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Tham khảo tài liệu 'drilling and associated cutting tool technology industrial handbook_4', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 114 Chapter 3 Inlet for Cutting GUN-DRILLING . ILLUSTRATING THE LARGE LENGTH-TO-DIAMETER RATIOS OF A TYPICAL TOOL. Shank Workpiece Steady Workpiece Bushing Shaft Support Connector for Non-Rotating Tool Inlet for Cutting Drill Tube Support Figure 58. Deep-hole drilling operations such as a gundrilling b double tube ejector drilling and c single tube ejector drilling. Courtesy of Sandvik Coromant Drilling and Associated Technologies 115 Minimal fluid-flow disturbance - giving consis-tent regular flow-rate to drill-head Minimum of fluid-turbulence - allowing chips to be easily evacuated from the cutting region. Typically cemented carbide heads have an external V-shaped chip-flute which extends along the shank the angle of this chip-flute has been experimentally-determined to be 110 providing the following advantages Optimum flute cross-section - allowing the most rapid cutting fluid return and chip transportation Facilitates an extra support pad - this is necessary when drilling through crossing holes Provides optimal torsional strength - important for workpieces having very long length-to-diameter ratios Facilitates tool clamping - enabling the tool to be held in a three-jaw chuck for convenient regrinding on a suitable cutter-grinder. Gun-Drill Failure One of the main reasons for Gun-drills to fail in operation is through an excessive misalignment of the drill bushing and this will be in relation to the drill s rotational axis i.e. see Fig. 58a . This type of alignment failure mode is termed a balk-crash - causing the tool to fracture into numerous pieces33. If the drill is rotated rather than the workpiece the stress is re-applied to differing portions of the tip and at the weakest point namely the drill s corner the tip will most likely fracture in this region. A potential failure mode is related to the Gun-drill s length which has its rigidity decreased with increased length34. The shank of a longer Gun-drill will not transmit a large amount of bending force to .
