Efficient machining methods for complex deep holes


Machining complex deep holes is becoming increasingly challenging. Parts often require additional features such as very tight hole finishes, internal chambers, hole diameter variations, contours, recesses, threads, and variable hole orientation. Efficiently obtaining such holes with very tight tolerances requires not only extensive experience and R&D resources, but also engineering capabilities, application facilities, and substantial customer involvement.

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. At first certain deep hole part features often seem impossible to form, but off-standard tooling solutions designed by experts not only solve process problems but also ensure that they are executed with some degree of efficiency and error-free characteristics.
The growing demand for complex holes and the urgent need to reduce machining times have contributed 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 begun to emerge as a bottleneck.

Success in this field is now often based on a mix of standard and specialized tool elements that have experience in designing tools for specialized deep hole machining. These tools are equipped with extended, high-precision shanks with support functions and integrated reamers, and combined with the latest cutting edge groove and insert materials, as well as efficient coolant and chip control, the required high-quality results can be achieved with the highest penetration rates and machining safety.
Fig. 1
Figure 1 A part to be stopped for deep hole machining requires first a very deep hole to be drilled and then often a variety of complex features to be machined. The success of deep hole machining is usually based on a mixture of specification and common tool components that have experience in designing non-standard tools. Such non-standard tools based on the T-Max 424.10 drill are part of the single-tube application.
In deep hole drilling small diameter holes below 1mm are machined with carbide gundrills, but for holes 15mm and above, welded edge drills are generally used, while for holes 25mm and above, indexable insert drills are used for very efficient drilling. Modern indexable insert technology and drilling tube systems also offer new possibilities for specialized tools for deep hole machining.
Holes are generally considered to be very deep when the hole depth exceeds 10 times the hole diameter. Hole depths of up to 300 times the diameter require specialized technology and the use of single or double tube systems in order to be drilled. The long process to the bottom of these holes requires specialized motion mechanisms, tool configurations and the correct cutting edges to complete the chambers, recesses, threads and cavities. Support plate technology is another important area that is also critical in deep hole drilling, and it is now making considerable progress as part of deep hole machining technology. This includes qualified tools suitable for this field that can provide higher performance.
Figure 2 

Figure 2 In deep hole machining, small diameter holes up to 1mm are machined using carbide gundrills, but for holes 15mm and above, welded edge drills are commonly used, while for holes 25mm and above, indexable insert drills are used to perform these processes very efficiently in single tube systems and Ejector dual tube systems.
Process Opportunities
Today's manufacturing requirements require a completely different solution for deep hole drilling (followed by a subsequent single-edge boring process that often has to be performed on other machines). Even on multi-tasking machines, single clamping requires this approach. For example, machining a hole several meters deep with a bore diameter of about 100 mm must be threaded at one end and have a large diameter for the inner chamber that penetrates deep into the hole. Typically, when drilling is complete, these features are subsequently added to the hole through a boring process after the part is moved to the lathe. Deep hole machining now combines the ability to perform subsequent processes with one tool and no machine adjustment limitations. This new tool technology has instead broadened its operating capabilities, allowing for more efficient machining of these demanding features within a smaller set of constraints.
An example of efficient feature machining using deep hole machining technology is an oil exploration part. Such parts are approximately 2.5m long and have a number of complex features with tight tolerances. To obtain small tolerances and excellent surface finish, the tooling solution first involves drilling a 90mm diameter hole, followed by a floating reamer for finishing. Then reaching a depth of 1.5m, the 115mm diameter hole was reamed and reamed. Another separation enters the hole about midway, then also reaming and reaming is performed, and the machining is completed by chamfering. Finally, boring and reaming is performed to form two chamfered (also reamed to finished size) chambers.
The Deep Hole Machining Global Center's common deep hole machining tool brings a non-standard disposal solution suitable for this power industry part. The cutting time was extended from more than 30 hours to 7 and a half hours. This non-standard tooling solution provides the small tolerances and surface finish required throughout the relatively complex hole. The process includes a single deep hole drilling and finishing with a floating reamer. Subsequently, a depth of 1.5m was reached and the 115mm diameter hole was stopped for reaming and reaming. Then the reaming and reaming of the shorter part in another deep hole is stopped and the chamfer is formed. Finally, the boring and reaming is stopped to form two chamfered (also reamed to finished size) chambers.
For conventional machining, the time required to complete this part on the machine is over 30 hours. A deep-hole solution with special tools can reduce the time to 7.5 hours.

Figure 3 

Efficiency gains
Completely different from multiple clamping operations, the use of deep-hole machining technology allows for productivity gains even with larger batches. It is not surprising that cutting times can be reduced by up to 80%. An example of demonstrated capability is the ability of proprietary technology in tool and insert design to maximize cutting edge load safety. Load balancing and optimized cutting action on an optimal 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 typical tolerances of 0.2 mm and 20 micron diameter tolerances.
Deep holes off centerline
Another example of the high demands 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-ton forged steel part with a hole nearly 5.5 m long and just over 100 mm in diameter in an asymmetrical manner to the shaft centerline. such deep holes must be drilled at an offset angle and must exit with a positional tolerance of 8 mm or less.
Drilling orientation, chip breakage and chip removal, and an absolutely scrap-free pre-machined shaft are critical to 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 time - from many hours to less than one hour - and has made many complex features machinable as well.