COST EVALUATION FOR WELDING PROCESS BY USING PREPARING FEATURES

INCOM'2006: 12th IFAC/IFIP/IFORS/IEEE/IMS Symposium Information Control Problems in Manufacturing May 17-19 2006, Saint-Etienne, France

COST EVALUATION FOR WELDING PROCESS BY USING PREPARING FEATURES Slah Chayoukhi, Zoubeir Bouaziz, Ali Zghal Unit of Mechanics, Solids, Structures and Technological Development, Ecole Supérieure des Sciences et Techniques de Tunis, B.P. 56. Bab Menara 1008 Tunis Tunisie

Abstract: This paper presents an approach for costing of weld preparation. The approach introduces the concept of preparing feature for the preparation of the welding joint edges. A generation of manufacturing process method is detailed. This method use production rules and respects the geometrical and technological parameters. A cost function uses an analytical method elaborated to formulate the time for weld preparation according to the different parameters of preparing feature. The estimating is based upon an evaluation of machine implementation, labour and consumable costs. At last, and to show the developed method interested and the efficiency of the model, we have treated an example of stiffened plate. Copyright © 2006 IFAC Keywords: Fabrication costs, Structural optimization, manufacturing feature, welded structures, material costs, labour costs, power costs.

1. INTRODUCTION

2001; Jong-Yun Jung, 2002) in particular in the estimate of machining cost of complex pockets of moulds (Bouaziz, et al., 2002), cylindrical parts (Seddiki, et al, 1995) and machined parts (Jung, 2002).

Costs’ studying is one of the basic imperatives of any decision making for enterprises working on request. In fact, the accuracy and the rapidity of cost estimation govern the transition to the effective order by the customer and help the enterprise to develop.

In the field of welding, the cost estimating tools are less numerous and have limitations as regards the welding preparation process. To remedy to this limitation, we elaborate, in this paper, a cost estimating tool based on the new concept of preparing feature.

At present, several data processing tools have been developed to evaluate the time and the manufacturing cost of mechanical parts, such as analogical method applied at machined parts (Duverlie, 1999), parametric method (Jimenez, 1995), etc.

As a first step, we detail the principle of the system which has been developed. Then we outline the modelling of the welding joints by the concept of preparing feature. After mentioning the means of manufacturing most used, we develop the generation procedure of manufacturing ranges of the welding joint.

The current evaluation of costs prediction methods consist in the use of the concept of feature. This concept aims at modelling expert knowledge, while taking into account geometrical and technological parameters of the feature. It consists in integrating simultaneously the product geometrical and technological characteristics which will be preserved during the whole manufacturing process.

In the second step, we introduce an analytical model for calculation of the preparation time and cost for each type of the features considered. To optimise the preparing range, we develop a data-processing tool to choose the range at minimum cost.

Modelling by the concept of feature is largely approached in various research tasks (Nagahanumaiah, et al., 2005; Jung-Hyun, et al.,

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Finally, we validate the procedure suggested by applying it in the welding preparation of a stiffened plate.

In fact the preparing feature is described by two groups of data (table 1). The first group contain the description of the form. The second group is the description of the associated manufacturing process. Consequently, modelling by preparing feature ensures the integration of the functions of design and manufacture by the association of a manufacturing process to the geometrical form.

2. COST ESTIMATION PROCEDURE The method of calculation is presented by the algorithm of figure 1. It consists of the following steps: - Decomposition of the structure in assemblages; - Establishment of the inventory of all the possible preparing features present in each assemblage; - Choice of preparing feature; - Cost prediction for different manufacturing process for each preparing feature; - Choice of the process corresponding to minimum cost; - Treatment of the following feature; - Calculation of minimal cost per assemblage thereof minimal cost of the structure;

The analysis of the forms and the specifications of different assemblies made within the framework of the model of feature construction led us to set up seven preparation features (table 2). It corresponding to the welding joints largely used in the various types of assemblies. Table 1: Data relating to the preparing feature Geometrical and technological specifications Material Type of material

Data structure

…… A1 Assemblage:

……..

