Productivity improvement of control cable manufacturing machine for the automotive industry

Abstract

Due to the high competitiveness that characterizes the automotive industry (AI), there is an intensive search for improvements that translates into significant impacts on the productivity, still guaranteeing the levels of quality required by the client. The requirement for flexibility is leading to a paradigm shift that arises from a constant change in the needs of consumers, who demand an ever-higher level of customization in the products. In recent years, the increasing use of automation in industry resulted in significantly improved product quality, better working environment, higher cost efficiency, and major improvement in process flexibility. The present work addresses the productivity improvement of a control cable manufacturing machine for the AI. The main objective is related to the development of a new stripping machine concept that allows the simultaneous coating removal from two cables, thus eliminating an existing bottleneck in the injection process of the cables’ first Zamak terminal. A prototype machine was built, and the new concept validated, and then implemented in the factory. An automatic cable extraction system was also designed, aiming to reduce the number of manual tasks performed by the operator, with the expectation of further improving the process productivity. The development of the two new mechanisms followed the Design Science Research (DSR) methodology. With the implementation of the new double stripping concept and automatic cable extraction system, a major increase in productivity (up to 87.8% compared to the current condition) and cost reduction were achieved.

Keywords:

• Automotive industry
• mechanical design
• finite element method
• control cables
• productivity

1. Introduction

Due to the high competitiveness that characterizes the AI and the enormous variety of processes to produce the most diverse components, there is an intensive search for improvements that significantly impact their productivity, still guaranteeing the levels of quality required by the client (Stief et al. 2022). The design of industrial machines is vital for the AI development (Qiu et al. 2023). Actually, with the implementation of new machines and technologies, it is possible to reach higher levels of productivity, quality, and efficiency. By increasing process automation, there is an inherent reduction in the needed manpower. However, this workforce can be redirected to tasks with greater added value and, in this way, it is possible to improve the response of companies to an increasing global market (Araújo et al. 2017). The AI and its component industry is one of the areas that most drives the world economy (Santos et al. 2019). According to the Organization Internationale des Constructeurs d‘Automobiles (OICA), worldwide, an average of approximately 60 million motor vehicles are produced every year. Moreover, there are over 50 million workers in manufacturing industries who, indirectly, perform functions for the AI (Fonseca and Domingues 2017). The AI is extremely competitive and is responsible for the emergence and development of numerous technologies (Fragoso-Medina and Velázquez-Villegas 2023), with innovation being one of its strengths over other industries (Silva et al. 2021). Car manufacturers tend to look for suppliers to whom they can delegate, or at least share, responsibilities in the design, development, and manufacture of components (Costa, Silva, and Campilho 2017). This trend puts pressure on suppliers to maintain competitive prices and manage to guarantee reduced delivery times, without compromising product quality (Pinto and Silva 2017). In short, this industry requires a very high production rate, combined with a high degree of precision, without compromising the existence of competitive prices, to ensure high levels of satisfaction on the part of consumers (Costa et al. 2018; Moreira et al. 2017; Rodrigues et al. 2020). The requirement for flexibility is leading to a paradigm shift that arises from a constant change in the needs of consumers, who demand an ever-higher level of customization in the products they intend to purchase (Silva et al. 2021; Costa et al. 2018). With automation, it became possible to provide cheaper and more reliable machines that can work 24 h a day (electric motors, computers, among others) (Huang et al. 2023). On the other hand, the pressure to reduce the price per unit led to production rates that could only be achieved with automation (Ribeiro and Barata 2011). The main reasons that lead companies to develop automation projects are the following (Costa, Silva, and Campilho 2017; Veiga et al. 2019; Groover 2015): productivity increase, cost reduction (despite the high initial investment), quality improvement (due to increased repeatability and compliance), shorter delivery times (less wasted time and faster processes), possibility of performing tasks impossible to be performed by human beings, waste reduction, and elimination of repetitive manual tasks.

Future trends in the AI have been identified in online technical sources that, by all indications, will mark the course of this industry in the coming years. Examples are (Sachs 2022; Radius 2020): use of increasingly lighter materials to improve efficiency and reduce fuel consumption, implementation of autonomous driving systems, widespread use of electric vehicles, use of additive manufacturing in the production of complex parts, and mass customization. Trends suggested by the scientific community include the application of new painting techniques, aiming to find more efficient solutions for one of the most energy-intensive processes in vehicle fabrication. For example, nano-ceramic pretreatment processes, unlike conventional processes, can be carried out at room temperature (Schulz 2013). This technique would result in less energy consumption and emissions, facilitate the use of water-based over solvent-based paints (since the former are more ecologically efficient), and enable the application of robots with electrostatically assisted guns to reduce paint waste and improve process repeatability. To accomplish all described goals in the AI, machine design is undertaken. There are possibly dozens of different solutions that manage to meet the defined requirements (Ashby and Johnson 2013). The big difference can be in the advantages and disadvantages of each solution, depending on the project specifications, such as production cost, ease of maintenance, or process stability. Therefore, developing a new machine entails several stages, from identifying a need, problem definition, and idealization of a design solution that is then perfected until its final form (Ashby and Johnson 2013; Budynas and Nisbett 2009). Moreira et al. (2017) developed an integrated system that combines different manufacturing and assembly operations of automotive metallic harness products in a single cell, and presented a production system flexible enough for the cell to produce more than one product reference. With this approach, it was possible to register productivity, quality, and flexibility improvements. Magalhães, Silva, and Campilho (2019) designed an automatic system to collect and reorient newly bent wires, used to fabricate cushions and suspension mats for automotive seats, providing the desired orientation for the next polymer over-injection process. The cycle time was shortened since the robotic wire bending machine does not need to stop the cycle so that another manipulator can pick up the wire before cutting and position it correctly. Santos et al. (2018) improved the bead production process for tires, through the automation of an APEX machine. The improvements significantly reduced machine downtime (by approximately 62%), and contributed to a higher overall equipment efficiency (OEE).

