Machining of composite materials

1. Dr. V. Auradi
Machining and cutting of coMposites

2. Dr. V. Auradi
Composite materials offer the benefits of part integration and thus
minimize the requirement for machining operations.
However, machining operations cannot be completely avoided and most
of the components have some degree of machining.
Machining operations are extensively used in the aerospace industry.
In a typical aerospace application, assembly and sub-assembly labor costs
account for as much as 50% of the total manufacturing costs of current
airframes.
A fighter plane has between 250,000 and 400,000 holes and a bomber
transport has between 1,000,000 and 2,000,000 holes; A typical wing on
an aircraft may have as many as 5000 holes.
Therefore, machining cost has become major production cost factor in
aerospace applications.

3. Dr. V. Auradi
Objectives/Purposes of Machining
To create holes, slots, and other features that are not possible to obtain
during manufacturing of the part.
For example, if a pultruded part needs holes and other features as shown
in Figure then machining of the part is unavoidable.
Machining is done to create the desired tolerance in the component.
For example, if a filament wound part requires the outside diameter to
have a tolerance of 0.002 in., then centerless grinding is done to get that
tolerance on the outer surface.

4. Dr. V. Auradi
Machining is performed to prepare the surface for bonding, coating, and
painting purposes.
In general, the outer surface is sanded to remove oils, grease, and release
agents.
Machining is performed to create smoothness on the desired surface.
To make prototype parts from a big blank or sheet of material,
machining operation is performed.
This process is very economical.
For example, if a designer wants to test glass/nylon short fiber
composites for a bushing application, he can machine a composite
rod/tube to develop a prototype part instead of making an expensive
mold.
Similarly, test coupons for tensile and bond testing are made from big
sheets of materials.

5. Dr. V. Auradi
Challenges during Machining of Composites
Machining of composite parts creates discontinuity in the fiber and
thus affects the performance of the part.
Machining exposes fibers to chemicals and moisture.
The temperature during cutting should not exceed the cure
temperature of the resin for thermoset composites to avoid material
disintegration. Glass and Kevlar fibers have poor thermal conductivity
and such high temperatures may lead to localized heating and
degradation. With thermoplastic composites, if the temperature comes
close to the melting temperature of the resin, it may clog the tool.
It is difficult to attain dimensional accuracy during the cutting of
composites because of differences in the coefficients of thermal
expansion (CTE) in the matrix (highly positive CTE) and fiber (slightly
negative CTE in carbon and aramid).
Drilled holes are often found to be smaller than the drill used.

6. Dr. V. Auradi
There is heat build-up in the cutting zone due to the low thermal
conductivity of the composite. A suitable coolant should be selected to
dissipate the heat from the tool and the work piece. In drilling metal
components, chips absorb 75% of total heat, whereas the tool and work
piece absorb 18 and 7%, respectively. In the drilling of carbon/epoxy
composites, the tool absorbs half of the heat and remainder is equally
absorbed by work piece and chips.
Tool life is usually shorter because of the abrasive nature of the composite.
For this reason, high-speed steel tools are coated with tungsten carbide or
titanium nitride to increase the life of the tool.
Obtaining a smooth cut edge is difficult with composites, especially aramid
composites. Aramid fibers are tough and absorb the cutting energy. Fiber
kinking or burr surfaces are obtained during cutting of aramid composites.
The effect of coolant materials on composites is unknown and therefore
any coolant material must be selected judiciously.

7. Dr. V. Auradi
Machining of composites causes delaminations at the cut edges of
continuous composites. The lay-up sequence and fiber orientations
have a significant effect on the amount of delamination.

8. Dr. V. Auradi
Cutting Tools
Cutting tools similar to those in metal machining are used for
composites as well.
However, high-speed steel (HSS) tools are coated with tungsten
carbide, titanium nitride, or diamond to avoid excessive wear on the
tool.
HSS tools without any coating performs reasonably well for a few cuts;
but after a while, the tool edge becomes dull and the cut quality
deteriorates.
In terms of tool life, carbide tools are superior, especially if carbide
grades of fine grain size are used. However, tool cost is considerably
higher.
Polycrystalline diamond (PCD) tools are extensively used for
machining glass and carbon-reinforced composites due to their high
wear resistance.
PCD tools cost about 10 times more than carbide tools.