Preparing feature: PFi1

Assemblage: An

Assemblage: Ai

…..

D. B Preparing features

Preparing feature: Preparing feature: PFij PFim

i = i+1

Range 1 J = j+1

i

Cost C j1

Range k

………

Range z i

………

Cost C jk

Cost C jz

No

Minimum preparation cost of the structure: n

C = ∑ C Ai i =1

Yes

Form of profile

I, X, V, U, ½ V, K, J

Groove angle

α (°)

Heel or height of the flat part

h (mm)

Volume of the removed matter

V (mm3)

Length of the feature

L (mm)

g (mm) Symmetrical, dissymmetrical, equal, unequal Manufacturing process Manufacturing Oxycutting, machining, mode forming, sawing, grinding, etc.

j
Minimum preparation cost of the assemblage (Ai) = (CAi)

t (mm)

Gap Topology

Optimal range of i the PF j

Inf Cijk Yes

Machines, constraints i

Thickness

Manufacturing ranges

i
No End

Fig. 1. Cost estimating

Tools

3. PREPARING FEATURE

Sequence of the operations of manufacture and definition of the means used. The list of tools for each manufacturing process.

4. PREPARING RANGES

With an aim of modelling knowledge in the preparing operations of the welding joints, we work out a feature oriented by manufacture called "preparing feature". Consequently, we define the geometrical and technological data relating to each feature to be associated with a manufacturing process. The preparing feature comprises on the one hand, the preparation of the edges. It consists in giving to edges a suitable geometrical contour and to eliminate any foreign body present on the surface which can affect the quality of the welding. On the other hand we realize the setting up of the edges (gap, alignment etc). A manufacturing process is associated with this geometry which expresses all that is necessary to the realization of the preparing feature.

4.1 Means used for manufacture of the preparing feature The production of preparing features approved difficulties, in particular the thick parts. They present problems of the heating effects, powers of the machines and automation. Thus, the knowledge of the resources proves to be essential, because on the one hand the means used influence the manufacturing cost and on the other hand it will be able to select with precision the manufacturing processes and to compare the performances for a given situation. For this work several means are used and are gathered in two modes: 814

- Thermal preparation (oxycutting, cut plasma, laser cut and electric arc) depends on several factors such as the type of material, the thickness of the part and the manufacturing cost. This mode can be modify metallurgical characteristics of metal.

Table 3: Preparing Ranges Preparing Range 1

Preparing range 2

Phase 10: Thermal cutting of part 1.

Phase 10: End milling

α

Table 2: Preparing features of our model

mill of 2 sizes

t = 2-8 mm h = e/2

part 1 Preparing feature in I α

part 1

Fall

t = 4-15 mm h =0-2 mm

α=40-90°

- Using a blowtorch - Plane chamfer of two stops of part 1. Phase 20: Thermal cutting of part 1

g = 1-2 mm Preparing feature in V α

t =10-40 mm h = 2-3 mm

Preparing feature in X α

α=40-60° g = 2-3 mm h =2-3 mm α=10-20°

Preparing feature in U α

- Using a blowtorch - Perpendicular cut: realization of the flat part. Phase 30: Edge surfaces grinding.

g = 2-3 mm t =10-20 mm h =2-3 mm α =10-20° g = 2-3 mm

Preparing feature in J α

part 2

t =4-15 mm

Using a mill of 02 sizes. - Plane chamfer of second stops of part 1 Phase 30: - Perpendicular cuts of an edge of part 1 to realize the flat part. - Using a mill disc.

part 1

α=40-60° g = 0-2 mm t =10-40 mm

Phase 40: Alignment and positioning of two parts.

h =0-3mm α=40-60° Preparing feature in K

mill of 2 sizes

part 1

h =0-2 mm

Preparing feature in ½V α

Phase 20: End milling

part 1

part 1

t =10-20 mm

- Using a mill of 02 sizes. - Plane chamfer of one stops of part 1.

part 2

g = 0-3 mm

Phase 40: Alignment and positioning of two parts.

