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The effect of Auxiliary Units on the Power

Consumption of CNC Machine tools at zero load cutting

Vincent A Balogun*1, Isuamfon F. Edem2, Paul T. Mativenga2

1*College of Engineering, Department of Mechanical & Mechatronics Engineering, Afe Babalola University, Ado Ekiti, Nigeria.

2School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL, United Kingdom

*Corresponding author: E-mail: balogunav@abuad.edu.ng

Abstract— Electricity consumptions have attracted global interest in recent times. This is attributable to the increasing technological advancement and new machines and materials development hence, an urgent global call for energy efficiency and sustainable manufacture. The electricity consumption in the manufacturing sector especially at the process level stages is an increasing trend. This is partly due to the energy demand of the auxiliary units and machine features incorporated into the machine tools at the design and manufacturing stages and on the other, as a result of increased production activities (increased product demand) during the use phase. This resulted in an increased embodied product energy that affects the cost and life cycle assessment of the product. In view of this economic and environmental objectives, it is paramount to investigate the energy consuming activities during machining (i.e. tip energy and zero load cutting energy) in order to optimize electricity demand at the secondary processing stages. In this work, the electrical energy demand of the auxiliary units and machine features of three different machine tools were investigated and characterized. This is required in order to encourage symbiotic and sustainable manufacture of products for resource optimization and also to determine specific areas for energy savings. It was observed that the electrical energy demand for non-cutting activities dominate the machining processes at more than 70% and the zero load cutting energy, which is machine dependent, is also about 14%. A step change in axes motor designs for CNC machine tools could facilitate energy reduction in this direction.

Index Terms— CNC machines, cutting, energy efficiency, process level, sustainable machining, tip energy, zero load cutting.

1 INTRODUCTION

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HE World Energy Outlook, WEO-2008 [1], reported that based on the trend of the global electricity consumption, especially the industrial electricity consumption, and that without any new energy policy, world primary energy de- mand will grow by 1.6% per year on average between 2006 and 2030 from 136,419.9 TWh to just over 197,826.3 TWh. This would lead to an energy demand increment of 45% between
2006 and 2030. Since carbon dioxide emission is attributable to electrical energy consumption, urgent action is required at all levels of electrical energy usage in order to cushion the impact of electrical energy consumption on the environment. The en- ergy demand for manufacturing is increasing due to the de- mand for scientific and innovative products. Manufacturing processes have been reported to be an energy intensive pro- cess and as a result, they have high environmental impact [2] that leaves behind carbon footprint. Dang et al. [3] reported that manufacturing industries consumed 37% of the world total electrical energy generated in 2006. In 2011, the Energy Information Administration (EIA) [4] reported that 42.6% of the world total electrical energy was consumed by the indus- tries. This therefore shows an increasing trend of electrical energy consumption within the manufacturing sector from
1971 to 2011. For example, in the United Kingdom UK, Digest of UK Energy Statistics’ (DUKES) [5] reported that in 2012, the
manufacturing industry consumed on average 17.9% (292
TWh) of the total energy consumption in the UK. Of this total,
machine tools and accessories (i.e. metal products, machinery
and equipment) being one of the most widely used processes
consumed on average 38 TWh. Machine tools have previously
been reported to have an efficiency of less than 30% [6] and
were also included in the European Union EU ECO-Design
directive [7] to be regulated in terms of its energy consump-
tion characteristics and efficiency. The global call has been to reduce the CO2 emission during the manufacturing processes. This will entail primarily, the optimization of the electricity consumption at the production and process level stages of
manufacturing and encouraging a zero waste manufacturing process through the six sigma approach.

1.1 Electrical energy modelling of the machine tools

The machine tools are one of the most widely used manufac- turing equipment globally [8]. Its production and consump- tion also varies especially within the developed countries. For example China accounted for approximately 33% of the world's machine tool consumption in 2013 followed by Ger- many at 9.3% and Japan at 5.3% [9]. The machine tools are equipped with varieties of components and accessories. These energy consuming components and accessories are called the

