Optimization of cutting parameters of machining thin walled ABS parts

Machining is one of the most important building blocks of manufacturing methods. With this method a lot of different materials can be produced in desired shapes and tolerances. With each day's work, new materials are being used and new methods are developed in the field of machining. Achieving the desired properties and always making machining production fit one step forward. In the light of new R&D studies, different materials are developed and produced and processed in the most appropriate way. Acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), polyamide (PA), polycarbonate (PC), polyoxymethylene (POM) and ultra high molecular weight polyethylene (UHMWPE) materials, which have recently come to the forefront under the name of engineering plastics, have found their usage areas in different sector and the fields. Engineering thermoplastics are a subset of plastics used in applications where high performance is often required in areas such as heat resistance, chemical resistance, impact, flame retardancy or mechanical strength. Engineering thermoplastics are referred to by virtue of their suitability for engineering applications to design components suitable for their intended use with properties that have one or more areas and which can exhibit much higher performance compared to commercial plastics. These developments of plastic materials, which have a particularly important share in the home appliance industry, have increased the desire and demand for them. Such new materials need to pass the test several times in order to be included in the designs. These tests require a large number of prototypes to be completed. In the production of these prototypes, additive manufacturing method is generally used. The disadvantage of this method is that it is not from the original material, but from similar materials that can exhibit the necessary properties of the original material. This situation is particularly difficult for the tests to be carried out in the last stage. In situations where this type of real material needs to be used, attempts are made to close the defects of layered manufacturing with machining. In machining, the use of real materials, low cost, higher surface quality, protection of anisotropic structure, ability to add operations and subsequent process are superior to additive manufacturing. This situation led to the attempt to produce engineering plastics by machining. When communicating with Arçelik R&D, it is stated that the most needed material is ABS parts. ABS materials are generally used in impact parts, chemical resistance and sensitive surface qualities. When the literature survey was conducted, there were not many studies related to the machining of polymer materials. There is no particular work done specifically for ABS materials. In this study, the most suitable processing method is determined by using different parameters and the optimization of these parameters is done. Firstly, different tool paths and cutting parameters are investigated in the premise studies. These tool paths examined are zig, zigzag, follow periphery and trochoidal. These cuts were made with 4 mm diameter uncoated WC tool. Pocket machining operations were carried out with 2 different cutting speeds and 2 different feed values. After the sections were completed, the results were examined in terms of surface roughness, dimensional accuracy and burr formation. Inspection of the surface roughness was carried out by means of optical profilometer. The surface roughnesses obtained in these studies are compared. Dimensional accuracy analysis was performed by ATOS GOM brand 3D scanner. The results from this scan were compared with the dimensions of the part, and the deviations were found. Burr formation was visually investigated. Zig and zigzag toolpaths leave uncut areas on the part. In addition, the zigzag tool path performs both up and down milling operations. At the up milling, the tool pushes the chips upward to provide the formation of burrs, while at the down milling, the tool pushes the chips to the workpiece so that it creates break off. Because of this, the zigzag tool path has left burrs on the edges of the walls. Follow periphery tool path cuts down all the walls by down milling to prevent the formation of burrs. The trochoidal tool path cuts off from the surface and causes defects on the surface of the part. Whereas the best result of surface roughness (1.6 μm) is obtained while cutting 100 m/min cutting speed and 0.1 mm/rev feed and "zig" cutting pattern, the worst surface roughness (6.46 μm) was measured at same cutting parameters pair and "follow periphery" cutting pattern. When the comparison of average surface roughness is done, "zig" tool path stands out as having lowest surface roughness; on the other hand, "follow periphery" has the highest average surface roughness. When the dimensional accuracy is compared, best wall thickness values was achieved with "follow periphery" tool path, also the worst wall thickness value was obtained with "follow periphery" tool path. This shows that selection of cutting parameters are important in terms of dimensional accuracy. Best results is obtained with 100 m/min cutting speed and 0.6 mm/rev feed values while the worst result is obtained with 200 m/min and 0.1 mm/rev. The best result is obtained by zig tool path in terms of surface roughness. However, since the machining time is twice as long as the cutting times of the other tool paths, it is more accurate to use the zigzag tool path, which gives results close to the zig tool path in terms of surface