030070104 Cebrail Yıldırım 6th week

Five-axis ball-endmilling (new&better) [Advanced milling Method]:

     5-axis ball-end milling has found application in various industries especially for machining of parts with complex surfaces.Additional two degree of freedoms, namely, lead and tilt angles

make it possible to machine complex parts by providing extraflexibility in cutting tool orientation. However, they also complicate the geometry of the process. Knowledge of the process geometry is important for understanding of 5-axis ball-end milling operations.

     5-axis milling is a geometrically complex process since there are two additional rotational degrees of freedom, namely lead and tilt angles, compared to 3-axis milling.They define the cutting tool orientation with respect to surface normal direction.Visualization of their effect on the process geometry is not straightforward; however, the understanding of the process geometry is a very important step in process modeling. In order to represent position and orientation of a cutting tool at an instantaneous point along a tool path, three coordinate systems need to be defined (Fig. 1(a)).
     The first one is a fixed coordinate system called the machine coordinate system (MCS). It consists of X, Y, Z axes of the machine tool, its origin is the home position. The second one is the process coordinate system (FCN). In FCN coordinate system F is the feed direction, C is the cross-feed direction and N is the surface normal direction.The origin of the FCN is at the ball-centre of the cutting tool, and thus it is a moving coordinate system. Moreover, F, C and N directions change along a tool path depending on the workpiece geometry and machining strategy selected. Tool coordinate system (TCS) is the third coordinate system and its origin is also the ball centre of the tool. x and y axes are transversal axes of the tool and z is along the tool axis direction. TCS defines the orientation of the cutting tool with respect to FCN. Lead angle defines the rotation of the cutting tool around C axis whereas tilt angle is the rotation of the toolF axis (Fig.2). Therefore, TCS is the rotated form of FCN with lead and tilt angles.

Some of the terminology in 3-axis milling is not directly applicable to 5-axis milling for definition of the process parameters. Due to the effects of lead and tilt angles, the tool axis is not parallel to the surface normal (Fig.2). Hence, the cutting depth term (a) is used to

define the depth removed from the workpiece in the surface normal direction instead of the axial depth term (Fig. 3(a)).

More comprehensive definition and visual demonstration.

(Erdem Ozturk, Erhan Budak, Tool Orientation Effects on the Geometry of 5-axis Ball-end Milling, Sabanci University, Istanbul, Turkey)

Five-axis ball-endmilling (previous):

In five-axis ball-endmilling, two additional orientation axes added to the machine allow the machining of very complex parts, which cannot be machined using three-axis machines.
Otherwise, cutting speed is zero at tool tip, making the tool cutting very unfavourable. This is because when ceramics or PCBN tools are used, typically failure is the fragile breakage of the tool tip. With five axes, milling can be performed avoiding the tool tip cutting.
Moreover, tool overhang, necessarily large when deep cavities are machined, can be reduced using five-axis milling. Therefore, tool stiffness is higher, which increases machining precision and reduces the risk of tool breakage. Tool stiffness is directly related to the tool slenderness factor L^3/D^4, so a tool length (L) reduction dramatically reduces tool deflection and the lack of precision due to this effect

(Davim J. P., Machining of hard materials, 2011, p. 67)


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Ion beam machining(new&better) [Advanced Machining Process]

Ion beam machining takes place in a vacuum chamber, with charged atoms(ions) fired from an ion source towards a target (workpiece) by means of an accelerating voltage. The process works therefore on principle similar to electron beam machining. An ion beam machine has three main components:

1.      A plasma source which generates the ions;

2.      Extraction grids for removim the ions from the plasma, and accelerating them towards the substrate (or specimen);

3.      A table for holding the specimen

       

    More concise definition fortified by a visual demonstration.

(     J. A. McGeough, Advanced methods of machining, pg. 35,36)

Ion beam machining (previous)

Ion beam machining (IBM) takes place in a vacuum chamber using charged ions fired from an ion source toward the workpiece by means of an accelerating voltage. The mechanism of material removal in IBM differs from that of EBM. It is closely related to the ejection of atoms, from the surface, by other ionized atoms (ions) that bombard the work material. The process is, therefore, called ion etching, ion milling, or ion polishing. The machining system, has an ion source that produces a sufficiently intense beam, with an acceptable spread in its energy for the removal of atoms from the workpiece surface by impingement of ions. Aheated tungsten filament acts as the cathode, from which electrons are accelerated by means of high voltage (1 kV) toward the anode. During the passage of these electrons from the cathode toward the anode, they interact with argon atoms in the plasma source, to produce argon ions.

Ar + e− →Ar+ + 2e

A magnetic field is produced between the cathode and anode that makes the electrons spiral. The path length of the electrons is, therefore, increased through the argon gas, which, in turn, increases the ionization process. The produced ions are then extracted from the plasma toward the workpiece, which is mounted on a water-cooled table having a tilting angle of 0° to 80°. Machining variables such as acceleration voltage, flux, and angle of incidence are independently controlled.

