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SELECTION OF OPTIMAL MACHINING
PARAMETERS IN DIAMOND CORE DRILLING PROCESSES

 
ABSTRACT
Diamond core drills are applied to cut difficult-todrill materials This paper intends to propose
basic understanding of ceramic drilling mechanics and to enable selection of optimal machining parameters affecting tool life, material removal rate (MRR) and hole quality. In contrast to normal metal cut drilling, core drilling processes make deep grooves for removal of machining materials. One of difficulties of the core drilling process is the evacuation of chips form the drilled groove. As the drilling depth increases, an increased amount of chips tend to cluster together and clog the groove. Eventually severe wear develops and diamond grits are separated from the drill body. To relieve the chip clogging problem and to evacuate chips from the groove easily, the helical drilling process is studied. Taguchi method is applied to select
optimal helical drilling conditions.
 
INTRODUCTION
Diamond core drills made of diamond grits brazed with nickel are widely used in IT, BT, construction and manufacturing industries. They are used to drill holes on the difficult-to-drill
materials such as ceramics, wafers, glasses, tiles, tungstens, and so on. In diamond core drilling of the difficult-to-drill materials, machining parameters are main factors affecting MRR, tool life and hole quality, it is important to select optimal machining parameters. However, it is
difficult to select them and to understand characteristics of them, because the diamond core drilling process has many factors acting on each other. [1-3]
In contrast to normal metal cut drilling, core drilling processes make deep grooves for removal of machining materials. One of difficulties of the core drilling process is the evacuation of chips form the drilled groove. As the drilling depth increases, an increased amount of chips tend to cluster together and clog the groove. Eventually severe wear develops and diamond grits are separated from the drill body. To relieve the chip clogging problem and to evacuate chips from the groove easily, vibration and helical drilling processes have been studied so far [4-5]. Huang and Wang [3] studied on affective machining parameters of the diamond core drilling process in rock machining. They derived mathematical models to predict the performance of drilling processes.
In this paper, diamond core drilling experiments are performed on porcelain tiles, one of difficultto-drill materials, to select optimal drilling conditions through the helical feed motion and
the Taguchi method. To analyze drilling characteristics and derive optimal drilling conditions, tool life, thrust force, and tool wear are quantified through the monitoring system equipped with a tool dynamometer and the machine vision unit. Multiple regression models are derived to estimate wear and thrust force in diamond core drilling processes.
 
 
figure
 

EXPERIMENTS-Experimental equipment

herical

Drilling experiments are conducted on the vertical tapping center with the maximum speed of 12,000 rpm . Workpieces, porcelain tiles, are clamped on a fixture, and a dynamometer is mounted between the fixture and the machine table. Fig. 1 shows the overall experimental setup. Diamond grits are brazed to steel body to make diamond core drills with an outside diameter of 6 mm and an inside diameter of 2
mm. Kistler model 9257B dynamometer is used to measure thrust forces. Charge signals from the dynamometer are converted into voltage signals by Kistler model 5010B10 charge amplifier. Labview is used to digitize the signal with 1KHz sampling rate.

Herical drilling method
Fig. 2 illustrates a drilling method using helical motion [5]. During helical drilling motion, loading is prevented from grits since the helical motion is applied to the conventional drilling process. It enables to aid distribution of drilling temperature through a gap between tool and the groove. In helical-feed drilling method, motion of spindle consists of a rotational motion for a cutting speed and a revolution motion forgeneration of the helical motion. Hence, a size of machining hole is given by
Design of experiments
Taguchi method is a robust design method to minimize distribution due to uncontrolled conditions into the value of characteristic. The characteristic is expressed as signal and noise ratio. In this paper, tool life is applied to lagerthe-better characteristics because the longer tool life is better, and thrust force and tool wear are applied to smaller-the-better characteristics because the smaller thrust force and tool wear
are better. Under these application, S/N ratio is calculated as follow, Eq (4) is lager-the-better characteristics and Eq (5) is smaller-the-better characteristics.
Design of experiments figure
 
Tool life assessment
On diamond core drilling process with porcelains, the trust force according to time is shown as Fig.
3. And variation of the gradient of thrust force according to time is denotes as Fig. 4. As the trust force gradually increase with time, it is possible to know that the thrust force increases rapidly after 150sec owing to tool wear and chip loading as shown in Fig. 3. In this paper, the criterion of tool life assessment is defined as the volume of cut with variation of the gradient of thrust force which is over 20 N/s. The volume of cut is given by
tool
Thrust force and tool wear
Fig. 5 shows the average of thrust force per spindle revolution versus time. In this study, the trust force is defined as the maximum value in the steady state stage. A measurement system for tool wear consists of halogen light source, LED lights, image processing computer and machining center which is attached a CCD camera. In this paper, the tool wear is defined as the volume of cut, 684.2 mm3 , with the variation
of height of diamond grit. A variation of height of diamond grit is measured by using CCD camera and image processing system [6]. Fig. 6 illustrates a variation of height of diamond grit [7].
 
EXPERIMENTAL RESULTS AND DISCUSSION
The statistical treatment of the data was made in two phases. The first phase was concerned with the ANOVA and the effect of the factors. The second phase allowed us to obtain the correlations between the machining parameters.
Analysis of Variance (ANOVA)
The ANOVA was used to predict the relative significance of the process factors and estimate the experimental errors. Table 2 shows the orthogonal arrays used. Tables 3-5 show the results of the ANOVA with the tool life, the thrust force and the tool wear, respectively. In table 3, the most important variable affecting the tool wear is the helical feed(P=66.35%), and the optimal machining parameters are identified, such as A3, B3 and C1. In table 4, the most important variable affecting the thrust force is
the helical feed(P=76.12%), and the optimal machining parameters are identified, such as A3,
B2 and C1. In table 5, the most important variable affecting the tool wear is the helical feed(P=96.59%), and the optimal machining parameters are identified, such as A3, B3 and C1.
 
Multiple regression analysis
Using multiple regression analysis, mechanical properties are estimated from machining parameters as follows:
figurs
 
table

CONCLUSIONS

In the diamond core drilling process, the design of experiments is applied to select the optimal machining parameters and to verify the relation of cutting conditions, tool life, thrust force, and tool wear. The results are summarized as follows:
(1) It is possible to select the optimal machining parameters about each result by using Taguchi
method.

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