Replaceable cutting head having external thread with concavely curved root and rotary cutting tool

A rotary cutting tool includes a replaceable cutting head and a tool holder. The replaceable cutting head includes a forward cutting portion and a rearward mounting portion. The mounting portion includes a male coupling member that includes an external thread defined by external inner and outer cylinders. The external thread has concavely curved external thread roots which each merge with a respective straight external loaded surface at a first external root point. The first external root point is spaced apart from the external inner cylinder by a first external radial distance. The first external radial distance is greater than a third and less than two thirds of the external thread height. The tool holder includes a female coupling member that includes an internal thread. When the rotary cutting tool is in a locked position, the external thread is threadingly engaged with the internal thread.

FIELD OF THE INVENTION

The subject matter of the present application relates to rotary cutting tools of the type in which a replaceable cutting head, having a male coupling member, is removably retained in a female coupling member, of a tool holder, by means of a threaded coupling mechanism.

BACKGROUND OF THE INVENTION

Rotary cutting tools can be provided with a threaded coupling mechanism, or “tool joint”, for securely retaining a replaceable cutting head within a tool holder.

The replaceable cutting head can include a male coupling member and the tool holder can include a female coupling member. The male coupling member can include an external thread. The female coupling member can include an internal thread that corresponds to the external thread on the male coupling member.

The external threads have external loaded surfaces for abutting corresponding surface on the internal thread. The external loaded surfaces are typically straight. The external threads have external roots which merge with a respective external loaded surface.

In some such rotary cutting tools, the external roots are substantially straight and have a small radius where they merge with the respective external loaded surface. An example of such a rotary cutting tool is disclosed in, for example, U.S. Pat. No. 6,485,220. A disadvantage of such external threads is that they are prone to stress fracture in the region where the external roots adjoin the respective external loaded surface (i.e. at the small radius).

In other such rotary cutting tools, in order to overcome such a problem, the roots are elliptical. Examples of such a rotary cutting tool are disclosed in, for example, U.S. Pat. Nos. 4,799,844 and 5,060,740.

In yet other such rotary cutting tools, the external threads have roots defined by one or more radii. Examples of such a rotary cutting tool is disclosed in U.S. Pat. Nos. 4,549,754, 6,196,598 B1, 7,997,842 B2 and 9,874,058 B2.

It is an object of the subject matter of the present application to provide a replaceable cutting head having an external thread with improved stress reduction.

It is an object of the subject matter of the present application to provide a replaceable cutting head having an external thread with improved distribution of stress concentration.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the subject matter of the present application there is provided a replaceable cutting head, having a head longitudinal axis extending in a forward to rearward direction, comprisinga forward portion forming a cutting portion comprising at least one cutting edge; anda rearward portion forming a mounting portion, the mounting portion comprising a male coupling member having an external thread and protruding rearwardly from a head base surface, the head base surface extending transversely with respect to the head longitudinal axis, and defining a boundary between the cutting portion and the mounting portion, wherein:the external thread comprises an external thread ridge extending helically about an external thread axis and comprising forward and rearward external flank surfaces and an external top surface extending therebetween;the forward and rearward external flank surfaces generally face in opposite axial directions and delimit a helical external thread groove that comprises an external bottom surface;the external thread has a constant external thread pitch;the external thread has a constant external thread height;the external thread of the male coupling member is a straight thread defined by external inner and outer cylinders, the external inner cylinder having a minor thread diameter and the external outer cylinder having a major thread diameter, andin a cross-sectional view taken in an axial plane containing the external thread axis:the forward and rearward external flank surfaces form a plurality of straight external loaded surfaces and a plurality of external non-loaded surfaces respectively;the external bottom surface forms a plurality of concavely curved external thread roots, each external thread root extending between first and second external root points and merging with a respective external loaded surface at the first external root point, the first external root point is spaced apart from the external inner cylinder by a first external radial distance; andthe first external radial distance is greater than a third of the external thread height HEand less than two thirds of the external thread height HE.

In accordance with a second aspect of the subject matter of the present application there is provided a rotary cutting tool having a longitudinal axis and extending in a forward to rearward direction, comprising:a tool holder having a holder longitudinal axis and a replaceable cutting head of the type described above threadingly engaged to the tool holder.

