40207_27

COMPOSITE MACHINING 27

Kent E . Kokkonen and Nitin Potdar

27.1 INTRODUCTION

The processes used to manufacture composite structures generally require that trimming and other machining operations be performed prior to assembly. Machining processes are required to produce accurate surfaces and holes to allow precision fitting of components into an assembly. Due to shrinkage during the curing stage of the composite structure it is not practicable to place holes in the part during the molding stage, therefore milling, cutting, drilling etc. are considered a post cure opera- tion.

Due to the toughness and abrasive nature of modern composites, there is a need for harder and longer lasting cutting tools. A large data- base of machining information for various high speed steel and carbide cutting tool materials exists for machining metal, wood and some thermoplastics. However, much of this data cannot be applied to machining mod- ern composites. Modern composites like graphite-epoxy, aramid-epoxy and carbon- carbon each have their own machining charac- teristics. Composites are not homogeneous or isotropic, therefore the machining characteris- tics are dependent on the tool path in relation to the direction of the reinforcing fibers. Metals or metal alloys have nearly homoge- neous properties throughout the workpiece, but each material in a composite retains its individual properties.

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

27.2 CONVENTIONAL MILLING

When milling graphite-epoxy with polycrys- talline diamond (PCD) the chips are formed as small particles of powder dust and fumes. The surface roughness is a function of fiber orien- tation, cutting direction and the angle between cutting direction and fiber direction. The sur- face may sometimes exhibit many small holes due to fiber pull out. When taking heavy milling cuts there is a greater tendency to break comers as the tool exits the material so it is advisable to first machine a step on the edge perpendicular to the final pass. A four fluted end mill will reduce cutting pressure on the laminate and keep it cooler. Climb milling helps prevent the fibers from separating from the matrix bond material.

Advantages of machining composites are:

0 improved surface finish unless part surface was directly in contact with the mold sur- face;

0 machined surfaces provide accurate mating surfaces for parts to be assembled;

0 eliminates the majority of the problems associated with part shrinkage and insert movement during the fabrication processes.

Tool life factors are:

0 PCD end milling cutters will perform sixty to one hundred times longer than carbide;

0 cutting speed does not have a great effect on the flank wear of PCD cutting tools. With increased cutting speeds, the feedrates can be increased and machining time decreased;

Mechanical drilling of composite materials 597

cutting speeds range from 244 surface m/min (800 surface ft/min) to 762 surface m/min (2500 surface ft/min) with PCD end mills; when cutting parallel to the fiber direction, the wear ratio on the cutting tool increases compared with cutting perpendicularly to the fiber direction; surface finish remains below 20Ra [arith- metical average roughness (see IS0 R488)] when cutting with PCD end mills and the flank wear is approximately 0.127 mm (0.005 in);

0 the surface finish deteriorates above 150 Ra when cutting with a carbide end mill and the flank wear has reached 0.127 mm (0.005 in);

0 roughing feedrates range from 0.23 mm/rev (0.007 in/rev) to 0.38 mm/rev (0.012 in/rev) and finish feedrates range from 0.076 mm/rev (0.002 in/rev) to 0.13 mm/rev (0.005 in/rev);

0 the depth of cuts should range from one quarter to one half of the diameter of the end mill cutter. Depth of cut will vary depending on the rigidity of machine ways, spindle and workholding devices. The disadvantages associated with milling

of composites include controlling the graphite chips (dust particles), confining them to a small area and having an adequate collection system. A second problem is controlling the outer lay- ers of the composite so that the fibers will shear instead of lifting up under the force of the cut- ting action and leaving extended fibers beyond the cut surface. Also when cutting perpendicu- lar to the lay of composite fibers, edge break-out can occur. This can be controlled by designing a backup structure in the tooling.

27.3 CONVENTIONAL TURNING

The turning of graphite composite is utilized to produce round surfaces that need to mate with either metal of graphite parts. The cutting speeds can be over 305 m/min (1000 ft/min) if the part can be held securely and PCD tool inserts are utilized.

Depth of cut will vary depending on the thickness of the part and the amount of mate- rial to be removed.

