Spindle unit

An apparatus for machining a hole in a work piece includes a spindle motor that is rotatable about a principal axis. The spindle motor includes a tool holder having a tool axis substantially parallel to the principal axis. The tool holder is rotatable about the tool axis. An axial actuator is configured for moving the spindle motor in an axial feed direction substantially parallel to each of the principal axis and the tool axis. A radial actuator is configured for adjusting a radial distance between the principal axis and the tool axis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and spindle unit for producing a 
hole or a recess in a work piece which may include flat or curved sheets 
of fiber-reinforced composite material, metal or combinations thereof. 
2. Description of the Related Art 
Structures for aerospace and other applications are often made up of thin 
curved shells of different material such as aluminum, titanium, stainless 
steel and fiber-reinforced composite materials. In structural applications 
different components are often fastened by use of bolted joints. Bolt 
holes for aerospace structures are typically about 4-20 mm diameter with 
high quality requirements to secure the integrity of the structure. 
Hole requirements are related to dimension and damage. Dimension 
requirements include, for example, cylindrical hole diameter, height of 
the cylindrical part of the hole, diameter and angle of countersinks, 
roundness, and alignment with the normal direction of the surface. Damage 
related requirements include, among other things, allowable burr height, 
surface finish and, with regard to fiber-reinforced composites, allowable 
delamination. 
Particular problems occur when drilling holes in fiber-reinforced 
composites. Polymer composite materials have been generally known since 
the 1950's. These materials are composed of a protective and binding 
polymer, either a thermoplastic or a thermosetting plastic, usually 
referred to as the matrix, together with fibers (e.g. glass, carbon or 
amide fibers), which may be regarded as a reinforcing material. The fibers 
may be continuous and oriented in specific directions, or they may be 
relatively short and arranged at random in the matrix. Composites with 
continuous and oriented fibers give products with mechanical properties 
superior to those of conventional polymer and metallic materials, 
especially as far as their weight-related strength and stiffness are 
concerned. Composites with shorter fibers find an application where rather 
less demanding properties are called for. One factor obstructing the wider 
use of composite materials is the absence of effective methods of cutting 
machining. The physical and chemical properties of the composite materials 
mean that known machining methods cannot generally be applied with 
successful results. 
Products consisting of composite material often contain holes for various 
purposes. These holes may be required, for instance, to permit the laying 
of service lines, assembly or inspection. Bolt holes are a particularly 
important category of hole. Structures for practical applications are 
often constructed from components joined together to produce a finished 
product. The purpose of the joint is to transfer the load from one 
structural element to another. One common form of joining is the bolted 
connection, in which the load is transferred by either shearing loads or 
tensile loads in the bolt. The strength of a bolted connection is 
influenced to a considerable degree by the quality and precision of the 
hole. Reference may be made to three particular problem areas when 
producing holes in polymer based fiber reinforced composite materials: 
1. Low interlaminar strength. When machining laminated composite materials, 
there is a risk of the layers separating (delaminating) because of the low 
interlaminar strength. Extensive delamination damage can jeopardize the 
strength of the laminate. 
2. Low resistance to heat and cold of certain thermoplastics. The heat 
generated during machining can cause the matrix to soften and block the 
tool, making further machining impossible. In order to achieve good hole 
quality, it is accordingly necessary to provide effective cooling of the 
tool/hole edge, and for the material removed by cutting (chips, splinters 
and grinding dust) to be removed continuously from the hole. 
3. High wear resistance of fibers. The cutting machining of the fiber 
composites causes severe wear of the tool because of the good wear 
characteristics of the fiber materials. This leads to high wear costs, 
especially when producing holes with a requirement for high precision. 
The methods used to produce holes in composite laminates are traditional 
drilling, boring, milling, sawing and grinding. The problem associated 
with these hole-forming methods as they are applied at the present time is 
that they are not sufficiently effective for various reasons from a 
technical/economic point of view. 
High wear costs are a general problem associated with cutting machining 
where high precision is required. Great care must be taken when drilling 
or boring to ensure that delamination damage is avoided on both the entry 
and exit sides. Special cutters are required in order to achieve the 
stipulated hole quality, and special procedures must be formulated. In 
order to avoid extensive delamination damage on the exit side of the 
laminate, local lateral pressure must be applied around the edge of the 
hole. Another previously disclosed method of protecting the exit side from 
damage is to provide the laminate with an additional protective layer. 
