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Examinations of Sliding Burnishing Using for Improving the Surface Quality of External Cylindrical Surfaces Dr. Gyula Varga, Prof. Illés Dudás, Department of Production Engineering, University of Miskolc, H - 3515 Miskolc - Egyetemváros, Hungary Contact

1. Introduction

The quality of a nachined surface is becoming more and more important in satisfying the increasing demands of performance and relibility. In the quality assurance of machine industrial product the so called "final finishing processes" have imporant role. During the last decades greater and greater the significance of the different cold-plastic forming methods. Such kind of effective and applied plastic forming method is the sliding burnishing using turning tool type tools with superhard inserts. This method can be used in piece production and in serial production using NC, CNC lathes or manufacturing centres.

Advantege of this method: simple tool, does not require special machine tool, suitable both hardened and low-carbon steel, etc. Because of the plastic deformatin the surface roughness of the turned (or grinded) surface is reducing. Generally the dimension and shape accuracy is improving, favourable change the feature of the surface layer determining the surface quality: hardness is increasing, developing of compressive (negative) stresses, etc. In this way the endurance limit, abrasive resistance, and resistance against other loads/stresses is increasing, which results the increase of the tool life of the part.

2. Manufacturing by sliding burnishing

Main feature of all the applied methods of the sliding burnishing method is, that in the contact zone between the working part of the tool and the surface to be manufactured occurs always sliding. Because of the effects of the pressure - beeing between the contacting surfaces - and sliding, the roughness features of the surface manufactured (turned, grinded, etc.) earlier will change (reduces) in consequence of elasto-plastic strain of the surface layer.
The dimension of the micro geometrical irregularities depends on - most of all - the pressing force of the tool F and the parameters of manufacturing, that is feed f_v and burnishing speed v_v. Other literature [3] deals, how the R radius of the tool has effect on the surface roughness.
The contemporary CNC turning centres have the capability to achieve the 6^th-7^th grade of tolerance. It gives the possibility to apply sliding burnishing directly after turning in the same clamping of the workpiece. Very important to ensure a flexible controlable pressing force of the working element of the tool when contacting to the surface to be machined. This solution can result different constructins using hidraulic and mechanical elements (e.g. springs) etc.
The value of the burnishing force necessary for burnishing can be adjusted by a calibratad scale. The programming and conrolling of burnishing is very similar to the fininsh turning.

3. Technological examinations

The goal of maintenance of piston pin by sliding burnishing is rentilization and reduction of expences. The method can be used in piece production in repair shop - where there is no high accuracy precision machine tools, but there is grinding machine, centre lathe, and burnishing tool. This reparing activity can be done easily and accurately.
Another goal was to create empirical formulas. Using them the effect of burnishing force and technological parameters on the surface roughness can be calculatein advance.

3. 1. Adjustments of parameters and experimental background
The preparing manufacturing of the weared piston pins was grinding, and the smooth sliding burhishing was done on a centre lathe type E400/1000. The clamping of the workpiece was done according to Fig. 1.b. The burnihing force was guaranteed by a calibrated burnishing tool having spring, which was clamped into the toolholder.
Input data:

burnishing force:	Fvmin = 125 N,	Fvmax = 150 N,
burnishing feed:	fvmin = 0,0203 mm/rev,	fvmax = 0,05 mm/rev,
burnishing speed:	vvmin = 60 m/min,	vvmax = 83 m/min.

Constant features were:

hardness of the hardened pistol pins:	HRC @ 55,
dimension of the grinded pistol pins:	Ć24,9 x 100mm,
the arithmetical mean deviation (Ra) and the maximum height of the profile (Rmax) of the profile of the grinded surface:
adjusting angle of the tool:	a @0°.


In case of burnishing the tool went only once through on the surface by use of intensive volume of oil coolants (lubricants). The examined - measured features - output data - were after burnishing the average values of the R_a,v and R_max,v surfaceroughnesses of the burnished surfaces. (Table 1)
These dimensions were measured on 8 different places on the periphery of the piston pins and the averages were calculated:

Surface roughness after grinding		Adjusted parameters			Measured surface roughness values after burnishing
Rmax,g	Ra,g	fv	vv	Fv	Rmax,v	Ra.v
mm		mm/ford	m/min	N	mm
2,66	0,33	0,0203	60		1,03	0,112
3,32	0,33	0,0500		125	1,92	0,260
3,39	0,37	0,0203	83		0,70	0,096
3,32	0,37	0,0500			1,40	0,180
2,79	0,37	0,0203	60		0,75	0,100
3,89	0,42	0,0500		150	1,18	0,160
3,53	0,43	0,,0203	83		0,97	0,130
2,95	0,34	0,0500			1,20	0,150
Table 1:

Beyond the measurements mentioned above we measured roundness error as well. The measurements were done by passameter, 3D measuring machine, Pert-o-meter, and Talyrund (for measurement of roundness error).

