LEAN MANUFACTURING PROCESS PLANNING FOR 5 AXES CNC DRIVEN MILLING MACHINE

The aim of this publication is to determine the OEE (Overall Equipment Efficiency) indicator for 5 axes milling machine found at Diehl Aircabin Hungary Ltd. for the present and future state. Based on this value, the utilization of the machine for the given production amount can be calculated. With the optimal choice of the right production parameters (the number of cuts, feeding, depth of cut, etc.) greater productivity can be achieved i.e. the machine main time (time of cutting) will be less. The possibilities of the reduction of the machine time will be analysed and calculated. Setting of the appropriate technological parameters the machine main time could be decreased. The calculation of the machine main time will be determined for the most frequent manufacturing technologies.


Introduction
LEAN is an organizational company conducting system which aims to produce its products and services by the most economical way. A LEAN company targets its activities on the customers' needs, and what they perceive as valuable. Those things which are not of value for the customer, and for which they are not willing to pay, LEAN system takes as loss, and increases the efficiency of the processes by ceasing these losses or reducing them to the maximum [4], [5]. a) Airbus A320 type Single Aisle b) A330/340 type Long Range    The milling machine has 2 work tables. Because of this, manufacturing activity and the fixing of the workpiece can be paralleled ( Figure 2) [1]. If we would like to use more than one tool, the automatic tool change system can change tool and it is possible to fix 12 tools in the tool storage system. ( Figure  3) [1], [2].

The calculation of OEE indicator
The OEE (Overall Equipment Efficiency) value shows percentage of the amount of the products are produced, of which the machine is being capable under optimal circumstances [4], [5] (Figure 4). OEE indicator is formed by 3 factors [5]:  The indicator of availability (A) refers to those time losses when the equipment could produce but due to some reasons it does not work. The losses that influence this factor: malfunction, change over time, and the time of tool change.  Performance indicator (P) includes those losses when the machine is operating but it is not producing at all or not producing with the appropriate amount. The sources of losses, which influence this factor: micro shut downs, loss of speed.  Quality indicator (Q): the equipment is running, producing but the product cannot be used because its quality is not appropriate. The sources of losses, which influence this factor: quality loss, start up loss. Sidewalls, doors and doorframes are the products produced by this machine [1]. For calculation the following data are needed:  number of produced pieces;  time schedule;  number of reworked workpieces;  number of scraps;  production time;  unplanned shutdowns.
The total amount is the number of the total produced pieces in the examined period. The steps of calculation of OEE indicator are the following [4], [5]: Total amount = number of produced pieces + number of reworked pieces+ + number of scraps (1)

Quality indicator (Q) = Number of produced pieces / Total amount
For 2015 (current state of affairs) based on data available and using the formulas in (1) -(5) the value of OEE indicator has been calculated, which equals 47%. So i.e. MAKA type CNC milling machine produces 47% of its number of products, of which it would be capable in optimal case [1].
For 2016 (future state) at Diehl Aircabin Hungary Ltd., customers' needs, number of produced products and their complexity are continuously growing. Based on data available and using the formulas in (1) -(5) the value of OEE indicator has been calculated, which is 159.3 %. The calculated OEE indicator is above 100%, which means if there is that high production volume, one MAKA type CNC milling machine will not be sufficient for production. Thus, for this reason 2 machines will be indispensable.

Making a computer program for OEE indicator calculator
For efficient calculation of OEE indicator, we have made a computer program in MATLAB developer environment. The aim was to ease the work of the company as with the help of the program OEE indicators can be calculated no only annually but also monthly. Input data of the program are the following [1], which should be provided:  [7], [8], [9], [10], [11]: Defining the duration of the operation elements [3], [7], [8], [9], [10], [11]:

