Cylindrical sleeve, typically made from high-grade tool steel, inserted into a jig fixture to guide cutting tools. There are three main types: renewable, used in liners that in turn are installed in the jig; press-fit, installed directly in the jig for short production runs; and liner (or master), installed permanently in a jig to receive renewable bushing.

These high-pressure coolants are also responsible for efficient chip evacuation from the cutting zone, preventing surface damage and chip removal strokes that may take time. Coolants allow the drilling process’s productivity and improve the hole quality.

The technological advancement of drilling equipment allows you to attain a certain depth-to-diameter ratio since a computer controls the process. Similarly, there are different deep-hole drilling techniques with varying modes of operation. Hence, choosing the one that fits your project demands best is essential.

Ejector drilling or a double-tube system is often used on machining centers and conventional machine tools. It provides drilling depths of about 100 x D. Despite being a BTA drilling system variant; it has no sealing to prevent the metal-working fluid at the workpiece from exiting. Therefore, this drilling technique is suitable for oblique boring or discontinuous parts since covering up the metal-working fluid circuit is impossible.

Powers also recommends using a bushing to support the drill. Contrary to conventional wisdom, however, he suggested it be made of brass or other soft metal and not hardened steel.

Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.

Product manufacturers and designers rely on different deep-hole drilling processes to achieve superior-quality final products while controlling costs. This section discusses the standard techniques used in deep hole drilling:

The typical drill bits for making deep holes in wood are Auger bits. These drill bits have a helical or spiral shape and comprise a sharp point on their tip, allowing drilling efficiency. This tip efficiently pulls the bit through the drilled wood to achieve the preferred depth.

Machine designed to rotate end-cutting tools. Can also be used for reaming, tapping, countersinking, counterboring, spotfacing and boring.

Kip Hanson is a contributing editor for Cutting Tool Engineering magazine. Contact him by phone at (520) 548-7328 or via e-mail at kip@kahmco.net.

This guide discusses everything you need to know about deep hole drilling, its benefits, applications, and standard techniques. You’ll also learn helpful tips for effective deep-hole drilling. Let’s dive in!

Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.

Manufacturers often use specialized equipment designed explicitly for deep-hole drilling procedures. These innovative equipment are set up to optimize the hole drilling operation for straightness and efficiency.

Deep hole drilling can be quite complex, irrespective of the sophistication of the machinery. Also, there are higher chances of errors when drilling a deep hole. However, here are some helpful considerations for effective hole drilling:

Deep hole drilling is a complex metalworking process that involves making long, straight holes in components. As such, workpiece deflection can occur when the workpiece is not properly held in position, resulting in inaccuracies. Therefore, it would be best to ensure workpiece stability when making holes.

The angled hole on this shaft required a delicate touch while being drilled on a knee mill. Image courtesy Washington Machine Works.

Generally, a normal drilling process has a length-to-diameter ratio usually smaller than 5. Hence, deep-hole drilling is characterized by a length-to-diameter ratio greater than 5.

The oil and gas industry benefits from deep-hole drilling because it helps make boreholes for oil and gas extraction. The process offers extreme depth-to-diameter ratios in long workpieces without compromising wall thickness or accuracy in roundness. Deep hole drilling is perfect for making holes with a depth-to-diameter ratio of up to 20:1.

Operation in which the cutter is mounted on the machine’s spindle rather than on an arbor. Commonly associated with facing operations on a milling machine.

Liquid used to improve workpiece machinability, enhance tool life, flush out chips and machining debris, and cool the workpiece and tool. Three basic types are: straight oils; soluble oils, which emulsify in water; and synthetic fluids, which are water-based chemical solutions having no oil. See coolant; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

Meanwhile, the defense industry is another primary field where deep-hole drilling is widely embraced. Manufacturers in this field use this drilling strategy to make accurate holes in the gun barrel with the preferred surface finish.

“One of our customers had an angled hole application and found that the chips would curl back in and get caught between the steel bushing and the cutter, causing it to bind,” he said. “By switching to brass, they improved chip evacuation and completely eliminated the problem. We’ve since validated their approach with our own in-house testing.”

However, full-face crown drill bits exhibit improved lifespan even though it is subjected to the hardness and rigidity of the material’s surface. Therefore, it is advisable to use the right drill bits to drill precise and accurate holes easily.

Solution: Use special fixturing mechanisms to steady the workpiece, mitigate deflection, and ensure drilling accuracy. Also, it is advisable to plan the drill entry point carefully to reduce impact and mitigate chances of deflection.

