Energy efficient process chain: The impact of cutting fluid strategies

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Peer-review under responsibility of the scientifi c committee of The 22nd CIRP conference on Life Cycle Engineering doi: 10.1016/j.procir.2015.02.056

Procedia CIRP 29 ( 2015 ) 360 – 365

ScienceDirect

The 22nd CIRP conference on Life Cycle Engineering

Energy efficient process chain: The impact of cutting fluid strategies

Nadine

Madanchi

a

*, Denis Kurle

a

, Marius Winter

a

, Sebastian Thiede

a

, Christoph Herrmann

a

a_{Chair of Sustainable Manufacturing and Life Cycle Engineering, Institute of Machine Tools and Production Technology (IWF), Technische Universität}

Braunschweig, Langer Kamp 19b, 38106 Braunschweig, Germany * Corresponding author. Tel.: +49-531-391-7639; fax: +49-531-391-5842. E-mail address: n.madanchi@tu-bs.de

Abstract

Machining processes can be realized using different cutting fluid strategies – flood, dry and minimum quantity lubrication (MQL). The choice of each strategy directly influences the need of peripheral units (e.g. filter units) and additional processes within the process chain (e.g. washing). Thus, selecting an energy and resource efficient cutting fluid strategy depends not only on cutting fluid as a resource, but also on its inherent impact on the entire process chain. Therefore, this paper presents a method for decision support, considering technical boundary conditions and effects on the process chain layout as well as its application in a case study.

Peer-review under responsibility of the International Scientific Committee of the Conference “22nd CIRP conference on Life Cycle Engineering.

Keywords: Cutting Fluid; Process Chain; Energy Efficiency;

1. Introduction

Besides economic objectives, the environmental performance of manufacturing systems is getting more into the focus of companies. In this regard, the environmental impact is mainly determined by the diversity of interacting energy and resource flows (e.g. demand, emissions) within a factory [1]. Numerous research investigations can be found on improving selected flows, e.g. improvement of electricity or compressed air demand (e.g. [2]). However, an isolated consideration of selected flows neglects existing relationships and interdependencies between multiple flows which may lead to problem shifting.

One example is the use of cutting fluids in manufacturing. Cutting fluids are widely used, especially in machining process chains. As previous studies indicate there is a significant direct and indirect influence of cutting fluids on up to 50% of the total energy demand [3] (see Fig. 1). Thus, the proper choice of a cutting fluid strategy bears many advantages for improving the energy demand of process chains. The general strategies like dry/flood machining or minimal quantity lubrication are each entailing a different

impact on the process chain as well as its respective configuration.

Fig. 1. Cutting fluid induced energy demand in a machining process chain ([4] based on [2]).

Against this background, the paper provides a method to predict the effect of different cutting fluid strategies on the energy demand of process chains. Thus, it can serve as decision support when setting up process chains to improve

0 10 20 30 40 50 Ma chin e to ol Cool ant f ilter sys tem Wa shing ope ratio n Coo ling de vice Dry Cl eani ng Asse mbly Exam inati on fo r tigh tnes s Othe r Machines types S h are o f el ect ric al en ergy co nsu m pt ion ( % )

Share of electrical energy consumption Coolant induced

S har e o f elec tr ical en er gy d eman d (% ) Machines types

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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the energy demand while considering existing interdependencies.

2. Research Background

2.1. Cutting fluids – Function and strategies

Functions of cutting fluids or generally speaking cooling and lubricating media in material removal processes are to lubricate, to cool and to clean. The fulfilment of these tasks depends on the cooling and lubricating media composition and enhances the process stability.

The lubrication capability of the cooling and lubricating media is required to minimize the friction between the tribological partner (tool and workpiece) by creating a lubricating film. The creation of this film depends on the media composition. In the machining of metals especially polar or metal-active additives are needed to form the lubricating film on the workpiece surface by absorption and chemisorption processes [5]. The cooling capability of the cooling and lubricating media is important to prevent thermal induced form and accuracy deviations as well as deterioration of workpiece and tool properties. The media cooling capacity depends on its physical properties (e.g. specific heat capacity, heat conductivity and vaporization heat [6]). The cleaning capacity of cooling and lubricating media is needed to remove chips from the contact zone as well as to transport them to the filtration system. Further tasks are, for example, to support chip breaking in turning or milling and to prevent pore blocking of the grinding wheel or the honledge. The flushing capability depends on the media type, the viscosity and the specific surface tension [7].

