Methods for Reducing Interface Aperture Inconsistency During NC Orbital Milling of Aircraft Laminates with Coarse Pitch

，，，

1.College of Mechanical and Electrical Engineering，Nanjing College of Information Technology，Nanjing 210046，P.R.China；

2.College of Mechanical and Electrical Engineering，Nanjing University of Aeronautics and Astronautics，Nanjing 210016，P.R.China；

3.School of Mechanical Engineering，Nanjing University of Science and Technology，Nanjing 210094，P.R.China

（Received 21 July 2019；revised 8 February 2020；accepted 15 April 2020）

Abstract: The methods for reducing interface aperture inconsistency are studied in NC orbital milling（NCOM）of CFRP/Ti6Al4V laminates with coarse pitch.Comparative experiments show burr，aperture inconsistency and error are typical interface defects.Meanwhile，aperture inconsistency and error are more serious than burr in NCOM with coarse pitch.As one of the major causes of interface defects，axial force and radial force are intensively studied.Based upon the machining principle of orbital milling（OM）and the actual hole-making condition in laminated structures，NCOM experiments with coarse pitch are conducted on CFRP/Ti6Al4V laminates under different cutting conditions. Then，the effects of interlayer clamping，minimal quantity lubrication（MQL），twice milling instead of reaming，and interlayer speed change on interface aperture are analyzed. Research shows that interlayer clamping，interlayer speed change and MQL can effectively reduce out-of-tolerance of interface aperture. When making holes of different diameters with one cutter，axial feed has a greater effect on interface aperture precision than tangential feed.When making holes of the same diameter with different cutters，small diameter cutter will reduce interface aperture precision in a single processing. But the method of“twice milling instead of reaming”can improve the aperture precision effectively.

Key words：CFRP/Ti6Al4V laminates；NC orbital milling；coarse pitch feed；interface aperture

0 Introduction

As a high-end equipment of modern aviation in?dustry，the quality and efficiency of lamination as?sembly in large aircraft has a great influence on flight security and manufacturing cost［1-2］. Studies find that the machining quality of connecting holes has significant influence on laminated components，such as roundness，perpendicularity，position accu?racy，hole surface quality，burr and chip between layers，fatigue life，etc［3-4］. Due to the coupling ef?fect of various factors in machining process，it is dif?ficult to effectively control the interlayer defects.Now，interlayer defect control has become a techni?cal bottleneck when making holes efficiently and nondestructively in laminates，especially in typical CFRP/alloy laminates［5-7］.

Many scholars have carried out a lot of research on interlayer defect control when push drilling（PD）of CFRP/alloy laminates. Ramulu et al. found that the cutting heat generated in PD of titanium alloy with high-speed would burn the composite material at the interface，and the cutting parameter sets for more difficult-to-cut material of the two laminated components were more suitable for processing the interface connection area［8］. Since the upper layer CFRP was supported by the lower layer Ti，the de?lamination and tearing defects at CFRP exit were in?hibited［9］. During PD test of clamped aluminum al?loy laminations，Melkote et al. found that high cut?ting speed，low feed speed and twist drill with a small top angle could reduce interlayer gap and inter?layer burr. In addition，clamping position had a sig?nificant effect on both［10］. Bi et al. confirmed that ap?plying pressing force or changing lamination se?quence was a feasible method to reduce the height of interlayer burr［11-12］. Luo studied the defects of holes in CFRP/Ti laminates，and pointed out that the higher the cutter feed speed，the lower the work?piece stiffness，and the more serious the damage be?tween layers［13］. Gao revealed the generation mecha?nism of non-coaxiality in PD of CFRP/Ti lami?nates，and proposed process measures to effectively control interlayer gap and non-coaxiality［14］.

The above researches have played an important role in reducing interlayer defects when PD of CFRP/alloy laminates. However，with the trend of thin wall，large size and lightweight in modern avia?tion components，the weak rigidity of laminated structures has a more prominent effect on interlayer defects［14］. Refs.［15-16］found that the excessive ax?ial force in PD of CFRP/alloy laminates is more likely to cause interlayer gap. Therefore，it is neces?sary to study hole-making technology with small axi?al force.

