THREAD CUTTING TAP AND METHOD FOR TREATMENT OF A THREAD CUTTING TAP

Abstract

A thread cutting tap and a method of treating the thread cutting tap, which includes a thread cutting portion including a plurality of cutting lands, is provided. Each cutting land has one or more thread cutting teeth in the chamfered section. Each thread cutting tooth includes a primary cutting edge. The method includes immersing at least the thread cutting portion in a bed of granules and effectuating relative movement between the thread cutting portion and the granules for generating an edge rounding of a first magnitude at the primary cutting edge of each tooth, and then exposing a subset of the thread cutting teeth to a dynamic plastic deformation treatment. The subset includes all the thread cutting teeth in the chamfered section, so as to generate an edge rounding of a second magnitude, greater than the first magnitude, at the primary cutting edge of each tooth in the subset.

Claims

1. A method for treatment of a thread cutting tap, comprising the steps of: providing the thread cutting tap, the thread cutting tap having a thread cutting portion including a plurality of cutting lands separated by flutes, wherein the thread cutting portion includes a chamfered section extending from a front end of the thread cutting tap to a full profile section extending from the chamfered section towards a rear end of the thread cutting tap, wherein each cutting land has one or more thread cutting teeth in the chamfered section and a plurality of thread cutting teeth in the full profile section, each thread cutting tooth including a primary cutting edge; immersing at least the thread cutting portion of the thread cutting tap in a bed of granules and effectuating relative movement between the thread cutting portion and the granules for removing grinding burr and generating an edge rounding of a first magnitude at the primary cutting edge of each thread cutting tooth; and exposing a subset of the thread cutting teeth to a dynamic plastic deformation treatment, the subset including at least all the thread cutting teeth in the chamfered section, such as to generate an edge rounding of a second magnitude, greater than the first magnitude, at the primary cutting edge of each thread cutting tooth in the subset.

2. The method according to claim 1, wherein the subset includes, for each cutting land, at least the one thread cutting tooth in the full profile section being closest to the chamfered section.

3. The method according to claim 1, wherein the subset includes at most half of the teeth in the full profile section.

4. The method according to claim 1, further comprising immersing at least the thread cutting portion of the thread cutting tap in the bed of granules and effectuating relative movement between the thread cutting portion and the granules for removing flaking particles on the surface of the thread cutting portion.

5. The method according to claim 1, further comprising coating at least the thread cutting portion of the thread cutting tap.

6. The method according to claim 1, wherein the dynamic plastic deformation treatment includes a process in which particles are propelled in a stream of liquid, vapor or air towards the cutting teeth in the subset.

7. The method according to claim 6, wherein the particles are nonabrasive.

8. A thread cutting tap comprising a thread cutting portion including a plurality of cutting lands separated by flutes, wherein the thread cutting portion includes a chamfered section extending from a front end of the thread cutting tap to a full profile section extending from the chamfered section towards a rear end of the thread cutting tap, wherein each cutting land has one or more thread cutting teeth in the chamfered section and a plurality of thread cutting teeth in the full profile section, each thread cutting tooth including a primary cutting edge, wherein a plurality of the thread cutting teeth, are included in a cutting teeth subset having at least all the thread cutting teeth in the chamfered section, and wherein the primary cutting edges of the majority of the thread cutting teeth that are not included in the subset have an edge rounding of a first magnitude and a first superficial microhardness, and the primary cutting edges of the thread cutting teeth included in the subset have an edge rounding of a second magnitude and a second superficial microhardness, and wherein the second magnitude and second superficial microhardness are greater than the first magnitude and the first superficial microhardness, respectively.

9. A thread cutting tap according to claim 8, wherein the thread cutting tap is made of high speed steel.

10. Thread A thread cutting tap according to claim 8, wherein the second edge rounding magnitude is 20-50% greater than the first edge rounding magnitude.

11. Thread A thread cutting tap according to claim 8, wherein the first edge rounding magnitude is between 4.5 m and 14 m, and the second edge rounding magnitude is between 5.5 m and 21 m.

