July 2011 / Volume 63 / Issue 7|
By Christer Richt, Sandvik Coromant Co.
| A qualified approach to machining complex, deep holes.
Courtesy of All images: Sandvik Coromant
Very deep, complex holes are becoming increasingly challenging to machine as hole tolerances become tighter and surface finish requirements get finer. In addition, components often require unique features, such as chambers, profiles, grooves, threads and hole-diameter and hole-direction variations. Efficiently machining these types of part features with tighter tolerances requires experience, R&D resources, engineering capability, application facilities and meaningful cooperation between a shop and its cutting tool supplier.
Machining holes from 10 to hundreds of diameters deep is an area dominated by application-specific tools. Deep-hole machining takes place in several industries, but is more common in machining energy and aerospace components because they typically require many deep holes, such as in landing gears, oil exploration components and turbine axles.
Some deep-hole features seem impossible to generate, but special tools can simplify operations, ensure high productivity and provide a strong degree of machining security.
The development of modern deep-hole machining (DHM) technology came about because of a growing need for complex holes and the unacceptably long machining times that resulted. Deep-hole drilling with cemented carbide tools has been an efficient method for several decades, but machining down-hole features has become a bottleneck.
DHM success is usually achieved by applying a mix of standard and special tools. This base knowledge, combined with design and field experience, leads users toward tools engineered for DHM. For example, precision tools with expandable toolholders help meet the challenges of chamber boring. Integrated support functions for the tool and reamer capability are combined with the latest carbide grades as well as efficient coolant and chip management. Support functions range from simple support pads to complex, fold-out supports on DHM tools.Size Matters
When deep-hole drilling, holes below 0.040 " in diameter can be made with gundrills, while intermediate holes (from 0.040 " to 0.590 ") can be made with gundrills, ground carbide-tipped drills and exchangeable-tip carbide drills, depending upon hole depth. For 0.591 "-dia. and larger holes, brazed-carbide drilling heads are more efficient. For even larger hole diameters—0.984 " and above—indexable-insert drills are preferred. Modern indexable-insert technology and drill-tube systems have led to the development of engineered DHM tools.
Holes with depths hundreds of times their diameters require specialized single- or double-tube gundrilling systems. Machining a long way into and at the ends of these types of holes requires qualified-movement mechanisms, new tool configurations and the proper cutting edges to make and finish chambers, grooves, threads and cavities.
When machining deep-hole features, users must consider in-process tool stability. Therefore, support pad technology is an important factor.Process Opportunities
Today’s complex components and machining productivity demands require DHM solutions as opposed to deep-hole drilling followed by single-edge boring-bar operations, often performed on separate machine tools. Advanced DHM tools, combined with multitask machines, can complete deep-hole operations in one setup. This broadens machining capabilities, making it possible to machine demanding features more efficiently, while achieving tighter tolerances.
For example, consider the machining of an oil-exploration component. The part was almost 8.2 ' long, had complex features, such as grooves, compartments, tapers and profiles, and required tight tolerances and a fine surface finish.
To meet those specifications, the shop drilled a 3.54 "-dia. hole and finished it with a floating reamer. Then, a 4.53 "-dia. hole was counterbored and reamed to a depth of 4.9 '. Another compartment, about half way into the hole, was counterbored, reamed and chamfered. Ultimately, boring and counterboring were used to form two chambers, with chamfers leading in and out of the main bores. All features were reamed to their finished dimensions.
With conventional machining, machine time for this component was more than 30 hours. With tools engineered for DHM, the shop reduced machining time to 7.5 hours.
Larger batch sizes can also be handled with tools engineered for DHM, rather than using multiple operations and setups. An 80 percent reduction in machining time with DHM technology is not unusual. That can be achieved by balancing loads and optimizing cutting action with the optimal number of inserts to increase the penetration rate.
DHM technology is frequently applied on multiple-diameter bores with tight tolerances. In these applications, 70 percent of the holes typically have concentric bores with a 0.008 " concentricity tolerance and a 0.0008 " diameter tolerance.
Producing and maintaining the correct chip form and size, and efficiently evacuating those chips, is as critical for successful DHM as when deep-hole drilling. Therefore, efficient and dependable chip control is an integral part of insert geometry development.
Effective coolant and swarf management is also essential. Technologies such as high-pressure coolant can improve DHM.
In many cases, difficult-to-machine materials are specified. In addition to the previously mentioned requirements, those materials must be cut with tools having the right wear and toughness characteristics. Insert grades with proven performance cutting stainless steel, titanium and heat-resistant superalloys are needed to achieve predictable tool life and benchmarked cutting requirements.Off-Center Holes
Another demanding application involved making deep holes in giant generator axles for power stations. The Generpro Co., a power industry supplier based in Västerås, Sweden, produced a 90-metric-ton forged steel component with 3.94 "-dia. holes nearly 18 ' deep. The holes had to be made asymmetrically to the centerline of the part, drilled at an off angle and within a positional exit tolerance of 0.315 ".
Drilling direction, chip breaking and chip evacuation were critical. Also, there could be no scrap in the premachined axle manufacturing process because parts of the component were irreplaceable and the component itself was extremely expensive. The company used a DHM tool that included a custom drill head with a new type of support pad. The process was tested before being applied on the axle and proved to be efficient and reliable. Exit position was within 0.098 " of the target.
The requirements for today’s deep-hole applications are often more demanding than in the past, but tools engineered for deep-hole machining make the process more productive and profitable than conventional deep-hole drilling following by single-edge boring. CTE
[Editor’s Note: The Sandvik Coromant Global Center for Deep Hole Machining makes available development, design and testing resources for the development of new machining processes. In addition to low-volume part applications, the center works with industries that require holemaking in high-volume part production operations, such as in heat exchanger and billet production.]About the Author: Christer Richt is technical editor for Sandvik Coromant Co., Fair Lawn, N.J. For more information about the company’s cutting tools, call (800) SANDVIK or visit www.sandvik.coromant.com/us.
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