Thread cutter with spiral groove and continuously increasing helix angle

Abstract

Thread cutter (10) with a helical groove, comprising a body (12) with an axial front end (14) and an axial rear end (16), wherein the body (12) has a cylindrical shank section (18) adjacent to the axial rear end (16), a fluted threaded section (20) adjacent to the axial front end (14), a fluted unthreaded section (19) between the cylindrical shank section (18) and the fluted threaded section (20), and a central longitudinal axis (17), wherein the fluted threaded section (20) and the fluted unthreaded section (19) of the thread cutter (10) with helical groove (22) further comprise a plurality of helical grooves (22),which are formed with a helix angle HA with respect to the central longitudinal axis (17) of the tap (10) and the helix angle HA increases continuously and at a constant rate in a reverse direction over a predetermined distance from the axial front end (14) to the axial rear end (16).

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a thread cutter and, more particularly, to a spiral groove thread cutter having a grooved thread portion with a continuously increasing groove spiral or a continuously decreasing groove thread pitch. BACKGROUND OF THE INVENTION

[0002] It is known in the art to form grooves with a low lead angle (i.e., high thread pitch) or a high lead angle (i.e., low thread pitch). A "low lead angle" (or low spiral turn groove) is a groove that spirals around a cylinder at an angle of no more than 45 degrees. A "super" slow lead angle would be a groove that spirals around a cylinder at an angle of no more than 15 degrees. A "high lead angle" (or high spiral turn groove) is a groove that spirals around a cylinder at an angle of more than 45 degrees, while a "super" high spiral turn groove winds around a cylinder at an angle of at least 65 degrees. Low lead angle grooves are generally used for rough cutting, while high lead angle grooves are used for finish cutting.

[0003] A straight flute or a slow-pitch flute is often used in shallow blind hole drilling. These flutes have better mechanical strength than deep-pitch flutes and can typically produce tightly twisted chips. However, they cannot be used for deep-hole drilling due to their limited chip lifting force. A flute with a high helix angle (or low thread pitch) is necessary for chip evacuation but is often prone to nesting or chip coiling.

[0004] Approximately 60% of all thread cutting operations occur in blind hole applications, and most of these holes are cut with thread cutters with helical flutes. Long chip materials require a fast helical flute pitch angle (>30°), which also easily leads to chip nesting or chip wrapping. This is one of the most difficult problems in the thread cutting sector. None of the thread cutters currently available on the market truly solve this problem.

[0005] DE 11 2007 003 489 T5 discloses a spiral tap having a spiral groove with a front section having a constant thread pitch and an adjoining rear section having a gradually increasing thread pitch.

[0006] WO 2009 / 074 343 A1 discloses another tap and a method for its production.

[0007] US 2003 / 0118411 A1 discloses an end mill which has spiral grooves with varying pitch angles.

[0008] The present invention has been developed in view of the above SUMMARY OF THE INVENTION

[0009] The problem of nesting or chip wrapping is solved by providing a thread cutter having both a slow spiral flute angle and a fast spiral flute pitch angle. In particular, the present invention provides a variable spiral flute geometry thread cutter having a slow-to-fast flute pitch change from the axial leading end of the thread cutter to the axial trailing end, which is opposite to prior art fast-to-slow designs. The rate of the slow-to-fast flute pitch change is constant. In other words, the thread pitch, which is inversely proportional to the tangent of the pitch angle, decreases continuously from the axial leading end to the axial trailing end of the thread cutter. Tests indicate that nesting is successfully eliminated.

[0010] In one aspect of the invention, there is a spiral groove thread cutter comprising a body having an axial front end and an axial rear end, the body having a cylindrical shank portion adjacent the axial rear end, a grooved threaded portion adjacent the axial front end, a grooved unthreaded portion between the cylindrical shank portion and the grooved threaded portion, and a central longitudinal axis, wherein the grooved threaded portion and the grooved unthreaded portion of the spiral groove thread cutter further comprise a plurality of spiral grooves formed at a lead angle HA with respect to the central longitudinal axis of the thread cutter,and wherein the thread pitch of the plurality of spiral grooves of the grooved threaded portion continuously decreases in size in a rearward direction for a predetermined distance from the axial front end to the axial rear end.

[0011] In another aspect of the invention there is a helical flute tap comprising a body having an axial front end and an axial rear end, the body having a cylindrical shank portion adjacent the axial rear end, a fluted threaded portion adjacent the axial front end, a fluted unthreaded portion between the cylindrical shank portion and the fluted threaded portion, and a central longitudinal axis, the helical flute tap further comprising a plurality of helical flutes formed at a lead angle HA with respect to the central longitudinal axis of the tap, the lead angle HA increasing continuously from the axial front end to the fluted unthreaded portion at a rate between 0.5 degrees / mm and 4.0 degrees / mm.

