DEVELOPMENT OF A CNC MICRO-LATHE AND MICRO-MACHINED BONE IMPLANTS
DANIEL ANTONIO RANGEL CALDERON
UNIVERSIDAD DE LOS ANDES FACULTY OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEEERING BOGOTA D.C. 2011
DEVELOPMENT OF A CNC MICRO-LATHE AND MICRO-MACHINED BONE IMPLANTS
DANIEL ANTONIO RANGEL CALDERON
Undergraduate project presented as partial requirement for the Mechanical Engineer degree.
Advisor:
FABIO ARTURO ROJAS MORA, Dr. Eng. Mec Department of Mechanical Engineering
Universidad de los Andes
UNIVERSIDAD DE LOS ANDES FACULTY OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEEERING BOGOTA D.C. 2011
To my parents
ACKNOWLEDGMENTS
The author would like to acknowledge the LATEMM and the professor Rojas at
the department of Mechanical engineering at Uniandes for encouraging the
development of this prototype, the contributions made by the LabMec and
Randal with respect to manufacture process, the author also thanks Gabriel
Quiroga and Néstor Arteaga for the first approach of Micro-machine center, their
results and advises were key elements in the design of the LATHE-M.
CONTENTS
ABSTRACT_____________________________________________________ 1
1. INTRODUCTION_______________________________________________ 2
2. MICRO-LATHE SYSTEM________________________________________ 4
2.1 Design considerations________________________________________ 4
2.2 The concept________________________________________________ 5
3. BONE IMPLANTS______________________________________________ 7
3.1 Experiment design___________________________________________ 7
3.2 Experimental results__________________________________________ 8
4. DISCUSSION_________________________________________________12
5. CONCLUSIONS_______________________________________________ 13
6. REFERENCES_______________________________________________ 14
1
ABSTRACT
This work evaluates de development of a CNC Micro-Lathe concept in order to
machine controlled geometries from cortical bone, which will serve as Micro-
implants. The motivation behind the design and construction of this machine tool
was to achieve high precision and high performance. A robust machine was
accomplished taking into account the adequate size ratio machine/workpiece at
the operating conditions of the system, a rotation speed up to 300,000 rpm was
achieved, the cutting tool moves in two axis through step motors coupled to
worm gear reductions thus resolution of 1µm is achieved. The interpolator was
programmed based on DDA integration. The machine was set under a
stereoscope to visualize the machining operations with zoom up to 30X.
Precision machining was reached adopting NC control and it was possible to
characterize Micro-machined cortical bone samples.
Keywords: Micro-machining, Micro-implants, NC control
2
1. INTRODUCTION
Tissue Engineering has shown recent advanced techniques employed to
produce implants with controlled architecture that can satisfy bioactivity demands
and shaping requirements. Scaffold materials, pore size and pore fraction that
result in bone formation in vivo has also been studied. Many types of Micro-
machining process are presented to make such implants. A current development
of machine tool shows the advantages of the Micro-machining process
associated to removal by mechanical force.
Research related to bone/scaffolds has shown the impact of controlled geometry
in order to assists cells attachment and growth in the interaction surface, Micro-
implants has also been widely investigated for drug delivery, nano-spheres have
been produced using nano-precipitation and its biological behaviour has been
discussed in terms of the in vitro interaction and the in vivo bio-distribution in
some animal models. Some approaches of biomaterials design focus on the
production of scaffold, which can host the biological activities in a physiological
manner. Advanced techniques, such as rapid prototyping, stereo lithography,
electroforming, photo etching and some melting/vaporization methods are key
technologies related to the fabrication of Micro-components and Micro-implants.
The NC Micro-Lathe concept developed at Uniandes consists in a high-
speed/high-precision system employed to machine controlled geometries of
micro size, the machine tool was calibrated and set-up for a first test and
measurement analysis were made to improve posterior results.
The purpose of this research is to verify the machinability of cortical bone at
architectural level (100-500) µm, an experimental model to achieve micro pieces
3
is proposed based on knowledge of Macro-machinability of bone, examination of
the Micro-machined implants by SEM indicates porosity, surface finishing and
controlled geometries. Elements analysis must be implemented on the samples
to make sure they match the implantation aims and in vivo studies shall be
performed to verify the biocompatibility with the receptor and the rate of bone
formation.
