Efficient machining methods for deep holes

Deep Hole Machining (DHM)

is a class of machining areas dominated by tools designed for existing applications. Many different industries are involved in deep hole machining, but the most widespread applications are in the energy and aerospace industries. Initially certain deep-hole part features often seem impossible to form, but off-standard tool solutions designed by specialists not only solve process problems but also ensure that they are executed with a degree of efficiency and error-free characteristics.

The growing demand for complex holes and the urgent need to reduce machining times have led to the development of modern deep hole machining technology. For decades, deep hole drilling has been an efficient machining method using carbide tools, but bottom boring has started to emerge as a bottleneck.

Success in this field of machining is now often based on a mixture of standard and specialised tool elements that have experience of being designed as specialised deep-hole machining tools. These tools are equipped with extended, high-precision shanks with support functions and integrated reamers, which, when combined with the latest cutting edge groove shapes and insert materials, as well as efficient coolant and chip control, enable the required high-quality results to be achieved with the highest penetration rates and machining safety.

In deep hole drilling small diameter holes up to 1 mm are machined with carbide gundrills, but for holes of 15 mm and above, welded edge drills are generally used, while for holes of 25 mm and above, indexable insert drills are used in order to drill very efficiently. Modern indexable insert technology and drilling tube systems also offer new possibilities for specialised tools for deep hole machining.

Holes are generally considered to be very deep when the hole depth exceeds 10 times the hole diameter. Bore depths of up to 300 times the diameter require specialised technology and the use of single or double tube systems in order to be able to drill. The long process of machining to the bottom of these holes requires specialised motion mechanisms, tool configurations and the correct cutting edges to complete the chambers, recesses, threads and cavities. Support plate technology is another important area, also vital in deep hole drilling, and is now also making considerable progress as part of deep hole machining technology. This includes qualified tools for this area that offer higher performance.

Process opportunities

Today's manufacturing requirements call for a completely different solution to deep hole drilling (followed by a subsequent single-edge boring process which often has to be performed on other machines). Even on multi-tasking machines, single clamping requires this approach. For example, holes several metres deep with a bore diameter of approximately 100 mm have to be threaded at one end and the inner chamber deep into the hole has a large diameter. Usually, when drilling is complete, these features are subsequently added to the hole by a boring process after the part has been moved to the lathe. Deep hole machining now combines the ability to perform subsequent processes with one tool and no machine adjustment restrictions. This new tool technology has instead broadened its operating capabilities, allowing these demanding features to be machined more efficiently within a smaller set of constraints.

An example of efficient feature machining using deep hole machining technology is that of oil exploration parts. Such parts are approximately 2.5m long and have a number of complex features with tight tolerances. To achieve small tolerances and excellent surface finish, the tooling solution first involves drilling a 90mm diameter hole and then finishing with a floating reamer. Then a depth of 1.5m is reached and a 115mm diameter hole is reamed and reamed. Another partition enters the hole about halfway through, then also reaming and reaming is carried out and the machining is completed by chamfering. Finally, boring and reaming is carried out to form two chamfered (also reamed to finished size) chambers.

The Deep Boring Global Centre's common deep bore tooling brings a non-standard disposal solution suitable for parts in this power industry. The cutting time was extended from more than 30 hours to 7½ hours. This off-standard tooling solution provides the required tight tolerances and surface finish throughout the relatively complex hole. The process consists of a single deep hole drilling and a finishing stop with a floating reamer. A depth of 1.5m was then reached and the 115mm diameter hole was reamed and reamed. This is followed by reaming and reaming of a shorter part in another deep hole and forming a chamfer. Finally, boring and reaming is stopped to form two chamfered (also reamed to finished size) chambers.

The time taken to complete this part on the machine for conventional machining is over 30 hours. A deep-hole machining solution with special tools can reduce this time to 7.5 hours.

Efficiency gains

Completely different from multi-operation clamping, the use of deep-hole machining technology can also result in productivity gains at higher batch sizes. It is not surprising that cutting times can be reduced by up to 80%. An example of this capability is the know-how in tool and insert design that maximises cutting edge load safety. Load balancing and optimised cutting action on an optimum number of inserts allows for higher penetration rates and thus shorter machining times. In terms of accuracy, small tolerances are a specialty in deep hole machining, where 70% of holes have concentric internal diameters with a typical tolerance of 0.2 mm and a diameter tolerance of 20 microns.

Deep holes off centre line

Another example of the high demands placed on tools and application know-how when hole drilling is the machining of very deep holes in the shafts of power station generators. In this case, the power generation specialist Generpro had to machine a 90 tonne forged steel part with a hole close to 5.5 m long and just over 100 mm in diameter in an asymmetrical manner to the centreline of the shaft. such deep holes have to be drilled at an angle and have to be exited with a positional tolerance of 8 mm or less.

Drilling direction, chip breakage and chip evacuation and an absolutely scrap-free pre-machined shaft are essential for this application. The tooling solution consists of a special drill bit and a new support plate. Drilling tests are carried out prior to application on the shaft and the results confirm greater efficiency and reliability - and the exit position is within 2.5 mm of the target.

The use of modern holemaking technology in many cases has shown a significant reduction in machining times - from many hours to less than one hour - and has made many complex features machinable as well.