(Advanced Machining Processes, Hassan El-Hofy,Pages172-173)

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Spatial occupancy enumeration(new&better)[Solid Modelling Type]

Spatial occupancy enumeration schemes are a particular case of cell decomposition in which all of the cells in the scheme must be cubical and lie in a fixed spatial grid. When the size of the cube is reduced to a point, this method becomes the representation of a solid body as a set of contiguous points in the space. In order to model a solid object with spatial-occupancy enumeration, one suggestion is to represent this set of cubical cells by listing the coordinates of the centers of the cells. Therefore a solid object is a set of adjacent cells, where the size of a cell determines the maximum resolution of the model. Usually a specific spatial scanning order is imposed; the corresponding ordered sets of three-tuples are called spatial arrays. The advantages of representing a solid by spatial arrays are:

1.       It is easy to access a given point

2.       Spatial uniqueness is assured.

The disadvantage is that there is no explicit relationship between the parts of an object, and such schemes demand a large amount of data storage.

More effective definition.

(Phillip C.-Y. Sheu,Qing Xue, Intelligent robotic planning systems, vol. 3, Pg.9)

Spatial Occupancy Enumeration (previous)
Spatial Occupancy Enumeration is a type of solid modelling. The object is represented by a list ofthe cubical disjoint spatial cells that it occupies. This is the special case of the cell decomposition where the shape of the cells is cubical

(Nasr E.A., Computer-Based Design and Manufacturing, p. 85).

 

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Stress Intensity Factor(new&better)[Factor]

Stress intensity factor constitute the crucial parameters in engineering design against brittle fracture. The stress intensity factor is a function of both the geometry of the cracked bdy and the associated loading. In general the stress intensity factor is expressed by a relation of the form K_1/2/3=F*σ *(π *a)^0.5 , where F is a form factor controlled by the geometry of the cracked component, σ is the applied stress, and a is the crack length. There are many methods of determining the stress intensity factor. Analytical methods based on the theory of elasticity provide many basic results for ideal situations. Numerical methods based on finite elements (FEM) or boundary elements (BEM) are widely used for finite geometry. Experimental techniques such as photoelasticity and strain gauges have also been employed. In all these methods, the stress intensity factoe is usually calculated by examining the variation of normal stress on the crack line ahead of the crack tip, or examining the displacement variation behind the crack tip.

More sensible definition.

(K. R. Y. Simha,K.R.V. Simha, Fracture Mechanics for Modern Engineering DesignPg.57)

Stress Intensity Factor(previous)

For surface cracked plate under unaxial load, the stress intesity factor correlations for semi-eliptical surface cracks were obtained using finite element methods. For the current analysis, the stress intensity factors were were taken from, which presents the stress intensity factors along the crack front for semi-elirtical surface cracks within the range of 0<=a/c<=1 and for a plate geometry within 0<=a/t<=0.8 under general linear and nonlinear loads. Where a/c is crack aspect ratio and a/t is plate ratio.

(Fatigue and fracture mechanics: 34th volume , Steven R. Daniewicz,J. C. Newman,Karl-Heinz Schwalbe,ASTM Committee E-8 on Fatigue and Fracture,European Structural Integrity Society, p.249)
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Direct Shell Production Casting(new&better) [3D printing Process]

Direct shell production casting (DSPC) creates ceramic molds for metal parts with integral coves directly and automatically from CAD files. Soligen’s DSPC machine, the DSPC 300, includes; a powder distributor, which disperses a thin layer of powder; rollers, which compress each layer before binding; a print head, which sprays binder on each layer; a bin, which holds the mold. DSPC is the only rapid prototyping process that creates ceramic molds directly for metal casting. As a result, functional metal parts could be made directly from CAD data of the part

The DSPC technology is derived from a process known as three-dimensional printing and was invented and developed at the Massachusetts Institute of Technology (MIT). The process contains following steps;

 

More sensible definition.

 

(John M. Usher,Utpal Roy,H. R. Parsaei, Integrated product and process development: methods, tools, and technologies, pg. 176)

 

Direct Shell Production Casting

The Direct Shell Production Casting (DSPC) process, is similar to the 3DP process except that it is focused on forming molds or shells rather than 3D models. Consequently, the actual 3D model or prototype must be produced by a later casting process. As in the 3DP process, DSPC begins with a CAD file of the desired prototype.

Two specialized kinds of equipment are needed for DSPC: a dedicated computer called a shell-design unit (SDU) and a shell- or mold-processing unit (SPU). The CAD file is loaded into the SDU to generate the data needed to define the mold. SDU software also modifies the original design dimensions in the CAD file to compensate for ceramic shrinkage. This software can also add fillets and delete such features as holes or keyways that must be machined after the prototype is cast.

(Sandin Paule E., Robot Mechanisms and Mechanical Devices Illustrated, "Introduction" section p. xxvi)