Beside the aforementioned composition the fulfilment of the lubricating, cooling and cleaning function depends on the applied cooling and lubricating media strategy. These strategies differ in regard of the applied amount, the condition of aggregation and the type of media. With respect to the applied amount commonly flood lubrication, minimal quantity lubrication (MQL) and dry processing strategies can be distinguished. The term flood lubrication is used for a cooling and lubricating media supply greater than 2 litres per minute [8]. In case of MQL different values are reported in literature with 10 to 50 ml/hour [9] as well as 50 to 500 ml/hour [10]. In dry cutting no cooling and lubricating media is applied at all [11].

The cooling and lubricating media strategies can be distinguished by their condition of aggregation (solid, liquid or gaseous state). Combinations of aggregation states are possible as well, for example the application of gas (e.g. compressed air) as a carrier medium for solids or liquids. The solid state enfolds, for example, CO2 [12] or graphite [13]. In the liquid state commonly oils (e.g. mineral oil, vegetable oil), emulsions, dilutions and liquid gases are applied [14]. The gaseous state enfolds, for example, compressed air, Nitrogen (N2) or carbon dioxide (CO2) [14].

Based on a literature study Fig. 2 shows a classification of cooling and lubricating media regarding the application strategy and the function fulfilment while considering their condition of aggregation [9], [14], [15]. It can be seen that

there is not “the best” cooling and lubricating media. Every media has advantages and disadvantages in fulfilling the required functions, resulting in different application strategies. The reasons are the different physical and chemical properties.

Fig. 2. Classification of cutting fluids regarding the strategy application and the fulfilment function subject to their condition of aggregation.

In manufacturing processes mainly liquid cooling and lubricating media are applied. The solids and gaseous media are commonly used to enhance one specific function. They can be applied as a single media as well as in combination with a liquid media to compensate disadvantages.

The liquid cooling and lubricating media oils have commonly a very good lubricating and a good cleaning capacity, but a limited cooling capacity. Therefore, they can be applied within the flood and MQL strategy, if their specific functions are required. Water based emulsion and dilutions have a very good cooling and cleaning capacity with a comparably poor lubricating capability. Therefore, their application is most suited for situations, which require a particularly good cooling and cleaning function. Liquid gases generally show the opposite capabilities in terms of cooling and lubricating. Liquid gases and oils can be combined within the MQL strategy to enhance advantages while reducing disadvantages.

2.2. Cutting fluid relevant machinery and equipment

Besides the technological impact, the choice of strategy influences the need for additional peripheral systems as well. In order to filtrate and to supply cutting fluids, to ensure occupational health and safety as well as to clean the workpieces, peripheral equipment and separate process steps

Co ol in g L ub ric at in g C le ani ng Fl ood Dr y MQ L Condition of aggregation (cooling and lubricating

media examples) Strategy Function Solid Liquid Gaseous CO2snow Graphite Oils Emulsion / dilution Liquid gases (LN2, CO2) Compressed air N2, CO2 Degree of fulfilment

Inferior Average Superior

Degree of application

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are required. Since the flood strategy using oils, emulsions or dilutions is prevalent in manufacturing processes, many process chains need further peripheral systems like the exhaust air filtration system, the cutting fluid filtration system and the workpiece washing and cleaning system. These components are discussed further in the following.

Exhaust air filtration system

In machining processes the use of cutting fluid generates emissions consisting of oil mist and vapour. These emissions may cause significant health problems to the workers if inhaled such as respiratory problems or in extreme cases even cancer [16]. Moreover, oil emissions are easily flammable and a high concentration may lead to an explosion. For these reasons, it is not only necessary to enclose the machine tool but also to install an exhaust air system to ensure occupational health and safety [15].