The development of automated machines to fabricate control cables, also known as bowden cables, also falls in the scope of AI design. Control cables are mechanical elements that transmit movement between two or more systems. These cables are often found in vehicles from the most varied manufacturers, and operate different mechanisms (Martins et al. 2020). There are, additionally, recent studies that address the possibility of using this type of cables for medical applications, in the transmission of movement in exoskeleton limbs (used for the rehabilitation of human beings), due to the reduced associated mass (Herbin and Pajor 2021; Qingcong and Xingsong 2013). Given the constant evolutions in the AI, the complexity of bowden cables has been increasing to satisfy customer requirements. Consequently, the respective manufacturing processes are increasingly diverse and technologically advanced (Martins et al. 2020). It is normal for these products to require metal braided/twisted cables (with or without plastic coating), a plastic tube where the metal cable runs inside, which in turn is coated on the outside by a spiral metal tube, an external damping tube, two terminals injected in Zamak alloy at the cable ends, some plastic parts injected at the ends of the spiral, and one or more parts in plastic material for fixation to the chassis (Martins et al. 2020). Recent works are available in the literature on the improvement in the control cable production process, either by applying automation systems, or by improving the production process. Santos et al. (2019) developed a new automatic system for spiral transport, from the beginning to the end of the production process. With this system, it was possible to significantly improve flexibility, achieving setup time reductions of approximately 97%. Moreover, a chip removal system was also implemented, which reduced the risk of system jamming due to excessive chip accumulation in critical areas. In the work of Figueiredo et al. (2020), an automatic system was developed capable of integrating pickling operations, creating the open geometry at the cable ends (called mushroom), and over injecting the terminal in a single system, without wasting time between the various stages, even managing to strip both ends of the cable in the same operation. With this system, only a part-time operator was needed to feed the cable spools into the machine and adjust essential parameters. Martins et al. (2020) developed an automatic cell aiming to integrate the different manufacturing processes of vehicle cables in a single system. The designed solution consisted of a flexible machine capable of producing three types of cables, which managed to aggregate tasks that were originally carried out on two machines. The level of human participation in the process has also been reduced. It was possible to reduce the manufacturing cycle time by 25% and increase productivity by more than 30%. Pinto et al. (2019) aimed to eliminate the appearance of pores in the injection process of control cable terminals in Zamak. Using simulation tools, the authors concluded that the improvements should focus on the mold design, allowing for correct cooling of the injected metal. Adjustments were made to the injection process parameters, to avoid the appearance of defects in the produced components. In view of the current knowledge on control cable fabrication, many solutions exist for process improvement and automation, although different and innovative solutions are still required to increase the production rate.

The main objective and novelty of this work consists of the development of a new stripping machine concept, based on straightforward mechanisms and pneumatics, which allows the simultaneous coating removal from two cables, thus significantly increasing the production rate and eliminating an existing bottleneck in the injection process of the cables’ first Zamak terminal. A prototype machine was built, and the new concept validated, and then implemented in the factory. An automatic cable extraction system was also designed aiming to reduce the number of manual tasks performed by the operator, with the expectation of further improving the process productivity.

2. Methodology

The Design Science Research (DSR) methodology was adopted to develop the current project. This scientific design methodology provides specific guidelines for evaluation and iteration within research projects, and it is usually used to develop and improve new concepts, starting from existing ones (Abdullah, Abbood, and Hussein 2020; Sousa et al. 2022; Tamada et al. 2020). This methodology involves the detailed study of existing processes and the proposal of new solutions, which leads to a process improvement that can be used in different applications (Barbosa et al. 2022). Since this work aims to develop a new machine based on an existing cable production system, this methodology is considered suitable to guide the new design proposal. The development of the new concept is divided into the six DSR stages, individually described in Table 1.

Table 1. Description of the different stages of the applied DSR methodology.

#	Stage	Description
1	Problem identification	Analysis of the current concept, identifying problems and aspects to be improved
2	Objectives	Definition of objectives and requirements for the new solution
3	Design and development	New concept development
4	Demonstration	Implementation of the developed concept
5	Evaluation	Performance evaluation of the developed solution, verifying whether it complies with the established objectives and requirements
6	Conclusions	Final evaluation of the solution, comparing the current machine with the new concept

As it becomes clear in the depicted stages of Table 1, the project starts with the evaluation of the previous production concept, with a view to identifying possible problems or necessary improvements. The first two stages of the methodology (problem identification and objectives) are presented in section 3. The design and development stage (third stage) can be found in section 4, in which a preliminary design of the machine is carried out. The fourth stage, demonstration, is present from section 5.2.1 to section 5.2.3, and corresponds to the different phases of machine design. The fifth stage, evaluation, appears in sections 5.2.4 and 5.2.5, showing the prototype validation, and the evaluation of productivity gains for the new solutions. The last stage of the methodology, conclusions, corresponds to the presentation of the work’s conclusions, appearing in section 6.

3. Problem identification and objectives

3.1. Process description and problem characterization

The production process of a control cable begins with the arrival of a metallic cable in the form of a coil, to be unrolled and coated with Nylon using extrusion machines. The already coated cable is wound again in a spool, after the extruded material has cooled. After this procedure, the cable spools are directed to the cable cutting machines, which unwind the spool and cut the cable to the desired size, according to the references to be executed. The cut cables are stored in boxes that later go to the 1^st Zamak injection machines or ZIMs (Fig. 1), to inject the first cable terminal. The focus of this work specifically addresses this stage of the production process. To inject the terminal with the desired quality, it is previously necessary to strip the coating and to create the mushroom cable tip. The latter operation involves hammering the cable tip up to reaching a mushroom shape, to ensure a strong terminal/cable connection (Pereira et al. 2022). The machine that performs these tasks is installed at the injection station, included in the ZIM structure.

Figure 1. 1^st ZIM: A - cable stripping machine, B - mushroom machine, C, D - injection mold and sprue breaker, E - storage box for spiral assembly.

