experimental analysis of drilling damage in thin carbon epoxy plate using

Experimental analysis of drilling damage in thin carbon/epoxy plate usingspecial drills

R. Piquet* , B. Ferret, F. Lachaud, P. Swider

Laboratoire de Ge´nie Mecanique de Toulouse, Institut Universitaire de Technologie Paul Sabatier, De´partement de Ge´nie Mecanique et Productique, 50chemin des maraıˆchers, 31077 Toulouse Cedex 4, France

Received 11 August 1999; received in revised form 1 March 2000; accepted 22 March 2000

Abstract

The aim of this study is drilling with a twist drill and a specific cutting tool of structural thin backing plates in carbon/epoxy. Drilling with atwist drill of the bolts’ holes to fix a stiff plate reinforcement in front of the damage leads to defects and damages at the entrance, on the holewall and at the plate exit.

The possibility to manufacture carbon/epoxy with a conventional cutting tool was analysed and the limits of the twist drill were shown.Consequently we defined a specific cutting tool. Series of comparative experiments were carried out using a conventional twist drill and thisspecific cutting tool. The results showed the capabilities of the specific cutting tool because several defects and damages usually encounteredin twist drilled holes were minimised or avoided (entrance damage, roundness and diameter defects and plate exit damage).q 2000 ElsevierScience Ltd. All rights reserved.

Keyword: Drilling

1. Introduction

The using of composite parts is increasing within thedesign of plane structural elements. For such structureshigh performance carbon/epoxy material are mainly used.For definitive assembling or structural temporary repairs,the permanent joints are achieved using bolts and it hasbeen verified that the reliability of assembling is sensitiveto the quality of the bolt holes.

The possibility to machine with a conventional tool,generally used to metals, was examined. Afterward, thecauses of various damages, observed on a hole, wereanalysed. These observations allowed us to define a specifictool better adapted to this type of drilling. Then, the machin-ing behaviour of this specific tool was compared with that ofa double fluted conventional drill. Lastly, the holes drillingare observed.

2. Damages analysis caused by a twist drill

The use of a twist drill, for the drilling of the bolt holes inthin carbon/epoxy composite without backing plate, cause

damages. The failure behaviour of epoxy matrix and carbonfibres is brittle. In the general context of machining compo-site materials Ko¨nig [1] and Guegan’s [2] studies reveal thatthe general machining conditions applied to metals canbe applied to composite materials with a thermosettingplastic matrix. It is therefore possible to drill compositestructures using a conventional twist drill (Fig. 1).Moreover, given the possibilities of shocks particularlyin the case of carbon/epoxy, tools should be tough andwear-resistant [2–5]. Given the machining constraints,only conventional and “micrograin” tungsten carbide,natural diamond splinters and synthetic diamonds(PCD) can be used.

Drilling with cutting tools leads to defects and damage atthe plate entrance, exit and in the hole wall. The hole inletdefect (Fig. 2) is linked to the material, to the tool, and to thecutting conditions (FC andFZ). Guegan [2] and Ho-Cheng[6] show that there is a peel effect of plies along the edge ofthe major drill cutting edges which takes place the momentthe tool tip [7] encounters the first ply. This defect increaseswith the rake angle [7] (g) and tends towards ply detach-ment. The chip thus formed tends to turn backwards with theaction of the flank (Ag ) [7]. The resulting load pushes ontothe hole edge via the fibres which connect the chip to the restof the ply. At this instant, the bonding strength applied bythe matrix under the ply is the only force which can resist

Composites: Part A 31 (2000) 1107–1115

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* Corresponding author. Tel.:1 33-562-258-722; fax:1 33-562-258-747.

E-mail address:[email protected] (R. Piquet).

delamination. Delamination occurs if the localised peelforce is greater than that allowable for ply delamination.

To check the nominal drilling diameter Guegan [2]explains that the fibres are bent by the action of the drill’scutting edge. A facet appears along the cut fibre which isinclined in relation to the direction of the minor cuttingedges [7] of the tool. This bending is caused by elasticdeformation of the fibre akin to buckling. The return tothe initial position, after a brittle shear failure, causes tigh-tening around the drill. As a consequence, the drilleddiameter is less than the drill diameter (Fig. 3). The drillmaterial also contributes to this phenomenon, as the cuttingradius of the bit increases with wear.