Sawing is a distinctly unsuitable method for producing holes with high 
precision requirements. When producing holes by grinding, use is made of a 
cylindrically shaped tubular body, the machining end of which is coated 
with a wear-resistant surface layer. Holes are produced by grinding the 
surface of the material transversely while first causing the grinding body 
to rotate. The method is slow and gives low precision. 
It should be pointed out in this respect that hole-machining methods, in 
which a body driven rotatably about an axis of rotation is also caused to 
execute an orbital motion (i.e., the axis of rotation is displaced in such 
a way that the side is able to move relative to the edge of the hole), are 
generally familiar. SE 173 899 discloses a machine tool having a tool 
carrier rotating eccentrically about a principal axis, in which the 
distance between the tool carrier and the principal axis is determined by 
a guide component, which rotates about the principal axis together with 
the tool carrier. The guide component rotating together with the tool 
carrier is arranged perpendicular to the principal axis and is executed as 
a cam capable of rotating about it in relation to the tool holder, with 
the guiding profile of which cam the tool holder is in direct engagement. 
The advantages of this invention include, among other things, the absence 
of free play and the space-saving execution of the guide component. 
However, the offset between the axis of rotation of the tool holder and 
the eccentric axis of rotation is fixed and determined by the size of the 
cam gear. Thus, the offset cannot be easily adjusted without replacing the 
cam gear within the head. SE 382 506 discloses a rotatably driven, 
combined cutting tool for making holes in stationary work pieces, which 
holes can be provided with a conical starting chamfer. 
Disclosed in the U.S. Pat. No. 5,641,252 (Eriksson et al.), is a method for 
machining holes in a fiber-reinforced composite material which presents a 
significant step forward in the art. The central axis of the hole passes 
through a predetermined point on the surface of the work piece and is 
oriented in a certain direction in relation to the longitudinal directions 
of the fibers in the immediate vicinity of the point. The material is 
machined simultaneously in both an axial and a radial sense by causing the 
tool to describe axial motion and rotate not only about its own axis, but 
also eccentrically about the central axis. This method makes it possible 
to machine holes without causing delamination in the composite material. 
Aerospace and related structures are typically made up of different 
materials stacked together. Particular problems occur when co-drilling 
structures including several layers of different materials (material 
stacks). Such problems include burrs in between the layers, close up 
holes, and damage in filler material in between layers (liquid shims). 
Drilling holes using traditional techniques generates heat which may cause 
rapid wear of the tool. This problem is particularly pronounced when 
drilling holes in titanium. 
It is also known to mount a traditional spindle on a robot arm and to use 
the control system of the robot to dictate the movements of the cutting 
tool. A problem is that the precision and quality of the resulting hole is 
limited by the mechanics of the robot and its associated control system, 
which are designed for moving and positioning a heavy robot arm. Thus, the 
precision and performance of the robot mechanics and control system are 
not sufficient to machine, for example, high precision fastener holes at 
high speeds using the required motion. 
SUMMARY OF THE INVENTION 
The present invention eliminates the shortcomings and limitations 
associated with previously disclosed methods and permits the rational and 
cost-effective production of holes, free from strength-reducing damage and 
burrs, and, in so doing, guarantees repeatably good hole quality. The 
present invention provides a spindle unit for cutting a hole in a work 
piece having a cutting tool with a first axis of rotation, wherein the 
cutting tool is also movable about a second, eccentric axis of rotation 
which is adjustable using a mechanical drive element. The spindle unit can 
be mounted on a robot arm which is used to position and orient the end 
effector, with the precise machining being controlled by the spindle unit. 
The invention comprises, in one form thereof, an apparatus for machining a 
hole in a work piece including a spindle motor that is rotatable about a 
principal axis. The spindle motor includes a tool holder having a tool 
axis substantially parallel to the principal axis. The tool holder is 
rotatable about the tool axis. An axial actuator is configured for moving 
the spindle motor in an axial feed direction substantially parallel to 
each of the principal axis and the tool axis. A radial actuator is 
configured for adjusting a radial distance between the principal axis and 
the tool axis. 
The eccentric rotary motion is a strictly rotary motion, i.e., it is 
executed with a constant or continuously varying distance between the 
central axis and the axis of rotation of the tool. 
The apparatus exhibits a number of substantial advantages compared with 
generally familiar machines: 
1. The apparatus allows the radial offset between the axis of rotation of 
the cutting tool and the eccentric axis of rotation to be easily adjusted 
without the replacement of any parts. 
2. The apparatus enables the production of holes of higher precision and 
quality than can be achieved by using the control system of a robot arm. 