Figure 2 identifies in more detail the four parts of the machine responsible for carrying out the operations inherent to the preparation and injection of the first cable terminal. Table 2 summarizes the fabrication steps in the ZIM. After finishing the injection of the first terminal, the cable subsets are stored in boxes that will later go to the assembly lines corresponding to their respective reference. In parallel to the described steps, the spiral production takes place by winding laminated wire around an inner tube, and depositing it in a drum. Subsequently, the drum with the spiral is directed to the extruder machines, where the spiral is coated with polypropylene. After the coating process, the spiral is deposited in a drum. Finally, the spiral is cut to the desired size, according to the reference to be produced, and it is sent to the respective assembly line.

Figure 2. Machine identification for preparation and injection of the first cable terminal. A – Cable stripping machine; B – Mushroom machine; C – Injection mold; D – Sprue breaker.

Table 2. Fabrication steps in the cable fabrication.

Machine/step	Visual representation
Initial state
A - Coating stripping
B – Mushroom execution
C – Injection
D – Sprue breaker

On the assembly lines, the cable subset with the 1^st terminal is introduced into the spiral, the remaining accessories specified by the customer are assembled, and then the 2^nd terminal is injected, by repeating the same sequence executed in the first injection, thus obtaining the final set of a control cable (Fig. 3). The produced control cables are placed in cardboard boxes, positioned next to the assembly lines, and later transported to the warehouse, where they will be directed to the customer.

Figure 3. Fabricated control cable: A - cable terminal, B - spiral, C- metallic cable, D - plastic part.

The focus of this work is the 1^st ZIM. As mentioned, two cable preparation operations are carried out at these stations before terminal injection. Thus, all operations should have the desired production rate, to avoid loss of time and a creating a bottleneck. Considering that, currently, double molds (to simultaneously inject two terminals) and double mushroom execution machines had already been implemented in some ZIMs, the bottleneck was found in the single cable stripping operation. This feature significantly limits the ZIM production rate. It is known that the ZIMs with double molds have their productivity limited to 900 injections per hour. Thus, at most, using double molds, the injection stations would be able to reach a production of 1800 cables per hour. However, the current production values measured at the production line never exceeded the mark of 900 units per hour. Therefore, it is necessary to intervene in the coating stripping process, aiming to eliminate the bottleneck of the production process. Another verified problem, as reported by the ZIM operators, is the need to manually extract the produced cables after each injection cycle of the 1^st terminal. This requirement causes unnecessary fatigue in the operator who performs the tasks at that station. Moreover, the productivity diminishes due to the operator’s accumulated fatigue, and the consequent time lost to manually extract the cables, leading to an excessive cable feed delay in the preparation machines (stripping and mushroom machines).

3.2. Current machine

To enable the design of a new cable stripping machine, the current machine was initially evaluated. This analysis proved to be vital, as it immediately enables understanding the operating principle of the machine, thus leading to an understanding of the existing movements, the fundamental components, and their organization into various functional subassemblies. The current stripping machine is shown in Fig. 4.

Figure 4. Current stripping machine (a), and respective front (b), and side views (c).

By deconstructing the existing CAD models to evaluate the different components, three subassemblies were identified (illustrated in exploded view in Fig. 5a), which play a fundamental role for the correct machine operation and stripping process. These subassemblies, separately described in the next subsection, are as follows: 1. Stoppers subassembly; 2. Blades subassembly, and 3. Grip subassembly.

Figure 5. Exploded view of the three subassemblies (a), and blades subassembly nomenclature (b).

The operating principle/steps of the machine is as follows: (1) manual introduction of the cable, until it reaches the stop, (2) the stop triggers an inductive sensor, which gives a signal to start the stripping cycle, (3) the grips close and tighten the cable, ensuring its correct positioning throughout the process, (4) the blades close, in order to cut the cable coating, (5) the blades recoil and remove the coating chip, the (6) the grips open, allowing the stripped cable to be removed, and (7) all actuators return to their initial position, to initiate a new working cycle.

3.2.1. Stoppers subassembly

The stoppers subassembly (Fig. 5a-1) triggers the onset of the machine’s operation cycle. It consists of a mechanical spring stop, in which an inductive sensor with M8 thread is mounted. This subassembly also has a linear pneumatic actuator, whose function is to move this subassembly forward and backward, allowing it to move away from the blades. As a result of this solution, the removed coating chip can fall more easily, instead of being trapped between the blades and the stop. The assembly movement is guided by two ground body screws. All components are mounted on a support. This subassembly must adjust the distance between the stop and the blades, since this dimension defines the final length of stripped cable. Frequent problems have been reported, related to the premature damage of the inductive sensors. A detailed inspection to the machine operation showed that this problem was associated with the chosen stop, which allowed mechanical contact between the stopper shaft and the sensor itself, thus causing its failure. This problem will be considered in the design of the new double stripping system.

3.2.2. Blades subassembly

The blades subassembly (Fig. 5a-2 and 5b) has the most movements. This subassembly is primarily responsible for cutting and removing the coating in the intended area and, therefore, it requires two pneumatic actuators to ensure the movement of the components in two different directions. When the operating cycle starts (with the grips already fixing the cable in the desired position), the blades close, moved by the pneumatic actuator with parallel grips A1, cutting the coating. Then, actuator A2 moves back, causing the blades (still closed) to drag the coating chip, until it separates from the cable and ends up falling in a receiver box.

3.2.3. Grip subassembly

The grip subassembly (Fig. 5a-3) is made up of gripping tips of equal dimensions, whose function is to fix the cable in the desired position, to prevent it from slipping while the coating is removed. The movement of these grips is promoted by a parallel pneumatic gripper, which is fixed to the top cover of the stripping machine. The operating stroke of the tips is limited to less than 1 mm, to prevent the cable from slipping out of the intended cutting/fixing area when manually introducing the cable. Thus, the operating stroke of these components needs to be smaller than the diameter of the cable to be stripped, to ensure that process failures do not occur, and the cable does not present defects (such as a pinched coating).

