high speed drilling seminar repoort

7/29/2019 high speed drilling seminar repoort

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1.Introduction

1.1Objective

This paper reports an experimental investigation of a full factorial design performed

on thin CFRP laminates using K20 carbide drill by varying the drilling parameters such asspindle speed and feed rate to determine optimum cutting conditions. The hole quality

parameters analyzed include hole diameter, circularity, peel-up delamination and push-out

delamination.

Analysis of variance (ANOVA) was carried out for hole quality parameters and their

contribution rates were determined. Genetic Algorithm (GA) methodology was used in the

multiple objective optimization to find the optimum cutting conditions for defect free drilling.

Tool life of the K20 carbide drill was predicted at optimized cutting speed and feed.

1.2 Scope of topic

There has been a growing interest in using composite materials in place of

conventional materials in various structural application ranging from aircraft and space

structures to automotive and marine applications. Composites, carbon fiber reinforced

plastics(CFRP) in particular, are being widely utilized at a great extent due to superior

properties. CFRP is stronger than steel than titanium, while still retaining its lighter weight.

Carbon fibers which are commonly used to reduce the weight of structural components on the

aircraft result in improved fuel economy, reduced Emissions& increased load carrying

capacity of the aircraft.

In Boeing 787, more than 50% of the structural weight is made of composites.

Although composites are made to near net shape Assembly requires the drilling of large

number of holes. Low speed drilling of CFRP was successfully conducted.

As we all know the importance of high speed machining.In high speed machining the

cutting time is very low and at the same time the quality is also very high.The productionrate is also high.So high speed drilling of CFRP is having a large scope in the modern world.


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2. Literature Survey

2.1 Study of Topic

Of all the defects caused by drilling, delamination is one of the most critical defects

because it is responsible for the rejection of approximately 60% of the components produced

in the aircraft Industry Delamination results in poor assembly tolerance and reduces the

structural integrity of the material.

The two most significant delamination mechanisms are peeling of the lamina at the

top layer and pushing action on the thin uncut layer at the bottom. Delamination is primarily

influenced by thrust force developed during drilling. The delamination factors (peel-up and

push-out) increased with an increase in thrust force and vice versa.

The delamination associated with push-out is more severe than that of a peel-up. For

the same fiber shape, the peel-up and push-out delaminations of woven/epoxy composite are

lesser when compared to that of woven/polyester composites despite the two composites

possessing approximately the same thrust forces.

The delamination associated in drilling CFRP with helical flute carbide drill was

lesser when drilled using a four flute carbide drill.

Current research focuses on high speed machining (HSM) as it increases production

besides being cost effect. When drilling is done at high cutting speeds, the thrust force

developed is significantly less. Hence the spindle energy consumption is also reduced.

Although the material removal rate is high, tool wear is one of the major limiting factors in

HSM. Chen reported that combinations of low feed rates and high spindle speeds were

preferable as they generated lower thrust force. When drilling carbon/epoxy material at high

speeds the resultant drill wear is higher. This in turn results in an increase in the thrust force .

Karnik et al. analyzed delamination while drilling CFRP at high spindle speeds using

artificial neural network (ANN) and concluded that spindle speed, feed rate and point angle

of the drill affect the delamination of the drilled hole. It was proposed that a combination of

high spindle speed, low feed rate and low point angle would minimize damages that occur

due to delamination.


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Enemuoh et al. used a new comprehensive approach to select the cutting parameters

for damage free drilling in CFRP. The approach they used was based on combination of

Taguchis experimental analysis technique and a multi-objective optimization criterion. Kimand Ramulu studied the drilling of Gr/BiTi stacks and optimized the cutting parameters in

terms of machined hole quality and machining cost. Due to dissimilar machining

characteristics of the work piece materials, a multiple objective linear program was used to

optimize the process parameters with suitable weighting factors.

An analysis of literature reveals that not much work has been reported on the high

speed drilling of CFRP laminates. The current study attempts to fill the gap by reporting an

experimental study on high speed drilling of thin CFRP laminate using K20 carbide drills to

study the influence of spindle speed and feed rate on hole diameter, circularity, delamination

and thrust force. The results obtained are optimized using genetic algorithm to arrive at the

optimal solution.


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3.Technology used

Work piece details

The CFRP composite specimen used in the investigation was 2 mm thick. The

laminate was prepared out of 8 layers of0.25 mm thickness each. Woven fibers (T300) were

used in epoxymatrix and the maintained fiber volume fraction was 60%.The laminate was

prepared in acontrolled atmosphere and compacted using a vacuum pump. A mold for the

laminate was prepared and placed in a vacuum bagging and evacuated to 0.7 bar. Curing was

then carried out at180 degrees for 120 min, during which the pressure was maintained at 7

bar in an autoclave.


