cost estimation and conceptual process planning

COST ESTIMATION AND CONCEPTUAL PROCESS PLANNING P. MARTIN, J.-Y. DANTAN, A. SIADAT, X. HOUIN, Q. DANIEL Laboratoire de Génie Industriel et de Production Mécanique Ecole Nationale Supérieure d’Arts et Métiers (ENSAM) 4 rue Augustin Fresnel 57 078– Metz – France patrick.martinmetz.ensam.fr

Abstract Engineering cost estimation is now compulsory from the very first stages of design. The later a cost issue will be detected, the more it will cost. This paper tends to show how to define which information is needed to allow this estimation to be done. A taxonomy that builds a structure in this information has been created. Using this taxonomy, we designed a method to calculate manufacturing costs thanks to an expert system. The cost is determined by considering Cost Entities using pertinent inductors and parametric calculation methods.

1. INTRODUCTION Cost considerations are nowadays critical in the engineering field. We know how tiny the liberty of action and decision for companies is. We also know how to evaluate the costs engagement during a project. As it is shown by fig 1, during the conceptual design stage, the decisions we make engage a lot of money for the following stages of the future. This is the stage where most of the choices that imply a cost are made. However, it is also the stage where changing the decisions is the cheapest. When a company has to bring modifications on the design while in the industrialisation stage, it is very costly because all the design process has to be done again.

Costs

Design

Industrialization

Manufacturing Modification costs

Financial impact of decisions Time

Fig 1: Decisions influence upon costs during a project [1]

Therefore it is necessary to bring to designers as much information as possible so that they can take the accurate decisions. In this context, letting designers to have -1-

in the very first stages of design a tool to evaluate the costs can be very attractive. It can prevent an inaccurate design that would go to the industrialisation department, which would send it back to designers because it is too expensive to manufacture. We will then try to define a tool that could handle these tasks.

2. CONCEPTUAL PROCESS PLANNING/CONCEPTUAL DESIGN This chapter is based on research by Shaw C. Feng research [2]. It deals with the means to determine a Conceptual process design from the conceptual design. To evaluate the manufacturing costs, it seems obvious that we need to know exactly what processes will be used to manufacture the part. However, in the conceptual design stage, we have no idea of what the processes will be. The aim of this chapter is to show how we can define a conceptual process design in the very first stages of a project. a. Link between Conceptual Design and Conceptual Process Planning Conceptual design refers to a stage in the design procedure. This stage is the one where designers define rough part characteristics based on the part requirements. Information such as the material, the general part shape and main geometrical data is provided during this stage. However, data are still rough and do not allow to define extensively the manufacturing processes. Conceptual process design is a first estimation of what the process plan could be. Manufacturing operations are not very detailed but it allows engineers to have an idea of processes and attached resources. These data make a first cost evaluation possible. These results will allow designers to adapt their decisions regarding the costs, which is something they can not do for the time being. Now the problem remains how to define the conceptual process plan from the conceptual design. Fig 2. shows how the two stages can be performed simultaneously. Fig 3 exposes the detail of typical data exchange and processing between the two stages. After this stage, the project will continue with a detailed design stage. The interest in the method is to start the detailed design stage with a validated conceptual design. This will prevent the detailed design stage to be done several times. b. Conceptual Process Planning Method Shaw C. Feng proposed a method to generate the Conceptual Process Planning from the Conceptual Design data. He used IDEF0 diagrams (Fig 4) to detail his methodology. Three main steps can be found in this method. All of them are critical in the cost evaluation process.

-2-

Conceptual Process Planning

Conceptual Design

Requirement

Form/Structure & property

Functional Design Function

Process •Fabrication •Assembly •Inspection

Behavioral Specification Behavior

Estimated Time & Cost

Embodiment Design Form/Structure

Process Selection

Resource Selection

Equipment/Skill Time & Cost Estimation Time & Cost Detailed Process Planning

Detailed Design •Geometry •Topology •Tolerances •Dimensions •Surface Conditions •Materials

•Operation sequences •Process parameters •Setup/Fixture •Accurate time& cost •Manufacturing resource

