Tham khảo tài liệu 'drilling and associated cutting tool technology industrial handbook_2', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 96 Chapter 3 Web-thinning - as its name implies the chisel point is web-thinned notched by regrinding to reduce the width of the chisel point while slightly modifying the profile giving a partial self-centring action Fig. 47c Helical - the chisel point is ground to an S-shape which modifies both the chisel point and its profiled shape improving the drilling performance and self-centring action . NB On some drills a sophisticated grinding action has imparted drills without a chisel point which significantly improves their drill penetration rates into the workpiece but requires a complex drill regrinding operation to re-sharpen them when the edge becomes dulled . Not only are drills supplied with appropriate point geometries but for twist drills the twin spiral flutes of the drill can also be specified - from the tooling manufacturer as this gives the drill its equivalent of the rake angle as found on a single-point turning tool. On conventional jobber drills the normal flute angle is 29 - giving a relatively slow helix Fig. 47a and in the past typically being utilised for drilling most plain carbon steel grades. Conversely a drill with a quick helix angle Fig. 47d might be employed to drill soft materials such as certain plastics. Brittle materials on the other hand which might be utilised typically when drilling Cartridge Brass . 70Cu 30Zn composition require a zero or slightly negative helix. NB It is possible to temporarily modify the drill s helix angle by re-grinding termed drill dubbing which refers to lightly flash-grinding the flutes at the lips to decrease the effective flute helix angle. The main strength of a drill is via its web or its crosssection which can be changed and as a result will modify the flute s geometric profile . see Fig. 48 . In general drill cross-section are classified in three groups namely Axe-shaped - having well-defined margins Fig. 48 -top Rounded heel - with increased web but small margins Fig. 48 - middle Rhombic -
