Self-Advancing, Step-Tap Drill Bit

How It Works

Flight-certified, step-tap drill bits for threaded holes that stop at the required size step (click image to view larger version)

Drilling a test piece of previously damaged leading edge material with a prototype step-tap drill bit (click image to view larger version)

Crew training using the step-tap drill bit in the Pistol Grip Tool (PGT) drill driver on a Shuttle leading edge test article (click image to view larger version)

Co-inventor Charlie Camarda on STS 114 Return to Flight with step-tap drill bits (click image to view larger version)

The self-advancing, step-tap drill bit uses a cutting edge to simultaneously enlarge a hole and cut threads. The drill bit is stepped, enabling an increase in the diameter of the hole with each step. To prevent the threads from stripping caused by the required cutting forces between the drill bit and the work material, the thread pitch (number of threads per inch) and diameter increase for each step are adjusted for the material type and thickness. The diameters of the steps are in increments of 0.030–0.060 inch (0.8–1.5 mm). The tip typically has a diameter of 1/8–3/16 inch (3.2–4.8 mm).

The thickness of the work piece to be drilled and tapped determines the length of the pilot-drill section chosen, so that the pilot hole is completed before engagement of the first tap section. If the cutting-edge geometry of the drill bit is optimized for the material to be drilled, only a relatively small axial force (typically a few pounds) must be applied when drilling the pilot hole. Once the first tap section engages the pilot hole, no additional axial force is necessary because the thread engagement between the tap and the work piece provides the axial force, seamlessly advancing the drill bit. A stop-lip or shoulder at the shank end of the widest tap section prevents further passage of the drill bit through the hole.

Why It Is Better

Applied axial drilling forces for handheld drills can be quite large, often as much as 75 lb (about 330 N) when drilling holes up to approximately 1 inch (25 mm) in diameter. With non-self-advancing drills, an operator often must bear down with near full body weight to facilitate downward drilling. When using such non-self-advancing drills, vertical, overhead, and repetitive drilling is extremely fatiguing and limits worker productivity. In addition, applying a large axial force with a hand-held drill can be dangerous because the drill bit can grab the work piece, causing the work piece to spin or tear the drill and work piece from the operator’s hand.

Other drilling technologies include drill bits that make a single-sized hole, step drills that enable hole enlargement, taps that thread one size of a pre-existing hole, and a simple combination of a single-sized drill and a tap (for tapping after a hole has been drilled). Combining any of these technologies will not result in a functional, self-advancing, step-tap drill. Rather, the innovation developed at Johnson Space Center is a precise combination of step size, cutting angle, thread advance, and flute design, producing a drill bit that all but eliminates the need to apply external axial force while concurrently cutting and threading a hole. This significantly reduces operator fatigue, increases safety and efficiency, and enables drilling larger holes in thinner materials with a standard, commercially available drill.

Partners licensing and commercializing this innovative technology can manufacture and market the drill bits in several optimized designs and sizes for purchase individually or as a set. Marketable variations include:

A set designed to leave a tapped hole for threading standard-sized fasteners
A set to leave a hole through which to pass the shank of standard-sized fasteners
A set for large holes
Sets optimized for metal, plastic, and brittle ceramics

Patent

Johnson Space Center has received patent protection for this technology: U.S. Patent No. 7,357,606

The unique aspect of this drill bit is its self-advancing feature. Once the pilot drill section has started advancing (which requires only about 3-4 pounds axial force) the drill bit requires no additional axial loading, even when drilling holes larger than 1 inch, due to its self-advancing feature. While common drill bits require high axial loads and feature low torques, this drill bit requires minimal axial load and features higher torques.

This self-advancing feature forms the foundation for the patent granted to NASA: U.S. patent no. 7,357,606.

The drill bit simultaneously cuts threads as it enlarges the hole, enabling the drill bit to advance on its own (or “self-advance”), similar to a screw. This eliminates the need to apply large axial force to the hand-held drill.

Drilling the pilot section is the only time an axial force is required, which is typically less than 4 pounds, generally for less than 20 seconds.