It is understood that the above-said is a summary, and that features described hereinafter may be applicable in any combination to the subject matter of the present application, for example, any of the following features may be applicable to the replaceable cutting head or the rotary cutting tool:

The first external radial distance can be greater than five twelfths of the external thread height HEand less than seven twelfths of the external thread height.

Each external loaded surface can have an external loaded surface length measured along contour thereof. Each external thread root can have an external thread root length measured along the contour thereof between the first and second external root points. The external thread root length can be between three and six times greater than the external loaded surface length.

The minor thread diameter can be at least 75% of the major thread diameter.

The external thread can have between three and four turns in the axial direction.

The external thread can be a single start thread.

The external thread height can be greater than a third of the external thread pitch and less than half of the external thread pitch.

Each external thread root can be defined by a single external root radius.

The external root radius can be greater than or equal to 0.3 mm and less than or equal to 0.5 mm.

The first and second external root points can subtend an external root subtend angle at the circle center of an imaginary circle defined by the external root radius. The external root subtend angle θ can be greater than or equal to 90° and less than or equal to 160°.

The external non-loaded surfaces can be straight. Each external thread root can merge tangentially with a respective external non-loaded surface at the second external root point. The external root subtend angle θ can be greater than or equal to 120° and less than or equal to 140°.

The external root radius can be greater than a third of the external thread pitch and less than half of the external thread pitch.

Each external thread root can merge tangentially with the respective external loaded surface.

Each external thread root can merge with a respective external non-loaded surface at the second external root point.

The external non-loaded surfaces can be straight.

Each external thread root can merge tangentially with the respective external non-loaded surface.

The first and second external root points can be spaced apart in an axial direction by a point distance, the point distance can be greater than a third of the external thread pitch PE and less than half of the external thread pitch.

The external loaded surfaces can be inclined at an external flank angle with respect to a head radial plane perpendicular to the external thread axis. The external flank angle is in the range of 28°<α<34°.

In said cross-sectional view taken in an axial plane containing the external thread axis, the external top surface can form a plurality of external thread crests each comprising a radially outermost external crest surface, the radially outermost external crest surfaces can be parallel to the external thread axis and co-linear with each other.

In said cross-sectional view taken in an axial plane containing the external thread axis, each straight external loaded surface can have an external loaded surface height measured perpendicular to the external thread axis. The external loaded surface height can be greater than a third of the external thread height HEand less than three-fifths of the external thread height.

The replaceable cutting head can be made from a first material. The tool holder can be made from a second material. The first material can be harder than the second material.

The tool holder can have a female coupling member having an internal thread extending rearwardly from a holder forward surface, the holder forward surface extending transversely with respect to the holder longitudinal axis. The rotary cutting tool can be adjustable between: a released position in which the internal and external threads may not be threadingly engaged to one another, and a locked position in which the male coupling member can be removably retained in the female coupling member with the internal and external threads threadingly engaged to one another.

The internal thread of the female coupling member can be a straight thread defined by internal inner and outer cylinders.

The internal thread can have a constant internal thread pitch which can be the same as the external thread pitch.

The internal thread can comprise an internal thread ridge, extending helically about an internal thread axis, and comprising forward and rearward internal flank surfaces and an internal top surface extending therebetween. The forward and rearward internal flank surfaces can generally face in opposite axial directions and delimit a helical internal thread groove that comprises an internal bottom surface. The forward external flank surface and the forward internal flank surface can face in the forward direction. The rearward external flank surface and the rearward internal flank surface can face in the rearward direction. In the locked position, the rearward internal flank surface can abut the forward external flank surface.

In the locked position, the forward internal flank surface can be spaced apart from the rearward external flank surface. The internal top surface can be spaced apart from the external bottom surface. The internal bottom surface can be spaced apart from the external top surface.

In a cross-sectional view taken in an axial plane containing the internal thread axis, the forward and rearward internal flank surfaces can form a plurality of internal non-loaded surfaces and a plurality of internal loaded surfaces respectively, the internal loaded surfaces can be straight.

