HK1051339A - Thread milling tool having helical flutes - Google Patents

Description

Thread milling tool with helical flutes

Technical Field

The present invention relates to a cutting tool for milling internal or external threads in a metal workpiece or a workpiece constructed from other materials. More particularly, the present invention relates to a thread milling tool having one or more helical flutes in the cutting portion of the tool. The invention may be incorporated in a thread milling tool to mill, for example, straight or wedge threads.

Background

Thread cutting tools for cutting an internal thread in a pre-drilled hole of a workpiece or an external thread on the outer circumference of a workpiece are known. These thread milling tools are typically referred to as "thread mills". A typical thread milling tool has a shank portion and a thread cutting portion. Thread mills can generally produce high quality threaded workpieces compared to other tools used for threading, such as taps.

The thread cutting portion of the thread milling tool may include thread milling teeth and flutes. A typical thread milling tool may include 2-4 flutes, but for certain sizes and milling applications, a thread milling tool may include more than 4 flutes. Flutes may be arranged on the thread milling tool to provide cutting edges and to evacuate chips produced during milling out of the milled threaded hole. The flutes defined in the thread milling tool may be straight or may take the form of a spiral or helical pattern in either a right-hand or left-hand direction. The straight flutes are generally arranged parallel to the longitudinal axis of the thread milling tool and extend along the cutting portion from the end of the cutting portion. The helical or spiral flutes may be defined by a helix angle and surround the outer surface of the thread milling tool. As used herein, the helix angle is a constant angle between the central axis of the cutting tool or any line parallel to the central axis and the flutes. One of ordinary skill in the art can understand and readily determine the helix angle for a particular cutting tool.

The helical flutes defined in the cutting portion of the thread milling tool may act to lift debris generated during the threading operation. This lifting action forces the debris through the junk slots and out of the cut hole. Helical flutes may also be advantageous, for example, when the hole being cut is provided with splines or other interruptions. The helical configuration of the flutes allows the cutting teeth to gradually contact the interruptions, which allows for smoother cutting and less vibration.

The thread cutting teeth of the thread milling tool may be configured in a thread form defined by the ISO or NPT standards, or may have any other thread shape. The configuration of the thread cutting element is complementary to the configuration of the thread formed on the inner surface of the bore or the outer surface of the workpiece. The rows of cutting teeth are separated from each other by flutes defined in the cutting portion of the thread milling tool.

The shank of the thread milling tool can be mounted on the shaft of a milling apparatus, a multi-process automatic numerically controlled machine tool or a milling apparatus numerically controlled by a computer, for example. Thread milling tools are generally used in equipment that is movable along three axes, which allows the thread milling tool to move in a desired helical path during operation. The thread milling tool rotates about its longitudinal axis, rotates about the outer circumference of the workpiece to be threaded, and also moves axially relative to the workpiece.

Multiple chip groove thread mills work in a similar manner to threaded taps, but differ in their design and operation in many respects. The tap operates essentially like a drill, i.e., it can be quickly inserted into a workpiece to form threads. The thread profile on the outside diameter of the tap is in the form of a helix with a pitch equal to the pitch of the thread required on the workpiece. Thus, when the tap is screwed into a workpiece, it is actually pulled forward, and therefore, to remove the tap from a blind threaded hole, the direction of feed rotation of the tap must often be changed. Typically, two or three taps must be used to create a complete thread. Thread milling tools, however, are milling tools that remove material in the form of chips as they mill threads in a workpiece. The diameter of the thread milling tool is typically smaller than the diameter of the hole to be tapped in the workpiece. Although the thread profile is also based on the outer diameter of the thread mill, which is similar to a tap, the threads are arranged linearly with respect to each other rather than helically oriented. Therefore, the thread milling tool must be helically inserted into the workpiece to form the desired helical thread. Each thread cutting tooth of the thread milling tool has a cutting edge, and a rake angle is formed on the cutting portion of the thread milling tool.

Examples of known thread milling tools include a combination hole making and thread tool disclosed in U.S. patent nos. 5,413,438 and 4,761,844. The combination hole making and threading tool is arranged with a long straight base having: a handle at one end; a drilling portion; a thread making section along the working portion of the tool. The thread-forming portion of the tool has a maximum diameter equal to the maximum diameter of the drilling means so that the thread-forming portion can enter the hole without interference while drilling. The thread making portion of the tool may have straight or helically arranged flutes. The hole is formed by a drilling portion and then threaded by a thread forming portion by a combination of simultaneous rotational, axial and orbital movement of the tool. The orbital motion of the tool is the motion of the axis of the tool about the centerline of the hole, i.e., along a circle whose center is on the centerline of the hole. The combination of the rotary, axial and orbital motion of the combination tool produces an internal thread on the inner surface of the bore.

