Precise Through-Holes

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Implementing the right tools helps achieve perfectly deburred chamfered through-holes – and these tools benefit more than just jet engine manufacturers.

September 30, 2011

The requirement for blending chamfered holes is growing rapidly in the jet engine industry. Any aerospace company working in this field needs to know the best practices for chamfered honing to industry standards. Beyond knowing the practice, knowing the companies that actively provide the tools for this is necessary as well.

That is where Brush Research Manufacturing (BRM) enters the picture. BRM has many years of experience customizing the Flex-Hone tool, specifically for these types of demanding applications.

According to Mike Miller, BRM's vice president of global sales, "Currently, one of the biggest applications where the Flex-Hone tool is used is in the surface preparation for eddy current testing. Any time there is removal or replacement of an aircraft fastener, inspection of the hole for cracking and stress propagation is required before reinstalling the fastener. The Flex-Hone removes oxides and corrosion in order to produce a clean surface for the eddy current to test.

Combining Tools
Custom Flex-Hones can be manufactured according to exact specifications. Special sizes for overall length, brush part, globules, filament, and special minimum and maximum diameters are designed and manufactured in Los Angeles, CA.Miller explains that the U.S. Navy published a Quality Hole specification several years ago, and that it includes the Flex-Hone for this purpose. Many major manufacturers in jet aviation are adopting advanced, hands-off hole drilling, edge breaking, and deburring techniques. In so doing, they recommend, or require, that their suppliers incorporate them as well.

This is where the Flex-Hone tool comes in. Available in standard sizes as small as 4mm in diameter, with custom sizes and abrasives available, the small abrasive globules permanently mounted to flexible filaments characterize the tool. As a flexible, relatively low-cost tool, it is utilized in the manufacturing marketplace for ultra-fine surface finishing, deburring, plateau finishing, and edge blending.

In combination with the Flex-Hone tool, for the flawless finishes required, most shops should standardize use with a Heule DEFA precision chamfering tool and COFA universal deburring tool, prior to implementation of the miniature Flex-Hone tool. Based in Cincinnati, OH, Heule Tool of North America, a subsidiary of Heule Werkzeug AG, offers this solution. The DEFA precision chamfering tool is available in sizes from 0.157" to 1.750". As a double-bladed chamfering tool, it creates pre-adjusted front and back chamfers in a single pass without stopping or reversing the spindle. Using this tool, exact chamfer diameters can be set without trial and error.

Next in this process is the COFA tool blade, available in sizes from 0.157" to 1.614", it cuts a smooth tapered edge break from 0.005" to 0.020", based on the tool size. This is a patented design that incorporates a unique TiN- or TiAlN-coated carbide blade that allows for faster feeds and speed, providing exceptionally long tool life.

Gary Brown, vice president and general manager, Heule Tool of North America, explains that currently, "A major jet engine component manufacturer uses the Huele-BRM tool combination for finishing chamfered holes on rotating engine parts.

"They do this, not only because it is an efficient way to achieve the very stringent tolerances required, but also because there simply is not another method of doing the work that is sufficiently stable and repeatable."

Problems Answered
Large burr formations in stainless, Inconel 718, or titanium alloys are very difficult to deburr. The carbide radial COFA deburring tool will remove the burr, followed by the Flex-Hone, which rounds the transitions of the edge breaks.Miller relates their experience when contacted by the surface-finishing laboratory of a major jet engine manufacturer: "The problem at hand was to provide an edge radius at the transition of the chamfer and through-hole. The application soon expanded to removal of the micro-burr left on the surface at the major diameter of the countersink. This application became additionally complicated when BRM needed to provide a tool that could also pass through the hole and perform the same deburring and radiusing on the backside of the hole.

"To complicate matters further, the tool could only enter the front side of the part. To perform all the deburring and edge radiusing required, simultaneously, a special Flex-Hone was needed."