12. Thread A thread cutting tap according to claim 8, wherein the second superficial microhardness is at least 3% greater than the first superficial microhardness.

13. Thread A thread cutting tap according to claim 8, further comprising a coating covering at least the thread cutting portion.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0054] In the following, example embodiments will be described in greater detail and with reference to the accompanying drawings, in which:

[0055] FIG. 1 shows a thread cutting tap according to an embodiment of the invention.

[0056] FIG. 2 illustrates a part of a thread cutting portion of the tap illustrated in FIG. 1, wherein the chamfered section and a part of the full profile section are shown.

[0057] FIG. 3 is a magnified view of one of the cutting teeth of the thread cutting tap.

[0058] FIG. 4 is a cross sectional view of the primary cutting edge of the tooth as indicated in FIG. 3.

[0059] FIG. 5 illustrates a method for treatment of a thread cutting tap according to the invention.

[0060] FIG. 6 illustrates an example of a first step of the method wherein a thread cutting tap is immersed in a bed of granules and moved in relation thereto.

[0061] FIGS. 7-8 illustrate an example of a second step of the method wherein the thread cutting tap is exposed to a dynamic plastic deformation treatment.

[0062] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the respective embodiments, whereas other parts may be omitted or merely suggested. Unless otherwise indicated, like reference numerals refer to like parts in different figures.

DETAILED DESCRIPTION OF EMBODIMENTS

[0063] FIG. 1 illustrates a thread cutting tap 1 made of high speed steel, having a front end 7 and a rear end 8. The thread cutting tap extends along a longitudinal axis C from the front end 7 to the rear end 8. The thread cutting tap comprises a thread cutting portion 2, a neck portion 10, and a shank portion 11. The thread cutting portion includes a chamfered section 5 extending from the front end 7, and a full profile section 6 extending from the chamfered section 5 to the neck 10. Moreover, the thread cutting portion has three cutting lands 3 and three flutes 4 extending along a helical path around the longitudinal axis C. Each cutting land 3 comprises a plurality of thread cutting teeth 9, 9. The thread cutting teeth 9 in the chamfered section 5 can be considered as teeth of which, in a direction towards the front end, increasingly large portions of the tip has been cut off. Accordingly, the major part of the material that is cut away when threading a hole in a workpiece is removed by the teeth 9 in the chamfered section and the first thereupon following tooth 9 in the full profile section 6. The remaining teeth 9 in the full profile section 6 will mainly create an accurate and smooth surface of the thread, but not remove any significant amount of workpiece material. Some teeth 9 in the full profile section 6, in particular teeth close to the rear end of the cutting portion, may not be involved much at all in the cutting process since the thread cutting portion 2 may be back tapered starting from the first full profile tooth, creating a slight radial relief in the threads. Thus, the teeth 9 in the full profile section 6 has a profile corresponding to the thread to be cut, but all these teeth are not necessarily located on the same radial distance from the longitudinal axis.

[0064] The thread cutting tap according to this example embodiment has a major diameter D of 8 mm (i.e. for cutting an M8 thread according to the ISO 68-1 standard).

[0065] FIG. 2 shows a part of the cutting portion 2 in more detail, including the chamfered section 5 and a part of the full profile section 6. As best seen in FIG. 3which is an enlarged view of one of the teeth as indicated in FIG. 2each tooth includes a primary cutting edge 12 and a secondary cutting edge 13 as well as a crest edge 14. The chamfered section 5 transitions into the full profile section 6 at an axial location at which the crest edge 14 has obtained a final width corresponding to the crest width of the thread to be machined. According to the embodiment illustrated in FIG. 2, this transition is located somewhere between the last tooth 94 in the chamfered section 5 and the first tooth 91 in the full profile section 6. That is, of the teeth 91-94 that all have the same crest edge width corresponding to the thread profile to be cut, the tooth 91 is the tooth being located closest to the front end 7, and is therefore considered as the first tooth in the full profile section 6. FIG. 4 is a cross section perpendicular to the primary cutting edge, as indicated in FIG. 3, and thus illustrates the edge rounding r of the primary cutting edge.