3.3. Objectives

In this work, following the description of the current control cable fabrication process, different research questions were considered: how can the current bottleneck at the cable stripping station be eliminated? is it technically efficient and cost effective to replace the current single stripping machine by a double stripping machine? does the new system have the same quality in the process? does an automatic cable extraction system increase the productivity? are the solutions able to double the production rate? does the validation process show proper operation in a real scenario? In view of these questions, two main objectives were outlined. The first objective of the work is to increase productivity in the 1^st ZIM of control cables, by designing a new stripping machine. This machine should meet the requirements of its predecessor, but producing two cables in each production cycle. As a result, in machines with double molds, the negative impact of the existing solution on the output of the workstation is reduced. The second objective is the productivity increase of the workstation, by applying an automatic cable extraction system that relieves the operator from this task, and makes the process faster and controlled. Both objectives are achieved through completing the following tasks:

• Define the requirements and limitations to be respected in the design phase;

• Proceed with the preliminary design of the machine, to evaluate alternative solutions;

• Design a new dual stripping machine/automatic cable extraction system;

• Build and validate the designed machine in a factory environment;

• Measure productivity improvements and evaluate the economic return.

3.4. Requirements and limitations

The requirements and limitations of this project conditioned the design choices and operation of the new machine, to meet the desired functionality. The requirements and limitations for the double stripping machine and the automatic cable extraction system are described in Table 3.

Table 3. Requirements and limitations for the double stripping machine and the automatic cable extraction system.

Requirements	Limitations
Double stripping machine
Machine capable of simultaneously removing the coating from two cables; Ability to adjust the length of the stripped zone; Capacity to store coating chips; Possibility to replace coating cutting blades (either due to wear or due to the need to produce new references with a different wire rope diameter); Ensuring operator safety.	Keep the external dimensions, in width and depth, similar to those of the current machine; Ensure the same distance between holes to introduce the cable, compared to the mushroom device and the injection mold (16 mm between axes), for easier cable handling; Resort to pneumatics to animate the components that require movement; Give preference to catalogue elements that already exist in stock, to avoid increased costs and to facilitate/speed up the replacement of these components (if necessary), without having to wait for the delivery of new components.
Automatic cable extraction system
Ability to simultaneously support and transport two cables; Automatic cable removal after the injection process (after breaking the sprue); Ability to deposit the cables in the water-cooling container positioned at the desired location; System integration into the currently implemented machine, without expensive changes; Guarantee a shorter cycle time than the injection time; Ensure operator safety.	Resort to pneumatics to animate the components that require movement; Only use standard and available in stock pneumatic components.

4. Pre-design

In this phase, a brainstorming session was held between the present design team and the maintenance team. This session aimed to propose different ideas of possible solutions of the mechanisms to be designed. After these were identified, it was necessary to find a suitable method to distinguish the solutions and classify them. The best solution was determined by the selection matrix method (Ashby 2016). To apply this method, it is necessary to define criteria that distinguish the various ideas and assign them the respective relative weights, according to the importance that each of the attributes has in the final decision. The following selection criteria were selected, ordered from highest to lowest importance: functionality, ease of maintenance, cost, ease of assembly, and ease of execution. The most important criterion was functionality, since the main objective is to make a machine that, in addition to duplicating the stripping rate, is also effective and easy to use by the operator. The second criterion is ease of maintenance, since the replacement of wear components (such as blades) should be as quick as possible. The third criterion is the cost, as an unavoidable factor in machine design, to maximize gains and minimize the return on investment (ROI), while serving as an incentive to replicate the solution to other ZIMs. Ease of assembly is the fourth criterion, since a difficult to assemble solution causes unnecessary inconvenience and loss of time. The ease of execution is the least important criterion, since it is already partly accounted for the cost, i.e., complex execution increases the cost. Nonetheless, higher complexity of execution can imply longer delivery time from suppliers, which would delay part replacement due to an unexpected failure. In Table 4, relative weights are attributed to each criterion, by establishing quantitative comparisons with the most important criterion (functionality), based on human experience and overall project goals and functionality limitations. Thus, criteria pairing in the form of 1-x corresponds to the relative weights between the functionality criterion and criterion x, with 2≤x ≤ 5. As an example, functionality, when averaged against ease of maintenance (pairing 1–2), is valued at 55%, while ease of maintenance is at 45%. The relative quotient (Q) is calculated for each of the criteria and the weight of each criterion (ω_i) is determined, which is given by the quotient between Q_i and ΣQ_i. For each of the ideas, a quantitative rating (V_i) will be assigned, following a ranking between 1 and 5, as follows: 1-very weak, 2-weak, 3-ok, 4-good, and 5-very good.

Table 4. Calculation of the relative weight of each attribute.

Criteria and pairing	1–2	1–3	1–4	1–5	Qi	ωi
1 - Functionality	55	55	60	65	1	0.2603
2 - Ease of maintenance	45				0.8182	0.2130
3 - Cost		45			0.8182	0.2130
4 - Ease of assembly			40		0.6667	0.1735
5 - Ease of execution				35	0.5385	0.1402
					Σ = 3.8416	Σ = 1

Having calculated the importance of each criterion, the weighted ranking of each criterion (β) is given by: (1) $$\beta =\frac{V_i}{M{V}_i}\times 100,$$(1) in which MV_i is the highest V_i between all ideas. The final ranking of the idea for each criterion (Ω) is given by: (2) $$\Omega =w_i\times {\beta }_i.$$(2)

Thus, the final ranking for each of the ideas (γ) is given by: (3) $$\gamma =\Sigma (w_i\times {\beta }_i)$$(3)

4.1. Double stripping machine

4.1.1. Stoppers subassembly

The two ideas consisted of a single stopper for the two cables and two independent stoppers. The first idea is the simplest option to execute, and with the lowest cost. However, it is unable to guarantee that both cables would be well positioned at the beginning of the cycle, since the stripping process initiates when a single cable reaches the stopper. Moreover, it becomes impossible to control the stripping length. The second solution guarantees that the stripping length will be the same on both stripped cables at each cycle, thus ensuring the desired functionality, but at the expense of a higher cost.

