Zinc alloys can be drilled under a wide range of operating conditions using cutting fluids. Dry hole drilling is possible but flood lubrication is recommended especially for larger diameters.
The following parameters are recommended. The fast spiral twist drill design with standard geometry of 118° point angle and 12°-15° lip relief angle is appropriate for most operations. This geometry was designed for drilling non-ferrous alloys and has large, smooth flutes and thin webs that reduce friction and thrust forces and enhance chip flow away from the cutting edges. The general-purpose jobber drill design is a good second choice. Flatter points (up to 136°) can be used to minimise the tendency for drills to wander when drilling deep holes through heavy sections of castings where porosity may be encountered.
All cutting edges must be kept sharp. Resharpening by hand is not recommended; it should be done in appropriate jigs to ensure correct angles and geometries using fine grinding wheels. Modifying drills with flats or by web thinning is not recommended unless care is taken to maintain smooth flute surfaces.
Speed and Feed
Speed and feed rates for shallow holes, to a depth of three diameters, based on a surface speed of 90m/min are as follows:
Feed rate dwells during drilling should be avoided, as it can lead to galling and burring.
When hole depths exceed three diameters, feed rates should be reduced by 25%. When drilling holes deeper than six times the drill diameter, speeds and feed rates should be reduced to approximately 50% of the above values. For very deep holes, drilling should be staged with inter-mediate withdrawals for flute clearing and to allow cutting fluid to reach the drill tip for cooling and lubrication.
Countersinking removes burrs from drilled holes and sinks screw heads to reduce or eliminate protrusion above casting surfaces. Countersinking tools should have large cutting edges with generous chip space. Insert carbide tipped cutting edges are useful for large diameter work.
The optimum feed rate should be determined by trial; pressing and holding the tool quickly against the workpiece will achieve the best quality.