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Experiment:

The workpiece was mounted on the dynamometer which was fixed on the bed of

a vertical machining center and the drill was fed into the workpiece. Drilling trials were

carried out using 5 mm diameter tungsten carbide drills

Spindle speeds were selected such a way that it suited the requirements of high

speed machining. All the trials were conducted without the use of a coolant. The thrust force

and torque during machining were measured using a dynamometer. The proportional charge

output from the dynamometer was fed to a charge amplifier, thus producing a scaled voltage

output signal proportional to the applied load in a digital form.


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The dynamometer works on the strain gauge principle. The imbalance of the

wheatstone bridge circuit can be attributed to the thrust force and the torque values, which

produce a proportional voltage equal to the applied force and torque.

The output from the dynamometer was given as input to oscilloscope to analyze

the Force fluctuations.

The delamination was measured by using flat bed scanner technique .The

specimen was placed directly on the glass plate on the scanner. The drilled holes were

scanned at a resolution of 7200 ppi and saved as a bitmap image. The delamination factor

was measured based on the pixel density of the delaminated zone.The scanned images were

imported into the image processing software Image J. The color images were converted into

a binary file.

While the delaminated zone appears in white, the undamaged area remains

black. The threshold value for binary conversion was set by comparing the histogram of array


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values of delaminated zone with that of the undamaged area. The pixel density of the white

zone represents the damaged area (Ad).

The pixel density (area) of the hole (A) was measured in the color image

obtained by scanning. The ratio of the two pixel densities gives the delamination factor (Fd).


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Binary Image formed

Experimental Design

Experiments were conducted using full factorial design (3x4x3 = 36 tests). Each experimental condition was repeated thrice to ensure consistent value.

Spindle speed has 3 levels

Feed rate has 4 levels


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Analysis of variance (ANOVA)

Analysis of variance is a standard statistical technique which is routinely used to

analyze the experimental data. The technique does not directly analyze the data, but

determines the percentage contribution of each factor by determining the variability

(variance) of the data.

Degrees of freedom (DOF)

Degree of freedom is the measure of the amount of information that can be uniquely

determined from the given set of data. DOF of a factor equals one less than the number of

levels. The concept of degree of freedom can be extended to an experiment. An experiment

with n trials and r repetitions of each trial has n*r trial runs.

Then the total degree if freedom becomes,

Ft =n*r-1

Sum of squares (SS)

The sum of squares is the measure of the deviation of the experimental data from the

mean value of the data. Summing each squared deviation gives the total deviation. Thus,

Where ST is the sum of squares, yi is the observed data and Y is the

Average value of yi.


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Variance measures the distribution of the data about the mean of the data. Since the

data is the representative of only a part of all Possible data, DOF is used.

Mean sum of the deviations squared

where T is the sum of deviations from the target value, S is the mean

sum of squares of the deviation and n is the number of trials.

Features of ANOVA

ANOVA is used in the analysis of comparative experiments, those in which only the

difference in outcomes is of interest. The statistical significance of the experiment is

determined by a ratio of two variances. This ratio is independent of several possible

alterations to the experimental observations: Adding a constant to all observations does not

alter significance. Multiplying all observations by a constant does not alter significance. So

ANOVA statistical significance results are independent of constant bias and scaling errors as

well as the units used in expressing observations. In the era of mechanical calculation it was

common to subtract a constant from all observations (when equivalent to dropping leading

digits) to simplify data entry. This is an example of data coding.


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Results and Discussions

Effect of cutting variables on thrust force

In high speed drilling experiments the torque values were found to be less than 0.1Nm and negligible when compared to values at low speed drilling hence the thrust force is

alone considered for analysis. The thrust force obtained for various speed and feed

combinations during the high speed drilling of CFRP.

It can be observed that the thrust force increases with an increase in feed owing to

increase in uncut chip thicknessand higher impact of the fibers as the feed rate is increased.

In addition, at higher feed rates, there is an increase in self-generated feed angle which

significantly reduces the working clearance angle of the drill resulting in rubbing against the

work material causing higher thrust.

It can also be observed that the rate at which the thrust force increases is high in the

range 0.01 mm/rev to 0.1 mm/rev that between 0.1 mm/rev and 0.3 mm/rev. Also thrust force

was found to decrease with an increase in spindle speed because of an increase in temperature

with Spindle speed.