The roundness error is due to the material’s anisotropy.For each angular position of the drill’s cutting edge in rela-tion to fibre orientation, there exists a different relative rein-forcement direction (Fig. 3). If we take the case of aunidirectional laminate ply, the drilling corner tool will, at

a certain moment in time, shear the material along the fibreaxis at 08, then following an increasing angle up to 908. Thiscycle is repeated four times per revolution. In addition toshearing (Fig. 3), the fibres in the 0–908 zone are loaded bycompression whereas those between 90–1808 are undertension. A difference has to be distinguished between thevarious types of fibre behaviour depending on the angularposition of the drill’s cutting edge. At 908, the fibres bendthe most and so their shrinkage by elastic deformation

R. Piquet et al. / Composites: Part A 31 (2000) 1107–11151108

Fig. 1. Conventional double fluted twist drill characteristics.

Fig. 2. Hole inlet defect.Fig. 3. Hole defects observed in a uni-directional plate, whereD is thediameter in practica,Dr the real diameter, andDth the theorical diameter.

caused by the action of the cutting edge makes the holenarrower, and it becomes elliptical. The angle between thelarge diameter of the ellipse and the fibre axis is 908. Thisphenomenon may be supposed identical in the case of thelay-up sequence of multidirectional plies, although thezones considered are different depending on material thick-ness. All these phenomena have been checked experimen-tally by Arola [8].

The pulled surface (Ra) is affected by the tool’s material,anisotropy and the type of loading. Ko¨nig [1] points out thatthe least favourable direction is to be found between 20 and458, as the fibres in this zone receive a compression loadingsuperimposed on the shearing action caused by the cuttingedge. The roughness is minimal when the fibres are between0 and 908.

The narrowing of the hole around the two minor cuttingedge create friction which causes localised heating as thecarbon/epoxy composite is thermically insulating as awhole. An abnormally high temperature of the hole cancause local damage to the epoxy matrix, if the temperaturereaches or exceeds the glass transition temperature (Tg). Inaddition, the Fibredux 914 epoxy matrix used in experi-ments contains thermoplastic nodules which, when affectedby high temperature, can become rubbery and stick to thecutting edge of the cutting tool. The transversal conductivity(l) of the carbon/epoxy can be calculated by using Spring-er’s [9] or Farmer’s [10] models. These models require thecalculation of l being the thermal conductivity of thecomposite perpendicular to the fibres,lm is the matrix ther-mal conductivity, l f the thermal conductivity of fibresorthogonal to their direction andVf fibre volume fraction.

The use of a conventional twist drill is limited whendrilling thin composites without a backing plate. The rela-tively large range of the non-cutting chisel edge [11] is itsmain drawback. When the active part of the drill approachesthe last laminate plies, beyond the critical thrust stressgreater than the ply cohesion force, cracking forms andthen spreads (Fig. 4). Crack propagation remains on theplane and frequently starts in a matrix-rich zone. Adhesionfailure in this matrix leads to bending and delamination of

the remaining plies. It is therefore extremely useful to tryto define a specific tool geometry to improve compositedrilling.

3. Definition of a specific tool

The reference tool is the double fluted twist drill [12]whose reference dimensions are given by the ISO 3002-2:E66-504 standard [7]. Although possessing a cutting radius(ra) [7] of the cutting edge greater than diamond, economicconsiderations have led us to use “micrograin” tungstencarbides which are tougher than ordinary carbides andequally hard. Tool selection is made from a range of two-figure numbers between 0.1 and 40, depending on the tough-ness or resistance to wear required. Machining carbon/epoxy plates requires the K20 rating as the tool must beboth tough and wear-resistant.