For most angled drilling, Pollock recommends reducing speeds and feeds by 30 to 50 percent when starting and exiting a hole. If a starter hole is required, he said many shops don’t drill one. Instead, they use a center-cutting endmill to create a flat the same size, or slightly larger, than the drill’s diameter. He added that a better option than endmilling a flat is to apply a dedicated pilot drill with a 180° point, because it has greater metal-removal capabilities and improves accuracy and tool life.

AT-Machining is the right machining partner for your CNC and deep-hole drilling needs. We leverage our range of hole-drilling machines with our machinists and product engineer expertise to achieve preferred tolerances and outstanding accuracy in your final products and prototype machining. We have years of experience offering reliable deep-hole drilling services across all fields.

Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

The flute design of the Walter DC170 drill creates support around the periphery of the tool to prevent chatter and drill walking when making angled holes, according to the company. Image courtesy Walter USA.

Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

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Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.

Here are some of the challenges you may encounter during deep hole drilling operations and helpful ways to overcome them:

Solution: Partner with a service provider with vast knowledge of the techniques and tools used in deep-hole drilling. Similarly, it is advisable to implement process documentation and quality control measures to mitigate human errors and detect mistakes early.

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Tooling usually considered to be a stationary apparatus. A jig assists in the assembly or manufacture of a part or device. It holds the workpiece while guiding the cutting tool with a bushing. A jig used in subassembly or final assembly might provide assembly aids such as alignments and adjustments. See fixture.

One of the biggest challenges with any angled-holemaking operation, Fettig said, is burr control. Because cutting fluid pressure around the drill tip falls to near zero when a drill breaks through into a cross-hole or out the back of the workpiece, chip evacuation is compromised while heat in the cutting area increases.

Someone who’s a firm believer in drill bushings—although probably not ones made of brass—is Anthony Fettig, CEO at deep-hole drilling system provider Unisig, Menomonee Falls, Wis. He said the company has “drilled some really strange holes with pretty significant complexity and close tolerances.” His first questions when quoting a job are often: What accuracy is required, and what burr condition is allowed?

Gundrilling relies on a bushing to guide a single-flute, noncenter-cutting drill into the workpiece. This bushing also acts to contain a stream of high-pressure cutting oil. Drilling an angled hole in an automobile crankshaft bearing journal, for example, requires a specially designed bushing with a concave face that conforms to the journal surface. Together with some clever grinding of the drill point, use of wear pads along the drill periphery and the proper application of speeds and feeds, holes can be predictably and accurately gundrilled by the hundreds of thousands.

That’s certainly the hope of Dave Fagan, shop foreman at Washington Machine Works, a Seattle job shop that specializes in low-volume, complex parts for the nuclear, marine and other industries. Fagan said the quantities at WMW are small enough that bushed drill jigs are rarely necessary. Instead, machinists rely on their sense of touch to guide drills during angled holemaking.

However, you might be able to perform the deep hole operations with a standard CNC gun drilling machine, depending on the hole’s depth-to-diameter ratio. However, you can use special tools to achieve higher precision. A specialized deep-hole drilling system can achieve larger diameters of about 400:1.

Luke Pollock, product manager at Walter USA LLC, Waukesha, Wis., said that regardless of the machine tool, one of the best solutions for angled holemaking is a four-margin drill. “A drill with two margins per cutting edge provides more contact between the tool and the workpiece material,” he said. “It helps guide the tool and prevents drill walk, especially as you break through the back of the part.”

Deep hole drilling is a process of machining a hole ten times deeper than the hole diameter. A deep hole is characterized by at least a depth-to-diameter ratio of up to 10:1. It is suitable for creating holes of desired depth in different CNC machining materials ranging from aluminum to the strongest super-alloys.

Because annular cutters remove only the outer periphery of the hole, horsepower and thrust requirements are far lower than traditional holemaking tools, making annular cutters a favorite for magnetic and hand-held drilling. Yet Allan Powers, technical applications lead at Hougen Manufacturing Inc., Swartz Creek, Mich., said Rotabroaches are a popular holemaking alternative on CNC machines as well, especially when spindle power is limited.

Moreover, you must feed the metal-working fluid through an annular space between the inner and drill tubes. Then, the fluid comes out from the side of the drilling head, flowing over the drill head to cool and lubricate it. The fluid pushes the chip through the internal cavity of the inner drill rod for efficient removal. Introducing some of the fluid into the thinner tube ring nozzle would be best.

Engineers and product designers in the automotive field rely on deep-hole drilling to achieve preferred tolerances in machining automotive parts and components. For instance, the fuel injectors in vehicle engines require small and highly accurate holes to inject fuel into the combustion chamber efficiently. Also, the crankshafts and cylinder blocks require precise deep holes to function effectively.