27.3.1 ADVANTAGES

Computer numerical controlled lathes (CNC) can be used to machine simple to very complex rotational parts. CNC machining produces accurate parts at a high production rate.

27.3.2 DISADVANTAGES

Delamination can also occur on a lathe (Fig. 27.1), therefore the part may require a finish cut moving from the largest diameter to the smaller diameter. Graphite chips are a serious problem. The spinning chuck creates a fan effect on the graphite particles. The exhaust system must be adequate to control the graphite chips. Also, the machine ways and the ball screws on the machine must have sealed protection to minimize wear. The com- puter control requires protection from the graphite chip particles.

27.4 MECHANICAL DRILLING OF COMPOSITE MATERIALS

Drilling holes in composites can cause failures that are different from those encountered when drilling metals. Delamination, fracture, break-out and separation are some of the most common failures. Delamination (surface and internal) is the major concern during drilling composite laminates as it reduces the struc- tural integrity, results in poor assembly tolerance, adds a potential for long term per- formance deterioration and may occur at both the entrance and exit plane. Delamination can be overcome by finding optimal thrust force (minimum force above which delamination is initiated). Figure 27.2 shows push out delami- nation at exit because at a certain point loading exceeds the interlaminar bond strength and delamination occurs. Figure 27.3 shows peel-up delamination at entrance

598 Composite machining

I I

Fig. 27.1 Machining direction for turning compos- ite parts on a lathe.

because the drill first abraded the laminate and then pulled the abraded material away along the flute causing the material to spiral up before being machined completely. This type of delamination decreases as drilling pro- ceeds since the thickness resisting the lamina bending becomes greater.

Among the variables to be considered for tool selection include the thickness of material, diameter of hole, tolerance requirements, hole finish requirements and the composite mater- ial being drilled. Tungsten carbide, micrograin tungsten carbide and drill tool materials are used for drilling composite materials.

Some commonly used composites are glass-epoxy, glass-graphite-epoxy, graphite- epoxy, graphite-epoxy with aluminum backup and graphite-epoxy with titanium backup. Other materials include the aramids (Kevlar@) with combinations of glass or

Fig. 27.2 Drill bit showing push-out delamination at exit.

graphite reinforcement materials. Each of these materials requires individual attention in the selection of cutting tool parameters. The composite materials with metal backup panels require separate drills with different geome- tries. Cutting speeds and feedrates vary in each of the various combinations of materials. Secondary drilling or reaming operations are required to hold tight tolerances or smooth surface finishes on the holes. Table 27.1 shows the drilling results when using four styles of drills.'

PCD tooling offers increased tool life, better hole quality, consistent hole size and higher machining rates. Drilling and countersinking

Peeling Action

4 I I I I

Fig. 27.3 Drill bit showing peel-up delamination at entrance.

Mechanical drilling of composite materials 599

Table 27.1 Summary of drill performance: mean hole quality measures as a function of point angle. Maximum recorded values of response parameters are shown in box brackets, [I (Reproduced from Ref 1 by permission of ASM Materials Week)

Criterion/drill Dagger 8-Facet 4-Facet Master NAS 907-1 HSS

Exit breakout (Rank least = 1)

Panel damage, D,

Microcrack density (Rank: lowest = 1)

Thrust force, N (1bf) Torque, Nm (ft lbs)

Surface finish, R,, Pm (Pin.) Hole diameter, mm (in)

Hole out-of-roundness, - (in) Drill point angle, deg.

1

1.96 (3.34)

1

114 [166] (25.6 [37.4])

1.29 [2.18] (0.95 [1.61])

0.4 [1.6]

6.354 [6.379] (0.25016

[0.25115])

(26 [641)

0.0061 [0.025] (0.00024 [ 0.001 01)

30

2 3 4

2.37 (3.18) 2.75 (3.62) 3.63 (5.54)

2 3 4

201 [378] 263 [428] 593 [969] (45.3 [85.2]) (59.3 [96.3]) (133.5 [218])

1.15 [2.0] 0.7 [1.64] 1.53 [2.2] (0.85 [1.5]) (0.50 [1.21]) (1.13 [1.61])

0.95 [2.2] 1.6 [3.0] 2.4 [4.12]

6.356 [6.369] 6.367 [6.395] 6.375 [6.397] (0.25022 (0.25067 (0.2510

[0.25075]) [ 0.251751) [0.25185])

(38 [88l) (64 L1.221) (96 11651)

0.003 [0.005] 0.0043 [0.018] 0.013 [0.03] (0.00012 [0.0002]) (0.00017 [0.0007]) (0.00051 [0.0012])

24,118 140 135

with a combination tool provides better hole quality. Tool life is normally determined by the extent of delamination and fiber break out. For machining graphite composites with or with- out aluminum backing, PCD tooling is suggested with the same speeds and feeds used for machining graphite composites with- out any backing.

