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Deep Hole Gundrilling using the 16 spindle Gundrilling centre developed by Mollart

Story added 11 February 2010.

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The first 18-axis, 16-spindle gundrilling centre able to produce up to 16 different holes simultaneously has been developed by Mollart Engineering at its Chessington facility for an exclusive Mollart machine overseas customer that already has some 144 spindles working in its factories. The new machine is capable of automatically producing up to 36,000 radial holes in a tubular die blank, at the rate of almost 1,000 holes/hr, to be used in the production of wooden pellets as part of a Biomass green energy project.

The dial-type, horizontal spindle machine has a central two-axis rotary table that is also programmable for positioning in the vertical Y-axis to create a rise and fall movement for vertically pitching each hole centre. Around the table are four outboard wing units each carrying four independent gundrilling spindles that will produce holes between 1.6 mm and 12 mm diameter up to almost 100:1 depth to dia ratio.

Each tubular die carried on the machine table can be up to 1,700 mm diameter by up to 500 mm deep and weigh up to 250 kg. Made from heat-treated die steel the wall thickness can be specified up to 150 mm which has to be drilled through by the gundrill tool.

Said Mollart’s technical director John Mercer: “This is the first ever deep hole machine development that will produce up to 16 holes simultaneously, which is able to create an increase of 30 per cent in productivity over previous machine designs. Through the Fanuc 30i control, it has the added flexibility of allowing the user to program any combination of spindles to be run from a single unit to the maximum of 16.”

He follows on to explain how the software development team at Chessington played a key part to provide fast change-over through the programming of the control unit to suit the different combinations of die size and hole specifications. “Our software allows automatic calculation of the appropriate machining cycle and creation of the program purely by inputting the diameter of the die, tool length, the number of rows of holes required, the number of holes in each row and the appropriate pitch between each hole.

The Biomass green energy project will make use of scrap wood that is initially shredded and mixed with sawdust and water before being pumped into the centre of the die. It is then extruded under pressure through the wall of the die via the gundrilled holes to create pellets that are between 1.6 mm and 12 mm diameter.

Such is the level of productivity achieved by the Mollart machine, tooling and gundrilling expertise that 16 holes can be produced in under one minute. Also key to the success of the process is the use of carbide tooling to achieve high orders of size consistency and surface finish in the toughened steel which ensures the extruded material is easy flowing and its abrasive nature does not reduce the working life of the die.

Each of the four spindles is carried on a horizontal slide which will feed each drill through its own guide bush for maintaining the start position of the tool and a chipbox. High pressure 120 bar coolant passes through each gundrill which is independently monitored for oil flow, thrust on the tip of the tool and in service tool life. Tool life can be calculated either from the recorded number of holes produced or the combined total depth of holes the tool has created.

The neat oil coolant, supplied from a centralised system, is filtered on the machine to 20 microns and through a flushing system introduced into each chipbox this ensures there should be no build up of swarf and the machine can run uninterrupted around-the-clock.

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