes, and why?

Robots are used when assembly costs are extremely high and there is a clear benefit in automating the process.

They are typically employed in highly rigid automated lines and in production environments that require simple operations performed in a predefined sequence, thanks to the high specificity and automation of the robots.

Robots are not recommended when continuous adaptation to the part being assembled is required.

They are generally used for assembling very small components and replace manual labor, as they ensure consistent precision and higher speed in repetitive tasks.

Another important application area is when the operation is hazardous to human health, such as welding stations in the automotive industry and painting operations.

Difference between lines with stations and without stations

Lines with stations consist of automated stations designed to perform specific assembly tasks. They originated in the 1950s, and each station is suited for simple operations. These lines are used for large production batches and are highly rigid.

Lines without stations, on the other hand, rely on operators who perform assembly tasks. Although assembly time is generally longer compared to robotic systems, humans offer greater flexibility. In fact, labor can be reassigned very flexibly, even from one day to the next. These lines are typically used for small production batches.

Selective assembly and related cases. When is it convenient to use it?

Selective assembly consists of measuring components and creating optimal pairings. It is used when it is not economically convenient to reduce component variability (i.e., tolerances are already very tight or it is difficult to further reduce variability).

Cases:

• Equal process capability (range within which most of the production of A and B lies):

  All components are measured and divided into classes (a5, a4, …, b2, b1). Then, components from class a5 are assembled with those from class b5, and so on, ensuring that the required tolerance for assembly is met.

• Different process capability, with one equal to the required tolerance:

  It is necessary to define the classes and their size based on the worse process, and then produce multiple classes of components, according to that proportion, using the better process.

• Different process capability, both worse than the required one:

  In this case, it is necessary to produce multiple batches for both components so that the resulting distributions have the same variability. More batches of component B are produ