From a technological perspective there are three different filter principles that can be distinguished: mechanical filtration, centrifugal separators and electrostatic separators. Exhaust air systems using electrostatic separators are quite complex, but they have the potential to achieve the highest degree of precipitation [17]. Generally, the exhaust air system can either be installed as a single-supply system for one machine or as a centralised system for multiple machines.

Cutting fluid filtration system

To fulfil the primary tasks of cutting fluids, i.e. to cool, to lubricate and to clean the contact zone, it is required to supply the necessary quantity and quality of cutting fluid. In most cases cutting fluids are contained in a circulating system to either supply an individual machine tool or multiple machines. Within these circulating systems contaminated fluid from the machine is filtrated in a filter unit and returned to the machine tool. In machining processes contamination of cutting fluids occurs due to dirt, metal fines, swarf or chips and also tramp liquids and results in a deterioration of fluid quality [17], [18]. In general, belt filters, magnetic separators, separators or hydrocyclones are used to purify the cutting fluids [15].

Workpiece cleaning system

Using cutting fluids in machining processes leads to a residual lubrication film on the workpiece surface. Especially, the viscosity of the cutting fluid and number of scooping elements (e.g. blind holes, tapped holes) has an influence on the residual fluid [19]. To avoid contamination of the used cutting fluid in a process chain with tramp fluids, it is required to clean the workpiece before the next process step [18]. Thus depending on the used cutting fluid cleaning systems are either installed between machining operations, between different processes (e.g. machining and hardening) or at the end of the manufacturing process. Similar to the machine tools a recycling system supplies the washing system with clean washing fluid. However, also in case of a dry strategy a clean workpiece without residual chips or swarf is required and usually achieved with the use of surface air cleaner.

3. Method

3.1. Impact of cutting fluid strategies on the process chain The previous sections showed that there are several strategies to apply cutting fluids in machining processes. However, it was pointed out that the major focus is on strategies using oils, emulsions or dilutions, as they are mainly used in manufacturing processes. Moreover, it was shown that the choice of cutting fluids entails an economical as well as environmental impact on the process chain. Based on a literature review and the results shown in section 2.2, Fig. 3 presents a classification of the strategies regarding the need for peripheral systems and additional cleaning processes.

This overview considers the conventional flood strategy as well as a flood strategy with multi-functional oils (MFO), because the latter bears an additional impact on the cleaning process. As there is no risk of contamination with tramp fluids, there is no need to wash the workpiece between different machining processes. With respect to dry strategies there is also a distinction between completely dry machining and dry machining involving cutting fluids to remove chips and metal fines from the machine tool. In contrast, the latter requires the use of a filter system.

Fig. 3: Classification of cutting fluid strategies regarding the requirement for additional machinery and equipment

Fl oo d Fl oo d (M ul ti -fu nc ti on al o il) MQ L D ry ( flu id s to rem ove chi ps) Dr y decentralised centralised Exhaust Air System

decentralised centralised washing fluid compressed air washing fluid compressed air washing fluid compressed air

Low Average High

Cleaning (end of the process chain) Degree of necessity Cutting fluid relevant machinery and equipment Strategy Filter System Cleaning (between machining processes) Cleaning (between different processes)

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Regarding the cleaning steps, the workpiece can either be cleaned between machining processes to avoid contamination, between different processes such as machining and hardening processes or at the end of the manufacturing process for corrosion resistance. Moreover, there is a general distinction between washing with washing fluid and cleaning with compressed air as it is usually used to remove chips and dirt for dry machining. To reduce the complexity no further distinction between centralised and decentralised systems or between one-piece and batch cleaning systems is considered. 3.2. Relations within a process chain

The application of cutting fluids has an impact on the type, number and size of peripheral systems and additional process steps. The relations between them and the machine tools as well as the relevant material and energy flows in terms of cutting fluid are shown in Fig. 4. However, to decrease the complexity of this study only the energy and compressed air flows are considered in the following.

Fig. 4: Relations of machinery and equipment as well as material and energy flows within a process chain using cutting fluids

According to the previous results the exhaust air filtration system and the cutting fluid filtration system are assigned as peripheral equipment to the machine tool. Regarding the cutting fluid filtration system a circulating system is shown. In this system contaminated fluid from the machine is filtered and then returned clean to the machine tool. Additionally, the generated oil emissions are sucked off and purified through the exhaust air filtration system. As an important process step related to the use of cutting fluid the workpiece cleaning system is integrated as well. Similar to the cutting fluid the washing fluid is also being recycled.

