MINIMIZING TOOL WEAR BY

CONTROLLING THE DRILLING PROCESS

PARAMETERS IN THE MACHINING OF

CFRP COMPOSITES

Presented by

K. Venkata Vimal

Akshay Keerthi

Siddhant J. Shetty

INTRODUCTION

Fibre-reinforced polymer (FRP) is a composite material made of a polymer matrix reinforced

with fibers. The fibres are usually glass, carbon, aramid, or basalt. The polymer is usually

an epoxy, vinylester or polyester.

FRPs are commonly used in the aerospace, automotive, marine, construction industries and

ballistic armour.

Drilling is one of the most important machining operations in the assembly of automobile and

aerospace structures. Many critical structural members are assembled by means of fasteners.

To assure the integrity of these fasteners, without defeating the advantages of composite strength

characteristics, the need exists for improved methods of quality hole production but, drilling in

FRP composites leads to damages such as spalling, de-lamination, edge chipping, fibre pullout,

crack formation on the drilled surface and excessive tool wear.

It was observed by a literature review that the process parameters that effect the drill tool wear

were, cutting speed and feed rate of the drill, drill geometry, point angle, the material(s) drilled

and the rigidity of the fixture.

ADVANTAGES OF COMPOSITES

High Specific strength (Strength/Density);

High Stiffness;

Light Weight;

Corrosion Resistance;

Better Fatigue Life;

Heat Resistant.

TOOL WEAR IN COMPOSITE DRILLING

Drilling is the most frequently employed operation for fibre reinforced materials owing to

the need for structure joining and assembly.

Drilling tools become blunt after a certain working time and require regrinding or

replacing. The consequences of tool wear while machining may change the dimension,

tolerance, shape and surface finish of a part or product.

It has been found out experimentally that the wear of the drill bit depends on the various

Tool factors, machining factors (process parameters) as well as the Material Parameters.

With an optimized combination of the various Parameters of the Materials and the Tool, the

Wear of the Drill Bit can be minimized.

The process of drilling of composites is economically important since the extremely

abrasive nature of the fibres limits the drill life.

SIGNIFICANCE OF TOOL WEAR

Tool wear influences the machining operations in the following ways:

increased cutting forces

increased cutting temperatures

poor surface finish

decreased accuracy of finished part

May lead to tool breakage

Causes change in tool geometry

From a cost factor point of view:

For an aircraft around 10, 00,000 to 20, 00,000 holes have to be drilled.

Billions of holes are drilled annually with a total cost, in the United States alone,

estimated to be over 300 million dollars.

Through advancement of the drilling processes, the life of the Drill Bit could be

extended by a modest 10% and the industry would save 30 million dollars.

SUMMARY OF LITERATURE REVIEW

Study was conducted on the effects of tool geometry and cutting force variations in CFRP

laminate experimentally during drilling. It was observed that the flank surface temperature

increases with increase in cutting speed but decreases with increase in feed rate.

Evenly and smoothly distributed abrasion wear on the cutting edge is due to the highly

abrasive nature of the carbon fibres.

Another research observed that as composite materials are anisotropic in nature, machining

composite materials is a complex process especially operations involved in in drilling and

milling. Therefore, dry drilling operation on composite materials affects the performance of

the drill tool

Process parameters were studied namely, cutting speed, feed, point angle and land width in

drilling of glass fiber reinforced polymer (GFRP) composites. Results indicated that feed rate

is the most significant factor influencing the thrust force followed by speed, land width and

point angle; cutting speed is the most significant factor affecting the torque, speed and the

circularity of the hole followed by feed, land width and point angle.

OBJECTIVE

To optimize and identify the drill process parameters to control tool wear through

experimental study.

To analyse the micrograph images of drill tool surface to detect the different wear patterns.

To build the mathematical model to predict the tool wear for the chosen process parameters

and experimental validation.

