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Manufacturing cost estimation in tolerance optimization

1 introduction

tolerance design should take the lowest cost as the objective function under the constraints of use requirements and processing capacity. This cost should include the comprehensive total cost of multiple cost factors in the product life cycle, including manufacturing cost, quality loss cost, waste and defective product loss cost, inventory cost and opportunity loss cost. Manufacturing cost refers to the sum of all kinds of losses in the manufacturing process of products. It is the most direct and essential cost factor among all kinds of cost factors. Therefore, modeling tolerance and manufacturing cost is the basic problem in tolerance optimization

the modeling method of the relationship between design tolerance and cost should be different according to the different manufacturing system information that the designer has. When the designer can obtain a set of empirical tolerance cost data, the curve fitting method can be used for modeling, that is, the tolerance cost relationship curve can be fitted according to the known tolerance cost data, so as to obtain the cost value corresponding to any tolerance. In this direction, speckhart proposed the exponential model, Spotts proposed the reciprocal square model, Dong & Hu proposed the polynomial model, O et al proposed the natural spline model. Curve fitting method is based on empirical data, and its use does not require the tolerance designer to understand the actual processing process of the designed product. In other words, when using the curve fitting method, the tolerance designer has assumed that the existing factory processing environment is consistent with the processing environment when obtaining empirical data. When the processing environment has changed (which is very likely to happen in practice), the error caused by using the curve fitting method can be imagined

the cost data obtained by curve fitting method is indirect data obtained by fitting reasoning. If the tolerance designer can obtain the relevant information about the actual processing process of the product at this time, he can directly calculate the manufacturing cost and obtain the direct cost data. This requirement can be fully realized under the advanced manufacturing concept

virtual manufacturing requires the modeling of the whole manufacturing system, so that the designer can fit the whole manufacturing process of realizing a certain design scheme at the same time, so as to evaluate and modify the design scheme. The cad/cam integrated environment realized under this concept can carry out product design and process determination in parallel. Therefore, while designing the tolerance, we can obtain the relevant information of the period process in which the manufacturing utilization is not very wide, and directly estimate the manufacturing cost

in fact, enterprises now use the direct calculation method to calculate product costs. Therefore, it is worth considering whether this method can be used to directly estimate the manufacturing cost in tolerance optimization. This paper analyzes and compares the existing cost calculation methods with the requirements of tolerance optimization, explains that the former cannot adapt to the latter, and puts forward a cost calculation method based on tolerance optimization

2 tolerance cost calculation and cost accounting

when all enterprises calculate the product cost, they must pay attention to that enterprises generally follow the principles and methods of cost accounting to calculate the product cost, which is obviously different from the requirements of tolerance designers. In tolerance optimization, tolerance cost should be calculated. Tolerance cost is the sum of those cost factors related to tolerance design and sensitive to tolerance design. Many factors in cost accounting can be completely ignored in tolerance cost calculation

the product cost in cost accounting is formed by collecting all direct and indirect expenses during the production period onto the unit product, including material costs, wages of workers and managers, machine tool depreciation and repair costs, transportation costs, travel expenses, etc. The tolerance cost only considers various losses in the process of product processing, and calculates those losses related to tolerance design. The two are essentially different

in addition, working hours are often used as a measure of cost in enterprise cost calculation, converting additional requirements such as technical difficulties into working hours, so as to compensate workers' wages. Therefore, the working hours often cannot represent the time occupied by the actual processing. Because the tolerance designer should consider the opportunity cost and other factors, the concept of working hours should have practical time significance

therefore, according to the above characteristics, the author puts forward a cost accounting method - multi parameter formula method, which is different from the cost accounting method for tolerance design

3 multi parameter formula method

multi parameter formula method is based on the state of the processing system to calculate various losses related to tolerance in the process of product processing, and the sum of which is taken as the tolerance cost of the product. Multi parameter formula method is a modeling method for the relationship between tolerance and manufacturing cost when the state of manufacturing system is known, in which the state of machining system appears in the formula in the form of multi parameters. Using the multi parameter formula method to estimate the manufacturing cost, the designer is required to clarify the whole manufacturing process corresponding to the tolerance and various parameters and states related to various losses of the product in the manufacturing cycle, such as the preparation time and start-up cost of the machine tool, the depreciation rate of the machine tool, the wear rate of the tool, the fashionable mainstream tool, the processing time and wage coefficient, the basic consumption of the workshop, etc. This not only conforms to the concept of virtual manufacturing, but also can be realized in the environment of cad/cam integration

due to the difference of manufacturing system composition and product processing route, the loss in the manufacturing process will be different, so the specific form of multi parameter formula method will also be different. Here, only an example is given to illustrate the application of multi parameter formula method. For a part of a factory, the surface feature is

, where: I - the process sequence of the part feature, 1, 2,... N

toi - the preparation time of a single piece of the machine tool used in process I (toi= the preparation time of the machine tool/the processing quantity in preparation every two times), H

Ti - the actual processing time used in process I, H

Si - the hourly processing wage quota of process I, Yuan

oi - the hourly depreciation of the machine tool used in process I, Oi= (machine tool price residual)/machine tool life hours, yuan/h

mi - hourly loss amount of tools used in process I, yuan

b - hourly single piece workshop basic consumption (workshop basic consumption includes energy, management costs, etc.), b= daily average consumption of workshop (yuan)/(average daily output) × 24), yuan/piece · h

the above parameters are known quantities

formula (1) does not consider the material cost when calculating the cost, because the tolerance design hardly affects the selection of blanks. Equation (1) shows the single piece tolerance cost, even for a certain part feature. Ti in formula (1) is the actual working hours. The difference in technical content caused by different processes is reflected in the wage base Si rather than converted into Ti

the factory has updated and adjusted the machine tool in nearly one fourth of the latter period. In order to reduce costs, the design tolerance of parts has been adjusted, and a new optimal design scheme has been determined through practice. The multi parameter formula method is used to verify, and the results are consistent with the empirical results, which reflects the feasibility of the multi parameter formula method

the multi parameter formula method considers various losses of a part feature in the processing process, and is a modeling method for the feature tolerance and manufacturing cost. The influence of workshop management on cost is not considered in the multi parameter formula method. However, some workshop management factors, such as waste disposal and inventory management, are sensitive to tolerance design. The tolerance designer should consider these issues in addition to the manufacturing cost when calculating the total cost

4 summary

the author analyzes and compares the traditional calculation methods of manufacturing cost, and puts forward the estimation method of product manufacturing cost aiming at optimizing tolerance design when the state of manufacturing system is known, that is, the modeling method of design tolerance and manufacturing cost when the state of manufacturing system is known. The traditional curve fitting method determines the tolerance cost curve based on empirical data, so it does not need the tolerance designer to understand the actual processing environment of the product. This method is simple and effective when the processing environment is relatively stable or when it is impossible to investigate the processing environment. However, because the curve fitting method obtains indirect data after 2. The main motor source of the experimental machine has electrical reasoning, when the state of the manufacturing system changes, the curve fitting method cannot make corresponding adjustments, so there are large systematic errors. When it is possible to investigate the processing environment, the multi parameter formula method can obtain direct cost data, and modify the parameters in the formula can quickly adapt to the changes of the processing environment at any time. Therefore, the multi parameter formula method has stronger accuracy and flexibility. In addition, it should be pointed out that the multi parameter formula method is more suitable for the requirements of virtual manufacturing and concurrent engineering, and can be used as a powerful tool in cad/cam integrated environment and virtual manufacturing system. (end)

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