An experimental result of surface roughness machining performance in deep hole drilling

This study presents an experimental result of a deep hole drilling process for Steel material at different machining parameters which are feed rate (f), spindle speed (s), the depth of the hole (d) and MQL, number of drops (m) on surface roughness, Ra. The experiment was designed using two level full factorial design of experiment (DoE) with centre points to collect surface roughness, Ra values. The signal to noise (S/N) ratio analysis was used to discover the optimum level for each machining parameters in the experiment.


Introduction
Deep hole drilling is a complex machining process in which the ratio of hole depth to hole diameter exceeds 10 [5].Deep hole drilling is a relatively complex drilling production process due to the high hole diameter to length ratio which makes the tool shaft prone to vibration and results typically in low quality holes from geometry and surface roughness viewpoints [1].Deep hole drilling processes are used to generate drill holes with a high length to diameter ratio.A good holes, which are holes with better straightness, better roundness and smoother surface roughness, there are many factors need to be considered such as material, spindle speed, feed rate and tool diameter [9].According to Hayajneh (2001), the important machining parameters that effect hole quality is spindle speed and feed rate [6].Surface roughness, R a is an important aspect in evaluating the quality of products.R a is widely used as an index of product quality and in most cases a technical requirement for mechanical products [3,12].Normally, R a value is influenced by many factors such as machining parameters, cutting phenomena, workpiece properties and cutting tool properties [15,16].Based on previous literature, R a is one of the machining performance measurements frequently considered by researchers [4,11,13,14].
This study focuses on an experimental result of R a in the deep hole drilling process.The flow of the experiment and analysis of result also discussed.
The deep hole drilling experiment was carried out at Production Laboratory using CNC 3 Axis Milling Machine (MAHO MH500E2).Figure 1 shows the CNC 3 Axis Milling Machine.This machine was used to produce the holes on the steel for conducting the deep hole drilling process.The tool used for this study is High Speed Steel, (HSS) with twist drill bits and workpiece material is Steel.The HSS diameter is 5.0 mm and the illustration of HSS is shown in Figure 2. Table 1 shows the mechanical properties of the HSS based on the book catalogue of Drilling Tools (YG-1 Drilling Tools Catalog 2009/2010).

Deep hole drilling parameters and experimental design
Design of experiment (DoE) is a collection of powerful statistical analysis techniques used for modelling, developing, improving, and optimizing various manufacturing and other useful processes [1].It also for structuring and organizing method for determining the relationship between input parameters, which affect the process and the output of the process.DoE is an important step in the selection of machining parameters and their levels within an effective and suitable experimental run [4].
The Two Level Full Factorial Design with centre points is used as DoE in this study.Twenty experimental numbers were executed, which consists of sixteen data of two levels, which 2 k full factorial analysis where k is referred to number of machining parameters involved in these experiments with four centre points.
In this study, there are four machining parameters that had been put into consideration which is feed rate, f (mm/min), spindle speed, s (rpm), depth of hole, d (mm) and MQL, number of drops m (ml/hour).The machining parameters and their levels are shown in Table 2.The R a value of the machined work piece was measured using the Handysurf Profile Meter.Full Factorial Design is used widely because they are easy to design and analyse, efficient to run and full of information [7].Then it is simplest and most common type of factorial design [2].

Results and discussion
The experimental results for R a along with their computed S/N ratio values are shown in Table 3.
From the table, the minimum value obtained is 2.8 μm.The optimal values of machining parameters that contribute to minimum R a are 85 mm/min for feed rate which in level +1, 1100 rpm for spindle speed which in level +1, 60 mm for depth of the hole which in level +1 and 40 ml/ hour for number of drops of MQL which in level +1.The lower the better (LB) category of S/N ratio has been selected for R a and used to identify the optimal level of machining parameters [8].The calculation of LB category can be referred in Eq.1.
where, ‫ݕ‬ = the observed data (R a ) ݊= the number of observations The highest of S/N ratio value corresponds to the better machining parameters [10].The mean response table of S/N ratio for each level of the machining parameters is summarized in Table 4. Based on the Table 4, R a value has been identified as minimum at the Level +1 of feed rate(f), Level +1 of spindle speed (s), Level +1 of depth of hole (d) and Level +1 MQL (m) respectively.The symbol of '*' corresponded to the highest value of S/N ratio among their levels and then it is identified as the optimum level.Consequently, the level that has a higher value of S/N ratio determines the optimum level of each machining parameters.In Table 4 for feed rate level +1 (f +1 = -11.8609),spindle speed level +1 (s +1 =-11.5857),depth of hole level +1 (d +1 = -11.6277)and MQL level +1 (m+ 1 = -11.6529)has the highest S/N ratio value, which indicated that the machining parameters at that level produced minimum value R a .As a result, the optimum combination to get the minimum R a is f +1 s +1 d +1 m +1 as shown in Table 5.From the result, it was found that the combination value of R a is acceptable which is the level obtained in real machining is the same level in the S/N ratio analysis which is f +1 s +1 d +1 m +1 .

Conclusion
This study presents an experimental result of R a in deep hole drilling process.The results were analysed using S/N ratio.From the S/N ratio analysis, the optimal machining parameters for R a is determined as f +1 s +1 d +1 m +1 , which are 85 mm/min for feed rate (level +1), 1100 rpm for spindle speed (level +1), 60 mm for depth of the hole (level +1) and 40 ml/ hour for number of drops of MQL (level +1).The results show that the combination level obtained from the S/N ratio analysis is the same level with the machining parameters that produced minimum value R a .It was found to be statistically significant each other.Special appreciation to reviewer(s) for useful advices and comments.The authors greatly acknowledge the Soft Computing Research Group (SCRG), Research Management Centre (RMC), UTM and Ministry of Higher Education Malaysia (MOHE) for financial support through the Fundamental Research Grant Scheme (FRGS) vot No R.J130000.7828.4F721.

Table 1 .
Mechanical properties of the HSS used in the experiment.

Table 2 .
Levels of machining parameters for deep hole drilling.

Table 3 .
Experimental results for R a and S/N ratio.

Table 4 .
The mean S/N response table for R a .

Table 5 .
Optimum value of machining parameters for deep hole drilling.