part 1

part 2

- Mechanical preparation (machining, shearing, sawing, nibbling, grinding and forming). The mechanical cutting of sheets is generally carried out by shearing or nibbling, whereas the cutting of profiled and the tubes calls upon the techniques of sawing or slicing. For the material of high quality and for certain processes of welding such as the electron beam, the production of the preparation features is ensured by machining with an aim of giving a surface quality and regular chamfers. All the preparing ranges implemented industrially use these two modes, according to the desired quality and the availability of machines.

part 1

4.2 Generation of the preparing range The preparing range describes the complete manufacture process of a preparing feature which gathers the operations of metal cutting, polishing and cleaning of edges surfaces. It also defines the means 815

preparing feature, t [mm] is the thickness of the sheet, L [mm] is the cutting length, ηr is a coefficient relating to the blending radius. However the equation 4 does not take into account the cutting speed reduction. This can be expressed by a factor Kr estimated at 1.0 for perpendicular cuts [ α = 0° ], 1.25 for the cuts in angle [ α = 30° ] and 1.45 for the cuts in angle [ α = 45° ]. Thus, the oxycutting time can be formulated as:

used and the cutting parameters. In the generation of the manufacturing process we taking into account the limitations of the machines, the technological and morphological constraints (thickness of sheet, bevel angle, cutting speed, etc). For each preparing feature, we associate several manufacturing possible processes where the choice rests on the manufacturing time and cost criterion. Table 3 present an example of two possible preparing ranges of feature type K. the range 1 is carried out by two cutting modes. The first mode is the oxycutting of the plane chamfers and flat part. The second mode is the grinding of edges surfaces. The range 2 is carried out by machining, where we use a mill of two sizes to realize the plane chamfers (phases 10 and 20) and a mill disc for perpendicular cuts to realize the flat part (phase 30). The alignment consists in presentation of parts according to necessary geometry to make successful welding.

t O = C.t η r .L.K r

(5)

5.4 Grinding Time To carry out a good welding it is necessary that the edges of the parts to be welded should be smooth, uniform, and free of the cracks and without rust. Consequently the preparation by oxycutting is generally followed by an operation of grinding. The edge grinding time can be expressed by the formula:

5. THE COST FUNCTION

tG =

The cost function can be evaluated by using the next expression: C F = C M + C ML + C C (1)

LG Fl

(6)

Where LG is the grinding length [mm] and Fl is the grinding linear flow [mm/min]. For steel the grinding linear flow is given by the values of: 750, 500, 375 and 300 [mm/min] suggested respectively for the thicknesses of 10, 20, 30 and 40 mm.

Where CM, CML and CC represent the cost of machine implementation, the labour cost and the consumable cost (combustion gas, cutting tools and gas of cut).

5.5 Labour cost. 5.1 Machine implementation cost

The labour cost CML [$] can be estimated by using the following formula: C L = C HL .t l (7)

The implementation cost of the machine CM [$] can be calculated by using the following expression: C M = C H .t c (2)

Where tl [h] is the labour time and CHL [$/h] is the labour hourly cost.

Where tc [h] is the cutting time (machining, oxycutting or grinding time) and CH [$/h] the hourly cost of the machine (CHM for machining and CHO for oxycutting).

5.6 Labour time The labour time presented in this paper is given for skilled workers and it gathers cutting time, unproductive time and handling time. The unproductive time (e.g. set up times, teardown times and down times) may amount to as much as 30% of all the total production time. The handling time th [h] of fixing and centering a workpiece for weight ranges between 0–25, 26–40, 41–70 and 71–100 kg, is estimated at 2.50×10-2, 5.00×10-2, 5.83×10-2 and 6.66×10-2 h respectively in compliance with (Klansek and Karavanja, 2005). Labour time is then can be determined by using the following term: t l = 1.3(t c + t h ) (8)

5.2 Machining Time The machining time tM [h] is calculated starting from the volume of the matter to remove V [mm3] and volume flow of chip Dv [mm3/min].