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auxiliary units and machine features. The auxiliary units and machine features are incorporated into the machine tools to reduce manufacturing errors at the process level and during the man-machine interactions. Recently, Balogun and Ma- tivenga [10] classified the machining processes into three elec- trical energy states according to their operational characteris- tics during the use phase. These include the ‘basic’ state (a state at which electrical energy is consumed at zero load cut- ting for preparatory activities), ‘ready’ state (a state at which electrical energy is consumed to move the axes x, y and z to the about to cut position) and ‘cutting’ state (a state at which electrical energy is consumed for the actual cutting opera- tions). Each of these states is controlled by the basic machine function units when the machine is switched ‘ON’ and throughout the cutting processes.
Few researchers have modelled the electrical energy and power consumption of machine tools with much emphasis at reducing the electrical energy consumption during the ma- chining processes. For example Gutowski et al., [11] and Dahmus and Gutowski [12] reported that energy consumption of machine tools during actual cutting processes accounted for
only 14.8% of the total energy consumed by the machine tools throughout the machining process of a unit and the auxiliary units consume 85.2% as shown in Figure 1. The auxiliary units’ power consumption could be a reflection of the start-up, shut down, in-cycle and other spindle positioning processes which were not accounted for in the model. More energy is being used up during start-up and to maintain the machine tool in a ‘ready state’ [13]. Gutowski et al., [11] proposed the power model as stated in Equation (1) and stated that the idle power is a constant. This overstretched hypothesis needs to be investigated and tested.
(1) Where, P represent the total power consumed in W, Po is
the idle power requirement in W, k is the specific cutting ener-
gy of the material in J/mm3 and is the material removal rate in mm3/s
It can be observed that Po could be constant for a particular machine tool and its value will definitely affect the overall energy consumptions during the production stages especially in the manual and semi-automated machine tools. More anal- yses are needed to determine the value and the contributing units of the auxiliary power consumptions that will eventually affect the total power consumption of the machine.

Fig. 1. Electrical energy used as a function of production rate for an au- tomobile production machining line adapted from Gutowski et al., [12]

Also, Vijayaraghavan and Dornfeld [14] adopted the strate- gy of event streaming the energy demand in machining to predict the electrical energy requirement during the manufac- turing process. The authors stated that event streaming strate- gy; system monitoring and data analysis software could be adopted for automated machines. These could encourage prompt manufacturing decision and effective optimization for electrical energy resource. In their analysis, the machining processes were monitored with the ‘event clouding effects’ that has the capabilities of live streaming of individual task and events. The output data shows spikes in the electricity consumption during the start-up and preparatory stages of the machine. Three machine tool states were identified as idle, low and high energy consumption stages. At these three stages, machine tools exhibit start up, shut down, idle and in-cycle stages. These stages however, have great impact on energy consumption of machine tools. Diaz, Nancy et al., [15] also reported that among the auxiliary units, the servo and the spindle consumes the most power in the basic and idle stages on a Mori Seiki NV1500DCG milling machine hence the need for assessment of the energy demand using other machine tools. Kara and Li [16] modelled energy consumption for ma- terial removal processes of a unit process with a simulated software LABView. They reported that during the mechanical machining on the Mori Seiki NL2000MC/500 machining cen- tre, a spike occurred during machine start-up but what con- tributes to the spikes during the idle stages was not recorded. An energy model was proposed as shown in Equation 2.

(2)

Where SEC represents the specific cutting energy in J/mm3,

Co and C1 are the machine tool dependant constants and MRR

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is the material removal rate in mm3/s.

In another approach, Mativenga and Rajemi [17] analysed
the energy consumption of machine tools using the optimum
cutting parameters based on minimum energy footprint. The
authors reported that the idle energy can be disintegrated into
auxiliary units and each of these units contributes to the total
electrical energy consumption. Hence, they proposed a model as shown in Equation 3.

(3)

Where, E is the total energy consumption, E1 is the energy consumed by the machine during setup operation (Idle ener- gy), E2 the cutting energy, E3 the tool change energy, E4 is the embodied energy of the tool and E5 the embodied energy of the material.

E1 which can be equated as the idle energy of the machine or zero cutting energy represents the energy consumed during

the setup and idling. This approach lumped together the zero cutting energy of the machine tools. This assumption which need further study, could aid the understanding of the energy contributing units of the idle stages. The study also reveals that a spike occurs during machine start-up. These spikes need clarification and adequate modelling.
In an attempt to categorise the energy consuming units of the machine tool, Mori et al., [18] modelled the total power consumption during the manufacturing processes with respect to time. It was reported that the electrical energy is consumed in several processes that includes; positioning and acceleration of the spindle following a tool change, actual cutting process- es, returning the spindle to the tool exchange position after cutting, and stopping the spindle at machine stop. The authors also proposed an energy model as shown in Equation 4.

