The hardness of the surface after the flat lapping.

Cohal, Viorel ; Serb, Adrian

1. INTRODUCTION

The lapping operation, being a complex process because of the numerous influence factors that appears, it is hard to be overall analyzed, consequently, both theoretical and experimental research are imposed An objective factor for evaluation of the lapping process is the hardness value of the surface layer. Because of the contact temperature the abrasive particle cut onto a modified surface layer, whose properties are different from those that characterize the material. This fact explains the absence of the steel hardness influence onto the cutting workability that characterize the lapping operation, the principal factors that determine the properties of the surface layer, are the chemical composition of the steel and the properties of the surface layer after the previous operation (usually, the grinding operation).

Lapping consists in the final smoothing of previously grinding surfaces. This is done by means of certain fine abrasive particles impressed on the lap or freely interposed between the part to be processed and the lap. Abrasive pastes applied on the lap can also be used. Through the relative motion of the lap as compared to the part, in the presence of abrasive grains, particles are being removed from the processed material.

Most of the interactions between the workpiece and the granule occur when a load is applied by the lap. A variety of abrasive jet cutting and bead blasting processes rely on granule kinetic energy either shearing work material or initiating and propagating fracture. Mechanical aspects of the interaction of a granule and a workpiece surface have been widely studied. Significant insights are provided by a body of work in indentation and scratch testing, although considerable care needs to be exercised when applying such physical insights to lapping. In lapping, penetration depths tend to be much smaller than in most indentation and scratch testing, although this is not true for some of the recent nano-indentation studies. Workpiece surfaces exhibit property variation (subsurface damage) over a depth which will be a larger fraction of the penetration depth in lapping than in scratch testing. For polycrystalline work materials, the relationship between grain size of the material and penetration depth must also be considered. Combining this with insights from classical cutting mechanics, we can describe granule interactions with materials as a function of load and the mechanics of the contact. Simply stated, if the workpiece yield stress is not exceeded, the granule will simply slide across the surface imposing elastic deformation and removing no material. Larger loads form dislocations, plastically deform material. The extensive literature in these fields provide ample descriptions of large scale single grit interactions; the challenge is to define the contact conditions of granules used in lapping processes such that predictive kinetic models (rather than parametric correlations) can be developed.

In conventional lapping, the applied load is born by granules embedded in the lap system. As more granules embed into the lap, there are more sites, for material removal. However, the load per granule decreases as the granule concentration is increased. These effects cancel each other leading to the observation that in some circumstances the conventional abrasive lapping rate is independent of (granule) concentration. However, since the lap .area is finite, abrasive concentration extremes give different behavior. At low abrasive concentration, the individual abrasive particles may be pushed too deeply into the lap or the workpiece and become ineffective. And at high abrasive concentrations the lap can become saturated with granules and form a sliding surface with the excess material (Konig, 1990).

The recent literature regarding the interactions of the workpiece-lap-fluid triplet in chemical mechanical polishing consists of studies carried out over a broad range of material combinations under a variety of process conditions. As a result, models attempting to capture the resulting physical behavior consist of an equally broad range of assumptions and approaches. The range includes consideration of complete wafer-pad separation and the hydrodynamic modeling of the workpiece-lap-fluid contact to the case where there is pad and wafer contact in the presence of a fluid (Evans & Paul, 2003).

In lapping, the tooling allowance is minimum and it over tops only a little the height of the roughness resulting from previous grinding. That is why mechanical lapping should be accompanied by the selfcentring of the parts or of the tool and it cannot correct the geometrical shape obtained as a result of the previous operation.

2. EXPERIMENTAL RESULTS OF RESEARCHES

In the experimental frame were machined materials bearings hardened steel (RUL 1). The abrasive material was the mechanical paste of chrome oxide, having the precision FFF, which is characteristic for finishing operations. The experiments utilized a plane-lapping machine with eccentric.

The values of the hardness, before (t=0) and after the flat lapping operation are presented in fig. 1. In order to obtain a better visualization the experimental data were presented into a graphical way, the working parameters being beside specified.

The optimal regression function was chosen from the five types of regression functions (which are linear, logarithmical, power, polynomial and exponential), using the [R.sub.2] criterion. The value of the [R.sub.2] criterion is between 0 and 1. As close the [R.sup.2] criterion is to value 1, as close the drawn curve pass nearest the experimental points. When [R.sub.2] = 1, the regression function pass through all the experimental points. The presented mathematical model is valuable only for the considered time interval. During the lapping process on to the material bearings hardened steel the optimal regression function ([R.sub.2] = 1) is:

[FIGURE 1 OMITTED]

The rest of the working parameters that describe the plane lapping process were maintained constants, their values being settled on the basis of the previous experiments, into a manner that the roughness and the working efficiency to be as productive as possible. The hardness value of the surface layer during the lapping process decrease approximately with 60 hardness units of measure in the very first minute afterwards, the decrease trend continue less sharp until the value of working time is close to t=3 minutes, later the growth of the hardness is limited, having a maximum value for t=4,5 minutes. In this particular case the obtained regression function have a 5 degree polynomial form which on the considered interval of time is characterized by an ascending trend (Cohal, 1998).

This result of researches was analysis with NCSS (fig.1). Number Cruncher Statistical System (NCSS) is an advanced, easy-to-use statistical analysis software package. Regression techniques analyze the relationship between a dependent (Y) variable and one or more independent (X) variables. NCSS has regression procedures for many different situations.

The assumptions of the one-way analysis of variance (fig.1.) are:

1. The data are continuous (not discrete).

2. The data follow the normal probability distribution. Each group is normally distributed about the group mean.

3. The variances of the populations are equal.

4. The groups are independent. There is no relationship among the individuals in one group as compared to another.

5. Each group is a simple random sample from its population. Each individual in the population has an equal probability of being selected in the sample.

The assumptions of the Kruskal-Wallis test are:

1. The variable of interest is continuous (not discrete). The measurement scale is at least ordinal.

2. The probability distributions of the populations are identical, except for location. Hence, we still require that the population variances are equal.

3. The groups are independent.

4. All groups are simple random samples from their respective populations. Each individual in the population has an equal probability of being selected in the sample.

There are few limitations when using these tests. Sample sizes may range from a few to several hundred. If your data are discrete with at least five unique values, you can assume that you have met the continuous variable assumption. Perhaps the greatest restriction is that your data come from a random sample of the population. If you do not have a random sample, your significance levels will be incorrect (Hintze, 2007).

Given that the analysis of variance (ANOVA) test finds a significant difference among treatment means, the next task is to determine which treatments are different. Multiple comparison procedures (MCPs) are methods that pinpoint which treatments are different.

If the F-test from an ANOVA for this experiment is significant, we do not know which of the three possible pairs of groups are different. MCPs can help solve this dilemma.

3. CONCLUSIONS

Lapping is successfully applied in fine mechanics manufacturing, increasing the cutting tools lifetime and precision and also in finishing and superfinishing the ceramic workpieces.

The statement that during the lapping operation an improving of the material structure occurs, could be accepted only after a superfinishing of the surface and the removal of a part from the surface layer that is deformed and cold-harden.

4. REFERENCES

Cohal, V., Contributions about surfaces lapping. Doctor's degree thesis., Iasi, Romania, 1998

Evans, C.J., Paul, E., Material Removal Mechanisms in Lapping, Annals of the CIRP Vol. 52/2/2003

Hintze, J., NCSS Quick Start & Self Help Manual, Kaysville, Utah, USA, 2007.

Konig, W., Finish Processing, Ceramic Materials, Zunch, 1990