Deep Hole Tools

The BTA self-piloting drilling process can machine bores from 18mm in diameter with length-to-diameter ratios of up to 100 or more.
"The BTA process - also known as the single-tube system or STS - covers deep-hole solid drill heads, pull-boring and counterboring heads, step and form boring tools and trepanning heads."
These different tool head types - each providing a more effective deep-hole production solution than conventional drilling and boring - need to be appropriately configured and applied to achieve optimum results, which is why formed its BTA Application Centre.
Mr Barker says: "Due to its configuration and hole size capability, BTA drilling requires more power than gun-drilling; and while both drill from solid, using high-pressure coolant to directly lubricate the cutting zone and evacuate chips away from the head of the drill, there is a marked difference in the design of a gun-drill and a BTA tool.
Gun-drilling uses a solid-carbide tool head brazed to a tube, through which coolant is pumped under pressure to the cutting edge of the tool head.
Chips are evacuated via a deep V-shaped flute in the drill head extending back along the exterior of the tube between the drill and the drilled portion of the workpiece.
"The BTA system has a rigid drill head - with either brazed solid-carbide or indexable inserts - that is secured to a support and feed tube.
"Coolant is pumped under pressure to the cutting edge between the outside of the tube, the cutting head and the newly created bore in the workpiece material.
"This pressure forces the chips to be flushed back through the drill head, tube and machine spindle to be collected. In both BTA and gun-drill systems, guide pads in the drill head support the cutting action, enabling continuous feed rates to be applied straight, and round accurately sized holes with high degrees of surface finish to be achieved."

Machining stability
Mr Barker goes on to explain that because chips are evacuated through the internal bore of the BTA drill head and tube, fluting or grooving is not required; this provides a greater cross-section of tool that increases rigidity and hence stability under cutting conditions.
Meanwhile, pull-boring is a BTA-based process used to accurately enlarge existing through bores to maintain - for instance - a constant wall thickness, tolerance or surface finish.
The process uses a tensioned BTA boring head but with the wear pads set ahead of the cutting inserts, so that the cutting tool can be pulled back through the workpiece.
Counterboring is an effective use of BTA technology to open up existing pre-bored holes. Here, the BTA drill head can achieve a more-precise diameter or concentricity of a bore - or provide an additional feature such as a bearing or oil seal diameter.
While gun-drilling can be used to machine bores as small as 0.5mm in diameter, BTA holes start at 18mm due to the size required for the drill tube; and as mentioned earlier, the use of trepanning-style heads extends the BTA concept to bore diameters up to 1,000mm.
The trepanning operation also produces a central core of material that can be used to make further components, and it is particularly economic when processing high-value materials.
BTA drill heads are larger in diameter than the rigid tube into which they are screwed.
Moreover, the construction of the tool means it can be used for drilling difficult materials such as exotic alloy steels and stainless steels - and at penetration rates several times greater than conventional drill types.

An automatic tool change with long tools takes place using a pick-up magazine and prismatic tool changer.
With small drilling diameters, the coolant is fed through the milling spindle with a coolant pressure of max. 200 bar. With larger drilling diameters, the coolant volume is crucial.
Deep hole drilling is particularly difficult with materials such as Inconel or titanium, because the indexable inserts are subject to heavy wear. This requires that the machine manufacturer possess comprehensive expertise.
Thanks to the tool magazine in the machining centre, it is possible – without much extra work – to use different drilling tools at different stages when working on extremely deep holes.
Any number of holes can be made at any position in no time at all. This provides theflexibility to carry out different machiningsteps consecutively.
The most basic form of internal machining is drilling. In drilling terms, holes with a diameter between 0.2 and 500 mm and a drilling depth typically greater than three times the diameter are generally considered to be 'deep' holes.

Special challenges call for special tools
The perennial challenges associated with deep hole drilling are how to introduce cooling lubricant to the cutting edge, how to remove swarf at a steady rate, and how to create the straightest possible hole. With deep hole drilling procedures, the drilling head next to the actual main cutting element (mostly an individual cutting edge or one made up of interchangeable cutting inserts) consists of a secondary cutting edge and additional guide strips. This arrangement ensures the drill bit is supported at the wall of the hole, which increases accuracy and makes it easier to centre the drill bit during the process. The support provided to the drill bit also produces a smoothing effect, which improves the surface quality inside the hole.

Different processes
Deep hole drilling processes are divided into two main categories, depending on whether chips are removed externally or internally. External chip removal is mainly associated with gun drilling and, more rarely, double-lip deep hole drilling, whereby cooling lubricant is introduced to the cutting edge via inlet holes in the drill bit and the chip/cooling lubricant mixture is transported away along a V-shaped longitudinal slot in the tool. This process is typically used for drilling diameters between 0.5 and 40 mm. Another option, at drilling diameters of 16 mm and above, is the BTA process. This is one of the processes whereby the chip/cooling lubricant mix is transported away internally. The advantage with processes where chips are transported away internally is that outgoing chips no longer come into contact with the surface of the hole and therefore cannot damage it. Ejector drill bits, a special form of BTA bit, may be used at diameters of approximately 25 mm and above based on a two-pipe arrangement. These drill bits have additional cooling lubricant outlets around the edge of the drilling head, and some of the lubricant is fed directly into the internal pipe through a ring nozzle. This creates negative pressure in the front section of the drill bit, which accelerates removal of the chip/cooling lubricant mixture.

Integrated machining
With extremely deep holes or materials where machining is difficult, excessive tool wear often requires that drilling is performed in stages using tools of different lengths but the same diameter. WFL MILLTURN machining centres offer decisive advantages as far as these kinds of machining steps are concerned. For one thing, the various drilling tools used can be stored and kept ready in the tool magazine; this minimises interruptions and manual processes while significantly improving positional accuracy. Not to mention the fact that genuine six-sided deep hole machining can only be performed during one clamping operation or two at the most.

Automatic workpiece measurement and adaptive control
Due to the large tool projection, deep drilled holes are subject to hole centre deviation that increases with the hole depth. These machining errors cannot be fully eliminated, even when machining on MILLTURN machines. Hole centre deviations are measured by intelligent in-process measuring following the completion of deep hole drilling. This is performed either with an extended measuring probe or by means of ultrasonic wall thickness measurement, which involves measuring the thickness of the wall at various circumferential positions and calculating the middle of the centre hole. New clamping points are then produced on the workpiece concentrically to the defective deep drilled hole by means of turn-milling. This allows the realisation of all subsequent machining processes with very close form and position tolerance to the deep drilled hole. The benefits are clear. Set-up times are reduced enormously and any finishing work due to deformation resulting fro