METHODOLOGY

To identify through a literature review the different

drill process parameters which have an influence on

the drill tool wear. For the experimental study, the tool parameters have been used as the control

factors whereas the material parameters have been kept constant.

Control Factor Units Level 1 Level 2 Level 3

Spindle Speed (N) rpm 4000 5000 6000

Feed Rate (F) mm/rev 0.3 0.4 0.5

Tool Diameter (Dt) mm 6 8 10

Tool Point Angle(Ɵ) Degree(o

)

90 104 118

Preparation of the test specimen.

Conduction of drilling experiments

Measuring the response/output.

Analysis of response and developing mathematical model

DESIGN OF THE LAMINATES

The laminates were designed as per the ASME standards for fasteners

Distances and spacing Mandatory

Minimum

Recommended minimum

E1 1.2do 2do

E2 1.2do 1.5do

P1 2.2do 3do

P2 2.4do 3do

M 2do

SIZING OF THE CFRP LAMINATES USING

SOLIDWORKS

Number of Composite Laminates: 18

Number of Slabs required for each Diameter (6, 8 and 10 mm): 6

Number of holes to be drilled in each Slab: 60

Maximum Tool Travel Distance in X-Direction: 240 mm

Maximum Tool Travel Distance in Y- Direction: 160 mm

Cutting Allowances to be given: 9-10 mm.

Different ways of drilling 60 holes (Rows*Columns):

6*10 ----- Chosen

4*15

5*12

Slab for 6 mm tool diameter

133.2 mm x 86.4 mm

Slab of 8 mm tool diameter:

177.6 mm x 115.2 mm

Slab for 10 mm tool diameter

222 mm x 144 mm

MANUFACTURING OF THE CFRP

LAMINATES

Laminate Specifications

Reinforcement: Carbon Fibre

Matrix: Hinpoxy C

Reinforcement: Matrix Ratio 1:1 (i.e., Fibre Volume Fraction = 50%)

Area weight: 200 gram/m2

Weave style: Plain

Fabric type: Bi directional

Filaments: 3000 fibres per strand

Area: 10 m2

Steps involved in the manufacturing of the CFRP

laminates

Prepare a clean and polished surface (wood/glass).

Apply two layers of wax with an interval of 10 minutes between each layer.

Cut the carbon fibre according to the required dimensions using a scissor.

Weigh the carbon fibre on a weighing scale and note it down. Weigh a quantity of resin

equal to the weighed carbon fibre. Hardener is then mixed with the resin in 100:30 weight

ratio.

Then apply the first layer of the resin-hardener mix. Following this place the first layer of

carbon fibre sheet.

After this apply alternating layers of mixture and carbon fibre until the desired thickness is

obtained.

Laminate quality

When manufacturing a traditional metal structure, start with the material in its final form.

Some heat treatment or stress relieving might be needed, but the material is essentially

complete.

With composites, however, the material is made at the same time as the structure. The

material is not complete until the cure cycle has finished, and therefore it is critical to

carefully control conditions during cure.

The three ingredients necessary for cure are heat, pressure, and vacuum.

Pressure compacts the laminate, providing good consolidation and interlaminar bonds.

Vacuum draws out volatiles and trapped air, resulting in a low void content. Both help to

improve resin flow.

Applying Resin on each layer

Bag Sealant Tape

BST is a putty-like material which comes in rolls, usually 1/2 inch wide, with a

release paper on one side. Press the tape against the tool, leaving the release paper

on until ready to apply the bag itself. The tape usually goes on after the part is

layed up, especially if it is a wet layup.

Materials used in the vacuum bagging

process

Peel Ply

One the laminate is in place, it's time to apply the bag. The first item to go down is a peel

ply.

Peel plies are a tightly woven fabric, often nylon, and impregnated with some type of

release agent.

The peel ply will stick to the laminate, but it will pull away without too much difficulty.