4.1.2. Blades subassembly

The mechanism and geometry of the cutting blades is one of the key issues to solve, as these must simultaneously cut the coating of two cables, while keeping the current stripping quality. This problem was divided into two issues, i.e., the blade mechanism and the geometry. For the blade mechanism, three ideas were proposed (Fig. 6): pair of blades with horizontal opening (a), pair of blades with vertical opening (b), and two independent pairs of blades (c). The first option requires the fewest changes to the current mechanism, since the parallel pneumatic grippers can be the same. However, in this configuration, the cables need to be positioned vertically, oppositely to the horizontal positioning observed for the other ideas. The second idea is effective in achieving the outlined objectives, but it is slightly more expensive than the first, as it requires a guide system with shafts and linear ball bearings to ensure the blades’ alignment. The third idea guarantees that the force applied to each cut is independent of possible variations between the two cables, but it is considerably more expensive than the others, and it would increase the difficulty of execution and maintenance.

Figure 6. Ideas for the blade mechanism: pair of blades with horizontal opening (a), pair of blades with vertical opening (b), and two independent pairs of blades (c).

The blades’ geometry shall extend their useful life, without compromising the machine’s functionality, and without increasing the cost too much, which would cancel the advantage in increasing the number of cutting sides. The proposed ideas are the following (Fig. 7): cutting blade with one cutting side (a), cutting blade with two cutting sides, which enables reusing (b), and cutting blade with four cutting sides (c). From the three ideas, the second one appears to be the best, because it keeps the dimensions of a simple blade, and duplicates the expected blade life, without compromising the cost.

Figure 7. Ideas for the blades’ geometry: cutting blade with one cutting side (a), cutting blade with two cutting sides (b), and cutting blade with four cutting sides (c).

4.1.3. Grip subassembly

The possible solutions for the grippers were (Fig. 8): one actuator and vertical blade opening (a), and two independent actuators (b). The first idea is simple to implement and less expensive. However, the outer coating diameter has an admitted tolerance of 0.15 mm, and using a single actuator with a guided system does not guarantee that both cables are properly tightened by the grippers. The second idea solves this problem since each gripping tip has an independent actuator, i.e., it assures the necessary stroke until the cable is blocked.

Figure 8. Ideas for the grippers: one actuator and vertical blade opening (a), and two independent actuators (b).

4.1.4. Selection of the best idea

To choose the best idea for each subassembly, the classification of each idea is presented. Table 5 shows the selection table for all the discussed ideas. The cell layout for each idea and criterion, considering the variables V_i, MV_i and β, follows the format of the caption depicted at the top left corner of Table 5.

Table 5. Selection of ideas to apply for the double stripping machine.

Vi	β		CRITERIA										PERFORMANCE INDEX γ
MVi			1-Functionality		2-Ease of maintenance		3-Cost		4-Ease of assembly		5-Ease of execution
		IDEA	ω1=0.2603		ω2=0.2130		ω3=0.2130		ω4=0.1735		ω5=0.1402
Blades	Mechanism	1	1	5.21	5	21.30	5	21.30	5	17.35	5	14.02	79.17
			20		100		100		100		100
		2	4	20.83	5	21.30	5	21.30	4	13.88	5	14.02	91.32
			80		100		100		80		100
		3	5	26.03	3	12.78	2	8.52	2	6.94	2	5.61	59.88
			100		60		40		40		40
	Geometry	1	5	26.03	4	17.04	3	12.78	5	17.35	5	14.02	87.22
			100		80		60		100		100
		2	5	26.03	5	21.30	5	21.30	5	17.35	5	14.02	100.00
			100		100		100		100		100
		3	2	10.41	1	4.26	2	8.52	4	13.88	4	11.21	48.29
			40		20		40		80		80
Stoppers		1	1	5.21	5	21.30	5	21.30	5	17.35	5	14.02	79.17
			20		100		100		100		100
		2	5	26.03	4	17.04	4	17.04	4	13.88	5	14.02	88.01
			100		80		80		80		100
Grippers		1	1	5.21	5	21.30	5	21.30	5	17.35	5	14.02	79.17
			20		100		100		100		100
		2	5	26.03	4	17.04	4	17.04	5	17.35	5	14.02	91.48
			100		80		80		100		100

The highest γ for the blade mechanism corresponds to the pair of blades with vertical opening and one pneumatic actuator (idea 2). Idea 2 was also selected for the blades’ geometry, consisting of cutting blades with two cutting sides. This solution presents the best β for all criteria under analysis, consequently presenting the highest γ, and making this choice unquestionable. Idea 2 is selected for the stoppers subassembly, i.e., use of two independent actuators as cable stops. It should be noted that the major difference between the two ideas is the functionality, which has the highest w_i. The idea with the highest γ for the gripper is idea 2, i.e., use of two independent actuators (one for each cable). Although this idea has a slightly higher cost, and maintenance difficulty because of the larger number of components, it excels in functionality.

4.2. Automatic cable extraction system

The following alternatives were considered for the cable gripper: parallel pneumatic gripper (opening in vertical direction), parallel pneumatic gripper (opening in horizontal direction), and radial pneumatic gripper. Idea 1 ensures proper gripping, because the opening and closing movements are parallel. However, release into the water-cooling container by gravity fall when the gripper opens is unfeasible. Idea 2 solves this limitation since the cables easily fall into the cooling container, but it becomes necessary that the gripper holding mechanism moves in a third direction to grab the cables by its upper part. This feature negatively affects the ease of assembly, ease of execution, and cost of the solution. Idea 3 solves the difficulty of idea 1. Due to the 180° opening of the radial gripper, the cables will easily fall by gravity when the grippers are opened. The only downside of idea 3 relates to the complex design of the gripper tips, since a minimum of articulation must be guaranteed so that, when the clamps close, the two cables become correctly gripped by the mechanism. Without this articulation, one of the handles may slip due to lack of parallelism between the gripper tips. Table 6 shows the selection matrix for the cable extraction mechanism, and γ of each idea in the different criteria. The radial pneumatic gripper (idea 3) has the highest γ, and is therefore chosen for the design process that follows.