3. The method permits the production of holes to tight tolerances. The 
dimensional accuracy of the hole is determined substantially by the 
accuracy of positioning the tool relative to a central axis. The 
requirements imposed on the geometry of the tool are not particularly 
high, on the other hand, since every individual tool is simply calibrated 
before use.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings and particularly to FIG. 1, there is shown a 
spindle unit 10 including a spindle motor 12, a radial offset mechanism 
14, an axial feed mechanism 16 and an eccentric rotation mechanism 18. 
Spindle motor 12 includes a body 20 and a rotatable tool holder 22 
configured for holding a cutting tool 24 during rotation. Cutting tool 24, 
which defines a tool axis 26, can be designed for producing a hole (not 
shown) in a work piece such that the diameter of the hole is larger than 
the diameter of cutting tool 24. The hole can be machined, for example, 
using the method disclosed in U.S. Pat. No. 5,641,252 (Eriksson, et al.). 
Spindle motor 12 also includes a conduit 28 through which spindle motor 12 
can be supplied with electric, pneumatic or hydraulic power. 
The top of spindle motor 12 is rigidly attached to an annular attachment 30 
having a retaining pin 32 (FIG. 2) extending radially from the rear 
thereof. Retaining pin 32 is slidably held within a slot 34 in a bracket 
36 of spindle unit 10. Retaining pin 32 has a limited degree of freedom to 
move within slot 34 in any direction substantially perpendicular to tool 
axis 26. Even while sliding within slot 34, however, a longitudinal axis 
38 of retaining pin 32 invariably extends in a rearward direction, thereby 
holding fixed the orientation of spindle motor body 20 such that body 20 
always faces a same direction. This holding fixed of the orientation of 
spindle motor body 20 facilitates the supplying of power to spindle motor 
12. 
Radial offset mechanism 14, best seen in the enlarged view of FIG. 3, 
includes a sliding block 40 having an internally tapering, hollow tube 42 
extending into annular attachment 30. Two annular bearings 44 are snugly 
disposed between and interconnect tube 42 and attachment 30, with a 
locking device 46 on the distal end of tube 42 holding bearings 44 in 
place. Bearings 44 allow tube 42 to rotate, while the rotational 
orientation of annular attachment 30 remains substantially fixed. 
An axle 48 has a hollow interior 50 containing a radial offset needle 52 
defining a principal axis 54. Radial offset needle 52 has a 
conically-shaped end 56 with a distal tip 57 extending into a tapered 
interior 58 of tube 42. End 56 is angled to match the angle of an inclined 
surface 60 of tapered interior 58 such that the entire length of 
conically-shaped end 56 can contact surface 60. Inclined inner surface 60 
of sliding block 40 forms an orifice 61 with an inner diameter 
substantially equal to the outer diameter of distal tip 57. 
Axle 48 has a radially extending portion 62 with an annular rim 64. A 
spring 66 is disposed in opposing holes 68 and 70 in sliding block 40 and 
rim 64, respectively. Spring 66 biases sliding block 40 against radial 
offset needle 52. More specifically, spring 66 biases surface 60 of 
tapered interior 58 of sliding block 40 against conically-shaped end 56 of 
needle 52. 
Radial offset needle 52 is movable in an axial feed direction indicated by 
double arrow 76, as is apparent by a comparison of FIGS. 2 and 5. An 
externally threaded sleeve 78 surrounds and is concentric with offset 
needle 52 such that needle 52 may rotate freely within sleeve 78, as 
enabled by bearings 79. An annular, disk-shaped screw 80 has an internally 
threaded, central hole 82 which receives and is coupled with the external 
threads of sleeve 78. Disk screw 80 is driven by a radial offset belt 84 
which, in turn, is driven by a radial offset motor 86. 
An upper portion of sleeve 78 is received in a bushing 87. Sleeve 78 is 
fixed in the angular direction by a wedge 91 disposed within bushing 87. A 
washer-type spring 89, which is compressible in the axial feed direction 
indicated by double arrow 76, is disposed around and connected to needle 
52 immediately above bushing 87. Spring 89 has an outer diameter which is 
larger than the inner diameter of sleeve 78 in order to allow spring 89 to 
be pressed against sleeve 78 and to prevent spring 89 from being pushed 
into the area within sleeve 78. Within bushing 87 is sleeve 78; and within 
sleeve 78, in turn, is needle 52. Washer-type spring 89 is configured for 
resisting axial movement of needle 52 toward orifice 61 of sliding block 
40 when distal tip 57 of needle 52 is within a predetermined distance of 
orifice 61. The predetermined distance is approximately equal to the axial 
distance by which spring 89 can be compressed. 