The internal top surface can form a plurality of internal thread crests, each of the plurality of internal thread crests comprising a radially innermost internal crest surface which can be parallel to the internal thread axis, the radially innermost internal crest surfaces can be co-linear with each other.

Each of the plurality of internal thread crests can comprise a relieved internal crest surface extending between a respective radially innermost internal crest surface and a respective internal loaded surface. Each relieved internal crest surface can be oriented transversely with respect to the respective radially innermost internal crest surface and the respective internal loaded surface.

It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity, or several physical components may be included in one functional block or element. Where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the subject matter of the present application will be described. For purposes of explanation, specific configurations and details are set forth in sufficient detail to provide a thorough understanding of the subject matter of the present application. However, it will also be apparent to one skilled in the art that the subject matter of the present application can be practiced without the specific configurations and details presented herein.

Attention is first drawn toFIGS. 1 and 2showing a rotary cutting tool20of the type used for milling operations, specifically end milling, in accordance with embodiments of the subject matter of the present application. The rotary cutting tool20has a tool longitudinal axis L around which the tool rotates in a direction of rotation R when performing metal cutting operations.

The rotary cutting tool20includes a replaceable cutting head22that has a head longitudinal axis A, around which the replaceable cutting head22rotates in the direction of rotation R. The head longitudinal axis A extends in the forward DFto rearward direction DR. The replaceable cutting head22is made from a first material. The first material can be cemented carbide.

The rotary cutting tool20also includes a tool holder24having a holder longitudinal axis C. The tool holder24is made from a second material. The second material can be harder than the first material. The second material can be steel. The replaceable cutting head22can be removably retained in the tool holder24by means of a threaded coupling mechanism. Such a threaded coupling mechanism could possibly be advantageous for other types of rotary cutting operations than that stated hereinabove, such as, for example, reaming or drilling.

It should be appreciated that use of the terms “forward” and “rearward” throughout the description and claims refer to a relative position of the replaceable cutting head22to the tool holder24of the assembled rotary cutting tool20, as seen inFIG. 1. The terms “forward” and “rearward” may also be applied in a direction of the head longitudinal axis A towards the left and right, respectively, inFIGS. 3 and 4, and also in a direction of holder longitudinal axis C towards the left and right, respectively, inFIGS. 6 and 7. It is understood that the tool holder24also extends along its own forward to rearward direction along the holder longitudinal axis C.

Reference is now made toFIGS. 3 to 5. The replaceable cutting head22has a forward portion that forms a cutting portion26and a rearward portion that forms a mounting portion28. In accordance with some embodiments of the subject matter of the present application the replaceable cutting head22can be formed to have unitary integral one-piece construction. This provides an advantage in that the replaceable cutting head22has no detachable cutting inserts (not shown).

Referring toFIG. 3, the cutting portion26includes at least one cutting edge30,30b. In this non-limiting example shown in the drawings the at least one cutting edge30,30bcan include exactly four peripheral cutting edges. Each peripheral cutting edge30is formed at the intersection of a peripheral relief surface32, and a peripheral rake surface34. The peripheral relief surface32is located rotationally behind the peripheral cutting edge30and the peripheral rake surface34is located rotationally ahead of the peripheral cutting edge30, both in respect to the direction of rotation R. The orientation of the peripheral cutting edge30allows metal cutting operations to be performed.

In accordance with some embodiments of the subject matter of the present application, the cutting portion26can include at least one flute36for evacuating chips (not shown) that are produced during the cutting operation. One flute36is associated with each peripheral cutting edge30. The at least one cutting edge30,30bcan include one or more end cutting edges30bat an end face37of the cutting portion26. In this non-limiting example shown in the drawings, the at least one cutting edge30,30bcan include exactly four end cutting edges30b.

Making reference now toFIGS. 3 and 4, the mounting portion28includes a male coupling member38that protrudes rearwardly from a head base surface40. The head base surface40extends transversely with respect to the head longitudinal axis A and defines a boundary between the cutting portion26and the mounting portion28. That is to say, the cutting portion26is formed forward of the head base surface40and the mounting portion28is formed rearward of the head base surface40. In accordance with some embodiments of the subject matter of the present application the male coupling member38can be rigid. The head base surface40can be perpendicular to the head longitudinal axis A. The head base surface40is intended to abut a corresponding surface70(described further below) on the tool holder24when the rotary cutting tool20is in a locked position, as will be described hereinafter.