Determination of the machine main time in case of turning
The turning is continuous cutting with circle shaped chip and arbitrary feed motion which is perpendicular for the cutting direction. Its tool is lathe machine (Figure 7). The isolated chip is usually constant sectional. The cutting of this is occurring continuously [3], [8], [12], [13], [14]. The specific cutting force is written for the three force components (Figure 7) [3], [7], [12], [13]: The territory of the chip section ( Figure 7) [3], [7], [12], [13]: where (10) The formula of the cutting power is (12) In Figure 8 a thread cutting manufacturing process could be an example. The numbers (1 -6) are nominated as the manufacturing steps. The aim is to calculate the total machine time. The machine times of every manufacturing steps must be added.  The total machine time is (8) The time of the thread cutting manufacturing step depends on the thread cutting strategy (Figure 9). a) radial or symmetrical b) one sided c) two sided or edge changing Fig. 9 Designing of cutting processes in case of thread cutting technology [14] In case of radial or symmetrical thread cutting process (Figure 9.a) the two cutting edges of the cutting tool are loaded equally. The machine main time will be long. In case of one sided thread cutting process (Figure 9.b) only one cutting edge is loaded that is why this cutting edge will be wearing. The machine main time will be lower than the radial or symmetrical process.
In case of two-sided thread cutting process (Figure 9.c) two cutting edges are loaded. The direction of the cutting tool always changes.

Calculation of the machine main time for drilling
During drilling or counterbore technology interior cylindrical surfaces or other interior formed surfaces (cone -shaped, threaded, etc.) are manufactured. The cutting motion is rotation motion which are done by the tool or workpiece [3], [7], [8], [9], [12], [13], [14]. The feed motion is also done by the tool or workpiece. The depth of cut is determined by the dimension of the cutting tool. The technological figures of drilling and counterbore technologies could be seen in Figure 10 [3], [7], [8], [9], [12], [13], [14]. a) drilling technology b) counterbore technology Fig. 10 The theorem of drilling and counterbore technologies [3], [13] The territory of the chip section in case of drilling technology (Figure 10.a) [3], [7], [8], [13]: The territory of the chip section in case of counterbore technology (Figure 10.b) [3], [7], [8], [13]: The cutting force for one edge in case of drilling technology ( Figure 11): The necessary moment for drilling ( Figure 11): The cutting force for one edge in case of counterbore technology ( Figure 12): (13) The necessary moment for counterbore technology ( Figure 12

Calculation of machine main time for technology of plain milling
In case of plain milling the cutting motion is done by the tool and the feed motion is done by the workpiece. The rotation axis of the hob is parallel with the milled surface. It is crucial. The machined surface is plane but it could be cylindrical or shaped in rare cases [3], [7], [8], [10][9], [12], [13], [14]. The cutting force for one edge of the tooth of the milling cutter [3], [7], [8], [13]: Considering the ψ switching number the total cutting force could be calculated: During cutting the switching number is expressed by the number of the tooth of milling cutter [3], [12], [13], [14]: Based on (17), (18) and (19) the total cutting force is (20) The cutting power is [3], [7], [8], [13] (21) Based on Figure 15, the length of milling is: Based on (22) and (7) the machine main time is:

Calculation of machine main time for face milling
In case of face milling the cutting motion is done by the tool and the feed motion is done by the tool or workpiece [3], [6], [7], [8], [12], [13], [14]. The rotation axis of the milling cutter is perpendicular for the milled surface. The machined surface is mainly planar surface. The chip section is changing along the cutting path of the tooth of the milling cutter: the lowest is on the place of the entrance and the exit. The highest is on the symmetric plane of the feed direction of the axis of the milling cutter. The fz feed for one edge is the most important parameter of the face milling. The frφ is the radius directional feed [3], [6], [7], [8], [12], [13], [14]. The cutting force for one edge of the tooth of the milling cutter ( Figure 16) [3], [13]: Considering the switch number the total cutting force is ( Figure 16) [3], [12], [13], [14] (25) The switching number is ( Figure 16) [3], [12], [13], [14] (26)  L bw  The results have shown that the utilization of the machine is 47% in 2015. Although for making calculation for 2016, we obtained result that 2 CNC milling machines will be necessary for producing the estimated number of products. We have developed a computer program for the analysis and calculation of the OEE indicator. With the help of our computer program, OEE indicator values for any types of machine can be calculated easily and fast. This will facilitate the activity of the company. Machine main time for the main technological operations on the milling machine have been determined. During production to optimise the machine main times, the right technological parameters (feeding, depth of cut, rotational speed etc.) should be chosen. Due to these, a higher number of produced items during a given period could be obtained. Based on machine main times, norm time of the total production series can be planned.

Acknowledgement
The work/publication is partly supported by the EFOP-3.6.1-16-2016-00022 project. The project is co-financed by the European Union and the European Social Fund.