For machining graphite composites with titanium backing, it is not recommended that the same drill be used for both the titanium and graphite sections. Initially a hole should be drilled up to the titanium layer with a hydraulic depth sensing device at high speeds and feed. A second drill with lower speed and feed for machining titanium should be used. Finally finish reaming operation and counter- sinking should be performed for assuring hole quality.

A study carried out on carbon fiber-epoxy

(CFRP) and glass fiber-epoxy (GFRP) lami- nates using HSS and carbide tipped drills made the following observations. Both chisel edge and flank wear increased on the carbide drill with a higher ratio of wear between 200 and 400 holes (test sample 400 holes). The tool wear was greater in the CFRP laminates due to the abrasive nature of carbon fibers. Flank wear is more pronounced in GFRP when the feed was increased and the same effect is noted when speed is increased. The HSS drills lasted for ten holes in the graphite and twenty holes in the glass.

27.4.1 DFULL GEOMETRY

Drill point geometries influence the torque requirements. Lip relief and rake angles are determined by the application. The dagger drill is ideal to machine graphite composites

600 Composite machining

as it eliminates breakout when exiting the workpiece. The dagger drill has 35" included point angle and a 121" chisel edge angle. Twist drills with flute configuration to control metal chips are also used. Fully fluted drills with PCD tips brazed on a solid carbide shaft pro- vide the strength of carbide and hardness of diamond. Drill geometries are continuing to be experimented with to find ways to elimi- nate the problems associated with the hole making process in composites.

Drill cutting parameters are:

0 feedrates range from 0.025 mm/rev (0.001 in/rev) to 0.063 (0.0025 in/rev);

0 cutting speeds range from 30 surface m/min (100 surface ft/min) to 460 surface m/min (1500 surface ft/min);

0 high cutting speeds can burn the matrix material and reduce bond strength between the composite material and the matrix material.

27.4.2 COOLANTS

A water soluble coolant forced through a cold air blast unit is recommended when machin- ing most composite materials. However if the composite is hydrophilic in nature then a cold air blast unit in combination with dust or vac- uum collection system should be used.

27.5 GRINDING COMPOSITE MATERIALS

The grinding process has been used exten- sively for finishing composite golf shafts and fishing rods. Five hundred parts per hour can be produced on centerless grinders. Silicon carbide wheels are used with an open grain to reduce wheel galling. Surface speeds between 1219 surface m/min (4000 surface ft/min) and 1829 surface m/min (6500 surface ft/min) can be achieved. This equipment is specially designed for grinding and finishing compos- ites. Grinding accuraces within 0.0127 mm (0.0005 in) can be achieved with centerless grinding. Both straight and tapered shafts can

be processed. The grinding of polymer matrix compos-

ites (PMC) has a number problems. For example in the case of thermoplastic matrix, the surface of grinder becomes covered with melted thermoplastic. In the case of aramid fiber it is hard to get a clean cut surface because the grains cannot abrade the aramid fibers cleanly. Abrasive belts have been used on aramids with some success but dust collec- tion has been a major problem.

