Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
3High-Performance Cutting and Grinding Technologyfor CFRP (Carbon Fiber Reinforced Plastic)

HIDEAKI ARISAWA*1

SATORU AKAMA*1

HARUHIKO NIITANI*2

Since it is not only a high-strength, but also light-weight material, carbon fiber reinforced
plastic (CFRP) has recently been used in certain specific sectors including aircraft, medicalequipment and other applications, while attracting attention to the expected future increase in its
utilization volume for moderately mass-produced items, such as in the auto industry, as well. Thisis, however, a difficult-to-machine material which suffers easy delamination of fiber beds. It also
shortens tool life and, therefore, studies on tools and cutting/grinding technologies are now under
way at manufacturers and research facilities. We hereby report our successful materialization of amachining technology superior to any existing techniques in terms of machining efficiency and toollife.

|1. Introduction
Carbon fiber reinforced plastic (CFRP) is a composite material fabricated by impregnating
carbon fibers with resin and then curing and molding it. CFRP, a material boasting high strengthand light weight at the same time, started to be practically used first in golf club shafts, fishing rods
and other sports goods but in recent years, it has also become more widely used for industrial
purposes such as aircraft materials and medical equipment.
Of late, CFRP has started to be used extensively in aircraft parts, one of our productcategories. It was previously used only in structural portions, such as tail assemblies or floor
structural materials, but thanks to the higher reliability brought by such improved materials, it hascome to be used for entire structures as in the case of the latest B787 model.
There are a number of CFRP manufacturing processes, of which, for aircraft members, anauto-clave process of molding is more frequently used since it promises high strength and stableperformance.
One of its features is the ability to fabricate large complex-shaped cast parts such as main
wing panels and fuselage barrels, thus contributing to weight saving and reduced part counts.
On the other hand, feed material, which occurs in excess on the end face at the end ofmolding, needs to be removed. In addition, fasteners are used to assemble parts with each other andsince it is difficult to make holes for such assembling at the time of molding, trimming and drilling
become necessary in the post-process (Figure 1).

|2. Higher efficiency of trimming
2.1

Present problems

AWJ (abrasive water jet) or an endmill is generally used for CFRP trimming, but the former
has problems of limited shapes that can be machined and heavy initial investment/maintenancecosts while the latter has difficulty in raising the trimming efficiency due to easy delamination ofCFRP fibers, as well as short tool life (Table 1, Figure 2).

*1

Process Technology Center, Machine Tool

*2Manager, Process Technology Center, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
4Figure 1

Process to manufacture CFRP (Auto-clave molding)Auto-clave molding cures the resin under increasing pressure within the furnace for molding.
Since excess feed material occurs at the end of molding, trimming and other finish machining
are needed.

Table 1 CFRP machining method comparison
AWJEndmillAdvantages
・ Good quality of machined
surfaces・ Usable with general-purpose
equipment
Diamond-electroplated ・ Usable with general-purpose
toolequipment
・ Long life of tools

Figure 2Disadvantages
・ Limited shapes that can bemachined.
・ High cost (of initial investment
as well as for maintenance)
・ Easy delamination
・ Short life of tools
・ Narrow chip pockets preventhigh feed speed.

Problems with AWJ and endmill machining(a) AWJ is capable of through-machining only and, in addition, requires a catcher to be arranged
at the jet end, thus limiting the shapes that can be machined.
(b) Endmill trimming easily causes delamination since the cutting force works in the direction todelaminate fiber beds.

2.2

Solution

As a means to solve this problem, we focused attention on an electroplated tool, the samemachining equipment as an endmill. The electroplated tool is a tool on which base metal surface
fine diamond abrasives are fixed by electroplating them with nickel.
To prevent fiber bed delamination as a problem with endmills, shallow cuts are essential. If
an electroplated tool is used, since a large number of diamond abrasives serve as cutting edges, thecuts per edge can be made shallower than in the case of endmills. Therefore, even if the feed speed
is increased for higher-efficiency machining, the occurrence of delamination may be limited.
With respect to tool life, an electroplated tool also enjoys higher durability since its cuttingMitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)5edge is crystalline diamond, which is worn away more stably than coated and other thin films(Figure 3). Moreover, since machining of CFRP is less subject to loading than metalworking andthe abrasive coating hardly flakes off, it seems that an electroplated tool is suitable for CFRP
machining.