(4)

where, P is the total power hour or energy in Wh, P1 (W) is the constant power consumption during the machine opera- tion regardless of the running state, T1 (h) is the cycle time dur- ing non-cutting state, T2 (h) is the cycle time during cutting state, P2 (W) is the power consumption for cutting by the spin- dle and servo motor, which fluctuates with cutting conditions, P3 (W) is the power consumption to position the work and to accelerate/decelerate the spindle to the specified speed, and T3 (h) is the time required position the work and to accelerate the spindle. The report also stated that improvement is on in reducing the idle power P1 , by machine tools designers. It is therefore clear that P1 could vary depending on machine tools and the auxiliary units.

Rajemi et al., [19] investigated the power distributions for machine tools at different cutting speeds of 300, 400 and 500 m/min respectively. The data presented in this study suggest that machine modules are major power and electrical energy consumers during the use phase. The machine module and idle power consumption recorded are within the range of 34% to 38% for idle power and 25% to 36% for machine modules in non-cutting stages. This further shows the high consumption of energy in the idle mode which in this case goes up to 74% of
the total energy demand of machining processes. It is clear from this report that non-cutting operations dominate the power consumption during the machining process. The per- centage of power consumed by the actual machining process was 31%, 35% and 39% respectively for the cutting speeds in- vestigated.
It is therefore obvious the need to clearly define the auxilia- ry unit’s power consumption of machine tools to determine the contribution of each unit towards the total power con- sumption in a machining process. This is appropriate to enable adequate data gathering for life cycle analysis and evaluation of carbon footprints of manufactured products. This study will also provide adequate information on areas of further im- provement in terms of machine tool designs and the need to reducing energy consumption throughout the product transi- tions from the concept stages to the end of life of a product. Hence, the needs for this research work.

1.2 Aim and objectives

The aim of this work is to understudy the auxiliary units and accessories of machine tools in order to evaluate the effect of such additional units on the total power demand. In view of this, three machine tools i.e. the Hitachi Seiki VG-45, Roeders RFM 760 High Speed Milling and Mikron HSM 400 Machining centre were investigated during zero cutting and cutting loads capacities. The auxiliary units of these machines were disinte- grated into various sub-units and were studied more closely to ascertain the impact of the auxiliary units in the overall power consumption of the machine. Data were presented and ana- lysed and recommendations proposed.

2 EXPERIMENTAL METHODOLOGY

The machining tests were conducted on three different ma- chine tools i.e. the Hitachi Seiki VG-45, Roeders RFM 760 High Speed Milling and Mikron HSM 400 Machining centre. For cutting tests on the Roeders RFM 760 High speed milling ma- chine, a carbide flat end mill of diameter 8 mm and four flutes was adopted. While the on Mikron HSM 400 machining cen- tre, the cutting tool engaged was a 6 mm carbide end milling tool with two flutes. As the machines were switched ‘ON’, the electrical current was measured with the Fluke 345 power clamp meter and ELITEpro SPTM Power meter with set sample rate of 1 second for consistency and repeatability of the meas- ured values. This initial electricity consumption is called the
‘basic’ electricity demand. After the basic power measure- ment, the NC codes generated with the Depocam CAD/CAM software were slightly modified as shown in Table 1 to suit the machine tool controller that is associated with the machine. A cutting test was conducted on the three machine tools. The workpiece material AISI1045 Steel alloy of 150 mm X 50 mm X
10 mm dimension was surface milled. The cutting parameters are as stated within the NC codes in Table 1. The zero load cutting tests were repeated three times on all the machine tools under investigations.