As described there is a general distinction between single-supply and centralised circulating systems. Fig. 4 presents both alternatives where the machine tool and cleaning system can either be considered as single machines or as multiple machines. In both cases the supply through the peripheral equipment and the circulating systems remains the same.

3.3. Modelling

To predict the energy demand of process chains related to cutting fluid strategies it is required to merge the previous results. Fig. 3 presents the required peripheral systems and machinery whereas Fig. 4 shows their material and energy flows. These flows focus on cutting fluid relevant flows where the choice of strategy has a direct impact. They emphasise further on the energy flows in terms of compressed air and electrical energy.

Thus, the derived model considers the relations between cutting fluid strategies and possible configurations of process chains. Additionally, the model includes averaged energy demands for each system component to calculate the general energy demand depending on the configuration of the process chain. The calculation of the averaged energy and compressed air demand is based on measured energy load profiles and compressed air volume flows of typical production machines and equipment. The conversion of compressed air flows to energy demand is calculated based on an empirical factor for the respective compressed air network. This factor may deviate from system to system depending on the required pressure level of the compressed air network. This model is implemented in a software tool, which provides a fast overview of the effects of the cutting fluid strategy on the energy demand of process chains.

4. Case Study

A short case study shall demonstrate the applicability and benefits of the proposed model for choosing appropriate cutting fluid strategies. It further indicates the need of additional peripheral systems within process chains. The considered process chains slightly differ from each other in terms of their peripheral units due to technological constraints. The examined part in this case study is a ball hub as an integral part of universal shafts commonly used, for example in the automotive industry. The manufacturing of ball hubs can be realized by setting up a flood or a dry process chain. By applying the key ideas of the proposed method, the comparison reveals interesting trade-offs in terms of energy demand.

Both process chains require seven process steps (turning, gearing, raceway, hardening, tempering, raceways/deburring and washing) for the transformation of the initial workpiece to the finished ball hub. The seven general process steps as well as the realization as a flood and dry process chain respectively are shown in Fig. 5. In addition to the general setup of the process chains, the electrical power demand and converted electrical power demand for compressed air flow rates are visualized as Sankey diagrams.

Both process chains are capable of producing the same ball hub quality. The major difference between both process chains consists in the selected peripheral and auxiliary systems. Those major differences are further marked by blue and red frames around the respective processes in Fig. 5.

Regarding the first process step, i.e. turning, the dry process chain uses a dry turning process which requires no cutting fluid filter.

not considered Cleaning of Workpiece Exhaust Air Filtration System Cutting Fluid Filtration System parts parts parts co m pr es sed air el ec tr ic al en er gy co m pr es sed ai r elec tr ic al en er gy Recycling System considered ne w fl ui d cu tti ng f lu id air em is sio n cu tti ng fl ui d Machining elec tr ic al en er gy ch ip s w as hi ng f lui d pur ifi ed ai r ai r e m is si on circulating system cu ttin g flu id + c hi ps cu tti ng f lu id w as hi ng f lui d w as hi ng f lui d elec tr ic al en er gy ne w wa sh in g fl ui d el ec tr ica l en er gy

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Fig. 5: Structural setup of flood and dry process chain

However, it requires a surface air cleaner which entails an increased demand of compressed air for blowing off chips instead of dispatching them with a cutting fluid. Similar trade-offs can be observed with respect to the two following processes, namely broaching and soft-milling. The flood process chain uses a central cutting fluid filtration system, a conveyer belt workpiece washing systems and its associated washing water recycling units, whereas the dry process chain employs two surface air cleaners instead. However, the peripheral systems certainly require further power and resources for the production system due to the treatment of wastewater and waste cutting fluid in the flood process chain and an increased tool and subsequent conveyor belt wear because of residual metal chips as well as occurring leakages in the compressed air system of the dry process chain. For both strategies the use of compressed air for the machine tools and the actual process energy are neglected as well. Thus, it is indispensable to use more power and resources. However, the proposed method helps to give an overview on what system and what part of the value creation process energy is spent. The calculation of power and compressed air demand of the production machines and peripheral units are each based on averaged measured values.

