Drilling of long chipping materials

A new range of indexable-insert drills has been introduced by Stellram for the efficient production of holes from 13.5 to 63 mm diameter in ductile, long chipping materials such as stainless steels. Called Posidrill Plus, it is intended for producing short holes up to three times (optionally four times) diameter. The system is ideal for drilling stacked material, as the positioning of the inserts in the body renders all the machined material to chips, allowing a smooth transition from plate to plate by avoiding the formation of slugs that would otherwise impede the drill. The drills use popular WCMX-style inserts with three cutting edges in four sizes, with up to six inserts loaded on alternate flutes to cover the range of diameters. The inserts are available in grade SP4034, a wear-resistant, micrograin structure combining hardness and toughness. It has a titanium-aluminium-nitride PVD coating offering high lubricity and hence low friction cutting. Both carbide structure and coating process are proprietary to Stellram. The sharp, positive top rake of the -60 geometry affords a high density of carbide grains at the cutting edge where it is most needed for cutting strong, abrasive, ductile materials. Identical inserts may be used on the inner and outer positions of the drill body, as toughness for cutting the hole centre is combined with wear resistance for cutting the periphery. In each flute of larger diameter drills, multiple inserts may be used to break the cut into smaller sections, creating more manageable, easily evacuated chips. Another benefit is a reduction in power consumption of the machine tool, which is already reduced owing to the positive geometry, irrespective of whether one is drilling on a lathe or a mill. For example, when machining a 316 stainless steel valve body on a machining centre at 135 m/min surface speed using a 25 mm diameter drill, power consumption is only 10.4 kW. During comparative tests on the valve, drilling an 18 mm diameter hole to a depth of 27 mm, Stellram was able to increase feed rate by 50 per cent and surface speed by 84 per cent compared with the incumbent tool used for the application. The resulting cycle time reduction from 28.4 to 9.5 seconds represented a three-fold saving.