⎡ V ⎤ tM = ⎢ ⎥ ⎣ 60.DV ⎦

(3)

5.3 Oxycutting Time The oxycutting time depends on the thickness of sheet, the nature of combustion gas and length of cut. It can be expressed by the equation given by Jermai (Jermai and Farkas, 1999):

t O = C.t η r .L

5.7 cost of cutting tools The consumption cost of cutting tool can be expressed by the formula:

(4)

Where "C" is the factor of difficulty relating to each

816

C cto =

η η −1

.C HM .t M

ranges using machining, oxycutting and grinding manufacturing modes.

(9)

Where CHM is the hourly cost of the machine tool and η is the coefficient of Taylor (η=1.25 for the hard steel, η=0.25 for carbide and η=0.5 for ceramics).

6.1 Preparing range n°1 (table 3) This range is carried out by the following operations: - Tilted thermal cutting of the stiffener:

5.8 Consumption cost of gases

t O1 =

The consumption cost in oxycutting includes the cost of the natural gas CnG [$] (acetylene, propane, etc.) and the total cost of oxygen CO [$] (oxygen uptake of heating and cutting). These costs are defined as follows: C nG = C mnG .D nG .t O (10)

C O = C mO .D O .t O

K r .C.L.t η r ( h) 60

- Perpendicular cuts:

tO2

C.L.hη r = ( h) 60

- Edge Grinding, the grinding length is LG = 3.L

tG =

(11)

3.L (h) 60.Fl

- The time of alignment is considered in handling time.

Where CmnG and CmO [$/m3] are respectively the costs of cubic meter of natural gas and oxygen, tO is the oxycutting time, DnG [m3/h] is the natural gas consumption and DO [m3/h] is the total consumption of oxygen. These consumptions depend on the thickness of the sheet t [mm] and they are expressed by the following formulas (Klansek and Karavanja, 2005): DnG=−8.68×10−7t4+1.1×10−4t3−4.93×10−3t2 +9.19× 10−2t+ 4.12×10−1 (12) DO=1.43×10−7t6−1.83×10−5t5+8.88×10−4t4 −2×10−2t3+2.06×10−1t2−6.37×10−1t+2.21 (13)

(

)

K r .C.L. η r t + hη r ( h ) 60 t l = 1,3.(t O + t G + t h ) tO =

C M = t O .C HO C ML = t l .C HL C C = t O .(C mnG .D nG + C mO .D O ) + t G .C p .

Pgm

η gm

.k am

C F = C M + C ML + C C 5.9 Energy costs for edges grinding The energy cost for edges grinding can be calculated by employing the following formula:

C CG = C p .

Pgm

η gm

.k am .t G

(14)

Where CP [$/kWh] represents the electric price of power, Pgm [kw] is the power of the machine of grinding, ηgm is the effectiveness of power of grinding machine (the value suggested of ηgm is 0,85), Kam is the factor which considers the allowances at the machine time (the typical value of Kam is 1,09) and tG is the grinding time of the edges.

Fig.2. Stiffened plate 6.2 Preparing range n°2 (table 3) This range is carried out by: - Milling of two plane chamfers: t M 1 =

6. STUDY CASE

V1 ( h) . 60.DV

It is necessary thus to calculate the volume of the matter to be removed, starting from a geometrical modelling of the preparing feature type K (figure 3):

In order to interpret the proposed approach, this paper presents a simple numerical example illustrating the optimization of welding preparation for a stiffened plate shown in figure 2. The problem is to minimise the cost considering machining and thermal cutting. The present structure is built up of a plate onto which are assembly three stiffeners by fillet welds. The joints are modelled by three identical preparing features type K. Therefore it is enough to calculate the cost of one preparing feature to know the total cost of the structure preparation. We seek and compare the manufacturing cost of two preparing

V1

2 ( t − h) = L. tan(α )

4

- Perpendicular cuts using a mill disc to realize the

V2 ( h) 60.DV With V2 = h.ct .L and Ct is the thickness of the flat part: t M 2 =

chips.

