BARROWS ALAN L
BURNS STEVEN R
GREENE DENNIS J
US2202236A | 1940-05-28 | |||
US5222847A | 1993-06-29 | |||
US3767544A | 1973-10-23 |
The invented tap, manufactured to the lengths and shank diameters listed above, has advantages over taps whose length and shank diameter are both in accordance with ASME B94.9. For example, a prior art tap with a nominal shank diameter of about 1A inches would have a nominal overall length of about 2.50 inches when manufactured in accordance with ASME B94.9, but the tap 10 has a nominal overall length of about 3.15 inches when manufactured according to DIN standard 376 of the invention. Similarly, prior art taps with nominal #4 machine screw size and 12 mm shank diameters would have overall lengths of 1.88 inches and 3.38 inches, respectively, in accordance with ASME B94.9, but have overall lengths of about 2.20 and 4.33 inches, respectively, when manufactured according to DIN standard 376 of the invention. The additional overall length permits greater access to the part being tapped, particularly when the tap is used on automated CNC equipment to machine parts with complicated shapes and to tap holes in deep recesses. [0025] After the tap 10 is cut to length and the diameters of the shank portion 12 and/or the thread cutting portion 16 are turned, the tap 10 is hardened in either a salt bath or a vacuum furnace, and tempered to the final desired hardness and metallurgical structure by use of well- known heat treating processes. Normally, the shank portion 12 and major diameter of the thread cutting portion 16 is ground to a final diameter after heat treatment. [0026] Subsequently, the asymmetrical straight flutes 22 are ground or milled along and past the threaded body 20 in a direction generally parallel to the longitudinal axis, L, of the tap 10. The asymmetrical straight flutes 22 provide access of coolant to the tapered chamfer 28 and provide cutting during tapping after wear of the cutting edges of the tapered chamfer 28. [0027] The configuration of the asymmetrical straight flutes 22 of the tap 10 of the invention is shown in Figures 3-5, and the configuration of the straight flutes of a conventional tap, as shown in Figure 6. As shown in Figure 6, the form of the straight flutes of a tap with a symmetrical flute form is typically a full radius. An undesirable consequence of this design is a negative chordal hook, for example, an angle of about -5 degrees, as shown in Figure 6. When the tap cutting face is curved as shown in Fig. 6, the chordal hook angle is defined by the angle between a line from the tap cutting edge and the center of the tap and a second line between the tap cutting edge and extending to the intersection of the cutting face and the thread minor diameter. [0028] To reduce tapping torque and enhance tap life, it has been determined that the cutting edges should ideally have a positive rake or chordal hook angle between about 5 and 15 degrees for tapping steel alloys. As shown in Figure 3, the asymmetrical straight flutes 22 of the tap 10 have a more complex asymmetrical shape that is provided by a straight cutting face, oriented at the desired rake angle, and tangent to a small blending radius, Rl, that is tangent in turn to a larger radius, R2, that defines the full width of the straight flute 22. Formed in this manner, the asymmetrical straight flutes 22 of the tap 10 can be ground to have the desired rake or chordal hook angle. The selection of the desired rake angle or chordal hook angle may depend on the specific grade of steel being tapped. For example, these angles may be 5 to 7 degrees for tapping hardened steels, 8 to 10 degrees for tapping annealed alloy steel, 10 to 12 degrees for tapping annealed carbon steel, and 13 to 15 degrees for tapping stainless steel. For practicality, the rake or chordal hook angle of the asymmetrical straight flutes may be set at one angle within the range 5 to 15 degrees for tapping all material and only the rake or chordal hook angle of the spiral point 24 changed according to the material being tapped. When the tap cutting face is straight as shown in Fig. 3, the rake angle is defined by the angle between a line from the tap cutting edge and the center of the tap and the straight line defining the cutting face. [0029] After the asymmetrical straight flutes 22 are ground, one or more short angular flutes or spiral points 24 are ground at an angle to the longitudinal axis, L, of the tap 10. Designed in conjunction with the straight flutes, these short angular flutes or spiral points 24 form the primary cutting faces of the tap 10, and because of their angular orientation, the flutes 24 direct the chips ahead of the motion of the tap 10, thereby preventing the chips from interfering with the tapping action. As shown in Figure 4, the preferred rake or chordal hook angle of the spiral points 24, measured at a position past the tapered chamfer 28 of the tap 10 (indicated at line 4-4 of Figure 2), is generally about 5 to 15 degrees for tapping steel alloys, but depending on the specific steel alloy being tapped may be 5 to 7 degrees for tapping hardened steels, 8 to 10 degrees for tapping annealed alloy steel, 10 to 12 degrees for tapping annealed carbon steel, and 13 to 15 degrees for tapping stainless steel. As a specific example, the preferred rake or chordal hook angle of the spiral points 24 may be about 10 degrees for tapping steel alloys containing chromium and/or molybdenum. The preferred rake or chordal hook angle is achieved during grinding by the radius, R3, of the spiral point 24 shown in Fig. 5, and the offset, Ll, of the center of the radius, R3, from the centerline of the tap 10. [0030] Designed in conjunction with the asymmetrical straight flute 22 having a positive rake or chordal hook of 5 to 15 degrees, the geometry of the spiral points 24 may be further refined. It has been found that the useful life of taps in the range from about #6 machine screw size through about one-half (1A) inch diameter is maximized if three flutes are used. For three-fluted taps, it has been found that the tapping torque may be minimized