Us patent No.5,733,078 provides a drilling and thread milling tool in which the cutting portion of the tool comprises thread milling teeth on the outer circumferential surface and one or more end cutting edges on the distal end face of the tool. Each end cutting edge has a maximum diameter that is intermediate in length between the smaller and larger diameters of the thread milling teeth on the same tool. The drilling and thread milling tool can form a threaded hole close to the edge of the workpiece by reducing the minimum wall thickness required between the larger diameter of the thread produced and the edge of the workpiece.

For a thread milling tool to function effectively, when a hole is cut in a workpiece, the debris generated during the milling process must be effectively removed. The entrapment of debris, or the inability to effectively remove the debris, is one of the main causes of increased power consumption, reduced quality of milled threads, and tool wear. Thread mills for turning blind holes are typically designed with flutes so that the chips can be lifted out of the hole or, if there is sufficient clearance at the bottom of the hole, pushed towards the front of the cutting tool. Directing cutting fluid into a bore through an internal passage of the tool is also useful when the design of the flutes of the cutting tool can lift debris out of the bore by the lifting action of the flutes. This flow of fluid both cools the workpiece and the cutting tool and assists in the flow of chips out of the hole through the flutes. By improving the design of the chip grooves of the common thread milling tool, the milling efficiency can be increased and the thread quality can be improved.

It would therefore be advantageous to provide an improved design for a thread milling tool that more efficiently transports away debris generated during the milling process. In this way, the efficiency of the milling operation can be improved, and the quality of the thread produced can be improved.

Disclosure of Invention

The present invention provides an improved thread milling tool for producing internal and/or external threads in a workpiece. The thread milling tool of the present invention includes a proximal shank portion and a distal cutting portion. The cutting portion of the thread milling tool comprises at least one thread milling tooth. At least one helically oriented flute is defined in the cutting portion. The helix angle of the flutes defined in the cutting portion is greater than or equal to 20 °.

The thread milling tool according to the invention may further comprise a wedge-shaped region defined in the cutting portion, wherein the diameter of the cutting portion tapers in the axial direction of at least a portion of the thread milling tool. The proximal shank may be attached to a device such as a milling apparatus, machining center, a multi-pass automatic numerically controlled machine, or a computer numerically controlled lathe with milling capabilities that can convey tools along a helical insertion path to form the desired threads. The cutting portion may form an internal or external thread in the workpiece as it moves relative to the workpiece.

Compared with the spiral chip removal type thread milling tool in the prior art, the thread milling tool improves the milling performance in the thread forming process, and the chip removing groove of the spiral chip removal type thread milling tool in the prior art has a smaller helix angle. The thread milling tool of the present invention results in more precise thread parameters and thread profile, thus allowing smoother milling, including smoother chipping, and also results in a generally smoother surface on the threaded bore. The improved performance of the thread milling tool of the present invention reduces the cutting forces required and the vibrations generated during the thread milling process, thus extending the life of the tool and increasing the tendency of the thread milling tool to shed chips during the milling process.

Drawings

The features and advantages of the present invention may be better understood with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of one embodiment of a thread milling tool constructed in accordance with the present invention;

FIG. 2 is an end view of the thread milling tool shown in FIG. 1 showing the rake angle formed by the leading edge of the thread on the cutting portion of the thread milling tool;

FIG. 3 is an enlarged general view of region III of FIG. 1 showing the wedge angle of the terminal region of the threads of the thread milling tool shown in FIG. 1;

fig. 4a and 4b are photographs of chips generated during the milling of a hole in AISI type 304 stainless steel using a thread milling tool constructed in accordance with the present invention having flutes with a helix angle of about 30 °;

FIGS. 5a and 5b are photographs of chips generated during milling of a hole in AISI type 304 stainless steel using an HTM490-14NPT type wedge spiral flute thread milling Tool from Schmarje Tool Company having flutes with a helix angle of about 10;

FIGS. 6a and 6b are photographs of chips generated during the spin Cutting of holes in AISI type 304 stainless steel using a TM430-14NPT-HC wedge spiral flute thread milling tool from Scientific Cutting Tools having flutes with a helix angle of about 15 °;

fig. 7 shows the variation of threaded holes milled using a prior art thread mill and a thread mill constructed in accordance with the present invention, with respect to a desired size of threaded hole.