If peel ply is used, it will absorb a small amount of resin, and this must be accounted for. A

net-resin prepare may end up too dry.

Applying the Peel ply

Bleeder and Breather

At least one layer of bleeder cloth goes above the release film. Bleeder is a thick, felt-like

cloth. Its purpose is to absorb excess resin.

The bleeder also acts as a breather, providing a continuous air path for pulling the vacuum.

Applying the breather

Vacuum bag

The bag is the last item to be placed. It's a relatively thick plastic layer, available in

different amounts of conformability.

Applying the vacuum bag onto the sealant tape

DRILLING OF THE LAMINATES

Experimental procedure

1. Clean the drill bits and measure the initial land width using the tool maker’s microscope.

2. Mount the CFRP laminate on a suitable fixture in the vice of Vertical Machining Centre, and

ensure it is rigid.

3. Set up the drill bit in the drill tool holder of corresponding size. Check for wobbling of the drill

bit.

4. Set an offset of at least 20 mm and conduct a trial run to ensure the accuracy of the tool path.

5. Change the offset to zero and execute the program.

6. Once the experimental run is complete, remove the laminate and the drill tool. Collect carbon fibre

chips for further analysis.

7. Repeat the procedure for all combinations of laminate and drill bit.

8. Upon completion of all experiments measure the final land widths, and calculate the difference.

This value is indicative of the tool wear.

AMS CNC Vertical Machining Centre

RESULTS AND ANALYSIS

TAGUCHI ANALYSIS

Taguchi is a scientifically disciplined mechanism for evaluating and implementing improvements in the Product, Materials, Equipment and Facilities

The Experimental Design proposed by Taguchi involves using O.A (Orthogonal Array) to organize the Parameters affecting the process and the levels at which they are varied. Instead of testing all possible combinations like the Full Factorial design, the Taguchi method tests pairs of Combinations

Steps involved in the Taguchi’s Method

Formulation of the Problem Definition

Identify the Objective function to be optimized

Determine the Process Parameters

Design the Matrix Experiment (O.A)

Preparation for the Experiment

Conduct the Designed Experiment

Analyse the Experimental Data

Interpretation of the Experimental Results

Predict the optimum levels and performance

Conduct the Verification Experiment

Procedure followed for Taguchi analysis

There are 4 parameters involved in the analysis, namely, spindle speed, feed, drill-bit diameter, point angle. Each of these parameters consists of 3 levels. The total experiments for each combination of parameter and level comes up to 64.

Taguchi analysis on the Minitab software reduces the total number of experiments to 9. These are the key experiments which are vital to the study of the above mentioned parameters on the wear.

To account for the error, 2 trials were performed for each experimental run. The average land width values were measured and entered into the Taguchi table

Taguchi table

Experiment no Spindle Speed

rpm

Feed

mm/rev

Drill

Diameter

mm

Point Angle

Degrees(0)

Average of Land

width difference

mm

E1 4000 0.3 6 90 0.075

E2 4000 0.4 8 104 0.213

E3 4000 0.5 10 118 0.160

E4 5000 0.3 8 118 0.095

E5 5000 0.4 10 90 0.170

E6 5000 0.5 6 104 0.075

E7 6000 0.3 10 104 0.070

E8 6000 0.4 6 118 0.030

E9 6000 0.5 8 90 0.105

Signal to noise ratio

After carrying out the reduced number of experiments by Taguchi Method, the observed Experimental Data is transformed into a Signal to Noise Ratio (S/N Ratio)

The objective is to maximize the S/N Ratio for any of the three criterions given below.

There are three categories of quality characteristics in the analysis of the S/N Ratio:

The Lower the Better (Target Value=0);

The Higher the Better (Target Value=∞);

The Nominal the Better (Target Value=µ).

Signal to Noise ratio

The criterion chosen in this project is STB (Smaller the Better) because minimum Tool Wear is desired.