Axial feed mechanism 16 includes a stationary axial feed motor 88 rigidly 
attached to a fixed mounting plate 90. Motor 88 rotates a threaded output 
shaft 92 which is received in, coupled to, and carries an internally 
threaded ball bearing screw 94. Ball bearing screw 94 is rigidly attached 
to an arm 96 of bracket 36, with bracket arm 96 being secured by screws 98 
to an annular casing 100 surrounding axle 48. Bearings 102 interconnect 
casing 100 and axle 48, yet allow axle 48 to rotate relative to casing 
100. A pair of sliding blocks 104 (FIG. 4) interconnect mounting plate 90 
and bracket 36 and allow relative sliding movement therebetween. 
Eccentric rotation mechanism 18 includes an eccentric rotation motor 106 
which drives an eccentric rotation belt 108 engaged with axle 48. Belt 108 
rotates axle 48 around principal axis 54, and thereby, due to the offset 
of tool axis 26 from principal axis 54 created by radial offset mechanism 
14, provides a corresponding eccentric rotation of cutting tool 24 around 
principal axis 54. 
In operation, spindle motor 12, receiving power through conduit 28, causes 
a rotation of tool holder 22 and a corresponding rotation of cutting tool 
24. Axial feed motor 88 of axial feed mechanism 16 causes a rotation of 
output shaft 92, which causes ball bearing screw 94 to move up or down, 
depending upon the direction of rotation of output shaft 92. Ball bearing 
screw 94, through bracket arm 96 and the rest of bracket 36, is rigidly 
connected to spindle motor 12, radial offset mechanism 14 and eccentric 
rotation mechanism 18, along with the associated motors 86 and 106. Thus, 
the axial movement of ball bearing screw 94 causes a corresponding axial 
movement in substantially all of spindle unit 10, except for the axial 
feed mechanism 16 itself. The parts of axial feed mechanism 16 which are 
stationary relative to the axial direction include axial feed motor 88, 
output shaft 92 and mounting plate 90. Through the above-described 
operation of axial feed mechanism 18, a rotating cutting tool 24 can be 
advanced into a work piece (not shown) in order to machine a hole in the 
work piece. 
Radial offset mechanism 14 can be operated in order to create a radial 
offset between tool axis 26 defined by cutting tool 24 and principal axis 
54 defined by axle 48 and radial offset needle 52, as shown in FIG. 5. 
Radial offset motor 86 drives radial offset belt 84, which, in turn, 
rotates disk-shaped screw 80. Sleeve 78, which is threadedly received 
within a central hole 82 of screw 80, moves up or down in the axial 
direction relative to screw 80, depending upon the direction of rotation 
of screw 80. Sleeve 78 is axially coupled to radial offset needle 52 such 
that needle 52 follows any axial movement of sleeve 78. However, needle 52 
is still rotatable within sleeve 78. When screw 80 is brought into 
rotation, sleeve 78 is moved in an axial direction. In turn, sleeve 78 
transfers this movement to needle 52. Needle 52 is thus free to rotate 
along with the eccentric rotation. The arrangement also makes it possible 
to change the offset during machining, which is useful for machining of 
conical holes or complex-shaped axisymmetrical holes. 
Sliding block 40 moves radially in response to the axial movement of radial 
offset needle 52. Spring 66 biases surface 60 of tapered interior 58 of 
sliding block 40 against the conically-shaped end 56 of needle 52. Due to 
the matching angles of conically-shaped end 56 and surface 60, which 
physically interface with each other, a movement of needle 52 away from 
spindle motor 12 results in a sliding movement of sliding block 40 towards 
mounting plate 90, as shown in FIG. 5. Sliding block 40 slides relative to 
axle 48 while at the same time retaining the ability to follow the 
rotation of axle 48. As sliding block 40 slides, it pushes annular 
attachment 30 and spindle motor body 20 along with it. Thus, tool axis 26 
of cutting tool 24 becomes displaced or offset from principal axis 54. 