Recent research shows orbital milling（OM）has more advantages than PD in cutting difficult-tocut materials，since it has small axial force，low cut?ter wear and high hole-making precision［17］，which becomes a new research spot when making holes in CFRP/alloy laminates［18］. Denkena studied the ef?fect of tangential and axial feed on OM of CFRP/Ti6Al4V laminates，and indicated that high tangen?tial feed speed and low axial feed speed were condu?cive to reduce aperture deviation［19］. Gui found inter?layer clamping and sharp cutter were both helpful in inhibiting interface orifice burr and CFRP internal delamination during OM of CFRP/Ti6Al4V lami?nates［20］. Wang found that big laminated thickness and large number of holes might increase cutter wear when OM of aviation laminates，and further reduce the precision and quality of holes［21］. Pan pro?posed an online laminated interface identification method for making holes in CFRP/Ti laminates［22］.Gao et al. proposed an ultrasonic vibration assisted helical milling method，which significantly reduces the manufacturing defects，such as tearing and burr［23］. Hu installed OM unit on robot to carry out hole-making tests in CFRP/Ti6Al4V laminates，and found that the poor machining stability of robot system brought large aperture size of CFRP exit［24］.

Although a lot of research has been done on in?terfacial burns，hole surface quality and interlayer burrs of CFRP/alloy laminates，the research on in?terface aperture inconsistency is still lacking. The in?terface aperture inconsistency can affect the quality of lamination assembly，and even lead to the failure of lamination assembly. Presently，the combined process of“drilling and reaming”is often used to im?prove the interface aperture consistency. But its pro?cessing efficiency is very low. Although non-coaxial?ity in PD of CFRP/Ti laminates has been studied in Ref.［14］，few reports can be found on interface ap?erture inconsistency of OM. Affected by resultant cutting force and cutter revolution radius，interface aperture error occurs frequently in OM［25］. Limited by processing equipment and hole-making preci?sion，present OM mostly uses small pitch. With the wide use of CFRP/Ti6Al4V laminates in modern aircraft structures，the influence of spiral feed on hole-making efficiency becomes a significant prob?lem.

To improve the hole-making quality and effi?ciency in CFRP/alloy laminates，He et al. proposed a new machining method as OM with coarse pitch［26-27］. He discovered the processing efficiency of coarse pitch was about 3—18 times that of small pitch when OM of Ti6Al4V. During OM of CFRP，the efficiency is 2—10 times［28］. However，the in?crease of axial feed pitch brings larger cutting force and accelerates cutter wear. This paper attempts to explore methods to reduce interface aperture incon?sistency when NC orbital milling（NCOM） of CFRP/Ti6Al4V laminates with coarse pitch. The research is helpful to improve hole-making efficien?cy and precision when NCOM of laminated struc?tures.

1 Machining Principle of NCOM with Coarse Pitch

1.1 Machining principle

As shown in Fig.1（a），cutter rotation is the main cutting movement，and cutter spiral feed movement is composed of feed alongzdirection and linear interpolation motion alongxandydirections.In OM the bottom edge performs continuous milling and the side edge participates in intermittent milling.According to the relative motion between the cutter and the workpiece，the undeformed chip model is established in Fig.1（b）.

Fig.1 NCOM

The cutting thickness per tooth of the bottom edge and the side edge can be calculated by Eqs.（1）—（2）.

wherensis the cutter rotation speed；nωthe cutter revolution speed；ethe cutter revolution radius；Zthe number of cutter teeth；Pthe axial feed pitch；fzt，maxthe cutting thickness per tooth of side edge；fza，maxthe cutting thickness per tooth of bottom edge.

If the cutter and other cutting parameters re?main unchanged，NCOM efficiency increases whenPincreasing. Meanwhile，fza，maxand the axial cutting depth of side edge are also increased. Finally the in?crease of cutting forces of two cutting edges acceler?ates cutter wear.

However，studies show that optimizing cutting parameters（such as increasingns，decreasingnωand increasinge）can realize the optimization of cutting force［29］. Furthermore，under the same processing efficiency，coarse pitch feed can reduce cutter revo?lution speed，increase axial cutting depth of side edge，and decrease tangential cutting thickness of side edge. The increase of the contact length of side edge and the decrease of cutter revolution speed can enhance the machining stability of NCOM.

