PSI - Issue 53

Bruno Sousa et al. / Procedia Structural Integrity 53 (2024) 291–298

295

Author name / Structural Integrity Procedia 00 (2023) 000–000

5

In order to understand the influence of polishing time on the surface roughness evolution of the polished samples, preliminary tests were conducted in which milled samples were submited to each of the grinding papers and polishing pastes for increasing time. Figure 3a shows that even though samples were grinded up to 15 min (from a milled initial state), no significant changes were noticed after 3 minutes. For the diamond pastes (refer to figure 3b), this stabilization occurred at the 60 second mark. For that reason, the polishing times per step have been defined according to table 3. Polishing references (B1 to B4) refer to sequential cumulative polishing steps (Step A to Step D).

Table 3. Duration (in minutes) of each of the polishing steps. Polishing reference StepA

StepB

StepC

StepD

B1 B2 B3 B4

3 3 3 3

-

- -

- - -

3 3 3

1 1

1

2.3. Surface roughness evaluation

Surface roughness was measured after each process (machining and polishing) using di ff erent equipment. A mechanical roughness tester (TESA RUGOSURF 10G), with a contact probe, was employed for the machined samples. Measuring parameters were defined according to ISO 4288:1996 standard and even though two distinct directions were taken in consideration (perpendicular and parallel to feed direction), results focus on the perpendicular to feed direction (parallel to radial depth of cut). The low surface roughness resultant from polishing operations hinders its measurement through contact probes. For that reason with light interferometry (WLI) was employed. This procedure was performed using a 3D optical profilometer, Bruker’s NPFLEX. Using this equipment, not only is the analysis finer, but it enables the statistical analysis of surface roughness parameters. For these tests, Sa (arithmetical mean height) and Sz (mean height) surface roughness parameters were considered.

3. Results

3.1. Milling

The results regarding average surface roughness measurements and machining time can be observed in figure 4. By testing the parameters against these roughness indicators, it is possible to perceive how the chosen variables influence the milling results. It is noteworthy the parameters set A1 and A3 correspond to the lowest and highest surface roughness, respectively, and also to the highest and lowest milling time, respectively. This illustrates the influence of the radial depth of cut ( a e ) parameter on the surface roughness and milling time results. The Taguchi method allowed for the creation of a robust design window capable of testing a three-level variance of three factors (ap, ae and fz), using a L9 standard array. Enabled by Minitab software, figure 5 shows the e ff ect of each individual parameter on each analysed output, confirming the significant influence of radial depth of cut, a e , on both surface roughness and the required machining time. Additionally, the influence of axial depth of cut ( a p ) and feed per tooth ( f z ) on the selected variables seems to be negligible.