The male coupling member38includes an external (male) thread42. Referring toFIG. 3, the external thread42includes an external thread ridge44that extends helically about an external thread axis B. The external thread axis B is co-incident with the head longitudinal axis A. Thus, the external thread portion42and the replaceable cutting head22are co-axial. Referring toFIG. 4, the external thread ridge44includes forward and rearward external flank surfaces46,48and an external top surface50that extends therebetween. The forward and rearward external flank surfaces46,48face in opposite axial directions DF, DR, with the forward external flank surface46facing in the forward direction DFand the rearward external flank surface48facing in the rearward direction DR. The forward and rearward external flank surfaces46,48delimit an external thread groove52. The external thread groove52extends helically about the external thread axis B and includes an external bottom surface54. The external thread42has an external thread pitch PE. The external thread pitch PE is constant.

As shown inFIG. 4, in a cross-sectional view taken in an axial plane (that is, a plane that contains the external thread axis B) the external top surface50forms a plurality of external thread crests56and the external bottom surface54forms a plurality of external thread roots58.

In the same cross-sectional view, the forward and rearward external flank surfaces46,48form a plurality of external loaded surfaces59aand a plurality of external non-loaded surfaces59b, respectively. The external loaded surfaces59aserve the purpose to contact a corresponding surface on an internal (female) thread. The external loaded surfaces59aare straight. Each external loaded surface59ahas an external loaded surface length L1measured along the contour of external loaded surface59a. Each external loaded surface59ahas an external loaded surface height HSmeasured perpendicular to the external thread axis B. In accordance with some embodiments of the subject matter of the present application, the external loaded surfaces59acan be inclined at an external flank angle α with respect to a head radial plane RP1perpendicular to the external thread axis B. Preferably, the external flank angle α can be between 28°-34°, and more preferably is 31°. The external non-loaded surfaces59bcan be straight. The external non-loaded surfaces59bcan be inclined to the head radial plane RP1at the same flank angle α as the external loaded surfaces59a(but reflected about the head radial plane RP1). The external thread42defines an external thread form60.

In accordance with some embodiments of the subject matter of the present application, the external thread42is a straight thread. It should be appreciated that the term “straight thread” throughout the description and claims relates to a thread where the thread ridge and thread groove extends about a respective cylinder and thus all the thread crests56,88are equidistant from the thread axis, as are all the thread roots58,90. Thus, in a straight thread, both thread diameters (the thread major diameter d1of external thread ridge44, and the thread minor diameter d2of the external thread groove52) are constant in the rearward direction DFof the cutting head22. Such a straight thread may be formed by threading the hollow forward end of a cylindrical steel rod with an external turning insert. As the steel rod rotates and moves in the axial direction to form the external thread it does not move radially away from the ‘static’ cutting insert so that the thread has a cylindrical configuration. Specifically, the thread groove extends about an external inner cylinder EC1defined by the points where the external bottom surface54are closest to the external thread axis B. The external thread ridge extends about an external outer cylinder EC2defined by the points where the external top surface50are furthest from the external thread axis B. The external inner and outer cylinders EC1, EC2have the external thread axis B as their respective axes.

The plurality of external thread crests56define the thread major diameter (corresponding to external outer cylinder EC2) and the plurality of external thread roots58define the thread minor diameter (associated with external inner cylinder EC1) of the external thread42, respectively. The major diameter minus the minor diameter, divided by two, equals the external thread height HEof the external thread42. The external thread height HEis constant. In accordance with some embodiments of the subject matter of the present application, the minor thread diameter d2of the external thread groove52can be at least 75% of the major thread diameter d1of external thread ridge44. Thus, the overall strength and rigidity of the external thread42is not detrimentally affected. The external thread height HEcan be greater than a third of the external thread pitch PE. The external thread height HEcan be less than half of the external thread pitch PE. The external loaded surface height HScan be greater than a third of the external thread height HE. The external loaded surface height HScan be less than three-fifths of the external thread height HE.