1.2 Experimental design

The comparative experiment is design for mak?ing holes in CFRP/Ti6Al4V laminates. The size of the laminates is 150 mm×100 mm×13.8 mm. The thickness of CFRP is 3.8 mm and the thickness of Ti6Al4V is 10 mm，as shown in Fig.2（a）. Since laminated structures are not suitable to be cut in from the side of titanium alloy，CFRP is set on the upper side. Two four-tooth carbide end mills and one two-tooth carbide drill are used in experiment.The diameters of the two carbide end mills areΦ6 mm andΦ8 mm. The diameter of the carbide drill isΦ10.2 mm. All these cutters are coated by TiAlN. The shape of carbide end mill and carbide drill is shown in Fig.2（b）.

The hole-making experiment of NCOM with coarse pitch is carried on UPC710 machine center，as shown in Fig.2（c）. Cutting parameters are set as follows.ns=3 000 r·min-1，fzt=0.06 mm，e=1.1 mm，P=1 mm. In PD experiment，drilling speed is 800 r·min-1and axial feed speed is 80 mm·min-1. CFRP chip is removed by vacuum. Minimal quantity lubrication（MQL）is used when cutting Ti6Al4V.

Fig.2 Experimental system

The cutting force acquisition system is mainly composed of three-dimensional dynamometer Kis?tler 9275B and the corresponding charge amplifier Kistler 5019. Under the workpiece coordinate sys?tem，Kistler 9275B measures the cutting forces in three directions asFx，FyandFz. However，to study the influence of cutting forces on aperture pre?cision in NCOM，it is more recommended to ana?lyze tangential forceFt，radial forceFrand axial forceFain the cutter coordinate system. According to the kinematic relationship between the two coor?dinate systems，as shown in Fig.3，Fx，FyandFzcan be converted toFr，FtandFaby Eq.（3）.

whereθis the cutter rotation angle；?the cutter rev?olution angle；βthe cutter spiral angle.

Fig.3 Schematic diagram of cutter movement in NCOM

Coordinate measuring instrument Micro Hite DCC is adopted to measure the interface apertures of CFRP exit and Ti6Al4V entrance. Each aperture is measured in different circumferential positions for 3 times，and the average value is taken as the final interface aperture.

2 Experimental Analysis

2.1 Analysis of interface defects

The morphology of CFRP exit and Ti6Al4V entrance is photographed. The ideal diameter of the holes isΦ10.2 mm. Some of the holes are processed by NCOM with aΦ8 mm carbide end mill，the oth?ers are processed by PD with aΦ10.2 mm carbide drill.

Experimental results show the interlayer mate?rial at CFRP exit by two processing methods has no delamination，but still has some tear and burr，as can be seen from Fig.4（a）and Fig.4（c）. In PD，the high cutting temperature generated when drilling of Ti6Al4V brings some resin ablative trace at CFRP exit. While in NCOM with coarse pitch，burr and trace ablative are not obvious.

Seen from Fig.4（b），a large number of burrs accumulate at Ti6Al4V entrance in PD because drilling produces excessive axial force. The burrs at Ti6Al4V entrance in NCOM with coarse pitch are not particularly obvious，as shown in Fig. 4（d）.With the increase of cutter wear in NCOM with coarse pitch，a small number of burrs gradually ap?pear at Ti6Al4V entrance.

Fig.5 shows the apertures of entrances and ex?its when CFRP/Ti6Al4V laminates are processed by PD and NCOM with coarse pitch. The measure?ments show that the aperture precision of CFRP en?trance and Ti6Al4V exit reaches IT9 under two ma?chining methods，but the apertures at the interface have large dimension error. In Fig.5，the interface aperture precision of NCOM with coarse pitch is lower than that of PD. However，the interface aper?ture consistency of NCOM with coarse pitch is sig?nificantly better than that of PD. With the increase of hole number，the interface apertures might ex?ceed IT9.

Fig.4 Morphology of CFRP exit and Ti6Al4V entrance

Fig.5 Exit and entrance apertures of CFRP and Ti6Al4V

In a word，NCOM with coarse pitch could ob?tain better orifice burr and higher aperture consisten?cy at the interface than PD，and but low interface aperture precision. When the increase of axial feed pitchPwould sharply worsen interface defects，in?terface aperture inconsistency and error happen more serious than burr. In experiments of NCOM with coarse pitch，it is also discovered that interface aperture consistency and precision are not only af?fected by cutting force，but also by coupling effect of many other factors，such as cutter diameter and cutting stability，etc.