when the spiral point 24 is ground at an angle of approximately 12 to 15 degrees with respect to the longitudinal axis, L, of the tap 10. In addition, it has been found that the tapping torque may be minimized when the radius, R3, of the spiral point 24 is approximately 19 to 22 percent of the tap major diameter, and the offset, Ll, of the center of the radius, R3, from the centerline of the tap 10 is approximately 22 to 32 percent of the tap major diameter measured from the centerline of the tap 10, as shown in Fig. 5. The use of this geometrical combination may be used effectively on taps without limit to specific grade of high speed steel, tap length and shank diameter or coating. [0031] In the next step, the threaded body 20 is ground to form V-shaped thread flank surfaces 30, along with minor and major diameters, on a helix. Subsequently, the shape of the thread cutting chamfer portion is formed by grinding. The V-shaped thread flank surfaces 30 and major diameter replicate the internal screw thread that is generated during tapping. The tapered chamfer 28 allows entry into the hole of the work material to be tapped. [0032] As a final step in the process, the tap 10 is coated with a wear resistant, low friction layer or coating (not shown) of metal nitrides, carbides, carbonitrides, borides and/or oxides, wherein the metal is chosen from on or more of the following: aluminum, silicon and the transition metals from Groups IVa, Va, and Via of the Periodic Chart. This wear resistant coating is deposited as a single monolayer or in multiple, including alternating layers. Because steel alloy work materials do not adhere to these coatings, friction, and consequently torque, is reduced, and tap life is significantly extended. [0033] The use of metal nitride, carbide and carbonitrides coatings are not effective in non- ferrous work materials, such as aluminum, titanium, zinc, or the like. Such non-ferrous materials gall or adhere to these coatings and increase tapping torque. It has been found that a spiral pointed high speed steel tap with an asymmetrical straight flute coated with a layer of carbon, or carbon and one or more transition metals selected from the group IVa, Va and Via of the Periodic Chart can be effectively used in such non-ferrous work materials. [0034] A wear resistant, low friction coating scheme comprises two basic coating regions. One region is a top coating region. The surface of the top region is in contact with the work piece during the cutting operation. The top coating region typically includes at least one layer (or multiple layers) of tribological coating material that has good overall tribological properties (including low friction). [0035] The top outer region may comprise a single layer of molybdenum disulfide. As an alternative, the top outer region comprises a single layer of molybdenum disulfide and a metallic additive. Typical metallic additives include molybdenum, tungsten, chromium, niobium and titanium. The metallic addition may comprise a single metal or a combination of any two or more of these metallic additives. When the top outer coating region comprises a single layer, the thickness thereof is between about 0.1 micrometers and about 10 micrometers. [0036] As an alternative to depositing molybdenum disulfide and the metallic addition as one layer, one can deposit alternating layers of molybdenum disulfide and the metallic addition. One example is the deposition of alternating layers of molybdenum disulfide and titanium. Another example is the deposition of molybdenum disulfide and chromium, hi addition to titanium and chromium, other candidates for the metallic addition include molybdenum, tungsten and niobium. Each individual layer of this alternating layer coating scheme (molybdenum disulfide and a metallic addition) has a thickness that ranges between about 0.1 nanometers and about 500 nanometers. The total thickness of this alternating layer of molybdenum disulfide and the metallic addition ranges between about 0.1 micrometers and about 10 micrometers. [0037] As still another alternative to the above schemes for the top outer coating region, one may deposit a single layer of carbon. The single layer of carbon has a thickness that is between about 0.1 micrometers and about 10 micrometers. As an alternative to the single carbon layer, alternating layers of carbon and a transition metal such as, for example, either chromium or titanium may be deposited to form the top coating region. The thickness of each layer of carbon and chromium (or titanium) may range between about 0.1 nanometers and about 500 nanometers. The total thickness of the alternating layers of carbon and chromium or titanium ranges between about 0.1 micrometers and about 10 micrometers. [0038] As yet another alternative to the above schemes for the top outer coating region, the present invention contemplates a carbon nitride (CNx) layer or layers. The value of x may range between about 0.01 and about 1.00. The thickness of a single layer of carbon nitride may range between about 0.1 micrometer and about 10 micrometers. In the case of multiple layers of carbon nitride, the total thickness would range between about 0.1 micrometer and about 10 micrometers. [0039] As still another option for the top outer coating region, the present invention contemplates alternating layers of carbon and transition metal carbide wherein the transition metal is selected from Group IVa, Group Va and Group Via of the Periodic Chart, for example, tungsten carbide. The thickness of each layer of carbon and metal carbide (e.g., tungsten carbide) may range between about 0.1 nanometers and 500 nanometers. The total thickness of the alternating layers equals between about 0.1 micrometers and about 10 micrometers. [0040] As still another alternative for the top outer coating region, carbon and the transition metal carbide can be co-deposited to form a single layer that comprises the top outer coating region. In the case of a single layer of carbon and transition metal carbide, the thickness of that single layer may range between about 0.1 