Aplication of digitalization and simulation to coining.

Skunca, Marko ; Loncar, Damir ; Math, Miljenko 等

1. INTRODUCTION

Papers regarding particular technology like coining or minting are scarce (Ike & Plancak, 1998), (Ike, 2005). Reason to this is highly commercial nature of latter technology. Moreover commemorative medals and coins tend to exhibit increase in price (Ladany, 1981). Therefore monetary institutes keep there technology as secret as possible.

High complexity of medal surface within the hight span of [+ or -] 0.05 mm in present case, represents a great challenge to FEM modelling. Therefore a 3D FE coining simulation of the golden medal shown in was performed. (Fig. 1)

In order to reduce details (Buffa et al., 2007) have created a model of simplified coin surface geometry. In this paper no geometry simplification was made and simulation was performed over 1/12 of the coin showed as marked area. (Fig. 2)

2. DIGITALIZATION

Digitalization of coin geometry was performed at FSB Zagreb, using ATOS Standard 3D digitizer manufactured by GOM mbh (http://www.gom.com). ATOS measuring head with two CCIR-50Hz camearas 0,8 MPixel, was used with retro reflective illumination. Medal of 37 mm diameter was digitized using available measuring volume of 50x40 mm (one set of lenses: 35 mm projector lens, 50 mm camera lenses, calibration object 50x40 mm).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Implemented photogrametric triangulation technique resulted in high detail scan. Basic steps of digitalisation were as follows; After fixture on base plate, medal was sprayed with penetrant in order to neutralise surface reflection. Calibration and digitalisation were performed in short time. Once point cloud was recorded, ATOS software was used to digitize, process, visualize and export the measured data to Mentat for preprocessing and FE model preparation.

3. PREPROCESSING

3.1 Geometry

Large number of triangles were converted to the geometry of 7e5 surfaces modelling desired geometry. From this number, 3e4 surfaces was chosen to model one twelvth of the coin vers, shown shadowed in Fig. 2. Minimum surface sizes of 0.05 mm were taken as satisfactory modelling of the surface geometry. With a details up to 0.02 mm.

One twelvth of the rigid punch was modelled using 3e4 surfaces. Underneath the punch FE mesh was generated as 1/12* of the cylinder of radius 18.3 mm and height 0.68 mm. Divided into mesh of 0.17 mm element size, cylinder is made of 1e5 finite elements. Tip of the cylinder near axis of symmetry was cut off in order to avoid (well known bug) of node penetration trough wedge formed by two intersecting surfaces.

Two wedge plains were used to impose boundary conditions upon 1/12 of the coin.

Flow of the material at the outer edge was restricted by the cylinder surface intersecting wedge planes.

Base plane was modelled as a flat plane, in order to keep the model as small as possible.

In every case of defining rigid surfaces, non-uniform rational B-splines were avoided.

3.2 Material

Rigid plastic formulation of material was used in order to make model numerically as simple as possible. Stress-strain curve for gold was assigned to material after an internal data sheet not intended for publishing.

3.3 Contact bodies

As previously mentioned, besides deforming FE billet, there are six contact bodies in numerical simulation. All those six bodies were modelled as rigid bodies. For the practical reasons of limited calculation time, none of the bodies was divided into FE elements.

3.4 Boundary conditions

Posed by the use of the symmetric wedge planes enabled the simulation to run to its completion. Moreover, likewise posed BC's enable 3D remeshing that was excecluded from the first processing.

3.5 Loadcases

Only one, static, time independent loadcase was considered. Basic constant time step loading procedure was used for the same reason of simplicity. Regarding global stiffness matrix positive definiteness was required. Number of recycles was increased to ensure completion of the simulation. Convergence criterion was set using relative residual force.

3.5 Job

Job was set upon one and single loadcase using linear tetrahedral elements written for the updated lagrangian framework.

4. PROCESSING

Numerical simulation lasted for 60 hours on average desktop PC of the year 2004. It was performed in MSC Marc via input text file created in MSC Mentat. (Buffa et al., 2007) used DEFORM-3D[TM], but no FE simulation processing time was given.

5. POSTPROCESSING

MSC Mentat was use to perform psotprocessing. Sueccesful simulation was obtained using the simplest allowaable element type and avoiding B-spline surface description. FE simulation has pointed to critical stress areas. (Fig. 3) Qualitatively these areas coincide with the critical points identified by a engineers involved in technology. Although not included in simulation, part of the geometry thet caused the most problems during coining is identified by highly stressed area denoted by the arrow. (Fig. 3) Indded this area is a bit to the right at the right angled tip.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Force stroke diagram shows adequate calibration steep force rise at punch displacement of 0.22 mm. The order of magnitude is appropriate to the force used in a workshop to mint a medal.

6. CONCLUSION

As shown in this paper it is possible to create and perform 3D FE medal minting simulation. Besides the primarily qualitative accordance of numerical and simulation data, one has to be aware of the limits of the simulation. Capturing the fine details emerging in minting operation requires extremely large number of finite elements. Therefore only critical areas should be modelled, i.e. 'virtual reality' should be only partially applied.

More quantitative interconnections between experiment and simulation should be established. Force-stroke diagram should be recorded, surface hardnesses or even grain texture via MLI (mean linear intercept) should be introduced. Aim of all mentioned is creation of experiment / simulation / production integration, neccesarry for any serious technological advancement.

When considering large models, simple numerical methods regarding element types, surface representation and integration models should be used (MSC.Marc--Vol. A).

7. ACKNOWLEDGMENTS

The present work has been supported by The Ministry of Science, Education and Sports of Republic of Croatia.

7. REFERENCES

Buffa G., Fratini L. and Micari F. (2007).: The Relevance of the Preform Design in Coining Processes of Cupronickel Alloy, Proceedings of the 9th International Conference on Numerical Methods in Industrial Forming Processes, J.M.A. Cesar de Sa and A. D. Santos (Ed.), pp. 1005-1010, ISBN 978-0-7354-0415-1, Porto, Portugal, 17-21 June 2007, American Institute of Physics, New York

Ike H., Plancak M. (1998).: Coining process as a means of controlling surface microgeometry, Journal of Materials Processing Technology, Volumes 80-81, 1 August 1998, pp. 101-107, ISSN: 0924-0136

Ike H. (2005).: Nanoscopic surface texture formed by indentation and sliding of a smooth wedge tool, Wear, Volume 258, Issue 9, April 2005, pp. 1404-1410, ISSN: 0043-1648

MSC.Marc--Volume A Theory and user information (Version 2003) 2003 MSC.Software Corporation, 2 MacArthur Place, Santa Ana, CA 92707

Shaul P. Ladany (1981). : A minting policy for commemorative coins, European Journal of Operational Research, Vol. 8, No. 2, (October 1981), pp. 130-138, ISSN: 0377-2217