2.2 Analysis of cutting force

Cutting force is an important factor that directly affects interface aperture precision of CFRP/Ti6Al4V laminates. Fig.6 shows two kinds of cut?ting forces produced byΦ10.2 mm carbide drill andΦ8 mm carbide end mill. In PD，Fain the upper CFRP is about 350 N，andFain the lower Ti6Al4V is about 1 470 N. While in NCOM with coarse pitch，the values ofFain CFRP and Ti6Al4V are about 320 N and 820 N. When machining the upper CFRP，Faof NCOM with coarse pitch is slightly less than that of PD. In the lower Ti6Al4V，Faof NCOM with coarse pitch is about 50% of that of PD. At the moment when the cutter cuts into Ti6Al4V from CFRP，the effect ofFaon interface aperture precision in NCOM is less than that in PD.This maybe one of the important reasons that the in?terface aperture consistency of NCOM is better than that of PD.

Fig.6 Cutting forces of two hole-making methods

Fig.6 also showsFrof NCOM is obviously big?ger than that of PD when processing CFRP/Ti6Al4V laminates. The excessiveFrmight cause cutter relieving along radial direction，thus enlarg?ing aperture error. So the aperture precision of NCOM is lower than that of PD. As the processing depth of CFRP/Ti6Al4V laminates increases in NCOM，the increase of theFrdifference between Ti6Al4V and CFRP reduces the interface aperture consistency.

Seen from Fig.6（a），Fain CFRP increases slowly at first，then reaches steady state. Next，a sudden change appears inFa，which indicates that the cutter cuts into the lower Ti6Al4V. When mak?ing holes in CFRP/Ti6Al4V laminates and CFRP respectively by PD，the change ofFain the upper CFRP of laminates is obviously different from that in CFRP materials［30］. The reason is that the materi?al properties of CFRP in the laminated structures are affected by the drilling temperature. The con?straint of the lower Ti6Al4V can avoid cutter reliev?ing，so the axial force at CFRP exit is relatively sta?ble. However， in PD experiments on CFRP/Ti6Al4V laminates and Ti6Al4V materials，the variations ofFain two materials are very similar.

Experiments of NCOM with coarse pitch are carried out on CFRP/Ti6Al4V laminates，CFRP materials and Ti6Al4V materials respectively.Faamplitudes in the upper CFRP of laminates and CFRP materials are very close. WhileFain the low?er Ti6Al4V of laminates is significantly larger than that in Ti6Al4V materials. There are two possible reasons for this：the first is cutter wear during the machining of CFRP，the second is the CFRP chip which has not been sucked away in time. When Ti6Al4V is processed，the chip sticks to the rack and flank faces of the cutter and increases the fric?tion.

3 Control Methods for Reducing Interface Aperture Inconsistency

Based upon current research on hole-making technology in large aircraft，NCOM with coarse pitch is studied in this paper to reduce interface de?fects of CFRP/Ti6Al4V laminates by analyzing in?terlayer clamping，cutting parameters，lubrication，cooling，twice milling instead of reaming，and inter?layer variable speed，etc.

3.1 Interlayer clamping

Influenced by the shape and size of the work?piece and the axial force in hole-making process，in?terlayer gap occurs when making holes in aircraft laminates. After NCOM with coarse pitch，a small amount of chip accumulates in the interlayer gap，which produces burrs at the interface orifice. When the cutter cutting into Ti6Al4V from CFRP，the in?terlayer gap intensifies the cutter impact，and affects the interface aperture precision. To study the effect of interlayer clamping on interface aperture precision in CFRP/Ti6Al4V laminates， two methods of clamping and non-clamping are adopted in experi?ments of NCOM with coarse pitch to make holes ofΦ10.2 mm. The thickness of CFRP is 3.8 mm and the thickness of Ti6Al4V is 10 mm. AΦ8 mm car?bide end mill with four teeth is selected.nsis 3 000 r·min-1，fztis 0.03 mm，eis 1.1 mm，Pis set as 0.5，1，1.5 and 2 mm. The test results are shown in Fig.7.