In accordance with some embodiments of the subject matter of the present application, the external thread42can have between three and four turns in the axial direction. Advantageously, this permits the cutting head22to be manufactured with less material than other cutting heads (not shown) having more turns. The external thread42can be a single start thread.

In a cross-sectional view taken in an axial plane containing the external thread axis B, each external thread root58is concavely curved. That is to say, each external thread root58curves inwardly. Advantageously such a configuration reduces stress at the external thread root58. Each external thread root58extends between first and second external root points P1, P2. The first and second external root points P1, P2are the points on the external thread form60at which opposing extremities of each concavely curved external thread root58terminate and transition into adjacent non-concavely curved external loaded and non-load surfaces59a,59b, respectively. The first and second external root points P1, P2are further from the external thread axis B than a mid-portion of the external thread root58. Each external thread root58has an external thread root length L2measured along the contour of the external thread root58between the first and second external root points P1, P2. In accordance with some embodiments of the subject matter of the present application, the external thread root length L2can be greater than the external loaded surface length L1. In particular, the external thread root length L2can be between three and six times greater than the external loaded surface length L1.

Reference is made in particular toFIG. 5. Each external thread root58merges with a respective external loaded surface59aat the first external root point P1. The first external root point P1is spaced apart from the external inner cylinder EC1by a first external radial distance ERD1. The first external root point P1is spaced apart from the external outer cylinder EC2by a second external radial distance ERD2. The sum of the first external radial distance ERD1and the second external radial distance ERD2is equal to the external thread height HE. The first external radial distance ERD1is greater than a third of the external thread height HEand less than two thirds of the external thread height HE. Advantageously, combined with the concavely curved root, this provides a reduction of stress at the external thread root, thereby increasing tool life, while at the same time providing sufficient contact area between the external and internal loaded surface59a,91afor a secure threaded coupling. Further advantageously, this configuration provides enlarged clearance to allow threaded engagement with an internal thread having increased height. Preferably, in accordance with some embodiments of the subject matter of the present application, the first external radial distance ERD1can be greater than five twelfths of the external thread height HEand less than seven twelfths of the external thread height HE.

In accordance with some embodiments of the subject matter of the present application, in a cross-sectional view taken in an axial plane containing the external thread axis B, the first and second external root points P1, P2can be spaced apart in an axial direction by a point distance d. The point distance d can be greater than a third of the external thread pitch PE. The point distance d can be less than half of the external thread pitch PE.

In accordance with some embodiments of the subject matter of the present application, the first external radial distance ERD1can be less than the second external radial distance ERD2. Stated differently, the first external root point P1is further from the external outer cylinder EC2than from the external inner cylinder EC1. Each external thread root58can merge tangentially with the respective external loaded surface59a. Each external thread root58can merge with a respective external non-loaded surface59bat the second external root point P2. In such a configuration, each external thread root58extends between one of the external loaded surfaces59aand one of the external non-loaded surfaces59b. In the configuration where the external non-loaded surfaces (59b) are straight, each external thread root58can merge tangentially with the respective external non-loaded surface59b.

In accordance with some embodiments of the subject matter of the present application, each external thread root58can be defined by a single external root radius R. Advantageously, this provides an improved distribution of stress at the external thread root58. Further advantageously, such cutting heads22are easier to manufacture. The external root radius R can be greater than or equal to 0.3 mm. The external root radius R can be less than or equal to 0.5 mm. The first and second external root points P1, P2can subtend an external root subtend angle θ at the circle center O of an imaginary circle defined by the external root radius R. The external root subtend angle θ can be greater than or equal to 90° and less than or equal to 160°. The external root radius R can be greater than a third of the external thread pitch PE. The external root radius R can be less than half of the external thread pitch PE. In the configuration where the external non-loaded surfaces (59b) are straight and each external thread root58merges tangentially with a respective external non-loaded surface59bat the second external root point P2, the external root subtend angle θ can be greater than or equal to 120° and less than or equal to 140°.

In accordance with some embodiments of the subject matter of the present application, the plurality of external thread crests56each include a radially outermost external crest surface61a. The radially outermost external crest surfaces61acan be parallel to the external thread axis B and co-linear with each other. Thus, a radially outermost portion of the external top surface50can lie on the external outer cylinder EC2.

