1. Drill Bit By : B. Ramesh Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119 Ph.D. Research Scholar College of Engineering Guindy campus, Anna University, Chennai-25.
2. Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a chuck, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes. The drilling process is primarily used to produce cylindrical holes. Through various tools (twist drills, combination drills, spade drills, etc.) different hole shape can be produced as cylindrical holes, drilled and counterbored, drilled and countersunk, multiple diameter holes, etc. From top to bottom: Spade, lip and spur (brad point), masonry bit and twist drill bits
13. The body of a twist drill has two spiral flutes which usually have a 30° helical angle. These flutes act as a passageway for chip extraction from the hole and for coolant to enter the hole (however, cooling is not effective since chips and coolant move in opposite directions). The thickness of the drill between the flutes, also called the web , provides support over the length of the drill body.
20. Excessive clearance results in lack of support behind cutting edge with quick dulling and poor tool life. Clearance angle behind cutting lip for general purposes is 8º to 12º.
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30. Tool geometry Workpiece material Point angle Helix angle Lip relief angle Brass 90 to 118 0 to 20 12 to 26 Cast iron 90 to 118 24 to 32 7 to 20 Mild steel 118 to 135 24 to 32 7 to 24 Stainless steel 118 to 135 24 to 32 7 to 24 Plastics 60 to 90 0 to 20 12 to 26
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36. SDS shank The SDS bit was developed by Bosch in 1975 and the name comes from the German " S teck – D reh – S itz" (Insert – Twist – Stay). The SDS shank has the advantage of a simple spring-loaded chuck, so that bits can be chucked with a simple and quick hand action. Further, the shank and chuck are uniquely suited to hammer drilling in stone and concrete. The drill bit is not held solidly in the chuck, but can slide back and forth like a piston. The hammer of the drill acts to accelerate only the drill bit itself, and not the large mass of the chuck, which makes hammer drilling with an SDS shank drill bit much more productive than with other types of shank. So, SDS shanks are most often seen on masonry drills, for which hammer drilling action is most helpful.
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43. TiAlN Coating: Titanium aluminum nitride (TiAlN) has proven more effective than TiN or TiCN in protecting tools from heat, because it has a higher oxidation temperature (1450° F). TiAlN can withstand higher temperatures without breaking down and degrading the tool substrate. Furthermore, when TiAlN's oxidation threshold is exceeded, its outer surface transforms into aluminum oxide, which has excellent hot hardness, low thermal conductivity, and high chemical stability. As a result, most of the heat generated by the cutting operation flows into the chips rather than the cutting tool. Furthermore, TiAlN provides the increased lubricity typical of PVD coatings.