Fig.7 Influence of interlayer clamping on hole-making in laminated structures

From Fig.7（a），it is found that interlayer clamping is helpful for improving aperture precision and consistency of CFRP exit and Ti6Al4V en?trance. When the interlayer clamping is adopted，both apertures at CFRP exit and Ti6Al4V entrance gradually decrease with the increase ofP. And the aperture of CFRP exit is more and more close to the designed aperture，while the aperture of Ti6Al4V entrance is more and more deviated from the de?signed aperture. But the interface aperture consisten?cy of CFRP exit ant Ti6Al4V entrance barely changes. Without interlayer clamping，the aperture of CFRP exit becomes larger whenPincreases，and the aperture of Ti6Al4V exit decreases continu?ously with the increase ofP. Both of the two aper?tures deviate from the designed aperture. Under a certainPvalue，the aperture change of interlayer clamping is smaller than that without clamping.Fig.7（b）showsFaandFrof each hole. Therefore，the interlayer clamping can effectively improve the rigidity of cutting area and restrain the abrupt change of interlayer cutting force，alleviate cutter impact and vibration，and help to reduce the inter?face aperture defect.

3.2 Cutting parameters

Reasonably controlling cutting force by adjust?ing cutting parameters is one of the most convenient ways to improve interface aperture precision of air?craft laminates in NCOM with coarse pitch. Rele?vant studies show that axial feed and tangential feed have significant influence on cutting force. In this pa?per，single factor experiment of NCOM with coarse pitch is conducted to discuss the influence of axial feed and tangential feed on the interface aperture of laminates. AΦ8 mm carbide end mill with four teeth is used. Cutting parameters are set as follows.ns=1 600 r·min-1，e=1.1 mm，fzt=［0.02，0.04，0.06，0.08，0.10，0.12］mm，fza=［0.002，0.004，0.006，0.008，0.010，0.012］mm.

Fig.8 shows the variation of interface aperture withfzaandfzt. Under the samefzaorfzt，the interface aperture precision of CFRP exit is always higher than that of Ti6Al4V entrance. With the increase offza，the apertures decrease continuously in Fig.8（a）.As shown in Fig.8（b），with the increase offzt，the aperture of CFRP exit decreases first and then in?creases. The apertures of the Ti6Al4V entrance keep getting bigger.

Fig.8 Influence of feed on NCOM of laminates

To further analyze the influence offzaandfzton interface aperture in NCOM with coarse pitch，the relationship of cutting forces and cutting thickness per tooth is analyzed under the cutting parameters asns=1 600 r·min-1，e=1.1 mm，fzt=0.06 mm，the relation offzaand cutting forces are described in Fig.9.FrandFtshow an increasing trend with the increase offza.Faof Ti6Al4V shows a steady trend，andFaof CFRP shows fluctuation. The reason is the cutting thickness of bottom edge and the cutting depth of side edge both increase with the increase offza，soFrandFtalso increase accordingly. The un?certain change inFaof CFRP indicates thatfzais not the only factor affecting cutting force. Due to the characteristics of CFRP，such as anisotropy and low interlaminar bonding strength，Faof CFRP is oscillating. AsFrincreases in Fig.8（a），the aper?ture dimensions of CFRP exit and Ti6Al4V en?trance have a general trend of decreasing. Because of little change inFa，the interface aperture consis?tency of CFRP exit and Ti6Al4V entrance is not changing much.

Fig.9 Influence of fza on cutting forces

Fig.10 shows the variation of cutting forces withfztwhenfzais 0. 006 mm. The other two param?eters asnsandeare the same as Fig.9. According to Eqs.（1）—（2），nωincreases with the increase offzt，thus the cutting thickness of the side edge increases.However，the axial cutting depth of side edge de?creases withfztincreasing. Since the circumferential cutting of the side edge plays a big role，FrandFtshow a downward trend.Fais mainly from the continuous milling of the bottom edge. With the in?crease offzt，Favaries within a certain range. In Fig.8（b），the decrease ofFrbrings an increase ten?dency of apertures at CFRP exit and Ti6Al4V en?trance. The little change inFaleads to the little change in interface aperture consistency of CFRP exit and Ti6Al4V entrance.

Fig.10 Influence of fzt on cutting forces

3.3 Cutter diameter

In NCOM，carbide end mill with small diame?ter is used to reduce cutting forces. But the low rigid cutter might affect aperture machining accuracy. To study the influence of cutter diameter on interface aperture in CFRP/Ti6Al4V laminates，two fourtooth carbide end mills coated by TiA1N are used in hole-making experiments. The diameters of the two cutters areΦ8 mm andΦ6 mm. The test results are shown in Table 1. The apertures of CFRP exit and Ti6Al4V entrance produced byΦ6 mm end mill are less than that produced byΦ8 mm end mill，which can be explained by Fig.11. ButΦ6 mm end mill can get better interface aperture consistency thanΦ8 mm end mill.