From the ANOVA calculations, it can be inferred that the thrust force is primarily

influenced by the feed rate. The contribution rate of feed (82.82%) is greater than that of

spindle speed (13.07%). SN ratio of the spindle speed and feed for the thrust force

determined show that a spindle speed of 20,000 rpm and a feed of 0.01 mm/rev is optimum

because this combination has the higher signal to noise ratio.


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Effect of cutting variables on hole size

The hole size obtained for various speed and feed combinations during the high

speed drilling of CFRP are presented in Figure. It can be inferred that the diameter of the

drilled hole lies near the expected Value (5 mm) at higher feeds. Since the feed rates are

lesser Than the depth of cut, the self induced vibration decreases with an Increase in feed

resulting in holes with diameter closer to nominal Diameter. At high spindle speed and low

feed, because of frictional heating.

The cutting temperature goes up which result in higher hole diameter.

The diameter of the hole varied from 5.02 mm to 5.95 mm.

The specific cutting resistance Increases at lower feeds due to smaller uncut chip thickness


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resulting in higher shear forces which in turn increase the vibration. Italso results in larger

hole size at lower feeds and decreases at higher feeds.


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From the ANOVA calculations it can be inferred that the hole size is primarily

influenced by feed rate. The contribution rate of feed (66.69%) is greater than that of spindle

speed (16.15%).

SN ratio of the spindle speed and feed for the hole size determined show that a

spindle speed of 12,000 rpm and a feed of 0.30 mm/rev is optimum. If high spindle speed is

desired for Increase in production rate, it is preferable to go for a feed rate of 0.1 mm/rev.

Effect of cutting parameters on circularity

The circularity measured for various spindle speed and feed combinations during

the high speed drilling of CFRP. It can be seen that the circularity decreases with an increase

in spindle speed, while it remains almost constant for increase in feeds. The rotational

stability of the drill is better at higher speeds than at lower speeds. It explains the lesser

circularity error at high speeds. A low feed rate of 0.01 mm/rev creates greater circularity.

This could be because of ploughing and frictional heating.

From the ANOVA calculations it can be inferred that the circularity is influenced

solely by spindle speed (85.95%). The higher value of error (14%) could be due to theinteraction effects between spindle speed and feed. The SN ratio of the spindle speed and

feed for the hole size determined show that a spindle speed of 20,000 rpm and a feed of 0.10

mm/rev is optimum.

Effect of cutting parameters on peel-up delamination

The peel-up delamination factor obtained for various speed and feed combinations

during the high speed drilling of CFRP are presented in Figure. It can be seen that the peel-up

delamination

Shows no clear pattern in relation to feed. Also the delaminations at lower speeds

were much lower than those obtained at higher speeds.


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From the ANOVA calculations, it can be inferred that the peel up delamination is not

influenced by spindle speed or feed in the selected range.

Effect of cutting parameters on push-out delamination:

The push-out delamination factor obtained for various speed and feed

combinations during the high speed drilling of CFRP are presented in Figure. It can be

observed that the push-out delamination factor increases with an increase in feed rate and

spindle speed. As feed rate is increased, the thrust force also increases.

Velayudham and Krishnamurthy observe that the delamination ratio increases

with an increase in thrust force. Similar results were obtained in these experiments as well. In

the experiments,the delamination ratio increased with an increase in spindle speed and feed

rate. This could be because of smaller thickness of the CFRP laminates. At high spindle

speed the delamination may be initiated at lower forces because the heating of matrix

resulting


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in lesser stiffness.

From the ANOVA calculations it can be inferred that the push-out

delamination factor is primarily influenced by feed rate. The contribution rate of feed

(51.40%) is greater than that of spindle speed (35.42%). The higher value of error (13.18%)

could be because of the interaction effects between spindle speed and feed. The SN ratio of

the spindle speed and feed for the hole size show that a spindle speed of 12,000 rpm and a

feed of 0.01 mm/rev is optimum.

Multiple objective optimization

Since each of the above parameters give conflicting solutions, it is important to

arrive at a optimized solution. This is achieved by developing a normalized objective

function that includes all the hole quality parameters. The optimized value provides a

reasonable compromise without placing undue burden on any of the hole quality parameters.

A multivariable linear regression model wasused to predict the hole quality parameters which

are dependenton spindle speed (N) and feed (f).

This model represents the dependent parameter y (observed response) as a function

of the main effects of factors (xi), their interaction (xixj), and their quadratic components

(X2i). The equations for the individual output parameters were obtained through regression

analysis, and the same are presented. All the hole quality parameters namely diameter,

circularity and delamination factors need to be optimized simultaneously which necessitates

the evaluation of solution alternatives according to multiple criteria.

