Precision drill bits

[0002] The present invention relates to drill bits, such as bits for use in high-speed machining and/or with portable hand held drills.

[0003] A wide variety of twist drills have been developed over the years for utilization in various machining operations. The typical twist drill is characterized by a drill bit having one or more spiral groves formed in the shank of the bit. These grooves, or “flutes”, are useful to force lubricant to the surface being machined, which is particularly beneficial for metal machining. In addition, the flutes provide a means for moving chips from the path of the drilling operation.

[0004] One important object in the design of the drill bits is to eliminate the bunching of chips within the newly drilled hole. In many drilling operations, the bulk of the heat generated in the operation is contained within the chips. If the chips are removed, the heat generated at the point of machining is dissipated and the drill bit and part being machined stay relatively cool. On the other hand, if the chips bunch within the drilled hole, the frictional heat tends to build up, which can be damaging to the machined part, can decrease the life of the drill bit, and can even make the drilling operation more difficult.

[0005] Efficient chip removal also reduces the thrust force required to advance the drill bit through the machined material. Where the chips bunch within the drilled hole, a greater thrust force is required to continually push the bit through the material. When the drill bit finally breaks through the back side of the material, the break-through thrust tends to pull the drill bit into the workpiece, which can damage the workpiece, as well as the drill and bit itself.

[0006] It is known that for optimum chip ejection, the shorter the path that the chip must travel, the more efficient the ejection and the greater ability to prevent chip compaction. Thus, some drill bits provide spherical flutes having a lower angle helix(when measured relative to the longitudinal axis of the bit) to facilitate the chip removal. However, the lower angle helix has the disadvantage of having a lower penetration rate and lower energy efficacy in completing the drilling operation. Thus, at the other end of the scale, a higher angle helix cuts and penetrates the material much more quickly and efficiently at a given rotational speed for the drill bit.

[0007] Another important aspect of the typical rotary drill bit is the ability to maintain the bit at a desired location. A common phenomenon with a standard drill bit is that the bit will “skate” across the surface to be machined. Consequently, there is a need for a “self centering” feature on the drill bit that allows the drill bit to be centered at the desired location more quickly and efficiently. The ability to rapidly center the drill bit is very beneficial where a number of drilling operations are to be performed in a particular workpiece.

[0008] The present invention seeks to combine the chip removal and self-centering features in a single drill bit. Thus, in one embodiment of the invention, a drill bit is provided that comprises an elongated body having a tip at one end and an engagement shank at the opposite end. A first portion of the body adjacent the tip has a first diameter and a second portion between the first portion and the engagement shank has a second diameter that is greater than the first diameter. In other words, the bit includes a reduced diameter tip that improves the centering capability of the bit.

[0009] In certain embodiments, the reduced diameter tip can have a diameter about ⅔ of the second diameter. The body can define a step between the first portion and the second portion, with the step in certain embodiments defining an included angle of less than 180° relative to the longitudinal axis of the elongated body. In specific embodiments, the included angle is about 135°. The stepped diameter tip not only provides a self-centering capability, it also improves the starting proficiency for a newly drilled hole in a workpiece.

[0010] In a further feature of the inventive drill bit, the elongated body defines flutes on at least the second portion. Most preferably, the flutes continue across the first portion to the tip of the bit. In an important aspect of the invention, the flutes are formed at a helix angle relative to the longitudinal axis of the elongated body, where the helix angle decreases from the tip to the engagement shank. The flutes of the second portion can follow the second diameter, which can be substantially constant along the length of the second portion. The flutes can combine with the stepped diameter tip for improved drilling efficiency.

[0011] In a preferred embodiment, the helix angle starts at about 38° adjacent the tip and decreases to a helix angle at the engagement shank of about 15°. In accordance with a feature of the invention, the helix angle can decrease in three segments along the length of the bit. This in a first segment, the helix angle decreases from about 38° adjacent the tip to about 35° at a first distance from the tip. This first distance can be about 0.5 inches for a typical ⅜ inch bit.

[0012] In a second segment, the helix angle can decrease from about 35° at the first distance to about 17° at a second distance from the tip. This second distance can be about 2.0 inches. In the final segment, the helix angle decreases from about 17° at the second distance to about 15° at a third distance from the tip. In a preferred embodiment, the third distance can be at least 3.5 inches.

[0013] Preferably, the helix angle decreases substantially linearly over each of the three segments. The slope of the linear change can vary between segments. For instance, the slope of the helix angle change in the second segment (i.e., between the first and second distances) can be much steeper than the slope of the angle change in the other segments. The slope of the change in the second segment is preferably steep enough to provide for rapid chip removal, but not so steep as to increase the risk of chip bunching along the length of the bit. Likewise, the slope in the initial segment is preferably shallow enough to provide smooth movement of the chip away from the drilling site. Preferably, the difference in slopes between the first and second segments is not dramatic enough to risk chip bunching at the first distance.