[0012] In yet another aspect of the invention, a method for manufacturing a spiral groove thread cutter comprises the following steps: Grinding a cylindrical shank to form a cylindrical shank portion at an axial rear end of the tap and a thread body portion having an outer diameter at an axial front end of the tap; Grinding a spiral groove in the thread body portion to form a cutting edge; Grinding the thread body portion with a taper to form a chamfered grooved portion and Grinding the thread body portion to form a cutting thread, thereby forming the spiral groove with a lead angle HA with respect to the central longitudinal axis of the tap, such that the thread pitch of the spiral groove decreases starting at the axial front end of the tap to the axial rear end over a predetermined distance.

[0013] These and other aspects of the present invention will be better understood after a review of this specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Although various embodiments of the invention are illustrated, the illustrated embodiments should not be construed as limiting the claims. It is believed that various modifications and changes may be made without departing from the scope of this invention. Fig. 1 is a side elevational view of a spiral flute thread cutter according to one aspect of the invention; Fig. Figure 2 is an enlarged partial cross-sectional view of the threaded body portion of the axial front end of the spiral groove thread cutter of Fig. 1 and Fig. 3 is an enlarged partial cross-sectional view of the constant diameter grooved portion of the spiral groove thread cutter of Fig. 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to Fig. 1, a thread cutter 10 according to one embodiment of the invention is shown. The thread cutter 10 includes an elongated body 12 having an axial forward end 14, an axial rearward end 16, and a central longitudinal axis 17. The thread cutter 10 includes a cylindrical shank portion (clamp 18) adjacent the axial rearward end 16, a grooved, unthreaded portion (clamp 19), and a grooved, threaded portion (clamp 20) adjacent the axial forward end 14. The shank portion 18 may have a relatively larger diameter than the grooved, unthreaded portion 19 and the grooved, threaded portion 20. The shank portion 18 may optionally include a square drive (not shown) for securing the thread cutter 10 from rotation in a tool holder (not shown).The grooved portion without thread 19 and the grooved thread portion 20 of the thread cutter 10 include a plurality of spiral, helical grooves 22 formed with a lead angle HA with respect to the central longitudinal axis 17 of the thread cutter 10.

[0016] It is understood that the thread pitch is a function of the lead angle HA and the diameter D of the thread cutter 10 according to the following equation: Thread pitch=π*D / tan(pitch angle)

[0017] As shown in Equation 1, the thread pitch is proportional to the diameter D of the tap 10 and inversely proportional to the tangent of the lead angle HA. In other words, the thread pitch changes when the diameter D and / or the lead angle HA change.

[0018] The thread cutter 10 is operatively connected at its cylindrical shank portion 18 by a tool holder to a machining tool (not shown) or the like. The grooved threaded portion 20 has a chamfered grooved portion 24 that begins at the axial front end 14 and extends therefrom in an axial rearward direction. The chamfered portion 24 is adjacent to a constant diameter portion (or terminal portion) 26 that extends in the axial rearward direction and terminates at the junction with the non-threaded grooved portion 19.

[0019] Regarding specific thread cutting applications, helical flute taps with a right-hand helix pull chips out of the hole (right-hand thread) and are effective in blind holes. Left-hand helical flute taps guide the chip in front of the tap (right-hand thread) and are effective in through holes. Straight flute taps are typically effective in materials like cast iron that produce a short chip. In straight flute taps, the flutes transport the chips out of the hole by capturing them.

[0020] Now on Fig. 2 and Fig. 3, the construction of the thread cutter 10 and details of the tapered groove portion 24 will now be described. The tapered grooved portion 24 has a length 44 and is formed at an angle 46 with respect to the central longitudinal axis 17. The constant diameter grooved portion (or terminal portion) 26 includes a series of V-shaped cutting threads, generally shown at 28. Each cutting thread 28 has a cutting edge 29 and is defined by a crest 30 at the outer diameter 32 and a root 34 at the minor diameter 36. A pitch diameter 38, defined as the diameter between the outer and minor diameters 32 and 36, respectively, effectively defines the position of a left-hand thread flank 40a and a right-hand thread flank 40b with respect to the longitudinal axis 17 of the thread cutter 10.