27.6 MACHINING OF KEVLAR

Cutting, Trimming, Turning and Milling of Kevlar

Because of its inherent toughness, Kevlar is dif- ficult to cut, so sharp, heavy duty upholstery scissors will cut up to 170 g/m2 (5 oz/yd2) fab- ric of Kevlar. Woven roving and heavier fabrics can be cut using specially designed serrated

techniques and applications is shown in Table 27.2. For more information on cutting and machining of Kevlar refer to DuPont's Machining Handbook2.

scissors. An overview of cutting and trimmin g

27.7 ABRASIVE WATER JET MACHINING

Abrasive water jet (AWJ) is used for linear pro- file cutting, turning, milling and drilling operations in composite materials. Conventional tool machining is affected by fiber or particle reinforcements rather than the matrix material while AWJ machining is not. To make a circular hole 6.35mm (0.25in) in diameter in aramid 3.18 mm (0.125 in) thick, it takes about the same time for both conven- tional as well as AWJ machining. The cutting process parameters for AWJ include water jet pressure, velocity, abrasive grain size, abrasive material, standoff distance and jet impinge- ment angle. and some additional parameters. Water jets without abrasive are also used for cutting soft composites. Figure 27.4 shows the AWJ processes and machining parameters3.

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achining 601

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Abrasive water jet machining 603

27.7.1 LINEAR CUTTING WITH AWJ

Linear cutting is used to trim composite parts and to cut profile shapes on the inside of a part. The cut surface is normally smoother near the entrance surface then becomes wavy in the lower half of the cut toward the exit sur- face. In general, the composite material is sheared away by a high velocity abrasive grain. The width of cut (kerf) decreases as the feedrate increases and the waviness increases as the feedrate increases3.

Table 27.3 shows some of the observations made by Hashish3. The maximum cutting tra- verse rate is primarily controlled by the matrix material.

Table 27.4 shows results for some compos- ites with different speeds.

27.7.2 TURNING

In turning with AWJ, a workpiece is usually rotated while the jet is fed along all three axes. The material encountered by the jet is abraded away in the form of a very fine debris. Higher jet pressure produces a smoother surface with a higher material removal rate. Higher tra- verse rates combined with multiple passes are more efficient than deeper cuts with lower tra- verse rates. Surface finish is affected by unsteadiness in traverse rate or abrasive flow rate. The repeatability and accuracy of the AWJ turning process depends on control and steadiness. A 10% variation in rotational speed does not affect the surface waviness but a tra- verse rate variation over 4% will significantly affect the surface waviness. Some methods to improve surface finish are:

Table 27.3 Typical through-cutting traverse rates (in mm/s) with AWJs for different composites3

Material Thickness (mm)

0.79 1.60 3.18 6.36 12.7 19.1 50.8

Organic matrix composites: Plastic and composites 53 38 29 21 15 10 2.5 Carbon-carbon composites 42 32 22 13 7.5 4 0.85 Epoxy-glass composites 105 95 76 42 17 12 15 Graphite-epoxy composites 74 63 52 40 15 10 4.2 Kevlar (steel reinforced) 42 25 17 8.5 4.7 2.5 0.63

Cutting conditions: p = 345 MPa, d, = 0.299 mm, d, = 0.762 mm, garnet mesh 80 Sic abrasives

Table 27.4 Surface waviness and corresponding cutting traverse rates (in mm/s) for some composite materials3

Material Rh4S surface waviness (pm)

1.9 2.5 3.8 5 6 8

A1,OJSiC (20%) 6.35 mm - - - -

Mg/B,C (15%) 6.36 ITU~ -

0.29 - Toughened zirconia (6.36 mm) 0.15 0.2 - 0.4 - 0.5

8 Graphite+poxy composites (3.18 mm thick) - 4 8 12 20 30 Graphite-epoxy composites (18.5 mm thick) 0.6 0.85 1.7 2.5 3.4 4.25

Cutting conditions: p = 370 MPa, dn = 0.299 mm, d, 0.762 mm, garnet mesh 80

- 6 - 3

604 Composite machining

0 multipassing by traversing the jet without lateral feed;

0 use of finer abrasive and increasing number of passes;

0 to improve surface roughness, use softer abrasives like silica sand, copper slag etc;

0 finishing with slurried abrasive yields improvement in surface roughness.

27.7.3 MILLING

The main objective of AWJ in milling is to pro- duce a cavity with controlled depth. In this method, the jet material interaction is the depth determining factor. The production of kerf irregularity can be reduced by manipulating one of the factors, such as traverse rate, increas- ing the stand off distance or angling the jets.