Grinding the drill with the two conical sides of the clear-ance (DIN1897) gives a cutting angle (g f) and a clearanceangle (a f) which are variable along the two cutting edges[7]; the non-cutting chisel edge represents 20% of the drilldiameter. A two slope grinding of the clearance face enablesthe angular defects of the preceding drill to be corrected.The three slope, or cruciform grinding (DIN1412C) has thesame advantages, but brings into existence of a facet or thirdclearance linked to the machining of the chisel edge whichimproves cutting and increases ease of penetration. Thisdrill is usually used in industry to drill composite plates.These modifications are unfortunately insufficient to reducethe defects caused by drilling thin carbon/epoxy plates, inthe working conditions shown in Fig. 4.

From a consideration of the above points, and an analysisof the defects, it is possible to define a specific tool forcomposite drilling. To be more rigorously compatible withthe twist drill, this tool is made of “micrograin” tungstencarbide (K20 rating). To reduce, or eliminate the entrancedefect (Fig. 2) Ko¨nig [1] and Guegan [2] show that a smallrake angle (max 68) prevents the first plate ply from liftingup and tearing off. A greater number of cutting edges

R. Piquet et al. / Composites: Part A 31 (2000) 1107–1115 1109

Fig. 4. Exit defect in a hole drilled in a composite without a backing plate.Fig. 5. Specific tool geometry for OP1 and OP2.

increases the length of tool/part contact, thus facilitating theremoval of heat produced by unlubricated machining. Infact, the thermal conductivityl of the carbon/epoxy testplates is equal to 30 W m21 K21, and that of tungstencarbidelWC is greater than 120 W m21 K21. It is the toolitself that mainly removes the heat produced by machining.In this case, the highest angle (k r) [7] is reduced, andreaches a value close to 708 in order not to weaken thetool [2,11]. A maximum angle of 1188 is initially necessaryfor the main cutting edges which are not very deep then 708for the minor cutting edges. Having between three and sixcutting edges improves tool control without undue frictionof the chamfers on the hole wall. Three main cutting edgestogether with three chamfers of the minor cutting edge with-out flank, are enough to evacuate the heat produced and toimprove tool rigidity, thus preventing it following thedirection of the fibres. Hole circularity is likewise improved.The three secondary cutting edges are concave in shape inorder to decrease the pulled surface effect (Ra). When thedrill approaches the end of the plate, the last few plies bendand then tear off (Fig. 4). Ko¨nig [1], Guegan [2], Ho-Cheng[6] and Jain [14] have demonstrated that this defect is linked

to tool geometry and more specifically to the chisel edgedimensions. It becomes necessary therefore to eliminate thechisel edge completely by machining supplementary cuttingfaces.

From an investigation of various tool suppliers, we havebeen able to identify and select two tools with a cuttinggeometry close to the specific tool. These are designatedin Fig. 5 as OP1 and OP2. Both are made of K20 rated“micrograin” tungsten carbide, comprising three cuttingedges with a rake angle equivalent to zero which is likelyto cause vibrations during machining. The OP2 tool has acutting geometry similar to that proposed by Roy Meade[11]. OP1 tool: 3 cutting edge, 598 for the major cuttingedge and variable to 08 for the minor cutting edges, twistand rake 08, clearance 68, chamfer angle 08.

OP2 tool: 3 cutting edges, 598 for the major cutting edgeand variable to 08 for the minor cutting edge, twist and rake08, clearance 68, no chamfer.

Experimental procedure based on the Taguchi experi-mental plan has led us to choose between these two tools[15].

Fixed parameters are those linked to

• Cutting.no lubrication, and chips are vacuum extracted,the drill is controlled by an operator-held drilling gun,feed rate is manually and not controlled. Correspondingto drilling temporary structural repairs on aircraft orwhen making sub-assemblies.

• Drilling. pneumatic portable, rotation speed N is equal to3100 rpm.

• The material to be machined: a first plate of ten plies[908/ 1 458/08/2458/08]S, and a second of 12 plies[908/ 1 458/08/08/2458/08]S.

The variable parameters or factors are show in Table 1.Hole conformity criteria are the inlet defect, the diameter,

visualising of the cylindrical surface generated, the round-ness error and the exit defect.