micrometers and about 10 micrometers. [0041] The second inner coating region comprises a hard, refractory coating scheme. The hard, refractory coating scheme may, as one alternative, comprise alternating coating layers of titanium nitride and silicon nitride. The titanium nitride layer in this usage and in other usages mentioned herein has the formula TiNx wherein x ranges between about 0.6 and about 1.0. The silicon nitride layer in this usage and in other usages mentioned herein may have the formula SiNx wherein JC ranges between about 0.75 to about 1.333 or Si3N4. Each individual layer has a thickness that ranges between about 0.1 nanometers and about 500 nanometers. The total thickness of the alternating layers of titanium nitride and silicon nitride ranges between about 0.5 micrometers and about 20 micrometers. [0042] As another alternative, the hard, refractory coating scheme may comprise alternating layers of titanium aluminum nitride and silicon nitride. The titanium aluminum nitride in this usage and in other usages mentioned herein has the formula (TixAl1-x )Ny wherein x ranges between about 0.25 and about 0.75, and y ranges between about 0.6 and about 1.0. Each individual layer has a thickness that ranges between about 0.1 nanometers and about 500 nanometers. The total thickness of the alternating layers of titanium aluminum nitride and silicon nitride ranges between about 0.5 micrometers and about 20 micrometers. [0043] It should be appreciated that there may be some instances in which the use of the alternating layers of titanium aluminum nitride and silicon nitride may be appropriate in the absence of any other coating scheme that has good tribological properties. The properties of the alternating layers of titanium aluminum nitride and silicon nitride used in the absence of a coating scheme with good tribological properties provides good performance in various applications such as, for example, cutting tools. [0044] As still another alternative for the hard, refractory coating scheme, one may co- deposit titanium and silicon in a reactive nitrogen atmosphere to deposit a single layer (or multiple layers) of titanium silicon nitride. In this usage, as well as in other usages mentioned hereinafter, the titanium silicon nitride has the formula (Ti1-xSix)Ny wherein Λ; ranges between about 0.01 and about 0.30, and;; ranges between about 0.6 and about 1.1. The single layer may have a thickness that ranges between about 0.5 micrometers and about 20 micrometers. [0045] In addition to the top outer coating region and the hard, refractory coating region, there may be an adherence coating scheme. The adherence coating scheme is applied directly to the surface of the substrate. The adherence coating scheme may comprise one or more layers of metals such as, for example, aluminum, silicon, or a transition metal such as, for example, titanium or chromium. The adherence layer may also comprise one or more layers of a nitride of the above elements; namely, aluminum nitride, silicon nitride, and transition metal nitrides such as, for example, titanium nitride and chromium nitride. As an alternative, the adherence layer may comprise the metal layer followed by metal nitride layer. For example, a titanium layer may be followed by a titanium nitride layer. The thickness of the adherence coating region is between about 1 nanometer and about 3000 nanometers. [0046] An adherence coating scheme may also be present so as to be between the top coating region and the hard, refractory coating region. The compositions and properties of this adherence coating scheme are the same as those described hereinabove for the adherence coating scheme that is between the hard, refractory coating region and the substrate. [0047] As a further optional step, the optional wear resistant, low friction coating or layer may be coated with a second outer friction-reducing layer (not shown) comprised of molybdenum disulfide, molybdenum disulfide (to be consistent with previous references) and transition metal carbides, carbon and transition metal carbides, carbon and a transition metal, carbon, and carbon nitride. The second outer friction-reducing layer can be deposited as a single monolayer or in multiple layers, including alternating layers. The friction-reducing layer effectively further reduces friction and tapping torque, further extending tap life. [0048] Both the wear resistant, low friction layer and the outer friction-reducing layer may be applied by use of a vapor deposition technique, such as one of the well-known physical vapor deposition (PVD) techniques, for example, any high ion density process, such as ion plating, magnetron sputtering, arc evaporation, or the like. [0049] In order to create the desired form, it should be appreciated that other methods or sequences of manufacturing can be used than that described herein. [0050] The invented tap has advantages over current art through the combination of elements described herein. Tests were conducted to prove the effectiveness of the tap 10 of the invention. For example, when tapping 0.197 inch diameter through holes in 1A" thick 4340 alloy steel at 50 sfrn with spiral pointed taps designed with a symmetrical flute form, as shown in Figure 6, and not coated with a wear resistant layer, the initial tapping torque was 36.5 in-lbs and the life was 887 holes. In contrast, the tap 10 of the invention, as shown in Figures 1-5, had 29.4 in-lbs torque and the life was 2,296 holes. Further, by use of molybdenum enriched high speed steel, the invented taps have advantages of alloy availability and avoidance of difficulty during heat treatment of otherwise identical tungsten enriched high speed steel taps made to current practice. Likewise, the overall length of invented tap provides easier access to parts with complicated shapes or parts with recessed holes, and may be used with tap holders designed for shanks to ASME B94.9. [0051] The patents and documents described herein are hereby incorporated by reference in their entirety. [0052] While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.