To mill square pockets the traverse speed can be varied rather than angling the water jet head. In this case the nozzle can be manipu- lated over the workpiece with an oscillatory drive using a motor and an eccentric. A uni- form traverse rate and exposure time can produce a uniform depth cavity. A hard mate- rial pattern with the shape to be milled can be used to mask the target surface. This way the mask will allow jetting in the traverse zones where the traverse rate is uniform. Surface fin- ish variations can be achieved by using different abrasive materials or grit sizes. Harder abrasives can be used for higher mate- rial removal rates and softer abrasives for finishing operations.

27.7.4 DRILLING

Hole drilling can be performed in any of the following ways depending on the diameter and accuracy of the holes: piercing is suitable for small diameter holes; kerf cutting is suit- able for large diameter holes; milling is suitable for blind holes.

Techniques of hole piercing vary for each composite material. Piercing glass, acrylic and polycarbonate show that the general geometri- cal features of pierced holes are similar. Particle

velocity decreases as the depth increases which can be attributed to the effect of return flow which reduces particle velocity and interferes with the impact process. Pressures of 3040 MPa are common for piercing glass. High pres- sures are necessary to pierce brittle or laminated composites. The higher pressures may cause the following problems: fracture due to shock loading of water; hydrocracking due to hole hydrodynamic pressurization; delamination due to loading.

Holes larger than the piercing diameter of the AWJ are first pierced, then profile cut to the finished diameter being offset by the kerf amount. Hole shape variance depends on mix- ing tube length, target material, standoff distance, depth of hole and dwell time in the cut. Mixing tube length is important when drilling materials with high resistance. Increasing the mixing tube length improves the distribution of the abrasive with the water jet. This produces holes that are straighter and rounded.

Advantages offered by AWJ are:

0 suitable for wide range of composites; 0 can perform many operations like turning,

drilling and milling; 0 no thermal stresses; 0 high as well as low material removal rates; 0 no heavy clamping of workpieces required; 0 omnidirectional machining; 0 process can be automated; 0 optimal range of parameters available to pre-

0 fine holes of 0.5 mm (0.012 in) can be vent delamination, loading and splintering;

drilled.

Disadvantages:

0 dimensional accuracy is low; 0 temperature rise in cutting region may be

observed; 0 limited data is available with respect to

applications in metal and ceramic compos- ites;

0 not suitable for materials that are hydrophilic in nature.

Ultrasonic machining 605

27.8 LASER MACHINING OF COMPOSITES Advantages:

Lasers are used in various industrial applica- tions such as drilling, cutting, welding and heat treatment of metals, etc. In composites, polymer matrix materials are most suited for laser cutting. Laser cutting is a non-contact ablation process in which efficiency is deter- mined by thermal properties of the workpiece material. Two types of laser have been used in industry: Nd-YAG solid state laser and CO, gas laser. The Nd-YAG laser operates in the near infrared (IR) region of the spectrum while CO, gas laser operates in the far infrared region. The Nd-YAG IR region wavelength is not absorbed by glass and many plastics while the CO, far IR region wavelength is.

Applications of Nd-YAG solid state lasers extend from drilling fine holes in jet engines to welding implant devices for the medical industry. It has been determined that the Nd- YAG laser is very effective in cutting graphite-epoxy composite materials. The high power short pulses achieved with this laser vaporizes both the graphite and epoxy matrix before the epoxy resin can be overheated.

The CO, gas laser applications extend from drilling holes in baby bottle nipples to welding automotive components in assembly lines. CO, lasers operate in either continuous wave or pulsed mode. Pulsed mode is preferred because of high powers obtained and cool down time. Aramid fiber reinforced plastic (AFRP) has been cut very effectively by the CO, lasers. The general characteristics of a laser cut zone in composite materials are shown in Fig. 27.5.

The charred layer which includes a zone with fibers protruding from the matrix and as outer darkened zone in which the matrix has undergone some degradation4t5.

Figure 27.6 shows the relationship between kerf width and cutting speed. For three dimen- sional (3D) machining two laser beams are directed through an optical assembly to inter- sect in the plane of work piece to cut shoulders and vee grooves.

0 superior quality edges due to high temper-

0 vaporization of the material in cut zone; 0 extremely localized action; 0 sealing of the edge in the cut zone; 0 pulsed CO, has been demonstrated as the

best laser for processing Kevlar composites.