The degree of difficulty of the various parameters areshown in Table 2. The only significant interaction is thestate of the tool with plate thickness (parameters A andB). The size of Taguchi’s table is L8, as the number ofvariables is equal to seven. Changing a worn drill is the


Table 1State of variable parameters

Variable parameters Condition 1 Condition 2

Tool condition New WornPlate thickness (mm) 2.8 3.36Pre-drilled hole Ø 3.3 mm Yes NoHydraulic damper Yes NoOperator O1 O2

Table 2Level of difficulty of the variable parameters

Parameters Condition 1 Condition 2 Level of difficulty

A Tool condition New Worn Very difficultB Thickness (mm) 2.8 3.6 DifficultC Pre-drilled Yes No DifficultD Damper Yes No DifficultE Operator O1 O2 Easy

Table 3Using Taguchi’s L8 table to check drilled diameter

Tool condition Thickness Interaction Pre-drilled hole Operator Damper Diameter

1 1 1 1 1 1 1 8.5002 1 1 1 2 2 2 9.2503 1 2 2 1 1 2 8.5004 1 2 2 2 2 1 9.0005 2 1 2 1 2 1 8.7506 2 1 2 2 1 2 8.0007 2 2 1 1 2 2 7.0008 2 2 1 2 1 1 7.750

Average 8.344

most limiting factor as it implies a long and painstakingprocedure. The thrust force of drillFZ can be controlledby a hydraulic damper fixed to the drill. All the pre-drillingholes are drilled with a 3.3 mm wide twist drill to minimisethe possible chisel edge effect. In order to observe wearinfluence on the hole geometry, worn (OP1) and (OP2)tools are used. A plan of experiments is applied to eachtool (OP1, OP2) and to each variable factor. Taguchi’stable relative to the hole diameter drilled with the OP1tool is shown on Table 3. Each line of Table 4 contains amark out of ten given to each of the parameters to bechecked. OP1 tool has been selected as its average markis the highest. Planes and radius values of the specific tool(OP1) are not provides by the tool manufacturer. This tool isa product from the G.M.I company (9 rue Buffault 75009,Paris, France).

4. Experimental study during machining

The machining behaviour of this specific tool is comparedwith that of a conventional twist drill. Fig. 6 gives thedimensions of the carbon/epoxy test plates (T300/914) aswell as their positioning on the table of a CNC millingmachine. Machining stresses are measured by means of aKistler 9272 piezo-electric dynamometer with four compo-nents (FX, FY, FZ and MZ) connected to a Kistler 5019Amulti-input charge amplifier. The characteristics of thespecific tool OP1 and the twist drill with cruciform grinding(DIN1412C) designated as F2 are shown in Table 5. Forcertain tests, a pre-drilling hole was pierced using a twistdrill (DIN1412C) designated as F3 in order to monitor theeffect on the final hole quality.

In accordance with the experiments carried out by Jain[14], the spindle rotation speed is 1200 rpm. Three tests pertool were carried out with a constant feed rate throughoutthe plate thickness (Table 6). A supplementary test per toolwas carried out with a variable decreasing feed rate depend-ing on the tip on the position in the material thickness (Table7). Holes (Ø 4.8 mm) were drilled without lubrication withnew and used tools, with or without pre-drilling hole. Tables6 and 7 values are estimated with Jain [14] values.

In the case of drilling with a double fluted twist drill, thethrust forceFZ and the global momentMZ can be expressedby means of relations (1) and (2) [13]:

FZ � K·Kf f·D �1�whereFZ is the thrust force in N,K the corrective coefficient(effect of sharpening),Kf the specific cutting coefficient inMPa,f the feed rate per rotation in mm,D the drill diameterin mm.

MZ � KM·f·D2 �2�where MZ is the global moment in N m andKM the specificcutting coefficient in N m mm23. CoefficientsKf andKM aredefined experimentally.

MomentMZ results from machining momentMCZ inducedby the two main cutting edges of the drill and from momentMRZ, caused by friction between the two chamfers of minor


Table 4Experimental results for drilling hole

Tools OP1 OP2

Diameter 8.340 6.280Circularity 8.750 4.410Cylinder surface 8.910 6.100Entry defects 8.125 6.625Exit defects 7.375 6.625Average 8.300 6.010

Fig. 6. Experimental setup.

