Tests for lapping of flat surfaces of ceramic elements with forced dosing of abrasive slurry

The issues related to the forced dosing system of the abrasive slurry in single-disk lapping are presented. This system allows dosing of a suspension with different fluid viscosities. The innovative system developed was adapted to work on the Abralap 380. The advantages of this type of abrasive sling dosing system and the procedure for planning testing of lapping of flat elements from technical ceramics, on the example of Al2O3 treatment, are described. Selected results of defect investigations are presented, which were subjected to detailed statistical analysis.


Introduction
The lapping process for flat surfaces is mainly carried out on single-disc lapping machines [4,11,12]. The effectiveness of this finishing treatment depends on many factors. They are related to the machined material, type and properties of the lapping disk, kinematic conditions of the machine tool, exerted unit pressure on the lapped elements as well as the type and characteristics of the abrasive micro-grains used in the suspension. The dosage method of the abrasive slurry itself has a great influence [1,2,5].
In the case of free (continuous) feeding of the slurry, some of the abrasive micro-grains are removed very quickly from the active surface of the lapping machine (through the separating rings moving on it and the machined elements) and does not participate in cutting [3]. For this reason, at the Gdansk University of Technology, in cooperation with the University of Technology and Life Sciences in Bydgoszcz, a forced (periodic) method of dosing the abrasive slurry is developed [6,8,9]; selected results of these tests are presented in the paper. An example of one-sided lapping of ceramic elements was used.

Experimental research model
As input (examined) factors adopted in the study, there were following ( fig. 1 • CD3 -lapping kinematics, lapping material as well as the size and method of applying pressure to the elements (machining station), • CD4 -device construction for forced dosing of abrasive slurry. Interferences occurring in the process mainly concern: • UD1 -voltage drop in the electrical network, • UD2 -height spread of samples machined simultaneously. The result (output) factor was Ubl -linear loss of samples [µm] The study used micro-grain silicon carbide 98C F400/17, dosed to a cast iron lapping disk. The average lapping speed was: 0.58 m/s, and value of the unit pressure: 0.045 MPa. A mixture of machine oil and kerosene was used as the liquid substance in the dosed abrasive slurry.

Program and selected research results
General view of the dosing system used for the abrasive slurry is given in fig. 2. A static, determined, selective, multifactorial, quasi-rotational, second order [10] study plan developed by Box and Hunter [7] with a spherical distribution of information was applied in the experiments. It ensures constant estimation of the regression function in a certain environment of the central point of the PS/DS-P: λ plan [13].

Fig. 3. Linear loss of Al2O3 technical ceramics, lapped on a Abralap 380 single-disc lapping machine, with a forced abrasive slurry dosing system
As a result of analyses, it was found that the viscosity of the liquid substance in the abrasive slurry (in the test range) has no significant impact on the loss of processed elements. From a practical point of view, the most important impact is the dose and percentage of abrasive grains. The experiment adopted a range of this dose from 10 ml / 20 min to 90 ml / 20 min. The best results in terms of process efficiency were obtained at the dose of 90 ml / 20 min. The influence of abrasive grains content in the carrier substance was also analyzed. For the purposes of the experiment, the content range of silicon carbide grains in the carrier substance -prepared on the basis of machine oil and cosmetic kerosene -was determined from 5% to 25%. It was found that the best effects of linear loss of ceramic samples can be obtained if the content of abrasive grains in the carrier substance is about 15%.

Summary
Based on the research carried out upon the impact of forced dosage and application of abrasive slurry on the working surface of a cast iron lapping machine on the effectiveness of the lapping process of flat elements made of Al2O3 ceramics, it can be concluded that: • forced dosing system contributes significantly to savings associated with the consumption of abrasive slurry, • the applied dosage method of the slurry generates smaller amount of harmful substances that should be subjected to costly disposal, • significant impact on the efficiency of the lapping process is exerted by appropriate selection of the percentage of abrasive grains; in the analyzed case, the best results were obtained with a 15% share of abrasive grains in the carrier substance, • excessive content of abrasive grains does not increase the efficiency of the lapping process, • an important factor influencing the efficiency of the lapping process is the amount of abrasive substance per revolution of the lapping disc, • based on the conducted experiment, it was found that the largest material loss was obtained using a total dose of 90 ml / 20 min, • viscosity of the liquid components of the abrasive slurry has no significant effect on the linear loss with a forced dosing system (in the tested range).
Presented solution of the dosing and application system of abrasive slurry allows to significantly reduce the costs associated with finishing machining and contributes to the development of automation of the lapping operations. The designed and made system enables the application of substances of various viscosities. Its advantage is also an even application of the abrasive slurry onto the working surface of the metal lapping disk, so that tool losses are as low as possible.