[0021] Although Fig. 3 shows a thread 28 with an included angle 42 between the thread flanks 40a and 40b of approximately 60 degrees, it should be understood that other angles are possible. For example, the included angle 42 may range from approximately 30 degrees to approximately 90 degrees. After tapping, the thread flanks 40a and 40b and the thread crown 30 contact the newly created internal thread. Generally, the minor diameter 36 of the tap 10 is intentionally designed to be larger than the minor diameter of the hole (not shown) and therefore does not contact the internal thread.

[0022] The thread cutter 10 creates an internal thread form through the sequence of cutting edges 29 formed by grooves in the grooved threaded portion of the thread cutter 10. Material is removed from the wall of the bore until the final thread form is obtained with the first complete thread 28 on the constant diameter grooved portion (or termination portion) 26 of the thread cutter 10. Beyond the first complete thread 28 of the thread cutter 10, threads are used to correctly guide the thread cutter 10 and to enable resharpening of the thread cutter 10 after use.

[0023] Back on Fig.1, one aspect of the invention is that the lead angle HA of the helical grooves 22 of the grooved threaded portion 20 continuously increases in an axial rearward direction, starting at the axial front end 14 to the axial rear end 16 of the thread cutter 10, until the lead angle HA reaches a maximum predetermined value. In one example, the maximum predetermined value may be about 60 degrees. Specifically, the lead angle HA of the helical grooves 22 of the grooved threaded portion 20 at the axial front end 14 (i.e., the initial lead angle HA) is relatively smaller compared to the lead angle HA of the helical grooves 22 of the grooved threaded portion 20 near the grooved non-threaded portion 19 (i.e., the final lead angle HA).In other words, the lead angle HA of the spiral grooves 22 of the grooved thread portion 20 gradually and continuously increases in an axial rearward direction, starting at the axial front end 14 of the thread cutter 10, until the lead angle HA reaches a maximum predetermined value.

[0024] As previously mentioned, the thread pitch of the helical grooves 22 is inversely proportional to the tangent of the lead angle HA. Thus, the thread pitch of the helical grooves 22 of the grooved threaded portion 20, starting at the axial front end 14, is relatively larger than the thread pitch of the helical groove 22 near the grooved non-threaded portion 19. Note that although the diameter of the tap 10 decreases in the grooved non-threaded portion 19, the thread pitch in the grooved non-threaded portion 20 continuously decreases. In other words, the thread pitch of the helical grooves 22 of the grooved threaded portion 20 and the grooved non-threaded portion 19 continuously decreases over a predetermined distance in an axial rearward direction, starting at the axial front end 14 to the axial rear end 16 of the tap 10, until the thread pitch reaches a minimum, predetermined value.

[0025] It should be noted that the thread pitch of the spiral grooves 22 is also proportional to the diameter D of the thread cutter 10. It should also be noted that the grooved, unthreaded portion 19 has a relatively smaller diameter than the grooved, threaded portion 20. However, the thread pitch of the spiral grooves 22 continues to decrease in the grooved, unthreaded portion 19 of the thread cutter 10, although the lead angle HA of the spiral grooves 22 may decrease in the grooved, unthreaded portion 19.

[0026] In one example of the invention, the initial pitch angle HA of the helical grooves 22 in the grooved threaded portion 20 may be between about 0 degrees and about 40 degrees, and the pitch angle HA near the grooved, unthreaded portion 19 may be between about 20 degrees and about 60 degrees. In one embodiment, the pitch angle HA of the helical grooves 22 of the grooved threaded portion 20 continuously increases by between about 0.5 degrees / mm and about 4.0 degrees / mm in the axial rearward direction of the thread cutter 10. In other words, the thread pitch, which is inversely proportional to the tangent of the continuously increasing pitch angle HA, continuously decreases in a rearward direction, beginning at the axial front end 14 of the thread cutter 10.

[0027] In another example of the invention, the initial pitch angle HA of the helical grooves 22 in the grooved thread portion 20 may be approximately 30 degrees, and the final pitch angle HA may be approximately 50 degrees. In this embodiment, the pitch angle HA continuously increases by approximately 1.25 degrees / mm in a rearward direction, beginning at the axial forward end 14 of the thread cutter 10. It should be understood that the rate of change of the pitch angle HA of the helical grooves 22 in the grooved thread portion 20 need not be constant, and that the invention may be practiced with any rate of increase of the pitch angle HA.

[0028] Tests have shown that the continuously increasing helix angle HA of the fluted threaded portion 20 of the threading cutter 10 of the invention (i.e., continuously decreasing thread pitch in the fluted threaded portion 20 and in the fluted non-threaded portion 19) provides superior chip evacuation performance (i.e., less nesting or chip wrapping) and a reduction in cutting forces compared to prior art threading cutter designs. This reduction in cutting forces provides a significant increase in tool life and a reduction in nesting or chip wrapping compared to prior art threading cutter designs.