9. Dr. V. Auradi
Figure shows end mills and drills for machining composites and shows
inserts for turning, boring, and milling operations.
These tools are coated with diamond in a chemical vapor deposition
(CVD) reactor and the result is called CVDD (CVD Diamond).
The CVDD coating is pure diamond with no metallic binder. CVDD-
coated tools are less costly than PCD tools.
Usually, there is no coolant necessary while using CVDD tools because
of the lubricity of the diamond.
A coolant (such as 5% water soluble oil) can sometimes be used to
improve the surface finish and/or to enhance chip clearing.

10. Dr. V. Auradi
Choosing the correct diamond coating thickness is very critical to the
success of an application.
For example, to machine fiberglass composites (commercially available
G10), a 20- to 24- m thick coating works very well and it is 65 times
better in sliding and 30 times better in side milling as compared to
carbide tools.
A 10 to 14 m thick diamond coating does not work well for machining
G10 fiberglass composites.
During the slotting test, the carbide tool machined 192 linear inches
before three corners were worn away, whereas the CVD tool with a 20-
m thick coating machined 12,384 linear inches before one corner wore
away.
A CVD tool with 10- m thick coating machined 480 linear inches
before one corner wore away.

11. Dr. V. Auradi
Types of Machining Operations
Machining operations are performed to achieve various objectives
The operation involves cutting, drilling, sanding, grinding, milling, and
other techniques similar to metal machining.
Standard machining equipment similar to metal machining is used
with some modifications, mostly in the cutting tool and coolant.
In all machining operations, it is important to keep the tool sharp to
obtain good-quality cuts and to avoid delaminations.
During the machining of composites, the proper backing material is
required to avoid delamination.
Two of the major machining operations — cutting and drilling — are
discussed in the following sections.

12. Dr. V. Auradi
Cutting Operation
Conversion of a flat sheet and rod
into smaller pieces
The cutting operation is performed
to get the desired dimensions or to
make Several parts from one part.
For example, a large sheet of FRP is
cut into small rectangular strips, or
any other shape, as shown in Figure
Sometimes, the cutting operation is
performed to fabricate net-shape
parts.
Flashes, runners, shear edges, etc.
obtained during molding processes
are removed by cutting operations.
For example, during compression molding of electronic enclosures or
automotive parts, shear edges are trimmed using a file while the part is
still hot.

13. Dr. V. Auradi
Cutting operations are performed using hand-held hacksaws, bend
saws, circular saws, abrasive files, routers, and more.
The tool is diamond coated for increased wear resistance.
During machining, the cutting speed should be selected based on
the matrix, better cut quality.
The higher cutting speed means lower cut forces perpendicular to
the work piece and feed direction, which consequently reduces the
amount of manufacturing induced damages.
Now a days, waterjet cutting and laser cutting are gaining more
importance and is discussed here.

14. Dr. V. Auradi
Waterjet Cutting
Waterjet cutting is used for machining composites ; sheet metals made
of steel and aluminum.
In waterjet cutting, high-velocity water is forced through a small-
diameter jet.
As the waterjet impinges on the surface, it cuts the material by
inducing a localized stress failure and eroding the material.

15. Dr. V. Auradi
In waterjet cutting, water pressures up to 60,000 psi (414 MPa) are used
to cut the material.
Water speeds of 2600 ft/s (800 m/s) and nozzle diameters on the order
of 0.010 in. (0.25 mm) are typical.
For most composite applications, abrasive particles are added with the
water to increase the cutting speed and to cut thick composite
laminates.
A schematic diagram of commercially available waterjet cutting
equipment is shown in Figure
As shown, the water nozzle remains stationary and the sample material
travels by a hydraulic cylinder.
A 30-gpm (113.5l/min) hydraulic pump delivers hydraulic oil at up to
3000 psi (21 MPa) to an intensifier via a four-way valve.
The intensifier is a differential-area, double-acting piston type in
which a large piston is shuttled back and forth by the 3000 psi oil.