[0014] One benefit of the present invention is that it combines a self-centering feature with a variable helix bit. Another benefit is that the variable helix is calibrated to mutually optimize penetration rate and chip removal. Other benefits and objects of the invention will become apparent upon consideration of the following written description taken with the accompanying figures.

[0015] FIG. 1 is a side elevational view of a precision drill bit in accordance with one embodiment of the present invention.

[0016] FIG. 2 is an enlarged side view of the end of the drill bit contained within the circle designated 2 in FIG. 1.

[0017] FIG. 3 is a graph of helix angle versus distance for the drill bit of FIGS. 1 and 2 in accordance with one embodiment of the present invention.

[0018] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains.

[0019] A precision drill bit 10 includes an elongated body 12 defining an axis A. The body defines a penetrating tip 14 at one end and an engagement shank 16 at the opposite end. The engagement shank can be configured to be engaged by a conventional mounting chuck. In accordance with the typical twist drill designed, the drill bit 10 includes a number of spiral flutes 22 extending along a portion of the length of the body 12. In the illustrated embodiments, a single flute is depicted; however, it is understood that multiple flutes may be provided.

[0020] In accordance with one aspect of the invention, the body 12 of the drill bit 10 includes a first portion 18 at the tip 14 of the bit, and a second portion 20 disposed between the engagement shank 16 and the first portion 18. Shown in more detail in the enlarged view of FIG. 2, the first portion 18 has a reduced diameter of Φ₁relative to the diameter Φ₂of the second portion 20. Thus, the first portion 18 is defined by a step 24 from the second portion 20, and a land 26 extending a distance h from the step 24.

[0021] As shown in FIG. 2, the first portion 18 tapers to the pointed penetration tip 14 at an included angle γ. In a most preferred embodiment this angle γ is about 135°. It has been found that a tip 14 tapered at this dimension optimizes the centering capability of the tip with the ability to easily penetrate a workpiece during a drilling operation. In addition, it has been found that a first diameter Φ₁that is about ⅔ of the larger second diameter Φ₂also helps the drill bit 10 to penetrate the workpiece more quickly and easily at the start of the hole. In some respects, the first portion 18 operates as a smaller pilot drill bit for enlargement by the second portion of the bit.

[0022] Also shown in FIG. 2, the step portion 24 defines an included angle β relative to the longitudinal axis A of the body 12. It has been found that a step angle β that is less than 180° also increases the penetration rate for the drill bits over prior conventional helical drill bits. It has further been discovered that an optimum value for the step angle β is about 135°. Since the first portion 18 serves to pilot and center the drill bit 10 during use, it preferable that the height h of the first portion 18 be as small as possible, yet still long enough to provide a centering engagement of the workpiece. In a specific embodiment, the height h of the first portion 18 is about 0.062 inches.

[0023] In another important aspect of the invention, a variable helix angle is combined with the reduced diameter centering tip described above. Thus, in accordance with this feature of the invention, the helix angle α decreases from the tip 14 to the engagement shank 16. Most preferably, the flutes 22 commence at the tip 14 and extend along both the first portion 18 and the second portion 20. However, it can be appreciated that in certain applications the cutting flute 22 may commence with the second portion 20 only. At any rate, the variable helix feature of the present invention combines the high helix angle at the tip of the bit to provide for rapid and efficient penetration of the workpiece, with a lower helix angle along the length of the bit to provide a shorter path for chip ejection. These features combine to allow the drill bit 10 to progress more quickly through the workpiece while generating less heat build up and requiring less break-through thrust to penetrate the workpiece.

[0024] In accordance with one feature of the invention, the helix angle α of the flute 22 changes in discrete segments along the length of the bit. In the preferred embodiment, the helix changes over three such segments. In a first segment beginning at the tip 14 and extending a distance D₁from the tip, the helix angle has a value α₁. In a second segment extending beyond the first distance D₁to its second distance D₂, the helix angle has a value α₂. In the final segment terminating at the distance D₃with the end of the flutes 22, the helix angle has a value of α₃. In accordance one feature of the present invention, each of the helix angles can adhere to the following relationship α₂>α₁>α₃.

