By changing from a secondary operation using conventional twist drills following pre-turning when producing 3.2 mm diameter through coolant feed holes by 145 mm deep in M2 tool steel, a UK tooling manufacturer has adopted gundrilling as part of an integrated machining cycle on a Mazak J-200 Integrex Multi-Tasking turn-mill centre.

As a result, of adopting Mollart Engineering’s recommendation to incorporate gundrilling and change to a single cycle method, cycle and lead times have been significantly reduced plus quality has improved by totally eliminating rejects and scrap due to lack of straightness which is now held to within 0.25 mm TIR.

As part of method improvement, not only did Mollart’s application engineer recommend the change to single cycle operation for the production sequence using a Botek Type 113 solid carbide gundrill, but also introduced Mollart’s own development of Spraymist system.  As a result more efficient chip removal was achieved from the drilled hole and both the drill’s guide/support pads and the cutting edge zone of the tool were lubricated.

The Integrex process involved the initial drilling of a pilot hole 8 mm deep to provide a ‘bush’ support for starting the gundrill.  The gundrill was then fed into the pilot hole while counter-rotating at 50 revs/min until the drill point was within 2 mm of the bottom of the pilot hole ready for the Spraymist to be switched-on.  The spindle speed was then increased to 4,500 revs/min and a constant feed rate of 35 m/min applied to the final hole depth.  The spindle was then stopped, the Spraymist closed down and the gundrill retracted.
Botek Type 113 gundrills are available between 0.7 and 12 mm diameter and can be used for depth-to-diameter ratios up to 80:1.  Each tool has a special nose grind and has a highly polished flute with kidney-shaped coolant channel to ensure maximum evacuation of chips from the cutting zone.

The Mollart developed Spraymist system is highly efficient for deep holes, using compressed airline pressures up to 10 bar which absorbs precisely timed pulsed jets of vegetable-based cutting oil from an integrated reservoir.  The system can be retrofitted to existing machine tools and to illustrate the increased efficiency, when a gundrill is removed from the component, it will be immediately cool to the touch.

The oil is used to lubricate the tip of the tool and its guide/support pads which prevents seizure occurring, especially in extra depth holes.  Meanwhile the air effectively cools the cutting edge as well as the component and helps to force the swarf chips to evacuate the cutting zone through the vee grove in the tool.

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