Poly (methyl methacrylate) (PMMA) is a hard-to-cut material due to its mechanical and chemical properties, such as a high brittle fracture at temperatures above 80oC and a low elongation-at-break of 5%. This limits the ability to machine the material using traditional machining techniques. However, the material has attracted a large number of applications, especially in the medical field due to its desirable properties, such as high light transmission (&gt;92%), high tensile strength (&gt;48 MPa), high wear resistance, and high chemical stability. There is, therefore, a need to seek alternative methods of machining the material. Computer Numerical Control machining has gained insight recently due to its high precision, quality, and productivity. This study employed CNC machining with Benchmill 6000 CNC machine in milling the general-purpose grade of PMMA and obtained the optimal machining parameters for a better quality (low surface roughness) and high productivity (high material removal rate). The milling factors considered were cutting speed, axial depth of cut, and feed rate, each having four levels obtained through the experimental trial and error method. The levels for the cutting speed were 300 rpm, 700 rpm, 1300 rpm, and 2000 rpm. For the axial depth of cut, the experimental levels were 0.3 mm, 0.8 mm, 1.5 mm, and 2 mm whereas the feed rate levels were 50 mm/min, 100 mm/min, 200 mm/min, and 350 mm/min. The responses for analysis were material removal rate and mean surface roughness. The Taguchi technique was used in the design of the experiments and an L16 orthogonal array was used, where 16 workpieces of 60 mm by 60 mm by 5 mm were used. The workpieces were weighed before and after milling to obtain the change in mass. The milling processes were timed to obtain the machining time. From the change in mass and the machining time, the material removal rate was computed. The milled surfaces were cleaned using ethanol, and then the surfaces were investigated for the mean surface roughness using the TR200 handheld roughness tester. The material removal rate and the mean surface roughness were then analyzed using the Taguchi technique to obtain signal-to-noise ratios (SNR). These ratios were used to evaluate the impacts of the factors on each of the responses. The analysis identified that the optimal milling parameters for maximum material removal rate were a cutting speed of 300 rpm, an axial depth of cut of 2 mm, and a feed rate of 350 mm/min. This was confirmed experimentally through milling, which gave the highest material removal rate of 4.60 g/min. The optimal CNC milling parameters for the least surface roughness were obtained as a cutting speed of 2000 rpm, an axial depth of cut of 0.3 mm, and a 50 mm/min feed rate. From experimental verification using the optimal parameters, the least mean surface roughness of 0.031 µm was obtained. An analysis of variance (ANOVA) was also conducted to determine the significance of each of the factors on the responses. For the material removal rate, and using a confidence interval of 95%, it was observed that all the three factors were significant with p-values of 0.027, 0.001, and 0.000 for the cutting speed, axial depth of cut, and feed rate, respectively. The feed rate had the highest percentage contribution (48.8%) to the material removal rate, followed by the depth of cut (38.3%), and lastly, the cutting speed (9.8%). Consequently, an ANOVA for the mean surface roughness depicted that cutting speed and feed rate were the most significant factors with p-values of 0.002 and 0.034, respectively. The axial depth of cut was insignificant towards the mean surface roughness with a p-value of 0.067. In terms of percentage contributions to the mean surface roughness, the cutting speed had the highest contribution (59.9%), followed by the feed rate (19.4%), and lastly, the axial depth of cut (13.9%).Keywords: PMMA; Milling; ANOVA; Taguchi; Surface roughness; Material removal rate&nbsp;

Optimization of CNC Milling of General-Purpose Poly (Methyl Methacrylate)

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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