Under the same cutting parameters，aΦ6 mm end mill and aΦ8 mm end mill are separately used in NCOMΦ10.2 mm holes. The cutting forces are described in Fig.11. When aΦ6 mm end mill is used in machining，Fain CFRP is about 200 N andFain Ti6Al4V is about 450 N. Compared with Fig.11（b），Fain CFRP and Ti6Al4V is reduced by 37.5% and 45.12% respectively. With the decrease of cutter diameter，the material cut by the bottom edge is decreasing in continuous milling，and the material cut by the side edge is increasing in inter?mittent milling. Meanwhile，the increasing chip re?moval space is good for cutting heat dissipation and chip removal，thus cutter wear is reduced. Based up?on the above factors，Faproduced byΦ6 mm end mill is significantly smaller than that produced byΦ8 mm end mill. In NCOM withΦ6 mm end mill，the mutation ofFaat interface is relatively small.This might be the reason that the interface aperture consistency produced byΦ6 mm end mill is better than that produced byΦ8 mm end mill. Meanwhile，the data ofFrin Fig.11（a）is larger than that in Fig.11（b）. Since the stiffness ofΦ6 mm end mill is weaker than that ofΦ8 mm end mill，Φ6 mm end mill is more likely to be affected by impact andFr.Therefore，interface aperture precision exists in holes produced byΦ6 mm end mill is lower than that produced byΦ8 mm end mill as shown in Ta?ble 1. When cutting laminates with small diameter cutter，finish machining is needed to improve inter?face aperture precision.

3.4 MQL

To improve cutting performance of NCOM with coarse pitch，MQL is a good choice. A com?parative experiment with MQL and dry milling was conducted in CFRP/Ti6Al4V laminates to study the effect of MQL on interface aperture. The thick?ness of upper CFRP is 3.8 mm，and the thickness of lower Ti6Al4V is 2 mm. The hole diameter to be made isΦ5 mm. The cutter is aΦ4 mm carbide end mill with four teeth. Cutting parameters are set as follows.ns=5 000 r·min-1，fzt=0.06 mm，e=0.5 mm，P=1 mm. The test results are shown in Table 2.

Table 2 Actual aperture and aperture error

Seen from Table 2，MQL has little influence on interface aperture in NCOM with coarse pitch.Fig.12 describes the forces in MQL and dry milling.Faof CFRP and Ti6Al4V in dry milling is about 120 N and 340 N respectively. When MQL is adopt?ed，Faof CFRP and Ti6Al4V is about 110 N and 320 N respectively. Since the amplitude reduction ofFacaused by MQL is small，MQL has little effect on aperture precision.

Fig.12 Influence of MQL and dry milling on cutting forces

3.5 Replacing reaming with twice milling

To obtain high machining accuracy， twice NCOM are needed in hole-making process. The first NCOM is regarded as rough machining and the second NCOM is finish machining. The second NCOM can improve the aperture precision，thus re?alizing the function of replacing reaming with second milling. To study the effect of the second NCOM on interface aperture，aΦ6 mm carbide end mill and aΦ8 mm carbide end mill are used in NCOM exper?iments respectively to processΦ10.2 mm holes with coarse pitch. The first set of experiments includes twice NCOM by aΦ6 mm end mill. In the first NCOM，eis set as 1.9 mm. Then the second NCOM withe=2.1 mm finishes the hole-making process. The second set of experiments uses aΦ8 mm end mill in NCOM for one time withe=1.1 mm. The other cutting parameters of the two sets of experiments are the same.

The experimental results are shown in Table 3.The interface aperture precision and consistency of twice NCOM with aΦ6 mm carbide end mill is much better than that of once NCOM with aΦ8 mm carbide end mill. The reason is that the CFRP/Ti6Al4V laminates are mainly cut by the side edge in the second NCOM. Fig.11（a）shows the cutting force of the first NCOM byΦ6 mm carbide end mill. Fig.13 gives the cutting force of the second NCOM byΦ6 mm carbide end mill. Compared with Fig.11（a），it is found thatFain the finishing stage of twice NCOM is very small.FrandFtin Fig.13 are also greatly reduced. In the second NCOM，Fachanges very little when the cutter cutting into Ti6Al4V from CFRP. ThoughΦ6 mm end mill ob?tains less aperture precision thanΦ8 mm end mill in a single NCOM，the second NCOM withΦ6 mm end mill can improve the aperture precision，and fur?ther improve the interface aperture consistency.