Disadvantages:

atures;

0 beam divergence after its focal point; 0 material thickness of about 9.5 mm (36 in) is

the maximum thickness that can be cut with 1500 W;

0 heat affected zone of varying dimensions.

27.9 ELECTRIC DISCHARGE MACHINING (EDMI

Advanced composites can be cut by EDM as there is no physical contact between the elec- trodes or workpiece and the tool. In order to EDM a composite, it should have an electrical resistivity of less than 1-3 ohm/m. Polymer matrix composite manufacturers can add a small amount of copper in the matrix of the product to allow shaping by EDM. EDM can be used with conductive silicides, borides, car- bides, etc. The EDM process is more accurate than AWJ machining. Small holes of 0.25 mm (0.01 in) diameter can be drilled in SiC/TiB, composites. The EDM process is found to be slow for many production applications.

27.10 ULTRASONIC MACHINING

Ultrasonic machining (USM) incorporates a tool vibrating at 20 kHz and abrasive in a slurry to perform impact grinding of brittle materials. This technique is particularly useful for machining of ceramic matrix composites that are difficult to process by conventional methods. USM is a mechanical material removal process best suited for machining brittle materials like glass, ceramics, graphite and ceramic matrix composites. The process is limited to workpieces of size below 1OOmm

606 Composite machining

PROTRUDING FIBRES I

i

\ I \

I CHARRED LAYER \

,i I

\

0 L-

Fig. 27.5 Schematic of FRP laser cut. (Reproduced by permission of Marcel Dekker Ltd.) W,: kerf width at the beam entry side; W,: kerf width at the

'I ICROSS SECTION 1

,

4 beam exit side

-

0 2 -

Fig. 27.6 Kerf width as a function of cutting speed

of Marcel Dekker Ltd.) for (0/90), laminates. (Reproduced by permission '.'a m u ) Bo 80 Irn 120 1

' ' Cutting speed (mm/s)

Ultrasonic machining 607

(3.94 in) because of the limitation on the size of the tool. Some of the variables that influence USM for close tolerances are as follows:

Abrasive type and size

Abrasives contained in the slurry do the actual machining so they must be selected on the basis of the workpiece material and the surface quality needed. As in the case of AWJ, larger abrasive grains give higher material removal rates and rougher surfaces. The grain diameter cannot be larger than amplitude of the sonotrode as this would inhibit the injection of the grains to the machining gap. Common types of abrasive used are A1,0, oxide, Sic, BC and diamond.

Table 27.5 shows recommended abrasive for various materials. The grain diameter affects surface roughness, overcut and machining rates. When high removal rates are necessary with no high surface quality required, 180-280 mesh abrasive do the job. For finer surface finish 320-600 mesh abrasive is recommended. Table 27.6 shows surface roughnesses for different workpiece materials.

Sonotrode (tool) material

Tools with diamond tips have good material removal characteristics and very low wear but are difficult to machine. Table 27.7 shows accu- racy results of using a non-rotating steel sonotrode.

Ultrasonic vibrations

The ideal condition would be the amplitude of ultrasonic vibration to be equal to the grain mean diameter. If the amplitude is too small the abrasive cannot enter the machining gap, if too large it causes the grains to be incorrectly projected. A mixture of both the types of abra- sive may be used and a suitable amplitude selected to determine which size grain enters the machining gap.

Surface area

This factor influences removal rates and tool wear. With a small diameter, higher feed rate is obtained but also higher tool wear is noticed. This can be overcome by using a dia- mond tool or with a closed loop force sensitive

Table 27.5 Recommended abrasive for various materials6

Material Recommended abrasive

Graphite Silicon carbide Zirconia Ceramic matric composites Silicon carbide Metal matrix composites Boron carbide

Silicon carbide or boron carbide

Table 27.6 Surface roughness for various materials6

Workpiece material Surface roughness Ra @ m)

Graphite 1-2 Zirconia 0.75 Ceramic matrix composites 0.70 Metal matrix composites 0.90

608 Composite machining

servo system maintaining accurate machining pressures. Table 27.8 shows typical ultrasonic machining rates for a variety of materials6.