Figure 3

Post-CFRP trimming photo of cutting edge (abrasive) attrition

When trimming CFRP, an endmill suffers chipping at its edge section due to coating
delamination while the electroplated tool is subject to stable attrition of its abrasive edge.

2.3

Problems with an electroplated tool

We conducted experiments on our test electroplated tool and the result was successful
trimming without causing any delamination, even at speeds faster than endmill machining. But if
the feed rate was increased until it reaches a certain level, sparks came off, thus preventing any
more improvement of machining efficiency. We thought that the sparks were caused because theelectroplated tool was loaded.
2.3.1 Mechanism of loading
Let us consider the mechanism of how the tool is loaded before taking measures against
loading.
The tool is required to have chip pockets spacious enough for the amount of chips
derived from trimming but such pockets of ordinary electroplated tools only have an abrasive
protrusion size-equivalent of space (Figure 4 (a)). Chips from electroplated tool-machined
CFRP are in a powdered state, so their volume swells several times from before beingmachined. For this reason, we thought that the amount of waste chips acceptable by chippockets was also small, thereby causing the tool to be loaded immediately upon attempting
high-efficiency machining.

Figure 4 Chip pockets of an electroplated tool(a) A conventional electroplated tool has chip pockets only equivalent in size to anabrasive protrusion and is, therefore, vulnerable to loading.
(b) The developed tool has a dimpled surface to ensure enough chip pockets, making
loading unlikely.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)6

2.3.2 Measures against loading
In order to secure chip pockets spacious enough for our targeted-efficiency trimming on
an electroplated tool, we have developed a tool with a dimpled grinding surface (see Figure 4(b)). The optimum dimple shape (volume) is closely associated with the aforementioned
amount of chips derived and, unless a certain volume or more is secured, loading occurs. It
should be noted, however, that if the area of a dimple opening is made too large, the number ofworking abrasives becomes smaller, adversely affecting the accuracy of finishing and/or toollife.
Hence, for the developed tool, a parameter of volume ratio (volume of working
chip-pocket sections/amount of chips coming out per revolution of the tool) was establishedand its relationship with loading was determined by way of work tests. Although the larger thevolume ratio, the less likely loading is to occur, where dimples are arranged so as to secure chippockets, even though the volume ratio is unchanged, if dimples are spaced too far apart, loading
occurs to the area distant from the dimple, thus making it difficult for the dimples to have aneffect. Dimples need to be equally distributed all over the working grind stone surface and, forthe developed tool, consideration was also given to dimple spacing in determining the shapeand layout of dimples (Figure 5).

Figure 5

Relationship between chip pockets and loading

Unless the chip pockets are spacious enough for the amount of chips coming out and thespace between the dimples is narrow, loading occurs.

2.4

Result of machining with the developed tool

2.4.1 High-speed machining test
Machining tests were conducted by grooving a CFRP board (aircraft member: around
10mm in depth), namely, by trimming it to a cut depth of 100-% tool diameter, and theroughness of machined surfaces was evaluated.
When machined, using an endmill, the surface roughness gradually worsened withincreasing feed speed from its usual level of around 200mm/min until it surpassed Ra3μm at
1000mm/min. On the other hand, the developed tool permitted machining to achieve Ra1.5μm
even at 2000mm/min, assuring the availability of work with efficiency five or more times
higher than that of the endmill (Figure 6).
[Reference] AWJ-machined surfaces are good in quality free from delamination but the surface
roughness was Ra4μm or so.
2.4.2 Life test
For life tests, the aforementioned CFRP board was subjected to the trimming of its end
face at the feed speed of 2000mm/min via a repetitive reciprocating motion of up cutting anddown cutting into the depth of 50-% tool diameter and, as in the preceding subsection, theroughness of machined surfaces was evaluated. Once machining started, the surface roughness
gradually worsened until Ra2.4μm was marked at the end of 40-m processing (counted asone-round trip equivalent of distance) and thereafter, the CFRP board could be processed withalmost no change in surface roughness (Figure 7).