Fig 2: Interaction between Conceptual Design and Conceptual Process Planning [2]

i. Manufacturing processes selection [3] This activity can be divided in four activities. Its aim is to select processes that are compatible with the data we have obtained from the Conceptual Design stage. The different activities are the following: - Determine compatible processes based on materials - Determine compatible processes based on quantity - Determine compatible processes based on product shape - Determine compatible processes based on tolerances. For this selection process, we will consider as potential processes the total range of processes. Then the material will prevent some of these processes. So the outcome will be a narrowed range of compatible processes. For example, we know we will not propose to forge a plastic part. Thanks to these four steps in a row, we will come out with a reduced range of processes that will be proposed for the Conceptual Process Plan. Once these compatible processes are selected, we could rank them in terms of relative cost and finally let the user choose those he will consider for the upcoming stages. -3-

• •

Conceptual Design • • •

• • • •

Defining User Requirements Mapping Engineering Requirements Identifying functions and decomposing functions

•

Production Methods Manufacturing Process(es) Manufacturing Resources Requirements Cost Estimation

Material Form
Main Shape
Feature Forms
Feature Relations User Requirements
Quantity
Cost Limitation
Date of Delivery

Conceptual Process Planning • • • • • •

Selecting Material Processes Selecting Quantity Processes Selecting Product Form Process Selecting Tolerances Process Retrieving Manufacturing Resources Estimating Manufacturing Cost

Fig 3: Data exchange and processing [2] User Requirements

Tolerance Standards

Product Conceptual Design

Determine Manufacturing Processes

Selected Processes

A1

Select Manufacturing Resources

Selected Resources

A2

Manufacturing Processing Knowledge

Estimate Manufacturing Cost

Estimated Cost

A3

Manufacturing Resource Model

Cost Model

Fig 4: Level A0 [2] ii. Manufacturing resources selection Processes are now considered known. If we want to evaluate the induced cost, we also need to determine what will be the used resources. This is the aim of this stage. This selection has to be made at three scales: -4-

-

-

-

Machines selection: Select available machines in factories for manufacturing the designed product. This implies we know the capabilities of each machine in terms of compatible processes. Tool and fixtures selection: Based on the selected machines, select tools and fixtures that are necessary for supporting the selected manufacturing processes. Labour skills selection: Based on the selected machines and tools, select labour skills to operate the machines and use tools for fabrication.

iii. Manufacturing cost estimation Once the manufacturing processes and the manufacturing resources have been selected, we have the information to evaluate the cost. We have to keep in mind that this calculated cost will be a rough evaluation. The purpose is not to determine the actual manufacturing cost but to assess manufacturability and cost in the early design stage. For this estimation stage, we propose another method than Shaw C. Feng’s one. This method will be developed in part 3. As a result, through Shaw C. Feng’s method, we have shown how we can attribute processes and resources to a part at an early design stage. This attribution activity is compulsory to allow cost estimation.

3. COST ESTIMATION a. Cost evaluation methods There are several methods that exist for calculating costs. Table 1 presents the main techniques. Accordingly with this table, in the conceptual design stage, the best methods to apply are analogical or parametric. Indeed, we do not have much information on the product yet and a analytical method would be inappropriate. However, these methods might give too rough estimation to allow the designer checking the validity of the product he has started to design. These methods may not help the designer very much in his tasks. Therefore, we will adapt an analytical method to the conceptual design stage. As we have already shown above, we can determine what processes and what resources can be used. This means that from the Conceptual Design stage, we have deduced some information usually provided in the detailed design stage. Yet, the information is still not accurate enough to allow an usual analytical method.

-5-

Description

Best applicable for

Accuracy

Intuition method

Evaluation of cost regarding personal knowledge and intuition.

Preliminary stage

from -30% to 50%

Comparison

Evaluation using similar parts

Preliminary stage

from -30% to 50%

Analogical method Parametric method Analytical method

Case-based evaluation, definition of Conceptual from -14% main parameters for comparison with design to 30% previous cases One or several parameters are chosen Conceptual from -14% to be critical. They are used along with design to 30% coefficients to evaluate the cost Direct and indirect costs are from -5% considered. Each cost is calculated Detailed design and then they are all summed to get to 15% the product cost

Table 1: Main cost estimation techniques [4]

b. Adapted cost entity method The cost entity method was created by Dr H’Mida [1]&[4] in the context of a PhD at ENSAM Metz. It is an evolution of the well-known ABC method. These methods are analytical methods. The cost entity method separates the company in several activities. To each activity corresponds a cost entity. Fig 5 represents how this cost entity is defined. As a result, each activity is linked to cost entity. All these entities can be regrouped in what Dr H’Mida called a Costgramme. Here is an example of a costgramme (fig 6).