[0029] The thread cutter 10 is manufactured from either a substrate consisting of tungsten carbide cemented with cobalt or high-speed steel. The first step in processing the substrate is to grind the blank to precision cylindrical tolerances using processes such as center-on cylindrical grinding or centerless plunge grinding. During this step, a cylindrical shank is ground to size to form the cylindrical shank portion 18 at the axially rearward end 16 of the thread cutter 10, and the outer diameter 38 of the fluted thread portion 20 is formed at the axially forward end 14 of the thread cutter 10.Generally, the diameter of the cylindrical shank portion 18 is approximately equal to the nominal thread diameter, but the diameter of the cylindrical shank portion 18 may be smaller than the nominal thread diameter for large-diameter taps, and alternatively larger for small-diameter taps. One option may include grinding the blank as part of the shank at the axially rearmost end of the tap 10.

[0030] In the next step, one or more spiral flutes 22 are ground with a continuously decreasing thread pitch to provide cutting edges 44 in combination with the chamfer. The spiral flutes 22 can be either right-hand or left-hand in any combination with right-hand or left-hand threads. CNC tool grinding machines can be programmed to produce the invented shape.

[0031] In the next step, the fluted thread section 20 is ground to form V-shaped thread flank surfaces 40 along with the minor and outer diameters 36 and 32 on a spiral. The V-shaped thread flank surfaces 40 and the outer diameter 38 replicate the internal screw thread created during tapping. Depending on the tapping application, the thread flank surfaces 40 and the minor and outer diameters 36 and 32, respectively, may or may not be tapered.

[0032] The chamfered fluted portion 24 of the fluted threaded portion 20 is ground with a chamfer at an angle 46 to allow insertion into the hole in which a thread is to be cut, and with a chamfer surface of increasing taper from the cutting edge to a point where the taper reaches a maximum, at an angle less than the landing angle. Thereafter, the chamfer surface has a decreasing taper to create a small clearance at the base of the thread. Although it is possible to produce this shape on straight fluted threads using state-of-the-art grinders where the taper is created by cams, it is impossible to produce this shape on spiral fluted threads using this technique. CNC tool grinders can be programmed to produce the invented shape.

[0033] After grinding, the thread cutter 10 can be honed with abrasives or abrasive brushes to create a small radius on the cutting edges and other sharp corners. The resulting radius can be between 0 and 100 microns. This honing further increases the strength of these edges.

[0034] Next, the thread cutter 10 can optionally be coated with a wear-resistant layer (not shown) of metal nitrides, carbides, carbonitrides, borides, and / or oxides, wherein the metal is selected from one or more of the following: aluminum, silicon, and transition metals of Groups IVa, Va, and VIa of the Periodic Table. This layer is applied as a single monolayer or in multiple, for example, alternating layers. Low-friction layers can also be applied over these wear-resistant layers.

[0035] In the final step of the process, the bevel surface can be polished after the wear-resistant layer has been applied.

[0036] As described above, the helical flute thread cutter of the invention includes a plurality of helical, helical flutes formed with a lead angle HA that continuously increases from the axial leading end to the axial trailing end of the thread cutter (i.e., continuously decreasing thread pitch) until the lead angle HA reaches a maximum predetermined value. The continuously increasing lead angle HA produces a thread cutter with superior performance characteristics, such as longer tool life and chip evacuation (less nesting or chip wrapping), compared to prior art thread cutter designs.

[0037] The patents and publications referred to in this document are hereby incorporated by reference.

[0038] Having described presently preferred embodiments, the invention may otherwise be practiced within the scope of the appended claims.