USM is used in applications like drilling aerospace cooling holes in ceramic matrix composite turbine blades, slotting, irregular configurations in ceramics and composites, machining of phased array radar components, cutting tool inserts, superconductors, wire draw dies and extrusion dies. A CNC USM can cut through 6mm (0.24in) thick composite layers and produce a controlled depth up to 50mm (1.97in). The latter is important, as many composites have backing sheets that should not be damaged. The ultrasonic action reduces the amount of force required to sever the hard materials. This results in a better cut

on prepreg materials like glass fiber, carbon fiber and Kevlar with reduced fiber damage.

0 conductive and nonconductive materials

0 material hardness is not so important; 0 there are no chemical or electrical alter-

ations in the workpiece; 0 3D and complex shapes can be machined

easily and quickly; 0 no heat affected zone.

Advantages:

can be machined;

Disadvantages:

0 amplitude of ultrasonic vibrations are very important for proper machining;

0 limited sizes can be machined.

Table 27.7 Accuracy results with a non-rotating steel sonotrode6

Material Inlet diameter Outlet diameter Taper Roundness (mm) (mm) (Yo) (mm)

Graphite 10.23-10.25 10.07-10.10 3.00 0.03" 10.26-10.29 10.02-10.05 2.70 0.03b

Metal matrix composite 10.20-10.24 8.87-9.92 9.00 0.04b 10.09-10.12 8.85-9.90 6.60 0.05b

Ceramic matrix composite 10.11-10.15 10.00 3.50 0.04b 5.04 4.99 1.25 -

Zirconia 5.05 4.85 5.50 -

c

C

a Tool 1: Exponential, Diameter = 10 mm

' Tool 3: Exponential, Diameter = 5 mm Tool 2: Exponential, Tube D = 10 mm, ID = 7 mm

Table 27.8 Typical ultrasonic machining rates for a variety materials7

Drilling diameter = 5 mm Drilling diameter = 10 mm

Material Time Removal rate Time Removal rate

Graphite 1 164 1.25 224 Ceramic matrix composite 3.5 39 5.6 50

Zirconia 210 0.65 90 3.1

(min) (mm3/min) (rnin) (mm3/min)

Metal matrix composite 10 7.6 14 9.3

REFERENCES

1.

2.

3.

4.

5.

6.

Mehat, M., Reinhart, T.J. and Soni, A-H., Effect of fastener hole drilling anomalies on structural integrity of PMR-l5/GR composite laminates, Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Ill, 1-5 Nov. 1992. Kevlar Cutting and Machining Handbook, E.I. Du Pont de Nemours and Co. Hashish, M. State of the art of abrasive waterjet machining operations for composites. Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Illinois, 1-5 November 1992. Di Ilio, A., Tagliaferri, V. and Veniali, F. Machining parameters and cut quality in laser cutting of aramid fibre reinforced plastics. Materials and Manufacturing Processes, 1990,5(4), 591-608. Lemma, S. and Sheehan, B. Laser Machining of Composite Materials. Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Illinois, 1-5 November 1992. Gilmore, R. Ultrasonic machining of composite materials, Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Ill. 1-5 November, 1992.

FURTHER READING

References 609

Bhatnagar, N., Naik, N.K. and Ramakrishnan, N. Experimental investigations of drilling on CFRP composites. Materials & Manufacturing Processes, 1990, 5(4), 591-608

Geskin, E.S., Tisminetski, L., Verbitsky, D., Ossikou,V., Scotton, T. and Schmitt, T. Investigation of waterjet machining of compos- ites. Proc. Machining of Composite Materials Symy., ASM Materials Week, Chicago, Illinois, 1-5 November 1992.

Hochegn, H., Puw, H.Y. and Yao, K.C. Experimental aspects of drilling of some fiber-reinforced plas- tics. Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Illinois, 1-5 November 1992.

Krishnamurthy, R., Santhanakrishnan, G. and Malhotra, S.K. Machining of polymeric compos- ites. Proc. Machining of Composite Materials Symp., ASM Materials Week, Chicago, Illinois, 1-5 November 1992.

Lubin, G., ed., Handbook of Composites, 1982, New York: Van Nostrand Reinholt.

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