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2 L.(t − h ) . tan(α ) h.ct .L (h ) + 120.DV 60.DV t l = 1,3.(t M + t h ) C M = t M .(C HM ) C ML = t l .(C HL )

carried out. In this case, manufacturing cost is calculated as a summation of machine implementation, labour and consumable costs.

tM =

The model is applied to the preparing features. It calculates the welding preparation cost and time for each feature of a given assemblage. This model can constitute an aided decision tool to optimize preparing range by the experts according to a minimum cost criteria. It is namely therefore possible for him to make simulations.

⎛ η ⎞ ⎟ C C = t M .⎜⎜ C HM . η − 1 ⎟⎠ ⎝ C F = C M + C ML + C C α

The association of a manufacturing process at the geometrical form in the preparing feature integrate the functions of design and manufacture. This made it possible to respect the technical criteria of execution to reach the level of the desired precision.

plate stiffener

Finally, good results are obtained by this tool thanks to its adaptability to any welding enterprise which uses its own data and parameters.

Fig.3. Preparing feature type K 6.3 Numerical application

REFERENCES

- C=0,965.10-3 , ηr=0,25, η=1,25, α= 45°, Kr=1,45, L=4000mm, t=10mm, m=50kg, th=5,83.10-2 h, ηgm=0,85, Kam=1,09, CP=0,2 $/kwh, Pgm=1,10 kw, Ct=10 mm, h=2mm. - CHM=30 $/h, CHO=5 $/h, CHL=10 $/h, CHE=0,1 $/ kWh, CmnG=0,56 $/m3, CmO=1,6 $/m3, - DnG=0,94 m3/h, DO=1,4 m3/h, Fl=300 mm/min, DV =36*103 mm3/min.

Bouaziz Z., Younes J. and Zghal A. (2002). A Fast and Reliable Tool for Estimates for Plastic Blowing Moulds, International Journal of Advanced Manufacturing Technology, N°20, pp. 545-550. Duverlie P. (1999). Méthode analogique appliquée aux pièces usinées. Journal Travail et Méthodes ; N°553, pp. 20-24. Jarmai K. and J. Farkas (1999). Cost calculation and optimisation of welded steel structures, Journal of Constructional Steel Research 50, 115–135 Jimenez A. (1995). Méthode paramétriquestatistique: application aux pièces usinées, Journal Travail et Méthodes N° 522 ; pp. 1925.Klansek U. and S. Karavanja (2005). Cost estimation, optimization and competitiveness of different composite floor systems—Part 1: Selfmanufacturing cost estimation of composite and steel structures. Journal of Constructional Steel Research 19 August 2005. Jong-Yun J.(2002). Manufacturing cost estimation for machined parts based on manufacturing features, Journal of intelligent manufacturing, 13, 227238. Jung Hyun H., M. Kang, H. Choi (2001). STEP-based feature recognition for manufacturing cost optimization, Computer-Aided Design 33, 671686. Nagahanumaiah, B. R., N.P. Mukherjee (2005). An integrated framework for die and mold cost estimation using design features and tooling parameters, Internaional Journal of Advanced Manufacturing Technology, 1138–1149. SEDDIKI, A., A. MOISAN, G. LEVAILLANT (1995). Proposition d’un Système d’Assistance à l’Elaboration de Devis d’Usinage basé sur le concept d’entité (SAEDU), Journal Mécanique Industrielle et Matériaux , Vol. 48, N°5.

Table 4: Distribution of the preparation costs

tc (h) tl (h) CM ($) CML ($) CC ($) CF ($) Preparation cost of the structure ($)

Range 1 (oxycutting) 0,272 0,507 2,537 5,07 0,769 8,376 25,128

Range 2 (machining) 0,056 0,148 1,68 1,48 8,4 11,56 34,68

Table 4 presents the distribution of all the costs of preparation and illustrate that the manufacturing cost depends on the cutting modes (machining, grinding, oxycutting, etc.), manufacturing process and the used means. The cost comparison shows that the cost of the thermal cutting is much lower than that of the machining. But, machining is used to obtain high quality and avoiding metallurgical modification which can be caused by thermal cutting. 7. CONCLUSION In this paper, we have given a general idea about the model of welding preparation cost which we have

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