Table 3 Apertures at CFRP exit and Ti6Al4V entrance under different types of milling

Fig.13 Cutting forces of finish machining in twice NCOM coarse pitch by Φ6 mm carbide end mill

3.6 Interlayer speed change

Due to the difference in cutting performance of different materials，there are two main methods for PD of CFRP/Ti6Al4V laminates［31］. The first method adopts diamond cutter to drill the upper CFRP firstly，then uses carbide drill to process the lower Ti6Al4V，and finally uses carbide reamer to carry out reaming on CFRP/Ti6Al4V laminates.The second method adopts appropriate cutting pa?rameters for drilling of Ti6Al4V，and machines CFRP/Ti6Al4V laminates by once drilling. Al?though the former method is easy to be realized by online interlayer speed change of CNC machine tools，the frequent cutter change may reduce pro?cessing efficiency. The cutting parameters used in the second method might aggravate cutter wear and reduce processing quality. NCOM with coarse pitch can realize the interlayer speed change by using thez-axis quasi-stop function of 5-axis CNC machine center. Based upon reasonable cutting parameter，twice NCOM with coarse pitch can effectively ob?tain high aperture precision，which avoids the de?fects of the above two PD methods. Table 4 shows the cutting parameters of aΦ8 mm carbide end mill in conditions of changing speed and unchanging speed during NCOM.

Table 4 Cutting parameters under two control methods

Fig.14 shows the curves of the diameter andFawhen NCOM of CFRP/Ti6Al4V laminates with coarse pitch under changing and unchanging speed.From Fig.14（a），it can be seen that interlayer aper?ture precision and consistency under changing speed are better than that of unchanging speed. With the increase of hole numbers，the advantage of interlay?er speed change is more obvious in improving aper?ture precision. The reason is that the amplitude ofFais smaller under changing speed，as shown in Fig.14（b），which leads to better machining preci?sion.

Fig.14 Influence of interlayer speed change on interface ap?ertures and Fa

Fig.15 Influence of interlayer speed change on cutter wear

To further analyze the influence of interlayer speed change on the interface aperture precision of NCOM with coarse pitch，the effect of interlayer speed change on cutter wear is observed. Fig.15（a）gives the morphology of the bottom edge and the side edge before machining. Fig.15（b）shows the cutter morphology after the sixth hole has been pro?cessed with constant speed. Under the high tempera?ture and high pressure，the chips stick tightly on the cutter spiral groove and attach to the rake face of the side edge and the bottom edge. And severe wear oc?curs on the flank face of the bottom edge. Fig.15（c）gives the cutter morphology after the sixth hole has been processed with variable speed. Except for a small amount of chips，the side edge has no signifi?cant wear. The wear on the flank face of the bottom edge is lighter than that with constant speed. There?fore，the interlayer speed change can effectively re?duce the cutter wear when NCOM of laminated structures with coarse pitch，which is beneficial to improve the interface aperture precision.

4 Conclusions

The comparative experiment when NCOM with coarse pitch and PD of CFRP/Ti6Al4V show the following conclusions：（1）the interface aper?ture consistency and orifice burr of NCOM are much better than those of PD，further the interface aperture inconsistency and error are more serious than burr in NCOM.（2）NCOM with coarse pitch could reduce axial force of CFRP by about 9% and axial force of Ti6Al4V by about 42%，which is more suitable for machining the low rigid CFRP/al?loy laminates，but big radial force also enlarges in?terface aperture error.

To suppress the unfavorable effect of interlayer cutting impact，interlayer clamping can effectively improve the processing stability and aperture quality of the laminated interface. When NCOM of CFRP/Ti6Al4V with coarse pitch，interlayer speed change can effectively reduce cutter wear and cutting force，and inhibit interface aperture inconsistency. Be?sides，MQL can also help to improve interface aper?ture quality.

In NCOM，axial feed has more influence on cutting force and aperture precision than tangential feed when cutter diameter is determined and rota?tion speed is constant. Therefore，it is recommend?ed to choose large tangential feed and moderate axi?al feed. When NCOM with small diameter cutter，low rigid cutter will reduce the aperture precision at laminated interface. Especially for large holes and deep thickness of CFRP/Ti6Al4V laminates，it is more appropriate to adopt“twice NCOM with coarse pitch”.