Evaluation of design manufacturability in assembly processes by modified Lucas Design for Assembly method

The paper presents a methodology for the design of the production product from the point of view of manufacturability design criterion. The evaluation method from the point of view of machining, assembly and organization of production was discussed on the example of a modified method of assessing the technological character of the structure from the point of view of the assembly operation (Design for Assembly – DFA).

The development of machining technology (thanks to the automation and extension of the possibility of making objects of complex construction) in connection with a significant share of manual work in the assembly processes of finished products has led to a change in the approach to the production of new products [5].The design process of the new product is shown in Figure 1.* prof.dr hab.inż.Józef Matuszek, jmatuszek@ath.bielsko.pl,mgr inż.Tomasz Seneta, tomasz.seneta@zf.com,dr inż.Aleksander Moczała, amoczala@ath.bielsko.pl,Akademia Techniczno-Humanistyczna w Bielsku-Białej, Wydział Budowy Maszyn i Informatyki, Katedra Inżynierii Produkcji The design process should be determined from the point of view of different usability criteria.The assessment is based on: marketing and conceptual preparation; documentationconstruction, production and organization; implementation of the production process; distribution; conditions for the operation and decommissioning of the product.
Errors of the designed process generate large costs associated with eliminating defects at the production stage.In publications concerning large-scale production (e.g. in the automotive industry), much attention is paid to the issues of the construction's technical character from the point of view of assembly.
Methods of forming assembly processes, referred to by the abbreviations DFA (design for assembly) or PdM (design for assembly), are presented in the works [2][3][4][5][6]8].The most common method of DFA is the Lucas DFA method [6].

Modified Lucas DFA method
The traditional Lucas DFA technology benchmark is based on the determination of three indicators whose values are related to the relative measure of installation difficulty.As a result of the assessment, the aim is to reduce the number of assembled elements of the final product (functional analysis) and analyze the assembly operation in terms of difficulty, complexity and assembly time (maneuvering analysis and assembly method).
The project effectiveness indicator based on functional analysis is defined by the formula: where: Wep -project effectiveness indicator; LkA -number of A components (fulfilling the function of the product); LkBnumber of B components (characterized by a lack of performance of the product function, e.g.rivets, washers).

Assessment of maneuvering of assembled components of the product determines the maneuvering coefficient of Wman:
where: Iman -maneuvering index; LpA, LpB, LpC, LpD -sum of points from segments A, B, C, D according to [6].
The assessment of the course of assembly operations from the point of view of feasibility is defined by the formula describing the results of the analysis of the Wmon coefficient of assembly: The analysis allows to make structural changes to the product, improving the efficiency of assembly processes [6].The presented course of proceedings was developed for large-scale production.The study attempts to modify this method so that it can also be used in the case of mass production and small lot production.The Lucas DFA method -due to its simplicity in comparison with other methods -may be susceptible to such modification.
According to fig. 1, the technical character of the structure can be considered in terms of machining and the organization of machining and assembly processes.From the point of view of the production organization, it is important to unify and normalize the components of the product, defined by the unification indicator: where: LkUthe number of unified and standardized components; LkNU -number of non-unified and non-standard components.
The use of group machining plans, group operations and operations not requiring retooling of workplaces is associated with a reduction in the costs of preparing manufacturing processes.The process efficiency improvement can be determined by the Unification Indicator of Wuno Operations: where: LOG -the number of operations carried out that do not require changing of work positions; LONG -the number of operations requiring retooling of workplaces.

Example
The calculation in the example shows that the structure is non-technological.In the applied method, Wep = 23% (authors of publication [9] give 60% as a minimum), Wman = 2.80 and Wmon = 11.84 (according to [9] both indicators should be less than 2.5 ).
On the basis of the obtained values of ratios and coefficients, the structure of the designed transmission was changed (fig.2).The transmission belongs to a series of types, consisting of several gears of various sizes, in which the same solutions of the construction form were applied.The results of technological analysis of the new construction version of the transmission are presented in tabular form (fig. 3).The operations with assigned parameters resulting from functional analysis (in the form of components included in group A or B [6]), maneuvers (within segments A, B, C and D [6]) and assembly (according to indicators main activities A, B, C, D, E and F and additional operations according to the procedure given in [6]).The following indicators were again determined: Wep efficiency of the project, Wman maneuvers and Wmon mountability.

LkA
Based on the data presented in the table (fig.3) Lka = 18, and LkB = 9.After substituting for formula (1), it is obtained: On the basis of formula (3), the coefficient of mountability was calculated: The treatment unification index is:

Conclusions
Analyzing the values of the evaluation parameters of the gear assembly, it can be concluded that: • assessment of parameter values (fig.3) may be the basis for analyzing the technological character of the product structure; • the assessment should take into account many other factors related to sales, servicing, availability of spare parts, production line, available assembly techniques, the level of automation, cooperative services, the possibility of using commercial components, the technical culture of the crew, etc.; • the method can be successfully used also for smaller batches of manufactured products -in the case of production of a group of technologically similar products, e.g.generalpurpose toothed gears; • it is helpful when assessing the structure to standardize machining and assembly operations -it makes it easier to estimate the times of performing these operations; • presented procedure can be for: designers, people implementing new products for production and production engineers, carrying out rationalization work at the stage of improving and expanding the production range.

Fig. 3 .
Fig. 3. Results of the technological analysis of gear assembly (fig.2) after the technological assessment According to the calculations presented, it can be concluded that the results of the analysis for cast iron cast gearboxes are better compared to the results for welded gears.The Wep project efficiency ratio should have the highest possible value -the change was 67%.Maneuvering coefficients and the mountability of Wman and Wmon should be as small as possible -after the change they are 1.96 and 6.27 respectively.Component unification indicator for gear units from fig. 2 (only the body and cover are not unified -they are not included in other products) is: 10. Więcek D. "Określanie kosztów produkcji według rachunku kosztów działań".Zeszyty Naukowe Politechniki Śląskiej: Organizacja i Zarządzanie.42 (2007): pp.117-124 ■ Translation of scientific articles, their computer composition and publishing them on the website www.mechanik.media.plby original articles in Polish is a task financed from the funds of the Ministry of Science and Higher Education designated for dissemination of science.