Position of the superfinishing in the finish machining.

Siketova, Katarina ; Lipa, Zdenko ; Baranek, Ivan 等

1. INTRODUCTION

The finish machining methods create a special part of the machining methods. They were developed in order to give the highest quality parameters to machined surfaces. It does not matter on the way of material removal and the dimension of material removal. These all are the additional parameters. The main parameters are roughness, precision and required surface characteristics of the machined surface.

One of the important properties of machined surface is its ability to hold the oil lubricant film. Another is the request for the existence of pressure residual tension on the surface layer of machined surface. Further requests can be the strengthening of the surface layer and the removing of amorphous (damaged) part of the surface layer arosed by previous operations primarly by the grinding.

However, the biggest accent is given to the low roughness and the highest precision of machined surface. These requirements move the finish machining to the nanotechnologies.

2. ROUGHNESS OF THE SUPERFINISHED SURFACE

The highest parameters of machined surface are provided by the superfinishing. In the beginning of development the superfinishing was put in the context with the artificial run-in of consecutively moving functional surfaces of components (e.g. antifriction bearing).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The original intention was not to establish the new production system. The authors of superfinishing (Wallace, Swigert etc.) only wanted to improve the surface properties provided by the finish grinding. Finaly by using complicated kinematics and light grinding tool there was however created a new technology when authors of superfinishing found out the method of material removal not only the method of smoothing of worn surface.

Grinding generally brings adverse residual stresses to the surface layer of machined surface. This causes creation of cracks by the exploitation and also decrease of life of such surface. The superfinishing uses fine abrasive stones or fine abrasive tapes. The oscillatory motion of these abrasive stones and tapes increases smoothness (decreases roughness) of machined surface. The roughness of machined surface is then the consequence of the superfinishing kinematics, the grain size of the superfinishing tool, the pressure proportions between the superfinishing tool and the workpiece and the time of superfinishing.

This we could conclude to the empirical equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [C.sub.Ra] is the constant factor dependent on the tool material and the workpiece material, on the grain concentration in the tool, on the grain shape, on the character of moves by superfinishing and others.,

[Ra.sub.p] is the input roughness (Ra) to the superfinishing,

[v.sub.a] is the axial speed (speed of oscillating motion of tool),

[v.sub.o] is the peripheral speed of workpiece,

[p.sub.s] is the pressure between the tool and the workpiece,

[d.sub.z] is the granularity of superfinishing tool grains,

[t.sub.T] is the superfinishing time,

[K.sub.Ra] is the correction index for the other conditions that are the standard conditions.

[p.sub.s0], [d.sub.z0], [t.sub.T0], [tg.sub.[beta]0] are the standard values.

The ratio [v.sub.a]/[v.sub.0] = tg[beta] and designates so-called angles of footcrossing by the superfinishing [beta].

The small angle [v.sub.a] [much less than] [v.sub.0] (up to 20[degrees]) causes only polish effect but does not remove the damaged layer created by the grinding.

The middle angle, [v.sub.a] [approximately equal to] [v.sub.o] (around 45[degrees]) gives the best values of the material removal. Too big angle (around 60[degrees]) causes the decrease of geometrical properties. The grains crumble away and the roughness of machined surface gets worse just by uncoordinated activity of these crumbed grains.

For the roughness of machined surface we can use also differential equations and here we come to relation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where [Ra.sub.z] is the final roughness Ra reachable by the superfinishing,

[a.sub.1], [a.sub.2], [a.sub.3], [a.sub.4] are the constants, that need to be find out experimentally.

We can also write:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

The reached roughness of machined surface by superfinishing Ra is normally from 0,08 [micro]m to 0,12 [micro]m.

Only by smoothing it is possible to reach roughness Ra=0,025 [micro]m. Rarely there can be reached Ra=0,001 [micro]m=1nm by using the small size abrasive. This is already on the edge of the picotechnology (parameters under 1 nm). This could be reacheable in the future.

[FIGURE 3 OMITTED]

3. PRECISION OF SUPERFINISHED SURFACE

The precision of machining is established by degrees from 1 to 16. If geometrical variances of shape (position and dimension) are reached by previous operation maybe it would be possible to produce the component by lapping also in degree 1 (most precisely) or eventually in improved degree 01.

It is the same by the superfinishing if we use such fine tool that is used by the lapping. The superfinishing is in this case equivalent to the lapping however does not have such requirements on the previous operations. Therefore there can be prefered the superfinishing to the lapping.

The production inaccuracy reached by the superfinishing represent deviations of shape. Waveness, deviation of circularity and non-cylindricity given by the drawing or the component design maybe reached by the superfinishing easily. The deviation of circularicity becomes a problem as it is hardly improved by the superfinishing. This is because the functional cutting part of the superfinishing tool has the shape of cylindrical surface part. The tool moves by oscillating motion in direction parallel with the axis of this cylindrical surface that can cause the damage of the created conical surface even before the superfinishing.

To improve the taper it could be usefull to use the uncommon tool shapes or tool moves by the superfinishing.

The superfinishing is not the operation used for the removing of mistakes created by the previous operations.

4. STATE OF MACHINED SURFACE BY SUPERFINISHING

The superfinishing as a technology was assigned for the removing of amorphous (damaged) layer arosed by grinding. Such layer is created by existing residual tension stresses arosed by primarly temperature stress of machined surface by the grinding. Unremoving of such layer causes the crumbing away of material by the exploitation. There comes to an abnormal wear and decrease of the surface life. The superfinishing besides removing of this defected layer has tendency to bring the compression tensions into the machined layer. The life of such surface is increased.

The resistance to the chemical influence increases, too. The ability to hold the oil lubricant film supports this.

It is the domain of the superfinishing. The kinematical secure crossing of feets from the grinding grains enables better hold of the oil filter than the parallel or smoothed feets.

5. CONCLUSION

From the above mentioned results that the superfinishing stands on the top of the scale of the finish methods of machining. It is really the most accuracy method of mechanical machining methods.

At the same time it is still in development. The superfinishing supported by the ultrasound, the integral superfinishing (integration of grinding and subsequent superfinishing to one operation), the highly punctual superfinishing and the high-speed superfinishing (increasing of movement speed of workpiece and tool) are being researched. The superfinishing is perspective, too.

6. ACKNOWLEDGEMENT

This work was kindly supported by the project VEGA SR 1/4108/07 "New trends and next development of superfinishing technology".

7. REFERENCES

Lipa, Z. (1992). Formulacia a riesenie vybranych problemov teorie superfinisovania. Innaugural dissertation. MtF STU, Trnava

Lipa, Z., Baranek, I. & Moravcikova, J. (2008). Prispevok k teoretickej problematike superfinisovania. Vedecke prace c. 24/2008. MtF STU, Trnava

***Honer. D. (2009). Superfinishing machine (lathe mounted) D--Honer DS--60. Catalogue of products. Available from: http://www.dhoner.com/images/sup-fin-mc1.jpg Accessed: 2009-06-10

***Supfina (2009a). Catalogue of products. Available from: http://images.google.sk/imgres?imgurl=http://www. supfinamachine.com/Bilder/ausgleichwellen.jpg&imgrefurl Accessed: 2009-06-13

***Supfina (2009b). Catalogue of products. Available from: http: //www. perfecbore. ch/images/I_SUPFINA_07.jpg Accessed: 2009-06-15