Detailed Description

The present invention relates to a thread milling tool or "thread mill" for producing internal or external threads in metal or other workpieces, which thread milling tool improves the performance of prior art thread milling tools. One embodiment of a thread milling tool constructed in accordance with the present invention is shown in fig. 1 and is indicated generally by the reference numeral 10. The thread milling tool 10 is adapted to produce an internal thread in a hole in a workpiece, and the helical flutes of the tool, which will be described hereinafter, simultaneously produce a lifting action on the chips, thereby forcing all or a portion of the chips out of the hole being milled through the flutes. The thread milling tool 10 may be manufactured from cemented carbide material or from any other suitable material. Any other suitable material as described may include, for example, high speed steel, spring steel, cemented carbide material with a steel core, tool steel, or sintered steel. As is well known in the art, other materials from which the thread mill 10 may be constructed will be readily apparent to those of ordinary skill in the art, the suitability of a particular material being at least partially dependent upon the application for the thread mill.

The thread milling tool 10 includes a cutting portion 12 and a shank portion 14. The cutting portion has a wedge-shaped design and comprises a plurality of flutes 16. As is well known in the art, wedge threads are made in at least a portion of a hole of a work piece using the wedge design shape of the cutting portion. The specific wedge parameters of the cutting portion may be determined using National Pipe Thread (National Pipe Thread) or other industry standards. The shank 14 of the constructed thread milling tool 10 allows the thread milling tool 10 to be suitably mounted on a shaft of a machining tool, such as a milling apparatus or a multi-pass automatic numerically controlled machine tool. A locating slot, such as slot 18, may be provided in the shank 14 to ensure that the thread milling tool 10 is mounted in the correct orientation on the shaft of the tool. The shank portion 14 may have a standard design shape, such as a Weldon shank, to ensure consistency in the milling operation after tool changes. The cutting portion 12 includes a plurality of lands that cut the threads or teeth 20, which are separated from one another by junk slots 16. The junk slots 16 are helically positioned relative to a central rotational axis 22 of the thread milling tool 10. The configuration of the helical flutes 16 and cutting teeth 20 may be such that the respective flank arrangement of the teeth is helically oriented relative to the central axis of rotation of the thread milling tool 10 in a direction from the terminal end 24 of the cutting portion 12 toward the shank portion 14. The helical flutes 16 form a helix angle, such as angle 26, which is the angle between the flutes 16 and the central axis 22 of the thread milling tool 10, or a line parallel to the central axis.

The configuration of the flank of the cutting tooth 20 is used to delineate several thread forms 28, which may have different configurations defined by ISO or other standards. Each thread form 28 defines a rake angle, angle 30 in fig. 2, and a wedge angle, angle 32 in fig. 3. Referring to fig. 3, the wedge angle 32 of the thread milling tool 10 is determined by continuously decreasing the distance between the central axis 22 of the tool 10 and the crest of each cutting tooth 20, gradually approaching the terminal end 24 of the thread milling tool 10.

Thus, the thread milling tool 10 is made of cemented carbide material and has a wedge-shaped configuration with helical flutes defined around the cutting portion of the thread milling tool. As will be described further below, the thread milling tool 10 includes a novel helix angle that provides a significant improvement over thread milling tools having straight flutes (i.e., flutes substantially parallel to the central axis of the tool or having a helix angle of 0 °), and thread milling tools including helical flutes defining a common helix angle. In particular, the helix angle of the thread milling tool 10 of the present invention is greater than the helix angle of known helical chip removing thread milling tools, which are less than 20 °. The increased helix angle of the present invention results in improved machinability when the thread is milled into a workpiece. The improved machining performance is manifested by better control of the chips, better cleaning of the chips, smoother milling, longer tool life, reduction in power consumption, and improved quality threads produced on the milled workpiece. In contrast to "ideal" threads, which are determined by industry standards, threads made using the thread milling tool of the present invention have less variation than threads made using existing thread milling tools.

The number of helically oriented flutes arranged on a thread milling tool constructed in accordance with the present invention may be any suitable number, preferably 2-6. The actual number of flutes provided depends primarily on the diameter of the thread milling tool and its application. As mentioned above, the wedge angle of the thread milling tool of the present invention can be increased from 0 ° (no slope) to any suitable taper. The preferred situation is: a thread milling tool constructed in accordance with the present invention has a wedge configuration and has a wedge angle of no more than 5 °. Of course, the appropriate wedge angle depends on the gauge of the thread to be made, which is well known to those skilled in the art of thread milling. As mentioned above, the top rake angle may have any suitable rake angle known in the art, preferably from 10 ° to 35 °. The thread form may be any suitable design shape, many of which are determined in industry specifications, including BSPT, NPT, NPTF specifications, all of which are ISO standards.