4
2. MICRO-LATHE SYSTEM
2.1 Design considerations
The LATEMM has developed important research in Micro-machining and the
actual machine has taken advantage of experimental results from an early
prototype of machine center developed in (2004-2005) at Uniandes. Quiroga
made the first approach of Micro-machine in his work: Fabrication of Lyophilized
Bone Microimplants, he developed a computer-controlled lathe, which was set
under a microscope, he used step motors for the movement of the axis, although
the resolution achieved was not highly precise. Arteaga in his work: Development
of a micro machining center for the LATEMM, made some improvements in the
early prototype, these were: Addition of a third controlled axis, addition of a
milling spindle, improvement of the electronic board and standardize the
software. He machined surfaces with defined geometries but deviation from the
ideal shape was important at that level of machining. Some aspects were
relevant in the design of the NC Micro-Lathe due the failures presented in the
first Micro-machine center, thus a robust machine had to be fabricated in order to
isolate vibrations, better resolution in the axis movement had to be achieved, an
improved electronic board was implemented and a better interpolation method
was employed to accomplish controlled geometries from cortical bone.
The machining of human bone has been reported by Rojas; fabrication of
composite biomaterials, and the development of bone microcapsules that could
carry drugs such as insulin, antibiotics, or anaesthetics into the bone to release
them gradually into the body have appeared. The competitive advantages of
these products rest on their biocompatibility and the ease of fabrication.
5
The aim of this project was to build an NC Micro-Lathe oriented to the Micro-
machining of soft materials. The system is divided into three parts: The
interpreter, the interpolator and the mechanical structure.
2.2 The concept
The Micro-Lathe in Fig. 1 was designed in CAD software as can be seen in Fig.
2. The Micro-machine was assembled on an old stereoscope and a microscope’s
XY table, a high speed pneumatic turbine was selected to function as a spindle-
collet, two step motors were chosen to move the cutting tool in the X and Y axis,
and the XY table movement is transmitted through worm gears designed to
achieve accuracy of 1 µm per step. The robustness was an important element in
the design of the mechanical structure. A microcontroller programmed in C++
send the bits sequences to the step motors controlling the desired steps,
frequency and direction, the interpolation implemented is based on DDA
integrator. The entire system is connected to the USB port of a PC in which a
program with sequence movements is generated and compiled in Borland C++
5.5.
Fig 1. NC Micro-Lathe (LATHE-M)
6
Fig 2. Micro-Lathe (CAD prototype)
In Fig. 3 a cortical bone sample is mounted in the air-turbine spindle and the
cutting tool has performed a turning operation in which half sphere has been
made. The maximum diameter that the collect can handle is 1.58 mm and due to
the single attachment of the workpiece it is advisable to work with a relation
length/diameter up to 3. The working pressure for the turning spindle must not
exceed 30 psi and the rotational speed achieved in this condition is
approximately 300,000 rpm. A housing was design to fit in the electronic board
and was fabricated with rapid prototyping. The cutting tool used for bone
machining was a tungsten insert with sharp edge.
Fig 3. Turning operation (10X zoom)
7
3. BONE IMPLANTS
3.1 Experiment design
Cortical bone samples with controlled features (see Fig. 4) had to be adequate to
the correct size to be Micro-machined in the NC Micro-Lathe; these samples are
human cortical bone cylinders and were turned with the following instructions to
remain with the same chemical composition:
Cutting velocity: (3-32) m/min
Feed rate: (0.03-0.14) mm/rev
Depth: 0.2 mm
The cutting tool was also specified with incidence angle of 5º and sharp edge.
Fig 4. Cortical bone sample (Initial stage)
The Macro-machined samples resulted in cylinders of 1.58 mm of diameter and
10 mm of length. These pieces were Micro-machined controlling the operating
conditions:
8
Spindle speed: (1-3)x105 rpm
Feed rate: 300 µm/sec
Cutting velocity: (100-300) m/min
Depth: (5-50) µm
To achieve controlled geometries it was necessary to generate different tool
paths for each Micro-machined sample, External turning and spherical
generation were the operations implemented to demonstrate the machinability of
bone at this level.