As radial offset needle 52 advances toward the position shown in FIG. 2, 
wherein tool axis 26 and principal axis 54 coincide, conically-shaped end 
56 begins to physically interface with surface 60 of tapered interior 58 
around the entire 360.degree. of its circumference. If conically-shaped 
end 56 is advanced too quickly into this position, there is a danger that 
end 56 will become wedged or jammed into tapered interior 58, which is 
also referred to as "clamping." When distal tip 57 of end 56 approaches 
orifice 61, the axial force would increase dramatically for small axial 
displacements if there were no elasticity in the system. Such elasticity 
is provided by washer spring 89. When tip 57 approaches orifice 61, washer 
spring 89 makes contact with and begins to become squeezed against the top 
of bushing 87. Spring 89 starts to contract, resisting further advances of 
needle 52 and its tip 57 towards orifice 61. Thus, the risk of clamping is 
substantially reduced. 
The operation of eccentric rotation mechanism 18 causes cutting tool 24 to 
circularly oscillate or orbit around principal axis 54 while tool 24 
simultaneously rotates about its own axis 26. The radius of the circular 
oscillation is substantially equal to the radial offset between tool axis 
26 and principal axis 54. As sliding block 40 rotates along with axle 48, 
spring 66 also rotates. As the rotational position of spring 66 changes, 
the direction of offset of sliding block 40, and thus the offset of 
cutting tool 24 from principal axis 54, undergoes a corresponding change. 
When offset from principal axis 54, cutting tool 24 makes one full 
rotation around principal axis 54 for every rotation of spring 66 around 
principal axis 54. 
Retaining pin 32 holds the orientation of attachment 30 and spindle motor 
body 20 fixed such that spindle motor body 20 always faces in a same, 
predetermined direction and cannot rotate about tool axis 26. However, 
bearings 44 allow sliding block 40 to freely rotate about tool axis 26, 
even while attachment 30 cannot. The radial offset and rotation of sliding 
block 40 causes attachment 30 to oscillate in a circular path about 
principal axis 54. Retaining pin 32 slides as necessary within slot 34 in 
any direction which is perpendicular to axes 26 and 54 in order to follow 
the oscillation of attachment 30. Retaining pin 32 always points in a same 
direction within slot 34, i.e., is always oriented in the same direction 
as shown in order to fix the orientation of spindle motor body 20. 
Using the spindle unit 10, cutting tool 24 can be simultaneously fed in an 
axial direction, rotated about its own axis 26, and eccentrically 
oscillated about a principal axis 54 in order to produce holes having 
diameters greater than the diameter of cutting tool 24. In addition, by 
using radial offset mechanism 14 to adjust the radial offset of cutting 
tool 24 during the machining process, it is possible to produce conical 
holes or other types of axisymmetrical complex-shaped holes. 
The interfacing contact surfaces of needle end 56 and the tapered interior 
58 of sliding block 40 are shown as being conically-shaped. However, it is 
to be understood that the interfacing contact surfaces can be shaped other 
than conically. It is possible for one or both of the needle end and the 
interior surface of the sliding block to have a rounded shape, i.e., to 
following a non-linear or parabolic function of the axial position. With 
such a non-linear needle end or non-linear interior surface, the needle 
end does not contact the interior surface of the sliding block along the 
entire axial length of the needle end, except in the special case where 
the needle end and the interior of the sliding block have identical or 
complementary shapes and the needle is fully inserted into the sliding 
block. Rather, the needle contacts the interior surface of the sliding 
block at only one discrete point along the axial length of the needle end, 
with the discrete point being a function of the axial position of the 
distal tip of the needle. It is also possible for the rounded interior of 
the sliding block to be substantially wider than the rounded needle end. 
Another embodiment of the spindle unit is shown in FIG. 6. This embodiment 
is substantially the same as that in FIGS. 1-5 with the exception of a 
radial offset mechanism 110, best seen in the enlarged view of FIG. 7. A 
shaft 112 is oriented in the axial feed direction, indicated by double 
arrow 76, and has an end 114 with a pin 116 extending radially therefrom. 
End 114 of shaft 112 is received in a sleeve 118 having an inclined slot 
120 retaining pin 116. Sleeve 118 is tightly held within a sliding block 
122 such that sleeve 118 abuts against and interfaces with an inner 
surface 124 of sliding block 124. A movement of shaft 112 in the axial 
feed direction, as shown in FIG. 8, results in pin 116 moving along 
inclined slot 120, since sleeve 118 is fixed in the axial direction 
relative to sliding block 122. The upward movement of pin 116 shifts 
sleeve 118 and, in turn, sliding block 124 away from mounting plate 126, 
thereby radially displacing cutting tool 128 and its axis 130 from 
principal axis 132. Sleeve 118, pin 116 and inclined slot 120 rotate 
around with sliding block 122 in order to rotate the direction of offset 
around principal axis 132, thereby orbiting cutting tool 128 around 
principal axis 132. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.