This drill bit is best suited to drilling in thin and medium-thin (less than 3/8-inch) materials. It is especially useful when drilling high-cost, advanced composite materials, because the unique self-advancing aspect of the drill bit leaves a clean-cut hole with minimal breakout damage. The self-advancing feature eliminates de-lamination of composite layers, particularly back-side breakout, because the threading action places the drilled material in compression. Materials well-suited to this drill bit include:

Can the drill bit be used on harder materials such as titanium and stainless steel?

The drill bit has not been tested on these types of materials, and therefore cannot be recommended for hardened materials at this time.

The drill bit is designed for lower speeds. Due to the self-advancement being dictated by the thread pitch, fast RPMs with coarse thread pitches advance the cutting edge at too great of rate. For the NASA-developed drill bits with 18 threads per inch (TPI), about 200 RPM is about the maximum useful rate when drilling advanced composites. For use during a space walk for repair of the space shuttle leading edge, the maximum speed of the space-rated drill driver is 60 RPM, thus that is the maximum planned NASA speed.

With an 18 TPI bit, testers were able to successfully control speed by partially depressing the power trigger on a commercially available drill set at low range.

The drill bit can be made of typical materials such as oil hardened tool steel, cobalt steel, high speed steel, or carbide. Coatings such as diamond, titanium nitride, titanium aluminum nitride, titanium carbon nitride, and others can be added to extend the life of the bit and provide heat resistance and lubrication.

The diameter range for the step-tap drill bit is a user-specified criterion. For a step enlargement of 0.030- to 0.060-inch per step with a step length of 0.3-inch, the diameter range then determines the drill bit’s overall length. The inventors chose to divide their required diameter range between two drill bits, one from pilot hole to 0.5-inch, the second from 0.5 to 1.0625-inch. The 1.0625-inch step-tap created a clear hole 1 inch in diameter. Each of these drill bits were less than 6 inches long, including the drive shank.

There are two basic designs for the choice of step-tap hole diameters and choice of TPI. One is to leave a threaded hole in which to screw a threaded fastener with TPI that matches standard screws and bolts. This requires a different TPI pitch on the steps that match standard-sized fasteners (e.g., 1/4–20, 5/16–18, 3/8–16, 1/2–13, etc.). The other is to create a clearance hole, even though threaded, sufficient to pass the shank of standard fasteners. This type of drill bit can have a constant TPI pitch chosen to optimize the self-advancement of this drill bit.

Can very small versions of the drill be made, for example, with a maximum step size of 0.3 inch or smaller?

What is the overall length of the drill bit? They look quite large in some of your photos.

The length of the drill bit is determined by the thread pitch, step enlargement, and desired maximum step size. If very large diameters are needed and the drill bit operates in tight, enclosed areas, the diameter range can be split across two drill bits, allowing each bit to be shorter in length. The space-flight rated drill bits are 6 inches long. One drill features bits from a diameter of 0.25 to 0.5 inch, the second from 0.5 to 1.0625 inch.

No, the self-advancing aspect of the drill bit is achieved by concurrently cutting threads as the hole is enlarged. Eliminating the threading would eliminate the self-advancing feature.

For applications requiring only a through hole, the through hole will have the threaded profile. This can be an advantage when drilling composite materials for repair. When a damaged area is drilled out and a patch is installed over the drilled out area, the threaded profile can act as a high-surface-area zone to which the composited repair compound can bond.

How do you manage the drilling process to avoid running up against the stop lip with excessive force and possibly damaging the work material?

Because of the slower speed of operation, a stop lip is not typically necessary. NASA initially included stop lips on prototypes but found them unnecessary and removed them for the final production version.

Yes, because the drill bit leaves a threaded hole, you must reverse it out through the last step to avoid damaging the threads.

Clogging is not an issue, as step-type drill bits are typically used with thinner materials. Each new step starts with a fresh drill bit section. The chips tend to be pushed forward and accumulate in the flutes of the previous steps, rather than clogging the bit. As noted above, coatings can be applied to provide heat resistance, if needed.

Qualified, interested parties can execute a no-cost Prototype License which allows for limited manufacturing for prototype and testing purposes. Please contact for more information.

Self-Advancing, Step-Tap Drill Bit

For reduced fatigue, improved safety, and greater efficiency

Patent

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