Design and Analysis of Grass Cutting Machine By Using DFMA Method

This paper describes about the implementation of redesign the component of grass cutting machine by using the application of Design for Manufacturing and Assembly (DFMA) methodology. The scope based on the existing grass cutting machine and the appropriate of DFMA methodology. The data was analysed by using Boothroyd-Dewhurst Design for Manufacture and Assembly method to verify the design efficiency, handling ratio and fitting ratio to achieve. The new proposed design of grass cutting machine drawn using CATIA V5 software based As a result, the assembly time for redesign showed an improvement of 18.68% where the assembly time was reduced from 568.84 s to 462.59 s and design efficiency was increased 8.33% from 24.40% to 34.70%. The total part, handling ratio fitting ratio and cost of existing design is reduced. Eventually, the improvement of redesign grass cutting machine the best design with optimal value is accomplished.


Introduction
Design for manufacture and assembly (DFMA) is a combination of design for assembly (DFA) and design for manufacture (DFM). The term DFMA is defined as a set of guidelines developed to ensure that a product is designed so that it can be easily and efficiently manufactured and assembled with a minimum laborious effort, assemble time, and cost to manufacture the product [1][2][3][4]. DFA is considering and resolving the possible problems in the assembly process at the early stage of the design which can make sure the part will be assembled with high speed, low cost and productivity [5]. DFM is that by considering the limitations related to the manufacturing at the early stage of the design which is the design engineer can make selection among the different materials, different technologies and estimate the manufacturing time [6,19]. The quality of new product was increased with the good development period including design, technology, manufacturing, service and so on [7][8][9]. Simultanously, the cost also decrease, including the cost of design, technology, manufacturing, delivery and technical support. The short developing cycle time, including the time of design, manufacturing preparing, and repeatedly calculation [10][11].

Research Objective
The main objective of this research is to analyse design efficiency on current design and redesign of grass cutting machine by using DFMA method. By using the DFMA as the benchmark tool was used to increase the design efficiency and minimize the quantity of part in manufacturing and assembly costing [2][3][4]20,21].

CAD Drawing
CAD design is capable of producing the designer with extremely precise designs and prospects. CAD drawing also helps the designer in evaluating the design and redesigning the drawing. Figure 1 shows the first process in Boothroyd-Dewhurst DFMA method is to design the concept [12]. Figure 2 shown the features of current grass cutting machine by using CAD drawing. .

Design For Assembly (DFA)
Design for Assembly (DFA) is an approach to reduce the cost of the product and time of assembly by simplifying the product and process. There are two uses of DFA. It may use to redesign a product already in manufacture or product being remarketed or reversed engineering. Beside that DFA may also use for analysis of a product while it is in design [13,14,18,22]. The DFA method should be considered at all stages of the design process especially in the early stages [15,[23][24][25]. In this paper, type of assembly that must be considered is manual assembly by. The manual assembly can be measure by using DFA tool, which is the handling and insertion time table. Table 1 shows the current quantity parts used, handling time, insertion time, theoretical part count and assembly time for each part for current grass cutting machine. The parts should be consider the orientation of the part rotating (Alpha -Į and Beta -ȕ) from X and Y-axis. While handling time and insertion time are selected based on the table estimate time For Manual Handling Time and Manual Insertion Time. The theoretical part count states the important of the part for grass cutting machine. If the part is important to the main function of the grass cutting machine, the value of theoretical part count is 1 and the less important part to the function of the grass cutting machine which is that parts can be eliminated or redesign is 0 [15].

Design Efficiency
The important data in the DFA method is the used the "Efficiency of assembly" of the proposed design. Basically, there have two major factors affecting cost of product assemblies or subassembly are: i) Parts number. ii) The easy of manual insertion, handling and fastening. Based on the calculation above, the current design efficiency of grass cutting machine is 26.4%. Therefore, the redesign efficiency of grass cutting machine must be higher than the current design efficiency.

Design for Manufacturing (DFM)
Design for Manufacturing (DFM) is to make easier for manufacturing and concerned with reducing overall part production cost by minimizes complexity of manufacturing operations [16][17][18]. The DFM will be analysing the material and process used to ease the design for grass cutting machine. Table 2 shows the Manufacturing Process and Material used for Each Part of grass cutting machine.

Result and Discussion
The assemble and dissemble process of existing design of the grass cutting machine will be identified which parts to undergo redesign process and some part can be eliminated, minimizing, or combined together for achieving higher design efficiency. The parts of the grass cutting machine are evaluated to identify which part has a higher potential to be eliminate or combined. The parts are that not necessary for the existing product of grass cutting machine can also be reduced. Table 3 has shown the assemble of tank and assemble of exhaust before and after redesign. The shoulder frame was improving the design of addition support to hold the tank for purpose of minimizing the assembly time by eliminates 6 parts from current tank assembly. The tank holder was design for extra support to hold the tank. By doing so, the number of part count has reduced with minimize the time required for assembly. Thus, the design efficiency for the grass cutting machine will be increased.
Next, the improvements were made to the exhaust assembly. The current exhaust assembly was design with 4 parts and combination of three manufacturing process which is die casting, bending and welding. The design is also quite complicated and can be increase the production time of assembly and material cost. Therefore, the redesign of exhaust was made by reducing the manufacturing process with eliminate the bending process. The production time for insertion between parts and material costs were reduced simultaneously. There is also no impact on the function of exhaust. Table 4 shows the comparison for the part quantity, handling time, insertion time, assembly time, and design efficiency in between the existing design and improved design of grass cutting machine.  Table 3. Comparison between original design and redesign for grass cutting machine.

Original design Redesign
Assemble of Tank  Table 4, there is an increase in design efficiency of 8.33 %; this is due to 8 components part has eliminated during the redesign process. For the percentage of part count, the result is 12.50 % which considered as good outcomes as one quarter of total numbers of the components has eliminated via the redesign process. Based on calculation, the result shows that insertion time being minimized is more than the handling time, which are 87.05 s and 29.26 s respectively. For total assembly time, a total time of 106.25 s is being eliminated. Therefore, the redesign is able to reduce the total production time and minimize the cost by having less part count.

Conclusions
This paper presented the redesign of grass cutting machine by applying the manufacturing and assembly design (DFMA) method [26][27][28][29]. It can be conclude that the design performance for grass cutting machine of the redesign has been enhanced after the redesign process resulting in reduced part count and overall assembly time. The design efficiency has come out of 26.4 percent of the original design efficiency by 8.33 percent from the report. However, this is not the perfect possible configuration for grass cutting machine, there can be a lot of change. The design consideration of the function of the part and the vibration issue was important in order to prevent the performance of the grass cutting machine. Therefore, some of the screws and bolt can be modified and eliminated by consider the function of that parts.