[0025] In accordance with the preferred embodiment, the flutes 22 start at a helix angle α₁of about 38°. This helix angle decreases over the distance D₁to a helix angle α₂of 35°. The length of the body 12 of the drill bit 10 between the distance D₁and D₂can be referred to as an intermediate transition zone. In this zone, the helix angle decreases from 35° to 17°. In the final length of the drill bit between distance D₂and D₃, which can be referred to as the flute run-out zone, the helix angle decreases further from 17° to 15°. It has been found this controlled variation or decrease in helix angle optimizes penetration speed with chip removal features of the drill bit.

[0026] In a most preferred embodiment, the distance D₁over which the helix angle transitions from about 38° to about 35°, is about 0.5 inches. The second distance D₂, as measured from the tip 14, is about 2.0 inches. The third distance D₃depends upon the length of the flutes 22. Nominally, this distance is at least 3.5 inches from the tip 14, and can be preferably 3.67 inches.

[0027] The change in helix angle is reflected in the graph in FIG. 3. In this graph it can be seen that the helix angle decreases from an initial value of about 38° to value of 35° at a distance D₁, from 35° to 17° from D₁to D₂, and 17° to 15° from distance D₂to D₃. Preferably, the change in helix angle in each segment is linear, meaning that over the length of the bit between the tip and distance D₁the helix angle changes substantially linearly from 38° to 35°, and likewise over the remaining distance segments.

[0028] In accordance with one aspect of the invention, the change in helix angle from the tip 14 to the distance D₁is at a first slope, the angle change from distance D₁to distance D₂is at a second slope, and over the final segment from distance D₂to D₃at a third slope. Preferably, each of the slopes is different, and most preferably, the second slope over the intermediate segment is greater than the other two slopes. It is in this region that the bulk of the chip transfer from the drilling site occurs. The first slope is less than the second slope so that the change in helix angle is more gradual from the tip 14 than over the intermediate segment of the bit length. A more gradual change in helix angle is preferable so that the bit has the opportunity to establish a bite into the workpiece before the chip is conveyed up the bit. A more severe slope can compromise the initial cutting action of the flute and risk bunching of the newly formed chip immediately at the drilling site.

[0029] On the other hand, the slope of the last segment is much shallower than the slope in either of the prior two segments. Thus, in the last segment from distance D₂to D₃the helix angle changes gradually so that the continuously generated chips can continue generally unbroken along the remaining length of the bit, even as the drill bit penetrates further into the workpiece.

[0030] In a specific embodiment, the slope in the first segment can be about 6° per inch, the second segment about 12°/inch and the last segment about 1° every inch. Preferably, the second segment slope is approximately double the helix angle change rate at the tip of the bit, and is significantly greater, about ten times or more, than the last slope.

[0031] The illustrated embodiments shown in FIGS. 1 and 2 correspond to a representative {fraction (3/8 )} inch drill bit. It is understood that certain of the specific dimensions would be scaled up or down depending upon the size of the particular drill bit. However, it can also be appreciated that the relationships between dimensions can remain unchanged, regardless of the size of the bit. Preferably, the change of flute helix angle from about 38° to 15° would adhere regardless of the cutting diameter of the drill bit. Moreover, the distance D₁-D₃at which the helix values change would build on the same each other. For instance, in the illustrated embodiment, the ratio of the distance D₁to the total length of the flute 22—i.e. distance D₃—is about 0.14. Similarly, the ratio of distance D₂to the flute length distance D₃is about 0.55. In a preferred application of the invention, if the drill bit is lengthened, and the flute length correspondingly increased, the distances D₁and D₂would maintain the same dimensional relationship to the flute length distance D₃.

[0032] A precision drill bit configured with in accordance with the present invention has been found to provide significant advantages over currently known helical bits. For instance, the combination of the variable helix with the steeped point configuration has led to an increased penetration rate of the precision drill bit over conventional bits. In one comparison, a standard split point bit having a 30° helix penetrated a particular test piece at a rate of 0.493 inches per minute. A known bit having a 38° helix penetrated at a rate of 0.589 inches per minute. However, a drill bit constructed in accordance with the present invention exhibited a penetration rate of 0.623 inches per minute. This increase in penetration rate represents a significant improvement in time savings in performing a number of drilling operations, as well as increases in drill bit life.

[0033] In these same test, it was found that the drill bit in accordance with the present invention was more prone to generate continues chips that can be readily and easily removed as the chip advances up the variable helix flute. In contrast, the prior drill bits intended to generate small flake-like chips which remain in the drill hole and lead to increased heat build up and increase thrust force requirements. All of these improvements are more significant when the drill is a portable hand-held drill, since most of these improvements help reduce the amount of time needed to drill through a workpiece. This reduced drilling time increases the number of drilling operations that can be preformed before the portable drill must be recharged.

[0034] While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.