Claims

[1] A spiral groove thread cutter (10) comprising a body (12) having an axial front end (14) and an axial rear end (16), the body (12) having a cylindrical shank portion (18) adjacent the axial rear end (16), a grooved threaded portion (20) adjacent the axial front end (14), a grooved, unthreaded portion (19) between the cylindrical shank portion (18) and the grooved, threaded portion (20), and a central longitudinal axis (17), the grooved, threaded portion (20) and the grooved, unthreaded portion (19) of the spiral groove thread cutter (10) further comprising a plurality of spiral grooves (22),which are formed with a lead angle HA with respect to the central longitudinal axis (17) of the thread cutter (10), and the lead angle HA increases continuously and at a constant rate in a rearward direction over a predetermined distance from the axial front end (14) to the axial rear end (16). [2] A thread cutter (10) with a spiral groove (22) according to claim 1, wherein the thread pitch of the plurality of spiral grooves (22) of the grooved portion (19) without a thread continuously decreases over a predetermined distance. [3] Thread cutter (10) with spiral groove (22) according to claim 1, wherein the lead angle HA of the spiral grooves (22) at the axial front end (14) is between 0 degrees and 40 degrees and the lead angle HA near the grooved portion (19) without thread is between 20 degrees and 60 degrees. [4] Thread cutter (10) with spiral groove (22) according to claim 1, wherein the lead angle HA of the spiral grooves (22) at the axial front end (14) is 30 degrees and the lead angle HA near the grooved portion (19) without thread is 50 degrees. [5] A thread cutter (10) with a helical groove (22) according to claim 1, wherein the lead angle HA varies from the axial front end (14) to near the grooved portion (19) of the threadless thread cutter (10) by 1.25 degrees / mm. [6] A thread cutter (10) with a helical groove (22) according to claim 1, wherein the grooved threaded portion (20) includes a chamfered grooved portion (24) and a constant diameter portion (26), the chamfered grooved portion (24) extending from the axial front end (14) and terminating at the constant diameter portion (26), the constant diameter portion (26) including at least one cutting thread (28) and a cutting edge (29) formed at an intersection between the helical groove (22) and the at least one cutting thread (28). [7] Thread cutter (10) with spiral groove (22) according to claim 6, wherein the beveled grooved portion (24) has a length and is formed at an angle to the central longitudinal axis (17). [8] A thread cutter (10) with a helical groove (22) according to claim 1, wherein the lead angle HA increases from the axial front end (14) to the grooved portion (19) without a thread at a constant rate between 0.5 degrees / mm to 4.0 degrees / mm. [9] Thread cutter (10) with spiral groove (22) according to claim 8, wherein a thread pitch of the plurality of spiral grooves (22) of the grooved thread portion (20) continuously decreases over a predetermined distance. [10] Thread cutter (10) with spiral groove (22) according to claim 8, wherein the lead angle HA of the spiral grooves (22) at the axial front end (14) is between 0 degrees and 40 degrees and the lead angle HA near the grooved portion (19) without thread is between 20 degrees and 60 degrees. [11] Thread cutter (10) with spiral groove (22) according to claim 8, wherein the lead angle HA of the spiral grooves (22) at the axial front end (14) is 30 degrees and the lead angle HA near the grooved portion (19) without thread is 50 degrees. [12] A thread cutter (10) with a spiral groove (22) according to claim 8, wherein the lead angle HA varies by 1.25 degrees / mm from the axial front end (14) to the grooved portion (19) of the thread cutter (10) without a thread. [13] A thread cutter (10) with a helical groove (22) according to claim 8, wherein the grooved threaded portion (20) includes a chamfered grooved portion (24) and a constant diameter portion (26), the chamfered grooved portion (24) extending from the axial front end (14) and terminating at the constant diameter portion (26), the constant diameter portion (24) including at least one cutting thread (28) and a cutting edge (29) formed at an intersection between the helical groove (22) and the at least one cutting thread. [14] Thread cutter (10) with spiral groove (22) according to claim 13, wherein the beveled grooved portion (24) has a length and is formed at an angle to the central longitudinal axis (17). [15] Thread cutter (10) with spiral groove (22) according to claim 1, wherein the thread pitch of the plurality of spiral grooves (22) of the grooved threaded portion (20) continuously decreases in size. [16] Method for producing a thread cutter (10) with a spiral groove (22), comprising the following steps: Grinding a cylindrical shank to form a cylindrical shank portion (18) at an axial rear end (16) of the thread cutter (10) and a thread body portion having an outer diameter at an axial front end (14) of the thread cutter (10); Grinding a spiral groove (22) in the thread body portion to form a cutting edge (28); Grinding the threaded body portion with a taper to form a chamfered grooved portion (24) and Grinding the thread body portion to form a cutting edge (28); whereby the spiral groove (22) is formed with a pitch angle HA with respect to the central longitudinal axis (17) of the thread cutter (10), such that the pitch angle HA of the groove (22) increases continuously and at a constant rate starting at the axial front end (14) of the thread cutter (10) to the axial rear end (16) over a predetermined distance. [17] The method of claim 16, wherein the lead angle HA of the spiral groove (22) at the axial front end (14) is between 0 degrees and 40 degrees and the lead angle HA near the shaft portion is between 20 degrees and 60 degrees.

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