Intersection ahead: Drilling Performance

Courtesy of Xebec Technology

A cross-hole deburring tool from Xebec.

Tools for finishing intersecting holes.

Every shop eventually faces the prospect of finishing parts with intersecting holes. Many inexpensive solutions exist, but the process begins with a clear definition of the need.

For example, Peter Gillespie, engineering manager at Shinn Fu Co. of America Inc., Kansas City, Mo., maker of hydraulic jacks, had the following scenario. A hole in the main jack cylinder and an internal seal passes over the edge of a hole, and sharp edges or burrs can damage the seal. “Is there is an economical way to smooth that edge to prevent or reduce seal damage?” he asked. “Because it’s on a curved surface, most of my ideas are not working.”

Assuming nothing Gillespie does while finishing the interior impacts the exterior, he needs a more complete definition for the intersection requirements.

The two most common requirements for hole intersections are deburring and providing a sealing edge at the intersection. For example, fuel injector holes must have no loose particles, burrs or rough edges that will negatively impact fuel flow, and hydraulic system holes require smooth surfaces for precise flow and minimal leakage around seals.

The author’s experience required finishing miniature holes in stainless steel, removing burrs visible at 30× magnification while assuring that the finished intersection had no more than a 0.001 ” radius. Each requirement necessitated different solutions.

Take it Easy

Normally, the size and tenacity of cross-hole burrs is the problem, but sometimes burrs are merely in the wrong hole. One solution is to create burrs in the most advantageous position. The best solution, however, is to prevent burr formation. If a hole is big enough, drill or punch from the inside out to put burrs in the easiest-to-remove location.

If you cannot prevent them, minimize burr size at cross-holes by slowing the feed rate before a drill’s corners start to exit into the larger or same-size hole. If the burrs are 0.005 ” thick, slow the feed 0.005 ” before the corners exit into the other hole. Drill cross-holes with a gundrill, which uses feed rates of ten thousandths of an inch, but rotates much faster than most drills, so the total cutting rate is still high. The small feed per tooth greatly reduces forces at the breakthrough hole intersection, and the resulting burrs are thin. Sometimes when appropriate based on part requirements, drilling a small hole first allows the larger tool to throw the burr into the smaller hole, so it does not have to be removed from the larger hole.

Simple Tool Solutions

When the object is to remove most of the burr (but not necessarily all of it) and the burrs are not thicker than 0.003 “, the following tools provide quick, low-cost solutions. Most of these tools can be used by hand and in drill presses, and some can be used on CNC equipment. They include swivel-blade tools, countersinks, reversible countersinks, back countersinks, rotary burs, flexible-shank cutters, flexible abrasive ball-tipped brushes, conventional brushes and whisk-style brushes.

Hand-held swivel tools are the simplest and lowest cost option for most deburring. They cost about $10 and blades sell for as little as $1 each. They can deburr and radius hole edges in a few seconds.

Courtesy of Heule Tool

A deburring tool from Heule Tool employs a rocking head to uniformly follow the contour of intersecting holes.

A variety of standard blade sizes and designs exist, including a clothespin style. Specials can also be ordered to fit specific needs. The blades are made of HSS or carbide.

A somewhat similar tool from Heule Tool Corp., Cincinnati, employs a rocking cutting head to uniformly follow the contour of the intersecting holes. It is designed for automated use, but can be applied manually on a drill press.

Inverted-cone rotary burs and stones allow a user to go through a small hole and manually traverse the cross-hole intersection. The backside of rotary burs can also provide that function. While the tools are largely limited to manual use, a 3-axis machining center can be used to boost production. For very small holes, inverted cones for the dental industry can be applied.

When a hole is big enough, 7⁄8 ” in diameter, for example, some shops use a right-angle head with a bur ball to finish an intersection. The right-angle head is also suitable for outward drilling of holes.

Courtesy of J.W. Done

J.W. Done’s Orbitool uses a smooth, round disc with bur-like teeth to finish intersecting holes.

Another cross-hole deburring tool option is the Orbitool from J.W. Done Corp., which uses a smooth, round disc to protect surface finishes in the holes from being damaged by its bur-like teeth. It can be used either manually or on CNC machines, and is reportedly popular for some automotive applications because it selectively targets the hole edge with the burr while preventing scarring of bore surfaces.

The flexible shaft allows the tool to wobble or rotate to accommodate an intersection’s uneven profile. This makes the tool flexible, which runs counter to the general notion that a tool must be rigid to perform. Michael Kapgan, president of the Hayward, Calif., company, noted that most potential users assume the tool is merely a rotary bur on a long stem, but the tool’s flexibility makes it react notably different than more traditional tools. That’s because the tool digs in less as it follows an intersection’s contour.

Courtesy of Brush Research Manufacturing

The Flex-Hone from Brush Research Manufacturing can reach inside parts to deburr intersecting holes.

The Orbitool costs about $125 for a typical 1⁄8 ” size. Part manufacturers report finishing 10,000 to 15,000 holes per tool in stainless steel and 20,000 to 50,000 holes in aluminum, according to the company. Cycle time is generally quick; for one automotive application it is 2 seconds. The finished edges can have a radius from 0.005 ” to 1⁄8 “. Producing a large radius, however, may take several passes.

Honing and Deburring

To both hone the surface and remove reasonable-size burrs, Brush Research Manufacturing Co. Inc., Los Angeles, offers the Flex-Hone tool. It is widely used on automotive engine blocks to create a surface finish texture that aids oil retention in cylinder walls. The tool has abrasive-laden balls on the ends of flexible fibers, which scrub the surface. When the bristles reach a cross-hole, they reach into the hole and drag across the intersection while entering and exiting. That provides deburring and radiusing.

While most users will use the larger hole to perform the deburring when intersecting holes have different diameters, the process can be performed through the smaller hole. The bristles can be short, thick and stiff or long, thin and more flexible. The abrasive balls are made of silicon carbide or diamond, and the various diameter balls are available with varying grit size. The tools are not designed to remove large, thick burrs. For those, use one of the previously mentioned tools first and then follow with the Flex-Hone.

Conventional wire brushes have been used for years when deburring intersecting holes, but they can easily scratch surfaces. Abrasive-filled nylon brushes can also be stiff or flexible, but they generally cut slower than the Flex-Hone.

For removing very light burrs or heavy burrs when surface finish is not an issue, flap wheels can be effective. They are available in many sizes and degrees of coarseness.

Finishing with Fiber Rods

Xebec Deburring Technologies LLC, Huber Heights, Ohio, offers two brush styles for cross-hole deburring and surface and edge smoothing. The ceramic stone types have an alumina fiber abrasive stone at the ends of the flexible shafts. Each tool uses a single stone on the tip, and the stone is shaped into a spherical ball or columnar cylinder configuration. The stone surface has many fiber cutting edges to remove material. The self-sharpening action of the cutting edge provides a stable and continuous grinding performance.

Nori Sumiyoshi, president and CEO of Xebec Technology Co. Ltd., Japan, which manufactures the tools, noted that the ceramic stone types remove burrs as thick as 0.2mm (0.008 “) at intersecting holes. They can be obtained in three different grades of abrasive and are used on metals up to 57 HRC. They can enter holes from 3mm to 6mm and finish intersections up to 40mm deep.