The S/N Ratio for different levels of a Process Parameter are calculated. The level which has maximum S/N Ratio is chosen as the most optimum level for a particular Process Parameter to get an Optimized Response.

Response table for signal to noise ratio (smaller is better)

Level Spindle Speed

rpm

Feed

mm/rev

Drill Diameter

mm

Point angle

Degrees(o)

1 18.29 21.10 23.12 19.3

2 20.38 20.94 19.26 20.76

3 22.46 19.08 18.74 21.07

Delta 4.17 2.02 4.38 1.77

Rank 2 3 1 4

As per the Delta Value of each factor from Table 4.3 i.e. the difference between the maximum and minimum S/N Ratio of Overall Tool Wear for a factor, the ranking of the Process Parameters in terms of significance is as follows:

a. Drill diameter

b. Spindle speed

c. feed

d. point angle

Analysis of S/N ratios

Referring to the plots above we come to the following optimised conclusions for each process parameter to obtain a minimum response:

-spindle speed=6000 rpm

-feed= 0.3mm/rev

-drill diameter= 6 mm

-point angle= 1180

Response table and plots for means

Level Spindle Speed

rpm

Feed

mm/rev

Drill Diameter

mm

Point angle

degrees (o)

1 0.12467 0.08833 0.07250 0.10917

2 0.09750 0.09800 0.10967 0.09383

3 0.08000 0.11583 0.1200 0.09917

Delta 0.04467 0.02750 0.04750 0.01533

Rank 2 3 1 4

As per the Delta Value of each factor from Table 4.4 i.e. the difference between the maximum and minimum Mean Overall Tool Wear for a factor, the ranking of the Process Parameters in term of significance is as follows:

a. Drill diameter

b. Spindle speed

c. feed

d. point angle

Main effect plot for means

Trends based on means plot

Drill diameter and Point Angle - As the diameter and point angle of the drill bit increases the area of contact increases, which leads to increased wear.

Spindle speed- Referring to Taylor’s tool life equation , it is observed that the feed rate is inversely proportional to the tool life. As the feed increases the tool life decreases, which happens with increasing wear.

Feed- As the feed increases the rate of work done by frictional force increases which leads to increasing tool wear.

ANOVA (Analysis of Variance)

ANOVA Table

Source DF Adj SS Adj MS F-value P-value %

Contributi

on

Regressio 4 0.007661 0.001915 11.72 0.018

Spindle

speed

1 0.002993 0.002993 18.31 0.013 36.55

Feed 1 0.001134 0.001134 6.94 0.058 14.04

Drill

Diameter

1 0.003384 0.003384 20.71 0.010 45.03

Point 1 0.000150 0.000150 0.92 0.392 4.37

error 4 0.000654 0.000163

Total 8 0.008315

S R-sq R-sq(adj) R-sq(pred)

0.0127832 92.14% 84.28% 60.39%

Trends based on ANOVA Table

The term R-sq in the table 4.5 is explained below:

R-sq: It signifies that 92.14 % of the data fits the linear model.

The percentage contribution of each Process Parameter is as follows:

1. Spindle speed- 36.55 %

2. feed-14.04 %

3. Drill diameter- 45.03

4. Point angle-4.37

From the above data drill diameter and spindle speed are the most sensitive to changes in values, that is if these values are changed there is a significant impact on the tool wear. This is followed by feed which has a comparatively lesser effect on the tool wear. The point angle has an almost negligible effect on tool wear.

Regression Equation

The regression analysis for overall tool wear (Land Width Difference) is:

Overall tool wear = Land width difference = 0.0995 -0.000022 Spindle Speed + 0.1375 Feed

+ 0.01188 Drill Diameter - 0.000357 Point angle

Micrograph Analysis

To observe the wear pattern, microstructure images of the land and chisel edge of the drill-bits were examined. The instrument used was the Trinocular Inverted Metallurgical Microscope

Custom made fixtures were used to mount the drill bits on the instrument. The images and the corresponding wear patterns obtained

By observing the images we can tell that spindle speed and feed rate are the major factors effecting the wear pattern more than the other factors such as point angle, drill diameter

Procedure for obtaining micrograph images

The drill bits required to conduct the micrograph analysis are selected.

