Undertaken at the company’s Chessington, Surrey subcontract facility, the trials involved the drilling of a 63.5 mm diameter through-bore machined in a 250 mm diameter by 2,180 mm long Super Duplex stainless steel bar.
Not only maintaining straightness, the process demonstrated a cycle time saving of 2.5 hours against the previous four-hour drilling cycle, which was undertaken on a Mollart Engineering Centeplex hybrid combination milling and gundrilling centre. To complete the whole part, however, the Centeplex is still required to complete initial features.
The 7-axis Acubore, developed at a cost of more than £550,000, is able to track the actual path of the drill point relative to the outside profile of the component as it penetrates the material by using non-destructive high speed laser ultrasonic technology (LUT). LUT then feeds the captured data to Acubore’s Fanuc 31i control. Within the control, specially developed algorithms process this data, resulting in a straightness correction factor that can be applied to the machine’s X-axis independent component steady support arrangement (two independently programmable three-point steadies), in conjunction with C-axis rotation of the part.
The cycle time reduction is achieved through a combination of LUT and Acubore’s greater power, torque and rigidity. The drill head of Acubore was developed for power drilling with a 30 kW motor able to create up to 666 Nm of torque with a top speed of 5,000 rpm, which enables the machine to accommodate smaller hole sizes, as well as freer machining material types.
Set to run at 450 rpm on the Super Duplex material with a feed rate of 80 mm/min, Accubore’s coolant pressure head, which is carried alongside the BTA Type 07 ‘chip-forward’ drill head, has a rate of flow of 190 lit/min. This ensures that the broken chips are removed effectively away from the drill head to pass back through the inner core of the single drill tube in order to prevent any build-up and to keep the cutting zone cool.
Due to the 2,180 mm overall length of the material, using the insert-based Botek BTA drill head the 63.5 mm main bore is drilled from solid to its halfway position. The component is then indexed around, relocated and the datum set to align and allow drilling of the main bore to break through. The 1.5-hour cycle includes handling, incidentally.
The change in method to produce the component’s central through-bore on Acubore still involves Mollart creating the initial machining datum and removing any inbuilt stress from the material by turning the outside diameter and skimming both faces to ensure they are square and parallel.
The part is then loaded to the company’s 5-axis Centeplex to produce a series of five, blind angle holes of 7.7 mm diameter set on a pitch circle diameter and drilled to a depth of 330 mm from the end face. Once completed, the component is then transported to Acubore for production of the main 63.5 mm diameter through-bore.