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3 RESULT AND DISCUSSIONS

The power consumption of machining operation according to CO2 PE! [20] and ISO 14955 [21] was divided into three groups including the basic, idle and cutting power. Each of these groups has an energy demand requirement on the machine tools. The basic and idle group are the power for zero cutting operations which is the scope of this study. The zero cutting operation includes the power required to switch ‘ON’ the ma- chine modules and auxiliary units (for example, computer and fans, hydraulic pump, etc.) without cutting. The basic power demand for switching ‘ON’ the machine modules are 2.55,
2.06 and 2.90 kW for Hitachi Seiki VG-45, Roeders RFM 760
High Speed Milling and Mikron HSM 400 Machining centre
respectively as tabulated in Table 3. Therefore, it can be de-
duced that the basic power demand is consumed at start up
and to maintain the operational capabilities of the machine
tool. It has been reported that at start-up, most of the auxiliary
units are powered and that the machine is brought to a point
‘just about to cut’ position (ready state) in preparation for the
actual cutting states.
Figures 2, 3 and 4 shows the power profile as measured with
the ELITEpro SPTM Power meter for Hitachi Seiki VG-45,
Roeders RFM 760 High Speed Milling and Mikron HSM 400

Machining centre respectively.

TABLE 1

SIMPLE CNC PROGRAM FOR CUTTING ON SELECTED MACHINE TOOLS

Fig. 2. Power profile for cutting on AISI 1045 steel alloy material on Hitachi Seiki VG-45 machine.

Fig. 3. Power profile for cutting AISI 1045 steel alloy material on the Roeders RFM 760 high speed milling machine.

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TABLE 3

ELECTRICAL ENERGY COMPARISM FOR THREE DIFFERENT MACHINE TOOLS

Fig. 4. Power profile for cutting of AISI 1045 steel alloy mate- rial on the Mikron HSM 400 machining centre.

It can be seen from Figures 2, 3 and 4, and as reported in literature [11], that the power consumption of the auxiliary units are comparatively higher compared to the actual cutting power consumption. This result is also a confirmation of liter- ature [12] that the share of the electrical energy for machining processes varies from 0 up to 48.1% depending on the cutting variables and machining load. The results also show an inter- esting fact, that machine auxiliary units and start-up, (the
‘basic and idle’ states) consumes the bulk of the electrical en- ergy of 80%, 80% and 69% of the total electrical energy de- mand on the Hitachi Seiki VG-45, Roeders RFM 760 High Speed Milling and Mikron HSM 400 Machining centre respec- tively as shown in Table 3. Thus switching ‘ON’ such a ma- chine tool has major impact on the electrical energy consump- tion at zero load cutting. Energy reduction in this direction could adherently create an opportunity to optimise the eco- nomic objectives of manufactured products and hence reduc- es the carbon footprint for the entire process. It further proves that the Hitachi Seiki VG-45, Roeders RFM 760 High Speed Milling and Mikron HSM 400 Machining centre should not be left in the basic and idle position for a considerable amount of time.

4 CONCLUSION

The research work investigated the effect of auxiliary units and machine features on the power demand of CNC machine tools at zero load cutting. It is shown that the auxiliary units and machine features dominate the power usage of the ma- chine tool. It is specifically clear that more electricity is con- sumed for start up and preparatory activities of the machines at switch ‘ON’. Thus switching ‘ON’ such a machine tool has major impact on the electrical energy consumption at zero load cutting. Therefore, electrical energy reduction in this di- rection would create an opportunity to optimize the economic

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objectives of manufactured products and hence reduces the carbon footprint for the entire process. This work, and in line with literature, also proves that the Hitachi Seiki VG-45, Roed- ers RFM 760 High Speed Milling and Mikron HSM 400 Ma- chining centre should not be left in the basic and idle position for a considerable amount of time else, electrical energy re- source could be wasted. Further conclusion deduced from the study is as follows:
1. Machine tool designers can improve on the design of
the auxiliary and machine features through techno- logical and innovative development to reducing pow- er consumption of the preparatory units. However more research is needed in this direction to improve power consumption of the auxiliary units.
2. A step change in the design of machine tools and oth- er manufacturing equipment will help to significantly bridge the gap in relation to energy efficiency of the machine tool at zero load cutting compared to materi- al removal processes.
3. The auxiliary and zero load cutting energy vary with
machine tools. The energy demand of the auxiliary units was 68.6%, 79.9% and 80.2% on Mikron HSM
400 machining centre, Roeders RFM 760 high speed
milling and Hitachi Seiki VG-45 machining centre re- spectively. While their corresponding zero load cut- ting energy were 3.3%, 4.6%, and 13.7% respectively. This further confirms the need for auxiliary units and machine feature design improvement with respect to electrical energy efficiency during the use phase.
4. Also a step change in the design of machine tool axes motor will facilitate energy optimization and waste reduction if properly harnessed during the machine design stages.

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