Influence of different types of sharpening in straight flute drills on burr formation

Drilling is among the most important manufacturing processes in modern industry. Information on the dynamics of the drilling process is very important to define optimum input parameters. The minimization of burrs is a great challenge in drilling process. Current analysis was carried out with stepped solid carbide drills with straight flutes in drilling of the aluminum alloy A306. Burr height was measured at hole exits and evaluated with ANOVA technique. Results showed that the feed rate and cutting speed variation exhibited significant influence while sharpening was the most important parameter on burr formation.


Introduction
Drilling is one of the most important processes in modern industry even though it is the last item performed within the manufacturing process.Traditionally, twist drills have been used in most manufacturing processes due to their high flexibility.According to Sambhav, Tandon, and Dhande (2012) geometric modeling of the tool is a very crucial part of tool design because its geometry affects surface roughness and burrs.Paul, Kapoor, and DeVor (2005) studied the optimization of chisel edge and cutting lip shape in drills and found a 40% reduction in thrust force and torque when drill point geometry is optimized.Thrust force and process dynamics are highly relevant to define input parameters and predict tool life.Gong, Li, and Ehmann (2005) studied the dynamics of initial penetration in drilling with twist drills and verified that cutting speed at the chisel edge is small when compared to the main cutting edges.Moreover, the effect on feed rate may no longer be neglected in this region.
The analysis of drill geometry is strongly linked to burr due to its influence on chip and burr formation.Kilickap (2010) studied the modeling and optimization of burr height in the drilling of 7075 aluminum alloy and stated that lower feed rates and cutting speeds are preferred.Aurich et al. (2009), state that burrs are sharp and may cause small injuries on finger to assembly workers.Furthermore, they may become loose during operation on a product and provoke damages.Some helpful devices have been employed to minimize burr height and improve the surface quality.However, many researches on burr formation, surface roughness, thrust force and torque have been carried out with twist drills, because straight flute drills are not frequently used in industries nowadays.
According to Jung and Ke (2007), the straight flute design, such as that in gun drills, adds strength to the drill and reduces the distance the chip must travel to escape the bore, when compared to twist drills.Current assay analyses drilling on SAE 306      large deformation in the plastic zone at the edge of workpiece.Furthermore, point angle controls burr height at the end of holes.Increase of point angle provides uniform burrs without drill caps.However, a significant increase of the point angle may generate crown burrs that are more complex to remove than uniform burrs.

Conclusion
Current study showed the influence of input parameters and drill geometry in the minimization of burr formation in the drilling of SAE 306 aluminum alloy.The main conclusions are: A) Feed rate and cutting speed variation affected significantly burr formation.Low feed rates and high cutting speeds provided the lowest burr height.
B) Two types of drills were used, uncoated and coated with TiN.However, results demonstrated that coating did not influence burr formation.
C) Sharpening was the most important and influential parameter on burr formation.Sharpening 'A' and 'R' showed the greatest rates for burr height.D) Sharpening 'N' demonstrated lowest burr rates.Drills with sharpening 'N' have a curved cutting edge and high point angles, which minimize burr formation.

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