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
7

Figure 6

High-speed trimming test of CFRP

Figure 7Life test of CFRP trimming

The developed tool permitted machining to achieve equivalent surface roughness even at afeed speed five or more times faster than that of an endmill.

The developed tool had almost no worsening of surface roughness for any longer machined
distance, permitting Ra2.4μm to be achieved even after 40m was machined.

|3. Higher efficiency of drilling
3.1

Present problems

When drilling CFRP, twist drills are commonly used. But in drilling, since the cutting force
is applied in a direction vertical to the material’s machined surface, fiber bed delamination anduncut fibers occur on the material’s surface, particularly, on the drill exit side, causing a great
qualitative problem and hampering the enhancement of machining efficiency (Figure 8).

Figure 8

Delamination, uncut fibers accompanying CFRP drilling

Drilling easily causes delamination and other problems on the drill exit side.

3.2

Solution

Delamination and uncut fibers on the drill exit side may presumably be caused by the process
where the uppermost fiber bed delaminates outside in the area wider than the drill diameter whenMitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
8the material becomes thin on the verge of drill penetration at the end of the work, and thedelamination remains even though machined after complete drill penetration (Figure 9 (a)). Also inthe case of when the point angle of the cutting edge on the outer corner of the drill that eventuallyfinishes the inner surface of a hole is large, the cutting force works heavily in the direction todelaminate fiber beds, inviting an easy-to-break-away condition (Figure 9 (b)).

Figure 9

Mechanism of delamination, uncut fibers accompanying CFRP drilling(a) The high feed speed just before drill penetration causes fibers to break away in an area wider thandrill diameter, leaving delamination and uncut fibers even after complete penetration of the drill.
(b) Although the point angle should be large since the drill point needs to be rigid, great force acts inthe direction to delaminate fiber beds on the verge of drill penetration if the point angle is large.

Due to the above reasons, the following two measures were taken to improve machining
efficiency:(1) The feed speed just before drill penetration should be made as low as possible to mitigate
processing load (Figure 10 (a)).
(2) The point angle on the drill’s outer corner should be made small and in two stages (Figure10 (b)).
Tool life is also based on whether or not the aforementioned delamination and/or uncut fibers
occur.
It is considered that a twist drill, until its life comes to an end, follows the process that
[Processing load concentrates on the drill's corner]⇒[Coating on the drill's corner breaks away]⇒[Starting from this, delamination or chipping of the parent material occurs]⇒[The cutting edge
becomes blunt]⇒[Delamination occurs on the drill exit side].
Measures were, therefore, taken to extend lifespan by:(3) Rendering the cutting edge shaped as to cut better and reduce cutting resistance, thereby
limiting load concentration on the corner.

Figure 10

Drilling tool, machining efficiency-related measures(a) Processing limits on the drill exit side should be kept lower to prevent delamination.
(b) The point angle on the drill’s outer corner should be made small and in two stages.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
93.3

Machining test result

Also in drilling work, the aforementioned CFRP board was tested for machinability as in thecase of trimming. Diamond-coated cement carbide twist drills from several tool makers were used
and any presence of delamination or uncut fibers on the drill exit side was examined as the base todetermine quality and lifespan.
As a result, with an optimally shaped twist drill, quality could be maintained until machining
efficiency became a maximum of 22 times higher than before. It was also ensured that lifespanmight well be extended as far as the number of holes that could be bored increased by four or moretimes from existing drills (Figure 11).

Figure 11

Effect of CFRP drilling improvement

A better tool for and method of drilling could improve (a) machining efficiency amaximum of 22 times and (b) tool life, 4 times.

|4. Conclusion
In order that trimming and drilling of CFRP can be performed with high efficiency, we havebeen engaged in development from both aspects of tools and cutting/grinding methods. Recently
obtained results on aircraft materials prove that efficiency can be several to several tens of times
higher than conventional rates, and tool life can also exceed existing levels. Toward the future, wewould like to further develop machining technologies that can be effectively used in industrial
fields other than aircraft and, at the same time, continue to offer optimum machining applications toour customers.