The helix angle of a thread milling tool constructed in accordance with the present invention may have any suitable angle greater than or equal to 20 °. The preferred situation is: the helix angle is in the range of 20-40.

During the cutting process, the thread milling tool simultaneously rotates about its central axis and moves axially from the workpiece being milled towards the milling chips. The process of forming threads using a thread milling tool is known to those skilled in the art and for this reason it will not be described in detail here. As one example, the thread milling tool of the present invention may operate as described in U.S. patent No.5,098,232 to form threads in a workpiece. The entire disclosure of U.S. patent No.5,098,232 is incorporated herein by reference. The design of prior art thread mills combines a wedge angle with a helically oriented chip groove. Such prior art designs include commercially available thread mills manufactured by Schmarje Tool Company, Muscatine, Iowa and scientific Cutting Tools, inc. However, the design of such thread milling tools of the prior art combines helical flutes with a relatively small helix angle of less than 20 °. Thread mills having helix angles greater than 20 ° have not been manufactured to date. One reason such tools have not been manufactured is: it is generally believed that such tools cannot be made with precise thread form configurations. However, as shown herein, the thread milling tool of the present invention can be unexpectedly made with a high quality thread, the useful life of the tool exceeding that of prior art thread milling tools.

The inventor has found through analysis and experiments that: a thread milling tool constructed in accordance with the present invention provides good machinability in terms of improved chip control and removal, extended tool life, reduced cutting power loss, and improved thread quality, incorporating flutes defined by a helix angle that is greater than the helix angle of prior art helical chip removing thread milling tools. The advantages resulting from a thread milling tool constructed in accordance with the present invention were experimentally demonstrated by comparing the performance characteristics of prior art thread milling tools with those of a wedge helix chip groove thread milling tool constructed in accordance with the present invention. The thread form of the thread mill tested in the present invention has a helix angle of about 30 deg. making 14NPT threads. The thread milling tool being tested is identical to the thread milling tool shown generally in fig. 1-3. This comparison was made by comparison with the HTM490-14NPT wedge helical flute cemented carbide thread milling tool from Schmarje ToolCompany, and with the TM430-14NPT-HC wedge helical flute cemented carbide thread milling tool from Scientific Cutting Tools. The three thread mills tested had the same ISO standard 14NPT spiral form. Three tools were tested by forming internal threads in the holes of AISI type 304 stainless steel workpieces using a Fadal Mill Model 906-1 milling tool at 20 horsepower, with the following milling conditions: cutting speed 266.1 ft/min; shaft speed 1637 revolutions per minute; linear movement speed of the workpiece was 20.05 inches/minute; the feed rate was 0.0031 inch/revolution.

Debris samples were collected to assess the quality of the debris formed using three different mills. Fig. 4a and 4b are photographs of collected debris when milling a hole in a stainless steel workpiece using a thread milling tool constructed in accordance with the present invention. FIGS. 5a-b and 6a-b are photographs of collected debris when milling holes in stainless steel workpieces using the Schmarje Tool Company thread milling Tool and the thread milling Tool of the Scientfic Cutting Tools, respectively. All of these photographs in fig. 4-6 are magnified at a magnification of 47X. The chips produced with the thread milling tool of the present invention are significantly smoother than the chips produced with the prior art thread milling tools. The smoother form of the crumb showed: the thread milling tool of the invention can perform milling operation more smoothly, thereby reducing power consumption, making milling more stable, prolonging the service life of the tool and improving the thread form.

Figure 7 shows the variation in the quality of the thread produced using three thread mills. A series of three sets of 10 holes each were made in an AISI type 304 stainless steel workpiece. A different set of 10 holes is milled using each of the three thread mills. One thread mill of each type was used to screw cut all 10 holes in each set without tool compensation for any of the three thread mills. In other words, the same settings and milling parameters are utilized for each tool. A standard 14-NPT plug is then threaded into each threaded bore and the height from the top of the plug to the top surface of the workpiece is measured. The stem cannot be fully threaded into the threaded bore so as to be flush with the top of the bore, indicating an undesirable change in the size of the threaded bore.