Table 5Description of tools used during the experiments

Name Ø (mm) Operation Flutes k r (8) g (8) a (8) Twist (8) Grinding

OP1 4.8 Final version 3 59 (major); 59–0 (minor) 0 6 0 SpecificF2 4.8 Final version 2 59 6 6 25 3 slopeF3 3.3 Pre-drilling 2 59 6 6 25 3 slope

Table 6Constant feed rate

Feed rate per revolution(f) (mm/rev)

0.05 0.125 0.2

cutting edges on the hole wall:

MZ � MCZ 1 MRZ �3�

The theoretical curves of forceFZ and momentMZ in rela-tion to the position of the tool’s active part in the test pieceare shown in Fig. 7. ForceFZ as well as machining momentMCZ are zero when the tip of the tool is inx1 (Fig. 7). Theyincrease in a non-linear fashion until reaching, when the tipis in x2, a maximum value which remains constant until thetip reachesx3. From then on, they decrease in a non-linearfashion to reach zero when the tip is inx4 and when the drillemerges from under the plate to reachx5, and during itsrapid return,FZ andMZ remain at zero until the tip of thedrill is in x1.

Friction between the two chamfers of minor cutting edges

and the hole wall creates a friction momentMRZ propor-tional to the cutting edge surface in contact with the holewall. This moment which appears when the drill tip is inx2

increases linearly until the tool corner it reaches inx4. Itremains constant until the tip is inx04 and decreases linearlyto reach zero when the tip is inx02.

Experimental curves (Fig. 8) relative to F2 drill show asimilarity with the theoretical curves (Fig. 7). In contrast,the experimental curve ofFZ relative to the specific toolOP1 (Fig. 9) shows no similarity with the theoreticalcurve. In fact, the tip of the drill had emerged from theplate even though its nominal diameter was not even atthe approach of the hole inlet (Fig. 10). A size control ofthe different holes was carried out using a comparator.

5. Defects and damages observations

For both OP1 and F2 tools, the slight construction in the908 direction appears in very few cases, and its value can beignored (5mm). This is due to the low value of the cuttingradius ra to the “micrograin” carbides used instead ofconventional carbides.


Table 7Decreasing feed rate

No. of plies 1–20 21 22 23 24

Feed rateperrevolution(f) (mm/rev)

0.050 0.025 0.009 0.002 0.001

Fig. 7. Drilling efforts; theoretical results.

Fig. 8. Experimental results (drill F2 andf � 0:05 mm=rev�:

Fig. 9. Comparison of the thrust force between OP1 tool and F2 drill�f �0:05 mm=rev and thickness plate of 3 mm).

5.1. Constant feed rate

With a new tool, macroscopic observation reveals that thedrillings obtained by an F2 drill cause damage at the plateentrance and exit (Fig. 11) which are reduced to a minimumby pre-drilling. The same is true for the surface state, fibreand matrix stripping on the hole walls. Only those holesobtained by pre-drilling are within the tolerance range.The specific tool OP1 causes no damage at the plateentrance and exit (Fig. 11). It enables the hole to be drilledwithin tolerance limits with an improved surface state. Forthis tool, a re-drilling hole is thus unnecessary. For the bothF2 and OP1 tools at a feed rate per revolution of 0.2 mm,maximum FZ stresses are high. These stresses, linked tomaintenance conditions, lead to major plate displacement,which are the cause of roundness errors generated by theminor cutting edges. The lateral cross section of the zonecomprised between point B and the nominal diameter (Fig.10) of the OP1 tool increases the elliptical aspect of thedefects. When the feed rate per rotation is 0.125 mm, corre-

sponding to the thickness of a unidirectional laminate ply,both F2 and OP1 tend to lift up the first ply in the plate. Thegreatest damage is caused by the cutting edge angle of theF2 tool.

5.2. Decreasing feed rate

The F2 drill gives better results. Pre-drilling improveshole quality. Fibres do not peel away at the plate exit andthe drilled hole diameter is within the tolerance range. TheOP1 tool is able to machine within the tolerance range,although a flash appears at the plate outlet. When the feedrate is very low, the extent of the contacts between thecutting edges and the material drilled creates a significantamount of friction leading to an abnormal temperatureincrease along the hole wall (Fig. 10).