quality. When the dimensional accuracy of the wall thicknesses is examined, the best result is achieved by follow periphery. Also follow periphery does not leave any burrs on the walls. After the preliminary tests were completed, repetitive tests were repeated with 5 different cutting speeds and 3 different feed parameters. During this groove milling procedure, force measurements were made by means of a dynamometer. In these tests, the zig tool path was used to examine the full immersion condition and to see the effect of the wall height on the thin-walled parts in 4 mm wide grooves. Zigzag and follow periphery tool paths were used in 8 mm wide grooves to examine the half immersion situations and find the best result. After completion of these tests, parts were examined for force measurements, roughness values of the base surface and wall surfaces, and dimensional accuracy. The force values in the zig tool path increase as the feed value increases. As the cutting speed values decreased, the force values increased. The maximum force value was obtained at a minimum cutting speed of 25 m/min and a maximum feed value of 0.1 mm/rev/tooth. Similar results were obtained with 10 mm deep and 20 mm deep grooves in this tool path. In follow periphery tool path, there is a full immersion and two half immersion situations. During these two half immersion operations, down-milling is performed. For the follow periphery tool path, we can see that the maximum and minimum of the Fxdelta and Fydelta results are obtained at the same cutting parameters for both full immersion and half immersion situations. The cutting speed and the effect of feed are also similar to the zig tool path in the follow periphery tool path. The increase of the cutting speed decreases the forces while the increase of the feed increases the cutting forces. Fxdelta values have fallen to almost half as expected in the case of half immersion, compared to full immersion. The Fydelta values are very similar. The zigzag tool path has exactly the same half immersion situation in the same way. One of these half immersion is down milling and the other one is up milling. The forces on this tool path are similar to those on the follow periphery tool path. Roughness values were investigated in two parts, base and wall surfaces. As the feed in the zig tool path decreases, the surface roughness values decrease. A significant reduction in surface roughness results has been observed with feed reduction in grooves with the same cutting speed. In grooves with the same feed values, the effect of the cutting speed is very low at high feed values, but very high at low feed values. The increase in the feed in the follow periphery tool path led to an increase in surface roughness. This increase is more important at high speeds. As in the other toolpaths on the zigzag toolpath, the increase in feed increases the surface roughness. This result is similar to the results obtained in the follow periphery tool path, but the minimum surface roughness value obtained in the follow periphery is more successful. The surface roughness of the walls cut through the down milling process in the zig tool path is low , except 75 m/min, 0.05 mm/rev/tooth and 100 m/min, 0.1 mm/rev/tooth cutting parameters. An increase was observed in the left wall surface roughness values as the cutting speed increased. Zig has only affected by high cutting speed of the surface roughness on the right walls. The maximum surface roughness value of the left wall in the zigzag tool path was obtained at the maximum cutting speed and maximum feed value. The increase in feed for the right wall usually caused a decrease in surface roughness at the same cutting speed. The results obtained in the follow periphery tool path are similar. On the zigzag tool path, the result is more balanced distribution in the down-milling direction, while the distribution of the results in the up-milling is much wider. When examined from the perspective of dimensional accuracy, the deviations of 10 mm deep grooves in the zig tool path are much lower than those of 20 mm deep grooves. It shows that the increase of the feed rate decreases the dimensional deviations. The deviation values of the grooves obtained by the zigzag tool path are within tolerance values. There are 3 parameters in the follow periphery tool path that exceed tolerance values. This tool path has produced close results and usually results close to the tolerance limit. After the results were obtained, the optimization process was carried out using width deviation, left wall deviation, right wall deviation, floor roughness, left wall roughness, right wall roughness and MRR variables. The main goal is to reduce deviations from these variables to zero, minimize roughness, and maximize MRR. In the first optimization, the weights of all variables were taken as 1 and the optimum cutting parameters were obtained as cutting speed 125 m/min, feed 0.0258 mm/rev/tooth and tool path zigzag. Then, when the weights of the variables were changed and optimized from scratch, the feed value was obtained as 0.0432 mm/rev/tooth when the cutting speed and tool path were constant. Optimal parameters were determined when right and left wall roughness were ignored. These parameters are 125 m/min cutting speed and 0.311 mm/rev/tooth. When the optimization was performed by changing the weights from one to another on these variables, the feedrate changed to 0.0447 mm/rev/tooth. As a result of this study, optimum parameters for the machining of ABS materials have been found and it has been shown which parameters can be worked according to desired properties.