Fig. 6 shows – based on the flood and dry process chain presented in Fig. 5 – the power demand of four different cutting fluid scenarios. The application of no fluid within a dry process (scenario (1)), the application of different cutting fluids within a decentralised flood lubrication (scenario (2)), a centralised flood lubrication with different cutting fluids (scenario (3)) and the application of a multi-functional oil (MFO) from a centralised filtration unit (scenario (4)).

The application of scenario (1) leads to the highest power demand followed by scenarios (2), (3) and (4). The figure indicates a high electrical power demand for the peripheral

systems in relationship to the overall power demand. The peripheral system of scenario (1) has an electrical power demand of 15 %, scenario (2) 35 %, scenario (3) 32 % and scenario (4) 31 %. However, scenario (1) exhibits a higher share of compressed air demand and therefore also a higher energy consumption (34 %) resulting from the compressed air demand of the surface air cleaner compared to circa 6 % of the flood lubrication strategies. The remaining power demand is due to the manufacturing process.

With respect to the flood process chain, it can be further deducted that centralised filtration shows a slightly lower energy demand due to a more efficient centralised filtration unit which functioning is yet subject to general allocation. Whereas decentralised filtration units might be more applicable for individual cases requiring a higher degree of flexibility and independency of general supply units.

Fig. 6: Share of energy demand for two alternative process chain, further distinguished between centralised and decentralised filtration

0 50,000 100,000 150,000 200,000 250,000 300,000 Dry Different Cutting fluids, Flood, decentral Different Cutting fluids, Flood, central Multi-function oil, Flood, decentral P o w er de m a nd in [ W ]

Machining, Electrical power Peripheral system, Compressed air Peripheral system, Electrical power

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This higher degree of flexibility comes with an increase in energy costs of circa 5.2 % (12 kW) of the total energy demand of the flood process chain. This power demand can be further reduced by the application of a MFO, which allows the omission of the washing process for the cleaning of parts between the broaching and soft milling processes. In total the flood process chain implies a power demand of 224.52 kW for strategy (4), 228.12 kW for strategy (3), 240.12 kW for strategy (2) and 271.36 kW for strategy (1). This gap basically results from the additional power requirements for the compressed air flow rate of the three surface air cleaner (180.8 Nm³/h per unit). Regarding the process power demand there is only a minor difference of 5 kW due to the comparison of the cumulated power demand of the two washing processes for degreasing slightly exceeding the electrical energy demand of the three surface air cleaner.

The identification and evaluation of the aforementioned trade-offs can directly be derived from the proposed method. In addition to that, an enhanced use of MQL could further foster the omission of central energy intensive cutting fluid filter units while entailing only marginally increased use of compressed air.

5. Conclusion and Outlook

This paper examines the effect of different cutting fluid strategies on the energy demand of process chains. Based on a theoretical description, it presents the variety of different strategies and their specific requirements for peripheral systems and additional process steps. It shows the significant influence of the chosen cutting fluid strategy on the process chains’ energy demand. Modelling the requirements of the selected cutting fluid strategies based on the best practice energy demand of relevant process elements (e.g. cutting fluid filtration system, cleaning systems) it is possible to predict the general energy demand of process chains. Using this approach, a case study presents the realization of one manufacturing process as a flood and dry process chain and analyzes the differences in terms of energy demand. This case study reveals trade-offs between the use of compressed air and washing fluid to clean the workpiece from chips.

In order to extend this approach in future studies the influence of different cutting fluids on the technological process result should be considered as well as a possible optimisation of the filtration systems. Furthermore, there should be a stronger focus on the material and resource flows to extend the calculation, for example, by wastewater and waste cutting fluid. The approach may also include further components along the process chain that are influenced by the choice of cutting fluid strategies. To improve accuracy and quality of the developed model more data for the energy demand of system components should be integrated as well. In the future, the model should have access to a database with a variety of components along with their technical details (e.g. energy demand, volume flow).

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