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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15

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