[0066] In the embodiment illustrated in FIGS. 1-4, a subset of the teeth 9, 9 are formed by all the teeth 91-94 in the chamfered section as well as the teeth 91-93 in the full profile section for which all teeth axially forward thereof in the respective cutting land is in the chamfered section. In other words, for each cutting land, the first tooth in the full profile section 6 is included in the subset. In this example, the primary cutting edges 12 of the teeth 91-94, 91-93 in the subset have an average edge rounding of a magnitude of 14.5 m, as measured using Alicona Infinite Focus optical 3D measurement equipment, and an average superficial microhardness of about 1000 HK0.3, as measured using Knoop equipment with minimal distance from the surface based on requirements of SS-EN ISO 4545-1:2018. The primary cutting edges 12 of the remaining teeth 94 . . . have an average edge rounding of approximately 11 m and an average superficial microhardness of approximately 935 HK0.3. Thus, according to this example, the second edge rounding magnitude is approximately 32% greater than the first edge rounding magnitude, and the second superficial microhardness is approximately 7% greater than the first superficial microhardness.

[0067] In the following, with reference to FIG. 5, a method for treatment of a thread cutting tap is described.

[0068] In step 51, the cutting portion of a thread cutting tap is immersed in a bed of granules, and a relative movement between the thread cutting tap and the granules is effectuated. The treatment is performed such that grinding burr is removed and the edges of the thread cutting teeth, and in particular the primary cutting edges, obtain an edge rounding of a first magnitude.

[0069] For example, a machine as schematically illustrated in FIG. 6 may be used. Using this machine, a plurality of thread cutting taps 1, clamped by independently rotatable holders 15, are immersed in a bed of granules 16 located in a rotatable container 17 (although the taps 1 are shown in an elevated state in FIG. 6, e.g. before being lowered into the container 17). The movement of the thread cutting taps 1 with respect to the granules 16 is achieved by rotating both the taps 1 and the container 17. The rotation directions may change between clockwise and counter-clockwise during one treatment cycle. As an example, the container rotation direction may be the same during the treatment cycle, while the rotation direction of the tool is changed multiple times. The tool and the container preferably rotate in opposite directions during at least part of the treatment cycle.

[0070] In step 52, a subset of the thread cutting teeth of the thread tap is exposed to a dynamic plastic deformation treatment. The treatment is performed such that the edges of the thread cutting teeth in the subset, and in a particular the primary cutting edges, obtains an edge rounding of a second magnitude which is greater than the first magnitude. For example, a dry blasting machine as schematically illustrated in FIG. 7 may be used, in which a dry blasting gun 18 is used for bombarding a part of the thread cutting portion (corresponding to the subset of thread cutting teeth that is to be treated) with particles 19 in a stream of air. A mask 20 is used to cover the thread cutting teeth that are not in the subset, such that only the teeth in the subset are exposed to the blasting. A rotatable holder 21 is used for rotating the thread cutting tap 1 such that the blasting will be evenly distributed over the treated surface, i.e. such that the cutting edges of each tooth in the subset will be exposed to similar treatment. The subset includes at least all thread cutting teeth in the chamfered section of the thread cutting tap. In the example illustrated in FIG. 7, the first tooth in each thread cutting land axially rearwards of the chamfered section, i.e. teeth corresponding to one spiral revolution in the full profile section, is included in the subset and thus exposed to the treatment.

[0071] For clarity, FIG. 7 illustrates the mask 20 as covering only a part of the thread cutting portion, namely the part corresponding to the cutting teeth that are not intended to be included in the subset. However, the mask 20 may be longer, covering the whole neck and shank of the thread cutting tap, and for example supported by the tool holder 21, such that only the top of the thread cutting tap will be positioned outside of the mask.

[0072] Due to difficulties of achieving a tight fit and an exact positioning of the mask at the thread cutting portion, the border between blasted and non-blasted areas may not be too precise. Thus, according to an alternative embodiment, with reference also to FIG. 2, the first teeth 91-93 in the full-profile section may not have obtained an edge rounding of the second magnitude, despite not being intentionally covered by the mask 20, and are therefore not included in the subset. In such case, the first teeth 91-93 in the full-profile section may be considered as transition teeth between the treated teeth in the subset and the non-treated teeth.