Table 6. Selection of ideas to apply for the automatic cable extraction system.

IDEA	CRITERIA										PERFORMANCE INDEX γ
	1-Functionality		2-Ease of maintenance		3-Cost		4-Ease of assembly		5-Ease of execution
	ω1=0.2603		ω2=0.2130		ω3=0.2130		ω4=0.1735		ω5=0.1402
1	1	5.21	5	21.30	5	21.30	5	17.35	5	14.02	79.17
	20		100		100		100		100
2	4	20.83	5	21.30	3	12.78	3	10.41	3	8.41	73.73
	80		100		60		60		60
3	5	26.03	5	21.30	5	21.30	4	13.88	4	11.21	93.73
	100		100		100		80		80

5. Design

5.1. Final solution

The double stripping machine and cable extraction mechanism final proposals are presented in Fig. 9. The double stripping machine allows, as intended, to simultaneously remove the coating chips of two cables at each work cycle, thus eliminating the bottleneck that conditioned the injection process of the 1^st terminal. This new concept follows the basic operating principles of the current machine, but has a relatively different construction in each of its subassemblies. The automatic cable extraction mechanism is applied in the final phase of the 1^st terminal injection process, after actuation of the sprue breaking mechanism. This system allows the two cables, with the injected terminal, to be removed from the transfer that supports them, and to be automatically deposited in the container with water. This concept increases the level of automation in the 1^st terminal injection process, while reducing the number of manual operations, which in turn increases the production rate, as the operator can focus on the stripping process.

Figure 9. Final solution for double stripping machine (A), and automatic cable extraction system (B).

The double stripping machine and automatic cable extraction system are controlled by a programmable logic controller (PLC) that operates the 1^st ZIM machine, following the input of a human-machine interaction (HMI) console. The HMI console provides the operator with relevant information about the machine’s operation. This system lets the operator know the machine’s status or any errors detected by the equipment so that these can be corrected. The console also allows entering data that the operator needs to pass on to the machine. The actuation of the pneumatic components of the equipment depends on the action of solenoid valves. These components operate electromechanically, i.e., they transform an electrical signal into a mechanical response. When they receive an electrical signal, the solenoid valves perform an action. This action depends on the signal emitted and can be opening or closing the valve, allowing air to pass through or not, or controlling the flow of air passing through. The PLC, the signal input and output cards, the transformers, the electrical components, and the auxiliary components are fundamental to the operation of the equipment. These components are stored inside the electrical panel, which is supported by a structure designed for the purpose and is attached to the equipment’s protection. Its function is to store, protect and organize the control components.

5.2. Design process

5.2.1. Double stripping machine

The first implemented improvement in the ZIM involved the replacement of the single stripping machine (currently installed), by the new double stripping machine, illustrated in Fig. 10a. The new system was developed based on the operating concepts of its predecessor machine. It is composed of three subassemblies (Fig. 10b): stoppers subassembly (1)(1) $$\beta =\frac{V_i}{M{V}_i}\times 100,$$(1) , blades subassembly (2)(2) $$\Omega =w_i\times {\beta }_i.$$(2) , and grip subassembly (3)(3) $$\gamma =\Sigma (w_i\times {\beta }_i)$$(3) . Despite having similar designations and functions, all these subassemblies show few similarities with those of the current machine.

Figure 10. Final CAD model of the double stripping machine (a), and exploded view of the three main subassemblies of the stripping machine (b).

The grip subassembly (Fig. 11) consists of a central immobile component, fixed on the upper and lower bases of the machine, and two side gripping tips with independent movements, promoted by two pneumatic actuators, fixed on the side supports of the machine. The central component ensures the guidance of the two side grippers, through the slots (identified by the red line in Fig. 11) where they are introduced.

Figure 11. Grip subassembly: A - central component, B - side gripping tips, C - pneumatic actuators, and D - slots.

The blades subassembly (Fig. 12a) moves in two orthogonal axes, similarly to the current stripping machine. This set consists of two blades and their respective supports (in Fig. 12a, the upper support is represented in blue, and the lower one in red). The lower support is fixed, while the upper support has its movement promoted by a pneumatic actuator, to guarantee opening and closing of the blades. Both the lower support and the pneumatic actuator are fixed on a connection base linked to another actuator, which moves the whole set when removing the coating chips.

Figure 12. Blades subassembly (a), and sectional view of air blowing detail (b).

To guarantee the absence of misalignments between the upper and lower blade (which would compromise the quality of the product and even the operation of the process), in addition to using a guided pneumatic actuator, a blade guidance system was also created, with shafts and linear bearings. The guiding shafts are bolted to the upper support, while the bearings are housed in the lower support. Additionally, the lower support features two holes targeting the cable coating area, to serve as output of the installed air blowing system, used to efficiently remove the coating trimmings (Fig. 12b). The air blowers are designed to prevent coating trimmings from sticking to the blades, due to their reduced mass.

The stoppers subassembly (Fig. 13a) follows the operating principle of the current machine, although with a different design. A mechanical limit plunger system is installed, which solves the problem of inductive sensor damage, as identified in the current machine. This subassembly consists of two independent M8 inductive sensors, mounted on the limit plungers from IFM, which are fixed on a bracket that enables tuning the stripping measurement. All these components are supported on a base. In this base, a pneumatic actuator is installed (Fig. 13b), with stroke of 10 mm, to move the stops away from the blades after stripping the cables, so that the coating chips are not retained between the stops and the blades. This movement is guided by a system of threaded shafts, which slide in bushings and are tightened on the connecting base of the blades subassembly.