As shown inFIGS. 3 and 4, the male coupling member38includes a forward bearing portion62. The forward bearing portion50is located on the forward side of the external thread42. The forward bearing portion62includes a forward head abutment surface64that tapers radially inwardly towards the head longitudinal axis A in a rearward direction DR. That is to say, the forward head abutment surface64has a conical shape facing radially outwards. It is noted that the forward head abutment surface64is intended to abut a corresponding surface94(described further below) on the tool holder24when the rotary cutting tool20is in a locked position, as will be described hereinafter.

It should be appreciated that use of the terms “radially inward/inwardly” and “radially outward/outwardly” throughout the description and claims refer to a relative position in a perpendicular direction in relation to the head longitudinal axis A and/or holder longitudinal axis C, towards and away from the respective axis, inFIGS. 3 to 4andFIG. 7.

Referring now toFIGS. 6 to 8, the tool holder24has a holder longitudinal axis C that extends in the forward DFto rearward direction DR. The tool holder24includes a holder peripheral surface71which extends about the holder longitudinal axis C. The tool holder24includes a female coupling member68that extends rearwardly from a holder forward surface70. The holder forward surface70extends transversely with respect to the holder longitudinal axis C. In accordance with some embodiments of the subject matter of the present application the holder forward surface70can be perpendicular to the holder longitudinal axis C.

The female coupling member68includes an internal (female) thread72. As shown in a longitudinal cross-sectional view of the female coupling member68containing the internal thread axis D (i.e.FIG. 7), the internal thread72includes an internal thread ridge74that extends helically about an internal thread axis D. The internal thread axis D is co-incident with the holder longitudinal axis C. Thus, the internal thread72is co-axial with the tool holder24. The internal thread ridge74includes forward and rearward internal flank surfaces76,78and an internal top surface80that extends therebetween. The forward and rearward internal flank surfaces76,78face in opposite axial directions DF, DR, with the forward internal flank surface76facing in the forward direction DFand the rearward internal flank surface78facing in the rearward direction DR. The forward and rearward internal flank surfaces76,78delimit an internal thread groove82. The internal thread72has an internal thread pitch PI. In accordance with some embodiments of the subject matter of the present application, the internal thread pitch PI can be constant. The internal thread pitch PI can be same as the external thread pitch PE.

The internal thread groove82extends helically about the internal thread axis D and includes an internal bottom surface84. In a cross-sectional view taken in an axial plane (that is, a plane that contains the internal thread axis D) the internal top surface80forms a plurality of internal thread crests88and the internal bottom surface84forms a plurality of internal thread roots90. The tool holder24has a holder thickness T measured in a radial direction between the holder peripheral surface71and the internal top surface80.

Reference is made in particular toFIGS. 7 and 8. In a cross-sectional view taken in an axial plane containing the internal thread axis D (i.e.FIG. 7), the forward and rearward internal flank surfaces76,78form a plurality of internal loaded surfaces91aand a plurality of internal non-loaded surfaces91b, respectively. In accordance with some embodiments of the subject matter of the present application, the internal loaded surfaces91acan be straight. The internal loaded surfaces91acan be inclined at an internal flank angle β with respect to a holder radial plane RP2perpendicular to the internal thread axis D. Preferably, the internal flank angle β can be 31°. The internal flank angle β can have the same value as the external flank angle α so that a continuous surface to surface abutment can be realized between internal and external loaded surface91a,59a. The internal non-loaded surfaces91bcan be straight. The internal non-loaded surfaces91bcan be inclined in an identical fashion to the internal loaded surfaces91a(but reflected about the holder radial plane RP2). The internal thread72defines an internal thread form86.

In accordance with some embodiments of the subject matter of the present application, the internal thread72can be a straight thread. Specifically, the internal thread ridge74extends about an internal inner cylinder IC1defined by the points where the internal top surface80are closest to the internal thread axis D. The internal thread groove82extends about an internal outer cylinder IC2defined by the points where the internal bottom surface84internal thread axis D. The internal inner and outer cylinders IC1, IC2have the internal thread axis D as their respective axes.

