Getting to Know the Drill on Drilling

Considerations include hole depth, material, accuracy, application, and tapping

Drilling is a critical metalworking process, especially to prepare a hole for tapping and threading. Reviewing some tips before selecting a drilling tool can save time and mitigate challenges.

One of the most important considerations is whether to use a multipurpose or application-specific drill. The answer depends on several factors, including:

A multipurpose drill for shops is designed to work on a wide range of materials, especially well suited for common materials ranging from carbon steels, stainless steels, aluminum, copper alloys, and, to some extent, certain nickel alloys.

However, due to their specialized design, application-specific drills may be a better choice for some more challenging issues. These include the use of materials such as titanium and nickel alloys, Inconel, and high-strength and hardened steels, as well as hard castings.

Multipurpose drills are engineered with versatility in mind, making them ideal for job shops with a higher mix and lower volume of parts. Look for drills that are constructed using specialized carbide grades that make the tools harder than conventional carbide grains for drilling, yet still retain the ability to withstand shock and chipping. The tool coating also plays a role, and it should have high hardness that reduces friction at elevated temperatures.

The latest multipurpose tools feature concave cutting edges, offering higher chip-shearing ability and a tightly controlled edge preparation process that provides consistent performance and long tool life. Tools are designed to help stabilize the drill in the cut, allowing rounder, more accurate holes.

For maximum chip evacuation, look for a drill web construction with a wide-open flute form. Self-centering tools will eliminate pre-spot drilling applications, and coolant-through capability is optimal.

Application-specific drilling tools are designed to provide options for the most challenging materials in order to maximize hole quality in longer production runs. These higher-performance tools with specialized geometries are the go-to choice for specific material groups or applications where cycle time is critical.

For example, based on the application and material, these tools can have a very long life and are ideally suited for a high volume and lower mix of parts. Application-specific drills usually have a higher price than multipurpose ones, however their value in gained productivity, especially in challenging applications and hard materials, can make them a better cost-performance alternative.

Specialized drills feature a double-margin design for improving hole quality, and guiding and adding stability during breakout, as well as a reverse-web taper and coolant hole channels that prevent chip packing and premature tool failure. Drills are designed to operate without peck cycles to reduce cycle times. For deep hole drilling, specially formulated, premium multilayer coatings such as TiALN are post polished.

Application-specific drills are designed for challenging ferrous materials such as steel, alloyed steel, martensitic stainless steel and nickel alloys common in aerospace parts manufacturing.

High-penetration-rate, solid-carbide tools that can drill and chamfer in one operation are another application-specific example. These drills save time and provide a more accurate hole-to-chamfer location, resulting in the most optimal hole preparation for tapping or thread milling. Tools feature a double-margin design on the minor diameter for the roundest threaded-hole size, and the web construction is adjusted for each diameter for maximum chip-evacuation efficiency.

Micro drills are an application-specific alternative that produce high-performance results in demanding materials at high feed rates. Solid-carbide micro drills achieve ideal results when machining alloyed steel, stainless steel, cast iron, and nickel. These drills are self centering, while operating at top-end cutting speeds and the highest feed rates to ensure the best hole quality. A unique flute and point geometry combination ensures outstanding surface quality and excellent tool life.

Deep drilling, which includes holes that are more than five times the depth of their diameter, presents its own challenges. When the drill diameter falls below 3.0 mm, the task is even tougher. Drill-flute and point-geometry options are limited in micro-drilling applications due to the size of the tool. For example, deep hole drills are most effective when the drill-web thickness is reduced, allowing for increased flute space for chip evacuation.

Micro drills, particularly carbide micro units, require heavier web thickness percentages than larger-diameter drills due to the fragility of the small tool.

The stronger web or core diameter is more robust, but the chip space is restricted.

High-performance, carbide micro drills address chip evacuation with the introduction of internal-coolant holes and parallel-web construction. High-pressure coolant is introduced at the cutting zone, which helps eject chips from the flutes.

The other benefit of using coolant-through drills is the ability to reduce and even eliminate peck cycles. The efficient evacuation of chips reduces the need for chip-clearing drill retractions. Point-geometry and web-thinning options are also limited due to the size of the tooling. Faceted-point grinds are the most common because they provide added cutting edge stability. Advanced coatings are necessary with high-penetration-rate drills if they are to be used in high-temperature alloys, stainless steel, and other materials that create high levels of heat during the drilling process. Coatings, such as special TiALN, aid in wear and heat resistance, and they are effective with carbide micro drills.

Emuge's new PunchDrill is designed for the fast, high-volume machining of cast aluminum alloys with at least 7% Si content and magnesium alloys—a growing material range due to its lightweight properties. PunchDrill doubles the feed rate compared to standard drills without increasing the axial force or spindle speed.

The drill reduces machining forces and optimizes chip breaking, producing cycle time savings of 50% or more, resulting in shorter machining times, fewer tool changes, and high metal-removal rates, in addition to higher productivity and reduced power consumption.

One of the most common mistakes a machinist makes when tapping a hole is using the wrong size drill. This isn't intentional, of course; it's just that most machinists are using outdated charts—designed in the 1950s when high-speed drills were the norm. To reduce the risk of thread failure, cautious design engineers often specified high percentages of thread height in tapped holes. The percentage of thread values that older tap drill charts provide is higher than needed in most cases.

Another reason why some tap charts are outdated is that most drills used for tapped holes were high-speed steel or cobalt when the charts were created. Many tap drill holes are now created with high-performance carbide drills, which generate more accurate holes than high-speed steel drills. High-speed drills typically cut larger holes than carbide drills.

Making the correct tap drill size choice will affect the machining operation. Tool manufacturers suggest using percentage-of-thread values between 60% and 70% for most pre-drilling applications. By increasing the pre-drilled hole diameter, the machinist can expect to increase the life of the tap by reducing the amount of force required to form the thread.

It's important to note that thread strength is not directly proportional to percent of thread. According to some sources, 100% thread specification is only 5% stronger than a 75% thread specification but requires three times the torque to produce. Tap life is greatly reduced in an effort to theoretically increase thread strength.

As an example, a 7/16-14 unified coarse (UNC) cut thread is usually denoted as a letter "U" diameter drill on most older tap-drill charts that equates to a 75% value for percentage of thread. In fact, a 9.4-mm drill might be a better choice. The slightly larger drill diameter still provides a 73% of thread value, which is more than acceptable. But that 2% reduction in thread percentage will reduce torque on the cutting tool and increase tap tool life.

As a general rule, the tougher the material, the less the percentage of thread is required to meet design requirements. In some harder materials such as nickel alloys, stainless steel, and hardened steels, it is possible to tap with as little as a 50% of thread value.

Roll-form threads require tap drill sizes that are larger than those specified for cut taps. A 7/16-14 UNC roll form thread will require a 10.25-mm minor diameter. Material is being displaced and formed instead of cut, requiring the pre-drilled hole to maintain the correct amount of material to be formed into the taps thread profile.

Choosing the correct tap drill size for an internal threading application is not as simple as looking at a possibly outdated tap drill size chart. Understanding how the values can affect the manufacturing process is an important consideration.

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