Thrust force and machining moment curvesFZ and MZ

(Fig. 8) caused byF2 and F3 drilling tools agree with thetheoretical curves shown in Fig. 7. This is not the case forthe OP1 specific tool, which possesses a higher number ofcutting edges, and a different geometry in the workingsection. Fig. 10 shows the different geometrical configura-tions of OP1 and F2. The drilling tip of OP1 (segment AB)has reached the lower edge of the thin plate whereas itsnominal diameter (point D) has not approached the upperedge, the thrust forceFZ of the OP1 drilling tool is less thanthat obtained for the F2 drill (Fig. 9). WithFZ at a maximumfor the OP1 drilling tool, the tip no longer pushes against thelast few laminate plies, thus reducing the risk of delamina-tion to a minimum.

In the case of decreasing feed rate, the chip machined byeach cutting edge is less than the minimum shaving, as theradial cut is linked to a relatively small point angle (Fig. 12)

The cutting edge of the drilling tool pushes more materialahead to a flash at the hole exit. Cutting conditions musttherefore take into account tool geometry, and feed rate foreach cutting edge and each revolution. This feed rate mustbe increased when point B on the main cutting edge is clearof the plate. Relation 3 gives the feed rate value to beprogrammed in (fP) as a function of the minor cuttingedge geometry (curved segment BCD) in order to maintainconstant shaving thickness (Fig. 12).

fP � fsinkr

·n �4�


Fig. 10. Geometrical comparison between F2 and OP1 working section.

Fig. 11. Inlet and exit hole defects.

wheren is the number of cutting edges andk r the cuttingedge angle of the drilling tool.

Two sets of the experiments have been carried out usingOP1 drilling tools:

• constant feed rate values between 0.5 and 62.5mm percutting edge;

• variable feed rate per cutting edge as a function of whenthe main cutting edges reach the under edge of the plate.

The last few plies are delaminated by the drill tip createsan identical maximum thrust in both case (Fig. 13). As soonas point B on the main cutting edge is clear of the plate, feedrate is varied and the thrust forceFZ drops by 50%. Whenfeed rate is variable, thrustFZ and momentMZ curves arecomparable to the theoretical curves, and new momentdistribution can be observed. A machining torque is addedto the friction torque, which reduces overheating andextends the life of the drilling tool. The both cases, theholes are within the tolerance range, with no evidence ofdelamination or flashing at the plate exit for a feed rate percutting edge per rotation of less than 45mm.

6. Conclusion

This research has analysed the effects of drilling toolgeometry on the drilling quality of thin carbon/epoxy plates.For certain given cutting conditions, it emerges that thegeometry of the drill’s working section plays a decisiverole in the defects and damage observed when drillingthin carbon/epoxy plates without a backing plate. For aconventional double fluted twist drill to give goodresults on these plates, it is necessary to pre-drill ahole in order to neutralise the chisel edge effect andto lubricate the machining process. A decreasing feedrate improves drilling quality, but leads to increasedwear and machining time.

The OP1 specific drilling tool defined from experimentalplans can give excellent results provided that drilling isdirectly carried out without the pre-drilling. Machiningconditions can further be improved by applying a variablefeed rate in relation to its geometry. This procedure, whichrequires the use of a CNC drilling machine, should helpresolve the problems related to the simultaneous drillingof metal/composite lay-up hybrids.


Fig. 12. Kr influence on the machining conditions.

Fig. 13. Experimental comparison ofFZ andMZ versus feed rate par cuttingedge.

Acknowledgements

We wish to thank R. Toppan, head of the S.I.D.M.I (Soci-ete Industrielle Aeronautique du Midi—Cornebarrieu—France) for his financial assistance, and also P. DUBOURG(IUT Paul Sabatier—31 Toulouse—France) for his invalu-able contribution to our study. This work was made during acontract with a manufacturer on structural temporary repairs(Centre Technique des Mate´riaux et Structures de la Direc-tion Generale de L’Armement).

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experimental analysis of drilling damage in thin carbon epoxy plate using

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