3.2 Experimental results
The resulting bone geometries were analysed by SEM. Some features such as
surface finishing, porosity, and concentricity are shown in Fig. 5-6. At levels
below 100 µm is evident that the size of the porous is close to the size of the
entire implant and this could affect its architecture and the behaviour of the
implant interacting with the receptor. Nonetheless it is clear that there is no
evidence of tool path until levels below 1 µm due to the high cutting speeds. The
difference between initial and final stage on the workpiece is also marked. The
sample in Fig. 6 was fractured due to the high stress presented in the bone and
due the runout of the spindle.
Fig 5. Micro-turned bone sample
9
Fig 6. Micro porosity
The concentricity error measured is the arctan (0.014), this should be considered
as a sum of calibration errors, in the alignment of the spindle and in the
parallelism of the cutting tool plane with respect to the turning spindle axis. There
is also important to take into account the effect of the cutting tool force, which
deflects the sample due to the high stress presented.
Micro-spheres were also Micro-machined in the LATHE-M. Software DDA
interpolator was used for the Micro-machining of two different size spheres. In
Fig. 7 half sphere of 250 µm in radius was made interpolating with an 8-bit
register, achieving accuracy of 1 µm (see Fig. 8), with an iteration frequency of 3
ms, the shape of a cone was generated later, starting at the end of the semi-
sphere.
Fig 7. Half sphere (500 µm in diameter)
10
In Fig. 9 half sphere of 600 µm in radius was made interpolating with a 7-bit
register, achieving accuracy of 10 µm per iteration (see Fig. 10), also the shape
of a cone was generated and finally external turning of 500 µm length was
performed.
Fig 9. Half sphere (1.2 mm in diameter)
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Fig 8. Circular interpolator (500 µm diameter sphere)
11
The tool path is more evident in Fig. 9 than in Fig. 7 due to the accuracy of the
interpolator, porous size also affects the geometry of the implant and the cutting
tool seems to be an important element since a small radius is needed on its edge
for machining at this level.
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Figure 10. Circular interpolator (1.2 mm diameter sphere)
12
4. DISCUSSION
The concentricity error obtained is important at the level of Micro-machining and
it is necessary to determine de source of this error, a ruler on the ocular of the
stereoscope would help to determine whether the pneumatic turbine or the XY
table is the problem. Measuring the movement of the axis with interferometer is
important to establish de parallelism between the working plane of the cutting
tool and the turning spindle axis. It is important to determine the speed of the
spindle while its in operation; some methods like Doppler effect solve the
problem of measuring the high speed of the turbine.
An Analysis of the workpiece as a cantilever beam was made, it results that
deflection is an important factor to take in mind for the tool path, depth of cut and
acceleration/deceleration methods for the cutting tool. It is also necessary to
acquire adequate cutting tools for this level of machining; a diamond insert would
be appropriate in these operating conditions.
A new design of machine tool has to be implemented in order to continue the
development of a CNC Micro-machine center; a milling machine could be joined
in the actual Micro-Lathe, high precision actuators must be used and a
positioning control system would allow better cutting operations, advanced
interpolation may also be employed.
Elements analysis has to be implemented on the cortical bone samples. Its
important to remain with the same composition in the Macro and Micro-machined
parts to fit the aims of implantation, if bone is burned in the Micro-machining, its
carbon percentage will increase. For the machining conditions presented
previously it is not expected to measure changes in elements composition above
10%.
13
5. CONCLUSIONS
The following conclusions can be drawn from this research:
• High speed Micro-turning has been proved as a method for the fabrication of
bone implants with controlled geometries.
• The most serious problem encountered during tuning the spindle was the
alignment with respect to the cutting tool and the cutting forces, which tend to
bend the workpiece; therefore small depth of cut and the correct tool path has
to be chosen in every operation.
• The machine developed meets the requirements for machining at micro size,
different geometries has been achieved and the machinability of bone at this
level is evident.
14
6. REFERENCES
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