The first step in this optimization scheme is the formulation of an objective

function after normalization. The normalization of the objective function of a parameter was

carried out by imposing constraints ai; desired and ai;max over the measured value ai [19].

The range of values obtained varied between zero and one for each parameter. The multi-


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objective function (F) consists of the sum of each objective (Zi) using different weight

coefficients (hi) for each criteria.

The weighting factors are assigned such that their sum was always equal to one. The

weighting factors were assigned to each parameter based on its relative importance [8,11]

which in turn depends on the deviation of the parameter from the desired value.

Using the regression equations and their corresponding weighting factors in the above

equations, the multiple objective function was obtained as:

The obtained equation was used to plot the objective function within the required

spindle speed and feed range (namely 12,00020,000 rpm, 0.010.30 mm/rev) in MATLAB

R2010a. The contour plot is shown. The genetic algorithm technique was used to determine

the operating condition form the desired working zone. The genetic algorithm tool in

MATLAB R2010a was


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Executed by varying the population size while retaining selection rules, crossover

rules and mutation rules as constant to obtain the optimized operating condition. The

convergence of computed values from continuous iterations was obtained at a population size

of 1000. The optimized operating condition was observed to be 12,000 rpm at 0.137 mm/rev.

Confirmation tests were repeated five times and the hole quality parameters measured along

with the expected values from the fitted regression model are presented.


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Confirmatory experiments

Tool life investigation

The tool life investigation experiments were carried out at the optimized spindlespeed and feed rate. A series of 150 holes were drilled for studying the tool life and the hole

quality parameters were analyzed as described earlier. Of the hole quality parameters,

diameter and circularity did not show much deviation. Nevertheless from the delamination

factors both peel-up and push-out significant results were extracted. Figure shows the

variation of the delamination factors against the number of holes drilled.

It can be observed that there is a steady increase in the delamination factor with the

number of holes drilled, but the rate of increase was not so significant in the initial stages.

The peel-up delamination was lesser compared to push-out delamination throughout the tool

life study.

From the first hole to the 30th hole, no significant delamination could be observed.

At the 120th hole the peel-up delamination factor was 1.015 and push-out delamination factor

was 1.045 and then increased rapidly thereafter.


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4. Future Enhancements

The major problems associated with high speed machining are:

Minute errors may lead to big consequences Good work and process planning necessary Need for expensive and special machine toolsThese problems should be rectified in the future for taking High speed drilling to another

level.

The CFRP materials is having a huge importance in the modern world because of its

properties .It is stiffer than Titanium and stronger than steel.It is having high strength to

weight ratio.

Carbon fibers which are commonly used to reduce the weight of structural components

on the aircraft result in improved fuel economy, reduced emissions & increased load carrying

capacity of the aircraft.

In Boeing 787, more than 50% of the structural weight is made of composites.

Although composites are made to near net shape assembly requires the drilling of large

number of holes.So the major problems associated with High speed drilling should be

rectified.


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5. Conclusion

The following conclusions can be drawn from the experimental results of effect of cutting

parameters on the cutting forces and hole quality.

Feed rate has a greater influence on thrust force, push-out delamination and diameter ofthe hole. While lower feed rates reduce thrust force and push-out delamination, higher

feed rates result in holes closer to the nominal diameter.

Spindle speed is one of the major determinants of the circularity of the drilled hole. Bestresults were obtained at 20,000 rpm.

Neither spindle speed nor feed rate had any visible influence on peel-up delamination,with in the tested range.

The optimized spindle speed and feed rate for drilling thin CFRP laminates at highspeeds were found to be 12,000 rpm and 0.137 mm/rev respectively.

In optimized cutting conditions, no significant tool wear was found till 150 holes.Whereas cutting edge rounding was seen. No significant delamination could be observed

till 30 holes. There was a steady increase in the delamination factor after 120 holes. The

peel-up delamination was lesser when compared to push-out delamination throughout the

study of tool life.


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6.Bibliography

[1].Vijayan Krishnaraj a,, A. Prabukarthi a, Arun Ramanathan a, N. Elanghovan a, M.

Senthil Kumar a, Redouane Zitoune b, J.P. Davim,Optimization of machining parameters at

high speed drilling of carbon fibre reinforced plastic (CFRP) laminates, University of Paul

Sabatier, Toulouse 31077, France University of Aveiro, Portugal 1791-1799,2012

[2].Ho-Cheng H, Dharan CKH. Delamination during drilling of composite laminates. J Eng

Ind, ASME 1990;112:2369

[3]. Zitoune Redouane, Krishnaraj Vijayan, Collombet Francis. Study of drilling of

composite material and aluminium stack. Compos Struct 2010;92:124655.

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high speed drilling seminar repoort