Figure 13. Stoppers subassembly: (a) general view, and (b) pneumatic actuator (A - mechanical limit plunger system, B - M8 inductive sensors, C - tuning bracket, D - base, E - pneumatic actuator, F - stops, and G - threaded shafts).

5.2.2. Automatic cable extraction system

The designed solution to increase the level of automation in the 1^st terminal injection process consists of applying a cable extraction manipulator after injection, shown in Fig. 14a. As described in section 3.1, after injection, the cables are manually removed by the operator in every cycle. As well as causing heavy wear and tear after hours of work, this procedure also negatively affects the production line’s operating speed, as the operator is unable to feed the stripping machine at the rate of the other operations. The new solution provides higher consistency in cycle times, increases the line productivity, improves the process control, and reduces the chances of faults or unexpected stops. This mechanism moves in two directions, both horizontally, allowing the cables to be extracted and then deposited in a water container (shown in yellow in Fig. 14a). Three pneumatic actuators (shown in Fig. 14b) are used to ensure the necessary movements: a radial gripper (G1), responsible for transporting the cables, and two guided linear units (A1 and A2), responsible for supporting and moving the gripper and the cables it carries.

Figure 14. Designed cable extraction mechanism (a), and identification of the pneumatic actuators (b).

A pair of gripping tips was designed for the gripper, which hold the cables while they are transported between the outlet of the sprue breaker and the container. The concept for the gripping tips (Fig. 15) is coherent with the selected idea in the pre-design (section 4). To ensure that both cables are correctly gripped, the system consists of a fixed gripping tip (bottom) and another tip with angular freedom (top), to provide gripping adjustment. The radial gripper enables the cables to fall freely to the desired location due to the action of gravity.

Figure 15. Gripper with gripping tips: (a) closed tips, and (b) open tips.

The working principle of this mechanism follows the sequence:

1. Gripper G1 closes, gripping the two cables that are currently positioned in the sprue-breaker stage;

2. Actuator A1 moves back, thus pulling the cables away from the ZIM and the transfer machines mounted on it;

3. Actuator A1 moves back, positioning the cables over the water container;

4. Gripper G1 opens 180°, allowing the cables to fall into the box due to gravity.

5.2.3. Safety elements

The safety elements used in the design of the new machine essentially consist of physical sheet metal guards (Fig. 16a). Since the side door allows access to the inside of the machine, a sensor from PILZ (Fig. 16b) has been applied to immediately interrupt the operating cycle when the side cover is opened, and to cut off the air supply from the pneumatic network. This procedure ensures that, when the operator has access to the inside of the machine, there is no risk of injury. The ZIM also incorporates a general emergency button which, if triggered by the operator, applies the same two actions to the machine.

Figure 16. Physical sheet metal protections (a), and sensor from PILZ for the stripping machine (b).

5.2.4. Prototype construction and validation

Once the design phase was complete, the parts needed to build the double stripping machine were acquired. On the other hand, the automatic cable extraction mechanism is not yet fabricated. The prototype of the new double stripping machine was built and tested. Fig. 17 shows images of the prototype’s subassemblies, still in the assembly and validation phase, which detail the machine construction. Fig. 17a details the blade subassembly, when it is recessed from the gripping tips, as well as the stoppers subassembly, both supported on the same base. Fig. 17b shows the positioning of the blade assembly when it is advanced (initial position of the cycle), highlighting how the blade supports accommodate the gripping tips to reduce their distance to the blades. The detail of the air blowers included in the blade holders, to ease the removal of chips resulting from the stripping process, is visible in Fig. 18a. The same image also shows the two cable stops, which are responsible to start the stripping cycle. Fig. 18b highlights the stoppers subassembly, respective fitting to the blade subassembly, and two inductive sensors that are responsible for triggering the machine’s operating cycle.

Figure 17. Side view of the blades and stoppers subassembly (a), and front view with the blades subassembly advanced to the gripping tips (b).

Figure 18. Detail of the blade clamping area, stops and air blower outlets (a), and of the stoppers subassembly (b).

Figure 19 shows the prototype under testing. Once the tests were carried out and the machine’s operating requirements verified, the concept was successfully validated, and the machine was installed in the ZIM production line (Fig. 20) and production began with the new machine. As a result, it was possible to evaluate the productivity gains that the implementation of this improvement would bring to the output of the cables’ production line.

Figure 19. Prototype being tested (a), and side view of the prototype being tested (b).

Figure 20. Stripping machine installed in the production line.

5.2.5. Return on investment

After finalizing the designs for the double stripping machine and the automatic cable extraction system, and the prototype of the stripping machine has been built and implemented, it is necessary to quantify the improvements made to the production process, by calculating the ROI of both designed systems.

5.2.5.1. Double stripping machine

To analyze the ROI of the double stripping machine, the costs of all the manufactured and catalogue components were collected. The manufactured components by machining processes were individually analyzed regarding the raw material, labor, and fabrication costs, leading to a total cost of 1623.15€. The catalogue components involved actuators, stops, bushings, connections, hinges, and handlings, totaling 812.48€. Thus, the total cost for the machine is 2435.63 €. This figure also includes the cost of assembling the machine. To assess the gains and the consequent ROI of the solution, productivity was measured in five different production hours, both in the current situation (single stripping machine), and in the new situation (double stripping machine). Table 7 shows the productivity, in cables/hour, of the two solutions.

Table 7. Comparison of productivity values before and after installation of the double stripping machine.

Measurement	Productivity (cables/hour)
	Single stripping machine	Double stripping machine
1	960	1324
2	866	1465
3	923	1644
4	960	1512
5	952	1623
Average	932	1514
Standard deviation	40	130
Coefficient of variation	4.3%	8.6%

It can be concluded that the implementation of the double stripping machine has led to significant improvements in the 1^st ZIM’s productivity. This increase in productivity mainly originates from the elimination of the bottleneck that initially existed in the stripping process. If the increase in productivity is calculated from the average production figures, the productivity gain is 62.4%. There was also an increase in the coefficient of variation from approximately 4.3% to 8.6%, which indicates a higher dispersion between the measured productivity values. This difference is explained by the measurements being taken on different shifts and operators, which are subjected to individualized and different adaptation periods to the new process. However, it is expected that this variation will stabilize as practice and skill in working with the new system improves.

