As mentioned in previous chapters, the rake angle of the cutting edge reduces towards the centre of the drill, and is actually negative at the chisel edge. Therefore, the cutting resistance increases. To reduce the cutting resistance at the chisel edge, it is possible to decrease the drill´s web thickness.
However, making the web thickness too small lowers drill strength and can cause vibration due to drill deflection. It can also lead to a shortened tool life and breakage, therefore there is a limit to the amount the web can be thinned. To reduce the cutting resistance without decreasing web thickness, a process called web thinning is carried out. Web thinning is also very effective for improving chip disposal.
Web thinning geometry and its influence
By combining a suitable cutting edge geometry and web thinning (X, XR, S or N type), hole accuracy can be improved and cutting resistance lowered.