Coolants are crucial in deep hole drilling because they ensure proper cooling and lubrication of the hole drilling operation. These coolants help to maintain moderate temperatures, ensuring longer tool life.

This deep-hole drilling method uses a machine with a thin cutting tool to achieve highly accurate and straighter holes in a workpiece. The introduction of gun drills transformed the drilling game because they offer higher accuracy than conventional-style drills.

Some angled holes call for complex, compound angles that have matching geometry with other machined features. A drill bushing ground with these special angles is not practical or economical in these cases. Image courtesy Unisig.

Drilling process using a self-guiding tool to produce deep, precise holes. High-pressure coolant is fed to the cutting area, usually through the gundrill’s shank.

Stringy portions of material formed on workpiece edges during machining. Often sharp. Can be removed with hand files, abrasive wheels or belts, wire wheels, abrasive-fiber brushes, waterjet equipment or other methods.

“A lot of our work is done on a Bridgeport,” he said. “It gives you a certain sense of touch, something you don’t feel on a big horizontal boring mill or other large manual machine, where much more iron is moved when you turn the handles.”

“Annular cutters are also effective on angled holes, provided the centerline is no more than 20° from perpendicular,” he said. “Unless it’s soft material, like aluminum, the lateral loads become too great, and you run the risk of breaking the cutter. We recommend reducing feed rates until the cutter is fully engaged, but, because the tool is largely self-supporting, there’s no need to slow down on breakout.”

Although deep hole drilling has extensive material compatibility, each material poses challenges. Certain materials, such as plastics, may experience deformation, while rigid materials like titanium may cause consistent machine tool wear during deep hole drilling.

Deep hole drilling is a versatile process that benefits various applications with stringent requirements and unique challenges in machining rigid materials with tight tolerances. This operation offers the preferred depth-to-diameter ratio for your hole-drilling projects. However, it would help to understand everything about the process to effectively exploit its benefits in achieving high-quality products.

Holemaking up to 20° from perpendicular is the rule of thumb for most annular-cutting operations. Image courtesy Hougen Manufacturing.

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For relatively shallow holes where the centerline is close to perpendicular, it’s often possible to drill without a starter hole.

Chips and cutting fluid flew everywhere. And because the drill would occasionally shatter on breakthrough, shrapnel was always a concern. It was mindless, tiring work, and there was an unspoken rule that all new employees had to spend their first week drilling submarines, upon which they could graduate to one of the NC mills or chuckers.

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Deep hole drilling is a widely used technique in many industry applications. However, it originated from the need to drill straight gun barrels with high accuracy in which the required depth-to-diameter ratio can reach 20:1.

The tools used in deep-hole drilling are usually durable since they are made of super-strong materials and have unique geometries. As such, these tools can withstand wear and prevent consistent tool changes. Hence, the extended tool life of deep-hole drilling machines helps to save costs while ensuring peak productivity.

Solution: Using drill bits with the correct geometry, material, and coating on the workpiece material would be best to avert this challenge. You can use drill bits with solid carbide or diamond coatings for better wear resistance. However, optimize feed rates, speed, and coolant selection to mitigate heat and ensure proper cutting frequency.

Using the appropriate drill bit to drill straight holes is essential to avoid drill deviation or inaccuracies. Drilling deep holes with incompatible drill bits can result in expensive mishaps. As such, manufacturing experts often prioritize the drill bit’s compatibility and durability.

Modern industries like aerospace benefit a great deal from deep-hole drilling. Here are the typical roles this operation plays in respective applications:

Deep hole drilling is a popular metal-working process in many fields, especially in producing firearms or heavy ordinances. This operation is perfect for most high-precision drilling projects because it provides highly accurate, repeatable holes with superior surface finishes. Therefore, it is crucial to understand everything about deep hole drilling to achieve the best results.

Even today, a drill press with a bushed fixture is a cost-effective way to make large numbers of holes, especially when they are not perpendicular to the workpiece surface. That’s because the bushing supports the drill, preventing deflection as it enters the workpiece, and the ability to feel the drill when it breaks through allows an operator to  manually reduce drilling pressure, hopefully avoiding breakage.

The Boring and Trepanning Association introduced the BTA drilling in the 1940s. This process involves inserting a special hollow tooling in the hole pushing pressurized coolant into the hole on the outside of the drilling tool during the operation. Then, it removes the chips through the hole within the drill.

Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

Another common problem, Fettig said, is inspecting intersecting holes, such as those seen in injection molds and a large number of automotive components.

Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

The gun drill comprises a straight flute cutting edge and a single internal coolant hole. As such, you can supply pressurized coolant through the inner holes while you remove chips through the single-flute gun drills within the shank. Hence, material removal occurs at the same time as the drilling.

Deep hole drilling techniques offer the tight tolerances and high accuracy required in machining aerospace components such as landing gear and turbine blades. Deep drilling processes produce holes exactly to tolerances to ensure optimal performance and low weight of these components.

Deep-hole drilling differs from regular drilling since cooling lubricant must be passed to the cutting edges in high volume and under high pressure, depending on the intended drilling technique and diameters.

Also, deep-hole drilling is pivotal in machining hydraulic cylinders for engineering vehicles and other industrial applications. These hydraulic cylinders are crucial for bulldozers, forklifts, excavators, etc. Deep hole drilling is ideal for making holes with the desired precision to ensure seamless motion.

Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.

Second, even specialty drills like the ones used in high-volume gundrilling are unable to cut effectively with only one side of the tool engaged, as this creates nonuniform drilling forces. The result is that workpiece material tends to extrude rather than shear, and a larger than normal burr is formed.

“When you’re drilling a pair of holes 0.080" in diameter that intersect 4" to 6" inside the part, there’s no real way to inspect them accurately,” Fettig said. “A CMM does a good job of projecting where the holes should meet, but there may have been some drill walk or deflection along the way. About the best you can do is use a borescope to inspect the hole intersection.”

We called them “submarines.” Tough, 17-4 PH stainless steel forgings shaped like miniature German U-boats, each requiring several ¼" (6.4mm) through-holes to be drilled at a 25° angle. Each sub was loaded into a jig that held it at the required angle, and a metal plate fitted with hardened steel bushings was then clamped to the top of the jig before the holes were produced with a drill press.

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Besides, the BTA tooling or single tube system comprises a hollow drill tube fastened to a threaded indexable cutter head, depending on the diameter and depth of the hole that requires drilling. The diameter range of the BTA tools ranges from 0.80 – 8.00 inches (20 – 200mm) and can attain depths of about 16.00 inches (400mm).

Annular cutters, originally designed for use with manual and magnetic drills, are increasingly used on CNC machine tools. Image courtesy Hougen Manufacturing.

“There are different ways to go about it,” Fagan said. “You could just as easily tip the part to the required angle with some sine blocks. What’s important is that you do the math and get the hole in the right spot. And you need a good feel for the drill as it breaks through, because it can catch and break. That’s why we like to use HSS, since it’s a lot more forgiving than carbide.”

“I wouldn’t do that on very tough materials or where high positional accuracy is required, but on depth-to-diameter ratios up to maybe 8:1, you can often just start drilling,” he said. “Another option is our DC170 drill, which, because of its flute design, offers support around the entire periphery of the drill point. It basically acts like a bearing surface, preventing any chatter or drill walk on exit.”

An annular cutter is another alternative to a jobber drill when making an angled hole. Also known as a Rotabroach, an annular cutter looks like a hollow endmill. It works by removing a ring of material the same size as the finished hole. The center portion inside of the ring remains intact and is ejected once the cutter is removed from the hole.

Deep hole drilling processes offer a better surface finish than conventional methods done with twist drills. Deep drilling involves the use of specialized tools and setups for high-pressure coolant flow and effective chip evacuation, ensuring the superior surface quality of the hole. Consequently, this excellent surface finish mitigates friction and wear, ensuring the durability and proper performance of moving parts.

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Later on, the technology advanced and extended to other applications, including making automotive parts like cam-shafts, crank-shafts, fuel rail, and fuel injector bodies, creating lo holes in injection molding, aerospace parts like landing gear, different parts in the oil and gas sector, general engineering components like hydraulic cylinders, etc.

Since deep hole drilling operations are automated, they allow precise and accurate depth-to-diameter ratios in drilled holes. Hence, deep drilling is ideal for attaining holes with dimensions that are exact to requirements in industries such as aerospace and defense.

Machinists often encounter common mistakes like misalignment or drill bit incompatibility, resulting in scrapped parts, poor surface finish, and excessive machine tool wear. Consequently, you may have to incur additional machining costs and experience delayed production.

One job that needed a delicate touch was a 316 stainless axle shaft for a cable drum. Fagan said WMW straddle-clamped the shaft directly to the Bridgeport table, tipped the head 45°, found the centerline and then spotfaced a small flat with an endmill at the required location. Next, a HSS jobber drill was applied to punch through to a cross-hole, into which a length of cable would later be fed.