The changes indicated by the detection process of the stopper rod with respect to the required thread form are compared. The curves shown in fig. 7 represent the dimensional changes experienced in each of the 10 holes milled with each thread mill. The process of milling 10 threaded holes is indicated by the horizontal axis in the figure, i.e. from the first to the tenth helical hole is milled with each tool. The resulting depth change of the plug rod in the helical bore is indicated on the vertical axis of the figure. As can be seen in fig. 7: the degree of dimensional change of the threaded bore generally increases with more holes milled, which may indicate wear of the tool or roughening of the threaded surface. The thread milling tool constructed according to the invention is shown schematically in fig. 7 with very little or no dimensional change in the threaded bore, in fact in the 6 front holes. In contrast, the thread mills of Schmarje Tool Company and the thread mills of Scientific Cutting Tools produce very large dimensional variations in the milled holes. It can be seen from the figure that: the undesired dimensional changes in the milled hole only after the first hole with the tools of the prior art become apparent. It should be appreciated that: contrary to the recognition in the prior art, the larger helix angle of a tool constructed in accordance with the present invention improves chip formation and removal, and generally improves helical milling performance, such as reduced tool wear, smoother thread surface of the workpiece, and greatly improves the quality of the thread produced.

During process inspection, it has been found that: with the tool constructed according to the invention, a loud audible sound is produced during milling with the prior art thread milling tool. As is well known in the milling art, the loud sounds of the tones represent rough cutting, tool deflection and thread deformation, and thus material can be more efficiently milled from a milled hole with less work consumption using the thread milling tool of the present invention.

It can also be observed that: the plug stem can be moved more easily and smoothly into the milled hole made with the tool of the present invention than the milled hole made with the prior art tool. This shows that the threads made with the tool of the present invention are better than those made with the prior art tools. It should also be appreciated that: the larger helix angle of the tool flutes constructed in accordance with the present invention allows for more efficient chip removal and thread formation without interference from chips trapped in the workpiece hole.

The milled hole size variation shown in fig. 7 shows: the geometry of the thread milling tool design of the present invention may result in better milling performance, including: more consistent thread dimensions and more precise thread configurations, and also smoother milling to produce smoother shaped chips and smoother surfaces on the milled holes. The good milling performance produced by a tool constructed in accordance with the present invention will improve the quality of the threaded connection, reduce the cutting forces required during the thread milling process, extend the life of the tool, and reduce the tendency for edge chipping on the tool during the milling process. The superiority of the tool of the invention shown in the results of the comparison was unexpected and significant.

It should be appreciated that: the foregoing has shown those aspects of the present invention to provide a clear understanding of the invention. Accordingly, aspects that would be apparent to those of ordinary skill in the art but which would not facilitate a better understanding of the invention have not been provided in this application for the purpose of simplifying the description of the invention. While the invention has been described in conjunction with a number of embodiments, it is to be understood that many modifications and variations may be resorted to that described above, those skilled in the art having the benefit of the teachings herein. All such modifications and variations of the present invention are intended to be covered by the foregoing description and the appended claims. Specifically, while only one embodiment of the present invention is provided herein having a helix angle of about 30 degrees, it should be appreciated that: the thread milling tool of the present invention includes any suitable helix angle greater than 20 degrees as provided by and included in the claims below.

Claims (13)

1. A thread milling tool comprising:

a proximal handle;

a distal cutting portion comprising at least one thread milling tooth; and

at least one helically oriented flute defined on said cutting portion, wherein: the helical flutes are defined by a helix angle of at least 20 °.

2. The thread milling tool of claim 1 wherein the diameter of the cutting portion tapers along an axial direction on at least a portion of the thread milling tool.

3. The thread milling tool of claim 1 wherein the cutting portion comprises a plurality of axially spaced thread milling teeth.

4. The thread milling tool of claim 1 wherein no more than six helical flutes are defined in said cutting portion.

5. The thread milling tool of claim 1, wherein the helical flutes are defined by a helix angle of less than 40 °.

6. The thread milling tool of claim 2, wherein the helical flutes are defined by a helix angle of less than 40 °.

7. The thread milling tool of claim 1, wherein the helical flutes are defined by a helix angle of less than 30 °.

8. The thread milling tool of claim 1 wherein said shank portion comprises a Weldon shank.

9. The thread milling tool of claim 1 wherein said shank includes a detent.

10. The thread milling tool of claim 1 wherein said shank is attachable to a device for transferring the thread milling tool to form a thread on a workpiece.

11. The thread milling tool of claim 10 wherein said means is a computer controlled digitally device.

12. The thread milling tool of claim 1 further comprising a bore through said shank portion and said cutting portion to introduce coolant into said cutting portion.

13. The thread milling tool of claim 10, wherein the threads made on the workpiece conform to international pipe thread standards.