16. Dr. V. Auradi
There are two small pistons attached directly to the large piston.
The small pistons have an area 1/20th of the large piston; thus it
converts the 3000-psi oil to 60,000 psi water.
Compressed water then flows out of the high-pressure cylinders to
the nozzle through a pair of check valves.
During waterjet cutting, the process parameters that affect cutting
performance include:
Waterjet pressure
Cutting speed
Laminate thickness
Nozzle orifice diameter (0.2–8 mm)

17. Dr. V. Auradi
Laser Cutting
In laser cutting, a concentrated monochromatic raw light beam is
focused on the work-piece into a spot size of 0.1 to 1 mm.
The cutting operation takes place by local melting, vaporization, and
chemical degradation.
Laser operation requires expertise because of the danger of high-
voltage radiation exposure and hazardous fumes.
The laser beam typically damages the resin in the areas of the cut and
may score the work-stand.
Proper ventilation is required while performing the laser cutting
operation.
Cutting of unreinforced thermoplastics and thermosets is much
easier than for reinforced composites.
Cutting thermoplastics results in local melting, whereas laser cutting
of thermosets results in local vaporization and chemical degradation.

18. Dr. V. Auradi
Once reinforcements are included into the resin, very high
temperatures are required to vaporize the fibers.
Vaporization temperature for carbon fiber is 3300°C, E-glass fiber is
2300°C, and aramid fiber is 950°C.
The high-temperature requirement for cutting reinforced plastic
results in local matrix degradation.
Laser cutting produces a sharp, clean edge with little discoloration at
speeds unrivaled by other cutting methods for prepregs (uncured
composites).
Continuous-wave 250-W CO2 lasers have produced cutting speeds up
to 300 in/min (127 mm/s) in single-ply uncured boron/epoxy and up to
400 in./min (169 mm/s) in single-ply Kevlar pre-pregs.
Cutting of cured composites requires higher laser power and cut edges
generally reveal a charring effect.
Charring can be reduced in thinner materials by increasing the speed.
A 1-kW laser can cut up to 0.2 in. (5 mm) thick Kevlar/epoxy

19. Dr. V. Auradi
Drilling Operation
The drilling operation is performed to create holes in a component.
Holes are created either for fastening purposes, such as riveting or
bolting, or for creating special features, such as a passage for liquid
injection or wire connection.
Drilling is performed similar to metal drilling but the tool used is usually
a tungsten carbide tool because of the abrasive nature of composites.
In metal drilling, drill tips are designed for metal-working, the tip
heating the metal to provide the plastic flow needed for efficient
cutting.
In composites, heat generation is kept low to avoid local matrix
degradation and/or to avoid tool clogging.
The chip formation in metal drilling is long; whereas in composites,
chips are dry and small, and can be easily removed.
If the drilling speed is high, then local heat generation makes the resin
sticky and produces a lumpy chip.

20. Dr. V. Auradi
Drilling creates delaminations in laminated composites, as shown in
Figures
delaminations caused
by drilling (a) upon
entry, and (b) upon
exit.
When the drill bit first enters the laminate, it peels up the uppermost
laminae (Figure a); and when it leaves the laminate, it acts as a punch,
causing delaminations on the other side of the laminate as shown in
Figure b.
Delamination on the other side can be minimized by supporting the
laminate at the back side using a plastic or wooden support, and also
by lowering the feed rate at the time of exit.

21. Dr. V. Auradi
A more pointed drill bit tip also lowers the amount of delamination,
because a pointed tip creates gradual penetration.
In drilling composites, a negative or neutral rake angle in the tool is
avoided.
A neutral rake angle tends to push the reinforcing fibers out in front,
requiring a great deal of pressure to penetrate the workpiece.
This pressure causes the fibers to bend, resulting in undersized and furry
holes. Moreover, this pressure produces excessive heat, which causes
galling and clogging of the tool.
A positive rake angle is preferred when designing the tool geometry for
composites drilling.
With a positive rake angle, reinforcing fibers are pulled into the
workpiece and sheared or broken between the cutting edge and the
uncut material.

22. Dr. V. Auradi
Positive rake removes more material per unit of time and per unit of
pressure than negative rake, but the more positive rake at cutting
edge makes the tool sensitive and fragile.
Fiber orientation and lay-up sequence affect the extent of
delamination during drilling.
Angle-ply laminates provide better-machined surfaces than
unidirectional laminates.
In unidirectional composites, fibers tend to pull out of matrix when
the local motion between tool and workpiece is parallel to the fibers
(0°).
The best surface quality is obtained when fibers are sheared off at a
right angle (90°),
while the worst surface is obtained when fibers are compressed and
bent, which occurs at intermediate angles (20° to 45°).