The drill bit is mounted on the custom made fixture.

The lens with an appropriate magnification is swivelled into position. The same magnification is selected on the software.

The section of the drill bit required for observation is brought into view using the lateral and longitudinal levers. It is then focused until a clear image is obtained.

The image of the wear pattern is observed and a snapshot of the same is saved.

Micrograph Analysis

The above image shows the drill tool wear at a spindle speed of 5000 rpm, feed rate of 0.3 mm/rev. At this level, abrasion wear in the form of parallel ridges due to ploughing can be observed. The abrasion wear is combined with adhesion wear as we can observe that the carbon fibre has adhered to the land of the drill bit.

The above image shows the drill tool wear while the spindle speed is set to 4000 rpm at a feed rate of 0.5 mm/rev. At this level the parallel ridges are not as deep as those of the higher speeds but scratches can be observed in the wear pattern due to high feed rate.

The above image shows the drill tool wear pattern at a speed of 6000 rpm and a feed rate 0.4 mm/rev. The ridges are the deeper than the lower speed levels with micro cracks, possibly due to overloading.

The above image shows the drill tool wear at a spindle speed of 6000 rpm and a feed rate of 0.5 mm/rev. At this level, we can observe abrasion wear with craters possibly due to diffusion of materials between drill bit and laminate. As a result we can observe that the ridges are not as deep as the levels with the same spindle speed and lower feed rates.

The above image shows wear of a drill tool of 8mm diameter at a spindle speed of 6000 rpm and a feed rate of 0.3 mm/rev. At this level, surface erosion due to fatigue can be observed. This is due to higher area of contact compared to the other levels.

CONCLUSION

1. From the experiments conducted the following conclusions were derived:

There was a decrease in the Land Width of the drill bits after drilling of the holes.

Powdered chips were observed for all the experimental runs.

Delamination of the Composite Material was found during machining operations.

2. For the Overall Tool Wear, it can be seen from Fig. 4.1 that the optimum levels for minimum

Tool Wear are N3 (6000 rpm), F1 (0.3 mm/rev), D1 (6 mm) and Ɵ3 (118°).

3. From the ANOVA Table, Drill Diameter has been found as the most significant Process

Parameter towards Overall Tool Wear with a percentage contribution of 45.03%, followed by

Spindle Speed (36.55 %), Feed (14.04 %) and Point angle(4.37 %).

4. So it can be observed that there will be a drastic change in tool wear when the drill diameter

and spindle speed change. Point angle has an insignificant effect on the tool wear, compared to

the other parameters.

5. For the experimental study, the Tool Parameters have been used as the Input Control

Parameters whereas the Material Parameters have been kept constant.

6. The primary process parameters affecting the wear patterns are spindle speed and feed rate. The mostcommon wear pattern is abrasion with parallel ridges. At higher spindle speeds and low feed rate abrasionwith adhesion wear has been observed.

7. At higher feed rates, scratches perpendicular to the parallel ridges in the wear pattern can be observed.

8. At higher spindle speeds deeper ridges and micro cracks due to overloading can be observed.

9. At higher drill tool diameters, surface erosion due to fatigue is observed

10. For future work, the various Material Parameters like fibre orientation, fibre material, materialthickness, fibre volume fraction, and specifications of the Drill Bits can be changed. These can then beused as the Input Control Parameters. Also the machining conditions like low-range and mid-range speedcan be varied.

11. Furthermore experimental validation can be conducted by choosing intermittent values of the processparameters and comparing the experimental wear with wear obtained from the regression equation(theoretical), where wear is the average land width difference. The percentage error can then becalculated.