The plurality of internal thread crests88define the minor diameter (corresponding to internal inner cylinder IC1) and the plurality of internal thread roots90define the major diameter (associated with internal outer cylinder IC2) of the internal thread72, respectively. The major diameter minus the minor diameter, divided by two, equals the internal thread height HIof the internal thread72. The internal thread height HIcan be constant. The internal thread height HIcan be greater than a third of the internal thread pitch PI. The internal thread height HIcan be less than half of the internal thread pitch PI.

In accordance with some embodiments of the subject matter of the present application, the internal thread72can have between three and four turns in the axial direction. The internal thread72can be a single start thread.

In accordance with some embodiments of the subject matter of the present application, the plurality of internal thread crests88can each include a radially innermost internal crest surface96a. The radially innermost internal crest surfaces96acan be parallel to the internal thread axis D and co-linear with each other. Thus, a radially innermost portion of the internal top surface80can lie on the internal inner cylinder IC1. The plurality of internal thread roots90can each include a radially outermost internal root surface96b. The radially outermost internal root surface96bcan be parallel to the internal thread axis D and co-linear with each other. Thus, a radially outermost portion of the internal bottom surface84can lie on the internal outer cylinder IC2.

In accordance with some embodiments of the subject matter of the present application, each of the plurality of internal thread crests88can include a relieved internal crest surface98extending between a respective radially innermost internal crest surface96aand a respective internal loaded surface91a. In a longitudinal cross-sectional view of the female coupling member68containing the internal thread axis D (i.e.FIG. 8), each relieved internal root surface98can be oriented transversely with respect to the respective radially outermost internal root surface96aand the respective internal loaded surface91a. The relieved internal crest surface98can merge with the respective internal loaded surface91aat an internal crest point P′. The internal crest point P′ is spaced apart from the internal inner cylinder IC1by a first internal radial distance IRD1. The internal crest point P′ is spaced apart from the internal outer cylinder IC2by a second internal radial distance IRD2. The sum of the first internal radial distance IRD1and the second internal radial distance IRD2is equal to the internal thread height HI. The first internal radial distance IRD1is greater than a quarter of the internal thread height HIand less than a half of the internal thread height HI. By increasing the internal thread height HI, the holder thickness T is also increased. It has been found that increased holder thickness reduces stress at the internal thread root90.

As shown inFIGS. 6 and 7, the female coupling member68includes a forward supporting portion92The forward supporting portion92is located on the forward side of the internal thread72. The forward supporting portion92includes a forward holder abutment surface94that tapers radially inwardly towards the holder longitudinal axis C in a rearward direction DR. That is to say, the forward holder abutment surface94has a conical shape facing radially inwards. The forward holder abutment surface94is configured and dimensioned to be complementary to the forward head abutment surface64discussed above.

Assembly of the rotary cutting tool20is known, for example, from U.S. Pat. No. 6,485,220 B2, which is hereby incorporated by reference in its entirety. It is noted that the rotary cutting tool20is adjustable between a released position and a locked (or assembled) position.

To adjust the rotary cutting tool20to the locked position the external thread42is screwed (i.e. turned) into the internal thread72.

In the locked position the male coupling member38is removably retained in the female coupling member68. Also, the external and internal threads42,72threadingly engage each other. Referring now toFIG. 9, the forward head abutment surface64abuts the forward holder abutment surface94. In accordance with some embodiments of the subject matter of the present application, the rearwardly facing head base surface40can abut the forwardly facing holder forward surface70. The rearward internal flank surface78can abut the forward external flank surface46. The forward internal flank surface76can be spaced apart from the rearward external flank surface48. The internal top surface80can be spaced apart from the external bottom surface54. The internal bottom surface84can be spaced apart from the external top surface50. By virtue of the foregoing, in a cross-sectional view taken in an axial plane containing the longitudinal axis L, each of the internal loaded surfaces91aabuts a respective external loaded surface59a. Each of the internal non-loaded surfaces91bis spaced apart from a respective external non-loaded surface59b. Each internal thread crest88is spaced apart from a respective external thread root58. Each internal thread root90is spaced apart from a respective external thread crest56.

Although the subject matter of the present application has been described to a certain degree of particularity, it should be understood that various alterations and modifications could be made without departing from the spirit or scope of the invention as hereinafter claimed.