3.2 Examination of microgeometrical features
The selection of the input paramteres, making them into right order by the use of Factorial Experiment Design can be done actively [4]. Output parameters the R_max.v and the R_a.v.
We write empirical functions to these paramteres, as response function as followes:

	(1)
	(2)

Applying the steps of the method to this concrete case, the contants C_vm and C_va, the a _m, g _m, d _m, and the a _a, g _a, d_a exponents were determined. The constants and exponents of R_max,v can be found as follows:

formula (1)

Cvm = e218,152

a m = (11,5073 - 0,2899 ln vv - 1,9766 ln Fv)

gm = - (41,9107 - 8,2493 ln Fv)

dm = - 42,7924

These formulas are a little bit sophisticated ones, but describe the results of the burnishing. However if we take into account, that the value of the surface roughnesses weren't the same, maybe we can accept these calculations with reservations.
Remark: the constants and exponents of R_a,v easily can be calculated, however we do not publish them here.
We can tell more about the improvement of the surface quality if we examine only the improvement/difference between the before and after burnishing surface roughness. So introduce the following inner parameters:

	(3)
	(4)


The effect of burnishing is the best when the calculated value of R_max,D and R_a,D are the maximum ones. For the improvement of the roughness we can create new formulas:

	(5)
	(6)


Applying the Factorial Experiment Design for the calculated values from Table 1 the konstant and the exponents of formula (5) are as follows:

formula (5)

Cr m = e-230,5053

a r m = 0,4515

g r m = (52,8366 - 10,6419 ln Fv)

d r m = 46.34


Remark: these formulas (1), (2), (5), (6) valid only for the intervals can be found in Table 1, for f_v, v_v and F_v.
The improvements in the maximum height of the profile (R_max) of the surface can be seen on Fig. 1.

Fig 1: Smooth sliding burnishing of external cylindrical surfaces on manufacturing centre (a) and on centre lathe. Workpiece (1), burnishing tool (2), burnishing insert (3), active element of tip (A).


Because the effects of the different parameters can be seen evidently, that is why only some remark follows. Remarks to Fig. 1.
• The effect of sliding velocity depends on the value of the burnishing force. In case of small value of burnishing force (e.g. F=125 N) the increase of the velocity causes improvement in the value of the R_max. Increasing the value of the burnishing force (e.g. to F=150 N) when using larger value of the velocity of the workpiece, the r_Rmax ratio will decrease.
• The decreasing of the feed increases the value of the r_Rmax ratio, so the value of the R_max will be smaller.

Fig 2: Characterisation of the rRmax relative feature of surface roughness as the function of the technological paramteres fv, vv and burnishing force Fv

4. Summary

Knowing the applied technological parameters (f_v, v_v) and the applied burnishing force, furthermore the values of featuring surface roughnesses (R_max,g, R_a,g) of the grinded external cylindrical surface before burnishing, the improvement of the R_max or R_a can be planned when a given R-radius of the working element and a given material to be burnished.
Before burnishing the surface requires rough grinding or smooth turning. The sliding burnishing can effectively and economically applied in piece production as well, when the used machine tool is engine lathe. The results of experiments done on CNC manufacturing centres shows, that in case of surface systems which requires high position accuracy, after turning the sliding burnishing can be performed immidiately. In this way the effectiveness of the production and manufacturing accuracy can be increased.

Acknowledgements

The authors would like to express their thanks to the OTKA Foundation, especially to the OTKA projects No.: T 025932 because of its finantial support.

References

1. Leskó, B.: Finishing manufacturing of piston pins by sliding burnishing, Research Report, University of Miskolc, 1988, (in Hungarian).
2. Przybylski, W.: The sliding burnishing working of shaft necks, IXth International Conference on Tools, University of Miskolc, Miskolc, Hungary, 3-5 Sept., 1996, 119-126.
3. Fridrik, L.: Selected chapters from the topics of experiment design in production engineering, Puslisher of Student Books, Budapest, 1986. (In Hungarian).

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