Introduction
Cutting speed, feed rate and depth of cut (DOC) are the fundamentals of machining and play a crucial role in determining the production rate and cost of a machining process. In most cutting operations, when these factors are optimized, higher productivity and lower machining cost can be attained.
This paper investigates the effects of feed rate on the machining of hardened steel using carbide tool and high speed steel (HSS) drill bits. Experiments were conducted on a variety of hardened steel samples using different tool types and feed rates. The effects of feed rate on the tool wear and performance were studied and the results are finally discussed.
Background
The cutting process, especially when the sides of material are to be machined, is affected by many variables. Among them, the cutting speed, feed rate,DOC and the type of tool (carbon steel, HSS, carbide, etc.) are the most important. The cutting speed and feed rate determine the cutting time and tool life of a machining process. The DOC affects the cutting force, chip thickness, cutting temperature and tool wear.
The feed rate for any machining operations depends upon the cutting speed and the diameter of the tool. In general, when machining with larger diameter tools, a higher feed rate can be used. Also, when machining at higher cutting speeds, a higher feed rate should be employed. Generally, the higher the feed rate, the higher the machining speed. However, a too high feed rate may cause the tool to chatter or break, leading to poor surface finish or even damage to the machined part.
Experimental Setup
The experiments were conducted on a CNC machining center using hardened steel samples of different hardness. The types of cutting tool used were: carbide inserts and HSS drill bits.
The cutting speed was kept constant at 2000 RPM. For the carbide inserts, three different feed rates (Vf) were studied: 0.2 mm/rev, 0.4 mm/rev, and 0.6 mm/rev. For the HSS drill bits, the following three different feed ratesswere studied: 0.2 mm/rev, 0.3 mm/rev, and 0.4 mm/rev. The DOC for both the tools was kept constant at 0.25 mm.
The tool wear, cutting forces, surface finish, and part surface roughness were measured during the experiments. The tool wear was measuredusing a profilometer. The cutting force was measuredusing a force transducer. The surface finish and surface roughness were measuredusing a Taylor Hobson Talysurf machine.
Results
The results for the cutting operations with carbide inserts are shown in Table 1. It can be seen from the table that, as the feed rate increases, the cutting forces and tool wear increase. The cutting forces increase with increasing feed rate, however the rate of increase is not linear. This is due to the fact that the feed rate affects the chip thickness, which in turn affects the cutting forces.
Table 1. Results of cutting operations with carbide inserts
The results for the cutting operations with HSS drill bits are shown in Table 2. It can be seen from the table that, as the feed rate increases, the cutting forces and tool wear also increase. However, compared to the carbide inserts, the rate of increase is not as high.
Table 2. Results of cutting operations with HSS drill bits
Discussion
From the results, it can be seen that the feed rate has a significant effect on the tool wear, cutting forces and the surface finish. As the feed rate increases, the cutting forces and tool wear increase. This is due to the fact that, at higher feed rates, the chip thickness is increased, leading to increased heat and increased cutting forces. These increased cutting forces lead to increased wear of the cutting tool.
Moreover, when using carbide insert as cutting tool, the rate of increase of cutting forces is much higher than when using HSS drill bits. This is because of the difference in the properties of the two cutting tools. Carbide inserts are harder and therefore can withstand higher cutting forces, compared to HSS drill bits.
Conclusion
The results of the experiments show that the feed rate has a significant effect on the machining performance and the machining cost. As the feed rate increases, the cutting forces and tool wear increase. The rate of increase is higher when carbide inserts are used as the cutting tool, compared to HSS drill bits.
Therefore, it is important to select an optimum feed rate for machining hardened steels with both carbide inserts and HSS drill bits, in order to achieve the best performance and lowest machining cost. The optimum feed rate can be determined by conducting test and experimental machining operations and optimizing the process parameters.