To calculate the monetary gains of the implemented solution, the annual production capacity in the current situation was initially calculated. Considering three eight-hour shifts for 240 days a year, the total period amounts to 5760 annual working hours. Thus, the current number of produced cables is 5760 × 932 = 5368320 units/year. After implementing the new machine, the same 5368320 cables will be produced in less time: 5368320/1514 ≈ 3546 h. Therefore, the yearly time gain is Δt = 5760–3546 = 2214 h. It is possible to calculate the monetary gain by multiplying the hourly cost of the working station by the difference between the number of hours worked in the final and current scenarios. The target hourly cost is 19.25 €/hour, and it covers the cost of direct and indirect labor, maintenance costs, production costs, and energy consumption. So, the calculation is as follows: (4) $$\mathit{\text{Monetary gain}}=2214\hspace{0.17em}\left(\mathit{\text{hour}}/\mathit{\text{year}}\right)\times 19.25(\mathrm{€}/\mathit{\text{hour}})\approx 42614\hspace{0.17em}\mathrm{€}/\mathit{\text{year}}$$(4)

The payback, or ROI, for this project is given by: (5) $$\mathit{\text{Payback}}=\frac{\mathit{\text{Investiment}}}{\mathit{\text{Monetary gain}}(1\hspace{0.17em}\mathit{\text{year}})}=\frac{2435,63\hspace{0.17em}\mathrm{€}}{42614\hspace{0.17em}\mathrm{€}/\mathit{\text{year}}}=0.057\times 365\approx 20.8\hspace{0.17em}\mathit{\text{days}}$$(5)

The calculation shows that, in less than a month, the investment in the double stripping machine is paid. The difference in investment between the new double stripping machine concept and the currently implemented concept is approximately €448.76, given that the cost of the current solution was approximately €1987.00.

5.2.5.2. Automatic cable extraction system

No prototype was built for the automatic cable extraction system to physically validate the solution. Thus, the following analysis consists of a forecast of the productivity improvement, and possible economic gains from its implementation. To estimate the productivity gains, measurements were taken on the shop floor, relieving the operator of the manual cable handling task. Under these conditions, the ZIM has no downtime, and it shows very close productivity figures to the ZIM capacity (900 injections/hour, i.e., 1800 cables/hour), as shown in Table 8.

Table 8. Productivity after implementing the automatic cable extraction system.

Measurement	Productivity (cables/hour)
1	1768
2	1692
3	1756
4	1792
5	1744
Average	1750
Standard deviation	37
Coefficient of variation	2.1%

An increase in productivity is witnessed compared to the sole implementation of the double stripping machine (approximately 15.6%). With the implementation of both solutions, a productivity gain of approximately 87.8% is estimated compared to the current situation. On the same basis as the calculations presented in section 5.2.5.1, a total monetary gain of approximately 51816€/year is obtained. The cost of produced parts for the automatic cable extractor totaled 308.95€, while the cost of catalogue components, including pneumatic grippers, actuators, and profiles, was 1305.34€. The automatic cable extraction system therefore costs a total of 1614.29€. Combining this figure with the cost of implementing the double stripping machine (€2,435.63) gives a total investment of €4049.92. The payback period is: (6) $$\mathit{\text{Payback}}=\frac{4049.92\hspace{0.17em}\mathrm{€}}{51816\hspace{0.17em}\mathrm{€}/\mathit{\text{year}}}=0.078\times 365\approx 28.5\hspace{0.17em}\mathit{\text{days}}$$(6)

6. Conclusions

After completing the new machine design, it can be concluded that all the proposed objectives were successfully met, although it was only possible to physical construct and validate the stripping machine. A new stripping machine concept was designed, capable of simultaneously cutting and removing the coating from two cables. The proposed double stripping concept, and respective design approach by the DSR methodology up to prototype validation, is considered the main novelty and contribution of this work, leading to higher production rates, and reduced operational costs. This approach can also be applied in different industrial processes, which require mass handing or intervention in series-produced components, to increase (ideally double) the production rate, while keeping the quality of the fabricated products unaffected. The double stripping machine was fabricated and applied to the control cables’ production line, and significant productivity improvements (≈62.4%) were obtained by applying this new mechanism in a 1^st ZIM station. Adding an automatic cable extraction system leads to an 87.8% productivity improvement over the current condition, and 15.6% over the intermediate double stripping condition. The impact of increasing productivity can be evaluated from two different perspectives. If, on the one hand, the costs inherent in the production of control cables are reduced, it is possible, from a future perspective, to attract new customers and win new client projects by reducing the proposed budget. On the other hand, it is also possible to reduce the investment needed to meet the production requirements of a new project, since the installed capacity of existing machines is being increased, thus making room to produce more orders without the need to invest in new machines.

The design of the double stripping machine considered the fulfillment of certain requirements imposed by the client, which were successfully met, as shown:

• Machine capable of removing the coating from two cables at the same time;

• Ability to fine-tune the length of the stripped area by adjusting the set of stops;

• Ability to store coating chips in a clip box that requires cleaning at the end of each shift;

• Possibility to replace cutting blades;

• Guarantee of operator safety through physical guards and a door opening sensor.

The automatic cable extraction system was also subject to requirements, which were met as follows:

• Simultaneous handling of two cables;

• Ability to automatically remove the cables at the end of the injection process (after sprue breaking);

• Ability to deposit the cables in the water-cooling container positioned in the desired location;

• Possibility to integrate the system into the ZIM without having to make changes to the other machines;

• Having a cycle time shorter than the injection time.

Disclosure statement

No potential conflict of interest was reported by the authors.