[0073] Accordingly, in such alternative embodiment, all teeth 91-94 in the chamfered section will have obtained an edge rounding of the second magnitude, and are thus included in the subset, whereas the teeth 91 . . . in the full-profile section are not included in the subset. The majority of the teeth 91 . . . in the full-profile section will have an edge rounding of the first magnitude, even though the first teeth 91-93 in the full-profile section, i.e. the transition teeth, may have an edge rounding somewhere between the first and the second magnitude.

[0074] FIG. 8 is a magnified view schematically illustrating the impact from the particles 19 when hitting the surface of the thread cutting tap 1, resulting in plastic deformation 22 on the surface. The particles 19 are shown as spherical, but may also have various other shapes.

[0075] In optional step 53, the cutting portion of the thread cutting tap is once again immersed in a bed of granules, for example using a machine as illustrated in FIG. 6. This time, however, the process parameters are selected such that the treatment is softer and has a polishing effect by removing flaking particles and other material that is partly loose or protruding from the surface, but without affecting the overall geometry. In particular, the treatment may be performed such as to not change the edge rounding of any cutting edge, at least not to an extent having any significance, or even possible to measure. Even if the edge rounding magnitudes would be affected, the difference between the respective edge rounding magnitudes of the cutting edges on the teeth in the subset and on the other teeth, would remain.

[0076] In optional step 54, the thread cutting tap 1, or the thread cutting portion 2 thereof, is coated. Post-coating-processing may also be applied. A coating applied to the thread cutting tap may change the respective edge rounding magnitudes. Nevertheless, the relationship between the edge rounding magnitudes is preferably not changed, i.e. the edge rounding magnitude of the primary cutting edges of the teeth in the subset is still greater than the edge rounding magnitude of the primary cutting edges of the other teeth.

[0077] According to an example embodiment, a thread cutting tap with the properties described with reference to FIGS. 1-4 was obtained using the method as described with reference to FIG. 5, as follows.

[0078] First, a treatment as described with reference to FIG. 6 was applied, in which the bed of granules included a mixture of SiC (212 m) and walnut shell granulate (0.8-1.4 mm) as abrasive media. Each thread cutting tap 1 was held in a holder 15 rotated with a spindle rotation speed of 2000 rpm repeatedly changing between clockwise and counter-clockwise rotation directions for intervals of 2 seconds, while the container was rotated counter-clockwise at a speed of 70 rpm for a total cycle time of 80 seconds.

[0079] Secondly, a treatment as described with reference to FIG. 7 was applied. The blasting media was ZrO.sub.2 with particle size of 70-120 m at a pressure of 1 bar and a treatment time of 5 seconds, wherein the blasting gun was directed at a 90 angle to the longitudinal axis of the thread cutting tap, at a distance of 30 mm therefrom, and wherein the thread cutting tap was rotated at 10000 rpm.

[0080] Thirdly, a treatment as described with reference to FIG. 6 was applied again, with a similar process as used during the first treatment step, but now with a total cycle time less than 60 seconds.

[0081] Tests were made in which a thread cutting tap treated according to the above (and thus having, before coating, first and second edge rounding magnitudes of 11 m and 14.5 m, respectively, and first and second superficial microhardness of 935 HK0.3 and 1000 HK0.3, respectively) and coated with TiAlN coating was compared to a reference tap with the same coating but produced by using a standard method comprising manual deburring followed by edge rounding by wet blasting using glass beads. Both taps were used for threading 16 mm long holes in plates of Impax Supreme steel (283-311 HB), with a threading speed of 15 m/min, using external coolant supply. A standard go/no go gauge was used to determine tool life. In this test, the number of threaded holes reached by the reference tap before failing the go/no go gauge test was 405, whereas the corresponding number for the tap treated in accordance with the invention was 1050. Hence, the tap according to the invention was found to have approximately 2.6 times longer tool life than the reference tap.