Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016

Editors J.F. Silva Gomes and S.A. Meguid

Publ. INEGI/FEUP (2016)

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PAPER REF: 6313

ANALYSIS OF CUTTING FORCE DURING THE MILLING OF

Co-28Cr-6Mo ALLOY FOR A FEMORAL PROTHESIS

Pedro Ferreira1,2,3

, Fernando Simões1,2,3(*)

, Carlos Relvas4

1Polytechnic Institute of Coimbra, ISEC, DEM, Portugal 2Polytechnic Institute of Coimbra, IIA, Applied Biomechanical Lab., Portugal 3CEMUC - Department of Mechanical Engineering, University of Coimbra, Portugal 4Department of Mechanical Engineering and TEMA, University of Aveiro, Portugal (*)Email: fsimoes@isec.pt

ABSTRACT

In a first step of this study, a regular shape of Co-28Cr-6Mo alloy was milled with cutting

speeds ranged from 50 up to 100 m/min. The maximum and the root mean square (RMS)

cutting force were evaluated and Fast Fourier Transform (FFT) method applied to relate the

forces recorded with the frequency of cutting process. Based in the results obtained and in

tool wear, in a next step, roughing and finishing milling operations were applied in a section

of a femoral prosthesis and evaluated the cutting forces.

Keywords: Milling, cutting force, femoral prosthesis, Co-28Cr-6Mo.

INTRODUCTION

Co-Cr-Mo alloys are widely used for medical prosthetic implant devices such as knee

implants, metal-to-metal hip joints and dental prosthetics due their high corrosion resistance,

hardness, fatigue resistance and finally due to its excellent biocompatibility. Usually this type

of medical prosthetic implants are cast or forged, and then machined to final dimension and

surface quality. The machinability of Co-Cr-Mo alloys in this final step is very difficult due to

material properties, such as high hardness and tensile strength (Bruschi, 2013 and Koike,

2009). As consequence, the cutting force is high. Furthermore, high temperatures are reached

at the tool nose, machined surface quality is poor and tool life decreases rapidly due to cutting

tool wear.

A regular and a complex geometry were milled in this work, in order to establish the cutting

force for Co-28Cr-6Mo alloy. The regular geometry was a rectangular shape with the

dimensions of 90 mm×18 mm×18 mm (length x width x height) and the complex geometry

used was a femoral prosthesis. A Computer Aided Manufacturing (CAM) software and a

three-axis vertical CNC milling machine were used to generate toolpaths and executing the

milling operations, respectively. End milling and ball nose Tungsten carbide tools were used

and three force components (Fx, Fy, Fz) and the moment (Mz) of the rotating tool are

measured by KISTLER 9123C piezoelectric dynamometer. The three cutting force signals

were combined using the eq. 1 to obtain the main cutting force F:

2 2 2

x y zF F F F= + + (1)

Topic_I: Biomechanical Applications

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Once the milling operation produce an interrupted cut, the components of the cutting forces

vary cyclically and therefore it is useful to calculate the Root Mean Square (RMS) of the main

cutting force. The RMS value can be understood as a statistical measure of the magnitude

variable cutting force during the machining, and can be calculated using the following eq. (2),

were Fi is the main cutting force in each instant of time and N is the number of data acquired:

2 2 2

2 1 2

1

...1 NN

i

i

F F FRMS F

N N=

+ + += =∑ (2)

To identify the frequency content of milling force signals, Fast Fourier Transform is

commonly used to transform the force values from time domain to frequency domain (Huang,

2013).

RESULTS AND CONCLUSIONS

The maximum of main cutting force, RMS and FFT cutting force were calculated for each

cutting speed applied in experimental procedure. The time domain establishes the nature and

level of static force magnitude change, whereas frequency analysis manifests the dynamic

force response to cutting conditions as well as accrued wear levels (Snr, 2000). The obtained

results are presented in fig. 1 and allowed to conclude that de cutting force increases as

cutting speed raises up to 80 m/min. and decreases after that. Concerning the tool wear, no

chip adhesion was observed. Just flank wear was detected and its value increases as cutting

speed raises.

Fig. 1 - Evolution of the cutting force (in domain time and frequency) and RMS in function

of the cutting speed, for a regular geometry of Co-28Cr-6Mo alloy.

Proceedings of the 5th International Conference on Integrity-Reliability-Failure

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Based on the experimental results obtained for regular shape, the 65 m/min. cutting speed was

adopted to mill a section of the femoral prosthesis in the Co-28Cr-6Mo. This velocity is the

one that has the best compromise between the tool wear, cutting force and surface roughness.

Fig. 2a) shows the magnitude of main cutting force in a section of the femoral prosthesis, and

fig. 2b) shows the result obtained after finishing mill operation, performed with a ball nose

tool.

(a)

(b)

Fig. 2 – a) Evolution of main cutting force during finishing mill operation; b) section of

femoral prosthesis after finishing mill operation, performed with a ball nose tool.

Topic_I: Biomechanical Applications

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REFERENCES

[1]-Bruschi S, Ghiotti A, Bordin, Effect of the process parameters on the machinability characteristics

of a CoCrMo alloy, Key Engineering Materials Vols. 554-557 (2013) pp. 1976-1983.

[2]-Koike Y, Matsubara A, Nakatsukasa Y, Yamaji I, Improving tool life in end milling of Cobalt

Chromium Molybdenum alloy, Proc. of LEM21, Osaka University, Japan, (2009), pp. 653-656.

[3]-Huang P, Li J, Sun J, Zhou J, Vibration analysis in milling titanium alloy based on signal

processing of cutting force, Int. J. Adv. Manuf. Technol., 2013, 64, p. 613–621

[4]-Snr D, Sensor signals for tool-wear monitoring in metal cutting operations-a review of methods,

International Journal of Machine Tools & Manufacture, 2000, 40, p. 1073–1098.