Miller states the solution was to provide a tool much larger in diameter than would normally be used in the through-hole diameter. The major diameter of the Flex-Hone used is large enough to treat the breakout burr on the surface of the part. To ensure that the tool could enter the through-hole and perform the same on the backside of the part, implementation of lighter-diameter filaments and lighter gauge stem wire was the plan.

"If we attempted to pass a standard tool through the small through-holes, the globules would have been sheared completely off," Miller explains. "With our custom design, we are able to handle combinations of through-holes with countersinks as large as 0.050" over the size of the hole. We also worked with the lab to develop operating parameters that maximized tool life."

The Flex-Hone provides the ideal surface finish in any type or size of cylinder. The oil holding crosshatch produced by the Flex-Hone creates a surface plateau
of more than 60%.
The Flex-Hone is available in a variety of abrasive types and grit selections to provide the optimum surface finish on any base material. BRM is constantly developing and testing new abrasive types to expand the range of applications. The newest abrasive material introduced to market is diamond, which is designed to finish harder materials like carbide, ceramic, and hardened tool steel.

Listed by the nominal diameter for its intended use, the general rule of thumb is to choose the next larger tool size if the bore to be finished is between our nominal sizes. This works fine in most cases, but occasionally a customer requires a specific diameter, which can help maximize tool life and avoid damaging nearby areas like threads or ports. Altering the filament diameter produces a change in the tool's cutting pressure. For example, a decrease in the filament diameter reduces the cutting pressure and allows the tool to pass through restricted openings to finish larger diameters. Increasing filament diameters will increase cutting pressure and is useful in cases where specifications call for making the surface rougher as in adhesive bonding.

The Flex-Hone tool is used to round the transitions of the edge breaks.The brush part (length where the actual globules are) can change to meet the requirements of the part to be finished. In some cases, the bore being finished is a short, blind bore, so only a small portion of the standard brush is required. In other cases, such as cam bearing bores, the brush part is increased to allow the tip of the tool to engage the next bearing saddle before completely passing through the prior bearing saddle. This allows the tool to be stroked in a smooth, continuous motion. BRM also makes adaptors and extensions for longer bores, which works well in most applications, however, customers sometimes want the tool to be a specific overall length without a coupling. It may be only a few inches or it may be several inches, and BRM routinely make tools up to 34" overall lengths.

Additional customization from BRM for the Flex-Hone is production in a variety of shapes to meet finish requirements in non-traditional applications. Whether it is spherical-end configurations, stepped or multi-diameter configurations for double diameters and counter-bores, tapered or cone shapes, segmented shapes, or combining Flex-Hone globules with other filament materials – customization is completely possible for the task.

Precise Touch
The Flex-Hone is characterized by the small abrasive globules that are permanently mounted to flexible filaments. A flexible, relatively low-cost tool, it is utilized for ultra-fine surface finishing, deburring, plateau finishing, and edge blending.The ball-style hone is commonly used to reduce Ra, Rk, and Rpk values while maintaining Rvk and Vo volume for oil retention. Using this tool for surface finishing allows the sizing tools to do their jobs quickly and accurately without adversely affecting surface finish.

BRM's Flex-Hone is also recommended for use in other sectors served by Heule cutting tools, such as automotive, semiconductor, and medical sectors, or wherever secondary burrs occur from a chamfer cutter.

"When you drill a hole you will also create a burr," Brown states. "We [Heule Tool] work with different manufacturers and suppliers in a wide range of industries to help them in their in-process hole cutting and edge breaking process…and we recommend the Flex-Hone as part of the process."

Miller concurs, stating that when customers are looking to implement the Flex-Hone, they need to consider three basic questions, the bore size, the material being finished, and the finish required.

After that, "We provide a tool recommendation based on this information and usually provide tools for initial testing along with estimated machining parameters. Overall, customers need to be aware of the wide variety of abrasive types available, as aerospace alloys can get quite exotic and the 'one abrasive fits all' approach definitely does not work."

That is why BRM and Heule offer customers a full range of smooth solutions.

Brush Research Mfg. Co. Inc.
Los Angeles, CA