Influence of the lead angle of an spherical diamond head on the roughness parameters of corundum ceramics

* Dr hab. inż. Jan Burek, prof. PRz, jburek@prz.edu.pl, https://orcid.org/0000-0003-2664-5248 – Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska, Rzeszów, Polska Mgr inż. Artur Szajna, a.szajna@prz.edu.pl, https://orcid.org/0000-0002-3820-7272 – Katedra Technik Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa, Politechnika Rzeszowska, Rzeszów, Polska The results of research on grinding of pre-sustained corundum ceramics with the use of a spherical diamond head are presented. In particular, the influence of the angle α of grinding wheel axis on selected surface roughness parameters at variable feed speed have been investigated. The distribution of the effective cutting speed in the contact zone of the grinding wheel with the object for the selected grinding depth have been also determined.


Introduction
The development of technology and as a consequence -increasing demand for more and more resistant and advanced materials has prompted interest in ceramic. Such materials are widely used in medicine as components osteoarticular systems, in dental implantation and also in aviation or automotive industry as engine components. The most popular one is corundum and zirconium ceramics [1,2]. Characteristics of these materials such as hardness, abrasion resistance and at the same time high fragility determine their methods of the treatment. Depending on the stage of ceramic manufacturing process, can be distinguished three states of that material, which limit application of different methods. While in green and partly white state (with the low pre-sintered level) machining is possible (e.g., turning, milling), for higher pre-sintered levels and after final sintered there is a need to apply grinding in order to form the final shape and appropriate geometric structure of the shaped part surface [3].
With growing degree of workpiece surface complexity, five-axis grinding machines are used with increasing frequency, on which high spindle speed is achieved, which is required for grinding with spherical diamond heads enabling geometrically complex surface treatment [4].
A significant problem during grinding by spherical diamond heads is their angular orientation in relation to the grinding surface. By analogy with the five-axis milling are distinguished leed angle α and angle of inclination β of the grinding wheel spindle that value has impact on cutting speed distribution in the contact zone of grinding wheel with grinded surface ( fig. 1).
The aim is to obtain such angular orientation of grinding wheel spindle in relation to the grinding surface to ensure the highest effective grinding speed [5,6].
Influence of angle α (both angle of advance and angle of delay) on distribution effective grinding speed vcef in the contact zone of grinding wheel with treated surface is presented in Fig. 2 and 3. The analysis was made for constant rotational speed of the grinding wheel ns = 25000 rpm, tool nominal diameter ds = 4,4 mm and grinding depth ap = 30 µm. The presented results demonstrate that angle α settings in both directions (delay and advance) effective grinding speed at already treated surface is identical for both leed angle variants. The difference in distribution of grinding speed develops in the area of contact the tool with upper surface of allowance for the grinding. Much higher values of effective grinding speed in the area of grinding is obtained by using angle of advance. Those values are smaller for angle of delay. This affects geometric structure parameters of the grinding surface.

Test stand and test conditions
Ceramic grinding tests were carried out on five-axis machining center Ultrasonic 20 linear Sauer company. ( fig. 4) based on Sinumerik 840D control system. The aim of this study was to verify the impact of effective grinding speed with positive and negative leed angle α value and feed speed on selected surface roughness parameters.
The samples were made from pre-sintered corundum ceramics. For grinding was used spherical diamond grinding wheel Dremel 7105 with nominal diameter ds = 4,4 mm. The samples are clamped in vise on tilting rotary table.

Test conditions
Six rectangular pre-sustained corundum ceramics samples were prepared. The frontal surface divided into seven parts, in which took place the change of rotational speed of the grinding wheel ns was grinded. The tests were carried out using two grinding wheel mandrel settings -advance and delay. The following parameters have been used: ■ variable rotational speed of the grinding wheel ns (tab. I), ■ constant grinding depth ap = 30 µm, ■ constant angle of advance/delay grinding wheel pin axis α = 25°, ■ constant angle of inclination β = 0°, ■ feed speed vf = 200 mm/min, 600 mm/min, 1000 mm/min, ■ constant width of cut ae. For the accepted rotational speeds of the grinding wheel were determined the distribution of effective grinding speed in the area of contact with treated surface (tab. I).

Tests results
Measurements of selected roughness parameters of grinding surfaces were performed by portable surface roughness tester MarSurf M 300. The obtained values of selected roughness parameters depending on lead angel α of grinding wheel (angle of advance and angle of delay) and feed speed and rotational speed of the grinding wheel are presented in tables II-IV.
The results prove that choice of angle direction α (angle of advance and angle of delay) has a significant impact on grinding surface roughness, because on this depends effective tool diameter, and from this -grinding speed.  The use of angle of advance allowed to obtain lower values of measured roughness parameters than use of angel of delay -both in grinding conditions with different feed speeds and with a changing rotational speed of the grinding wheel. Feed speed had a significant impact on the results. The best results in terms of quality of obtained surface -both angle of advance and angel of delay -achieved for rotational speed ns=25000 rpm and feeding speed vf=200 rpm.

Summary
After analysing surface roughness obtained during grinding tests it was stated that choice of an appropriate grinding wheel pin setting in relation to the grinding surface has a significant impact on the quality of achieved surface. After the application of leading angle has obtained lower values of analysed parameters. Significant impact on the obtained roughness parameters values has also grinding speed. Its increase has caused reduction of roughness parameters values.