Influence of grinding parameters on the surface roughness of steel 1.0562 in the softened state

Przedstawiono wyniki badań procesu szlifowania elementów konstrukcyjnych ze stali stopowej 1.0562 w stanie zmiękczonym. Przeanalizowano wpływ głębokości szlifowania ae i prędkości posuwu wzdłużnego przedmiotu vft na chropowatość powierzchni. Zaprezentowano wartości parametrów chropowatości Ra, Rz i Rt możliwe do uzyskania w przyjętych warunkach badań. SŁOWA KLUCZOWE: szlifowanie płaszczyzn, prędkość posuwu wzdłużnego, głębokość szlifowania, parametry chropowatości powierzchni

The correct course of the grinding process is related to the appropriate selection of technological parameters, which allows to meet certain construction requirements, mainly to obtain high dimensional accuracy and shape of the required surface geometry (WW) and achieve the desired condition of the workpiece (PO) [2,5].In addition to the low surface roughness and high grinding efficiency [1], the research also reduces the process energy consumption and carbon dioxide emissions [6].The grinding wheel and its conditioning parameters also have a significant influence on the process and technological effects [3].Proper selection of machining parameters with correct delivery of cooling-lubricating fluid (CCS) avoids damage in the form of micro-cracks and cures in WW-PO [4] and prolongs the service life of the workpieces, e.g.railway rails [5].
The results of the study along with the assessment of the effect of grinding depth ae and the longitudinal feedrate of the object vft on the values of surface roughness parameters of structural elements made of 1.0562 steel in the softened state, were presented.Obtaining Ra roughness value below 0.1 μm was one of the basic design requirements for plates (fig.1), which is a research hydraulic device.

Test conditions
Grinding tests were performed on a CNC grinding machine for SPG 25X60 planes with horizontal spindle axis.The panels of 256×68×28 mm dimensions (fig. 1) made of 1.0562 steel with hardness of 220 HB, tensile strength Rm = 490÷630 MPa and yield strength Re = 335 MPa, were machined.By default, the components of this material are heat treated to increase their hardness and wear resistance.In the research carried out for the purpose of the hydraulic equipment, in accordance with the requirements of the constructor, the plates were softened.The treatment was carried out with Norton Grinding Wheels at 38A60LVS and dimensions (D×T×H) 250×25×76.2mm.
The grinding was performed at constant wheel circumferential speed vs = 25 m/s, using coolant.The transverse feedrate of the grinding wheel followed the grinding wheel along the grinding surface and was 15 mm (60% of the grinding wheel).Prior to each test, the grinding wheel was conditioned with a diamond dresser with the following parameters set in the control program:  depth ae at one passage -0.01 mm,  number of crossing passages -4,  abrasive wheel circumferential velocity -vs = 23 m/s,  transverse feedrate of abrasive wheel -fa = 0.2 mm/rev.Statistically determined complete 3-value study plan -PS/DK-3 2 was adopted.Each grinding test (Table I) was repeated.
Less velocity of the longitudinal feedrate vft than the recommended ones [2] was assumed, because at higher velocities, the roughness and the waviness of the surface of the PO increased with visible inequalities in the form of bands.After each grinding and repetition test, 4 surface roughness measurements were takena total of 8 measurements.For this purpose, the contact profilometer HOMMEL TESTER T1000 was used.Measurements were made for an elementary section of 0.8 mm.

Test results
Fig. 2 and fig. 3 show mean values of roughness parameters from individual grinding tests (Table I), and as a measure of spread, the standard deviation was assumed.The highest values of roughness parameters, irrespective of the grinding depth, were obtained after samples 7-9 with the highest feedrate of vft = 1.8 m/min.The lowestvalues of roughness parameters such as Ra, Rz and Rt were obtained after grinding with vft = 0.9 m/min and ae = 0.004 mm.Tables II-IV present results of the study upon significance of the influence of grinding parameters on surface roughness parameters.The analysis was performed for assumed significance level α = 0.05.

Number of sample
In the case of the roughness parameter Ra, the vft and ae parameters as well as interaction between them were significantly affected.The parameter Rt was most influenced by velocity of the longitudinal feedrate of the object vft and interaction of this parameter with the grinding depth ae.Similar results were obtained for Rz parameter.

Conclusions
Based on the grinding of 1.0562 steel in the softened state, it has been determined that the longitudinal feedrate vft has a greater effect on the surface roughness than the grinding depth ae.The lowest surface roughness was reported for the grinding parameters No. 5.The smallest values of the Ra roughness parameterin the range of 0.06÷0.08μmappear to be the limiting values that can be achieved under the accepted conditions of grinding of low hardness objects.Increasing the vft speed over 1.8 m/min resulted in an increased surface roughness.At the lowest accepted feedrate, the best results were also not obtained.

Fig. 2 .
Fig. 2. Average values of surface roughness parameters Ra and Rq obtained after subsequent grinding tests

Fig. 4 .
Fig. 4. Dependence of roughness parameters: a) Ra, b) Rt, c) Rz for ground surface on the longitudinal feedrate vft and grinding depth ae

Fig. 4
Fig.4shows the values of roughness parameters Ra, Rt and Rz obtained depending on the feedrate of the object and the depth of grinding.It can be seen that within the range of assumed values of vft = 0.3 and 0.9 m/min, the Ra and Rz values were similarly independent of the grinding depth.Only for Rt, significant values were recorded for the smallest longitudinal feedrate of the object vft and grinding depth ae = 0.004 mm.

TABLE II . ANOVA analysis for parameter Ra
Legend: SSsum of squares of deviations; dfnumber of degrees of freedom; MSaverage square of deviations; Fcalculated value of F statistics; pprobability; F testcritical value read at significance level α = 0.05.