Input object

Driver

Resources

COST ENTITY

Cost

Output Activity Fig 5: Cost entity model [3]

-6-

Production

M anufa cturin g prep aration Process planning Cost Entity

NCprograming Cost Entity

Quality plan Cost Entity

Scheduling Cost Entity

Logistics Cost Entity

Release Cost Entity

…

Level 2

Handling Cost Entity

Tool Change Cost Entity

Setup Cost Entity

M achining Cost Entity

Quality Controle Cost Entity

Handling Cost Entity

…

Machine M2

Machine M1 Level 1 Operation Cost Entity (1,1)

Operation Cost Entity (1,2)

Operation Cost Entity (1,3)

Operation Cost Entity (2,1)

Operation Cost Entity (2,2)

Fig 6: Costgramme model [4]

Each cost entity is calculated by using equation 1. We can note that there is homogeneity between all the activities which is one of the advantages of the method. All activities can be considered identically in terms of cost contribution form. Cost Entity = d x ∑ (α αR x IRR) Equation 1 d: Unique driver chosen for the Cost Entity αr: Resource R consumption coefficient IRr: Resource R imputation rate *

Example : EC = 2€/cm )

0.8

CE driver

*

(

2kg

*

Part mass (cons coeff)

3€/kg

Mass rate (cost rate)

+

4cm

Length associated cost

To adapt this method to conceptual design, we will simplify the formula by selecting one driver and one linked coefficient for the whole cost entity. The coefficient can be a property of a machine or a material for example. As these -7-

coefficients may be difficult to evaluate, the company may use old cost estimations to determine them. Then the used formula reduces to equation 2. Cost Entity = Driver x Imputation rate + fixed cost (€) (type) (€ / type) (€) Equation 2 Roughly speaking, this method results in using the structure of a cost entity model with a parametric method. However here the parametric method is not applied to the part but to the cost entity. c. Considered cost entities To represent all the costs we will consider in our evaluation, we use a taxonomy. This representation details what are the components of the Part Cost Entity and what they are composed of. This drawing is presented in fig 7.

Fig 7: Cost Entity taxonomy

-8-

This scheme shows that we consider both direct costs and indirect costs with the same importance. Thanks to this cost entity, we will be able to evaluate the overall cost of the product while we are just in the conceptual design stage.

Fig 8: Manufacturing cost entity detail Fig 8 exposes the detail of the manufacturing cost entity. Here are explanations on the different subclasses: - Tool entity cost: represent costs due to the tools such as dies or drill spindles. - Volume conservation: This section represents mainly casting and forging processes. These are processes without volume loss. It represents mainly primary processes in a manufacturing process. - Volume loss: it contains mainly machining operations and cutting processes. These are the secondary processes. - Small volume variation: This represents mainly final treatments that can be applied to the product. We can note that all the manufacturing processes are considered for the cost estimation. It goes from the rough material to the finished product. The following table (table 2) show examples of cost entities expressions. Yet the table is not exhaustive and some additional research has to be done to determine every cost entity expression. At the end, the part cost would be the sum of all the calculated cost entities. As it was told above, some of these coefficients may not be available in the company. But they could be calculated by considering old projects. We could consider a part whose costs are known. We would apply the cost structure shown in the taxonomy and we could perform the coefficients calculations knowing the cost result and the inductor. However, this implementation stage may be painful and time-consuming for the company.

-9-

driver

description

part volume Rough material cost formula

Launching per machine cost

Cmaterial = Ir x Vb x ρ x Cunit driver Entity complexity index

formula Csetup=Ie x A x CM driver

tolerance Quality per tolerance cost formula

depends on

ρ material density (kg/m^3) Vb rough part volume (m^3) Cunit material unit cost per kilo (€/kg)

part

Ir : filling index (