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.
1. A replaceable cutting head ( 22 ), having a head longitudinal axis (A) extending in a forward (D F ) to rearward direction (D R ), comprising
a forward portion forming a cutting portion ( 26 ) comprising at least one cutting edge ( 30 ); and
a rearward portion forming a mounting portion ( 28 ), the mounting portion ( 28 ) comprising a male coupling member ( 38 ) having an external thread ( 42 ) and protruding rearwardly from a head base surface ( 40 ), the head base surface ( 40 ) extending transversely with respect to the head longitudinal axis (A), and defining a boundary between the cutting portion ( 26 ) and the mounting portion ( 28 ), wherein:
the external thread ( 42 ) comprises an external thread ridge ( 44 ) extending helically about an external thread axis (B) and comprising forward and rearward external flank surfaces ( 46 , 48 ) and an external top surface ( 50 ) extending therebetween;
the forward and rearward external flank surfaces ( 46 , 48 ) generally face in opposite axial directions (D F , D R ) and delimit a helical external thread groove ( 52 ) that comprises an external bottom surface ( 54 );
the external thread ( 42 ) has a constant external thread pitch PE;
the external thread ( 42 ) has a constant external thread height H E ;
the external thread ( 42 ) of the male coupling member ( 38 ) is a straight thread defined by external inner and outer cylinders (EC 1 , EC 2 ), the external inner cylinder (EC 1 ) having a minor thread diameter d 2 and the external outer cylinder (EC 2 ) having a major thread diameter d 1 ; and
in a cross-sectional view taken in an axial plane containing the external thread axis (B):
the forward and rearward external flank surfaces ( 46 , 48 ) form a plurality of straight external loaded surfaces ( 59 a ) and a plurality of external non-loaded surfaces ( 59 b ) respectively;
the external bottom surface ( 54 ) forms a plurality of concavely curved external thread roots ( 58 ), each external thread root ( 58 ) extending between first and second external root points (P 1 , P 2 ) and merging with a respective external loaded surface ( 59 a ) at the first external root point (P 1 ), the first external root point (P 1 ) is spaced apart from the external inner cylinder (EC 1 ) by a first external radial distance ERD 1 ; and
the first external radial distance ERD 1 is greater than a third of the external thread height H E and less than two thirds of the external thread height H E .
2. The replaceable cutting head ( 22 ) according to claim 1 , wherein the first external radial distance ERD 1 is greater than five twelfths of the external thread height H E and less than seven twelfths of the external thread height H E .
3. The replaceable cutting head ( 22 ) according to claim 1 , wherein:
each external loaded surface ( 59 a ) has an external loaded surface length L 1 measured along contour thereof; and
each external thread root ( 58 ) has an external thread root length L 2 measured along the contour thereof between the first and second external root points (P 1 , P 2 ); and
the external thread root length L 2 is between three and six times greater than the external loaded surface length L 1 .
4. The replaceable cutting head ( 22 ) according to claim 1 , wherein the minor thread diameter d 2 is at least 75% of the major thread diameter d 1 .
5. The replaceable cutting head ( 22 ) according to claim 1 , wherein the external thread ( 42 ) has between three and four turns in the axial direction.
6. The replaceable cutting head ( 22 ) according to claim 1 , wherein the external thread ( 42 ) is a single start thread.
7. The replaceable cutting head ( 22 ) according to claim 1 , wherein the external thread height H E is greater than a third of the external thread pitch PE and less than half of the external thread pitch PE.
8. The replaceable cutting head ( 22 ) according to claim 1 , wherein each external thread root ( 58 ) is defined by a single external root radius R.
9. The replaceable cutting head ( 22 ) according to claim 8 , wherein the external root radius R is greater than or equal to 0.3 mm and less than or equal to 0.5 mm.
10. The replaceable cutting head ( 22 ) according to claim 8 , wherein:
the first and second external root points (P 1 , P 2 ) subtend an external root subtend angle θ at the circle center (O) of an imaginary circle defined by the external root radius R; and
the external root subtend angle θ is greater than or equal to 90° and less than or equal to 160°.
11. The replaceable cutting head ( 22 ) according to claim 10 , wherein:
the external non-loaded surfaces ( 59 b ) are straight;
each external thread root ( 58 ) merges tangentially with a respective external non-loaded surface ( 59 b ) at the second external root point (P 2 ); and
the external root subtend angle θ is greater than or equal to 120° and less than or equal to 140°.
12. The replaceable cutting head ( 22 ) according to claim 8 , wherein the external root radius R is greater than a third of the external thread pitch PE and less than half of the external thread pitch PE.
13. The replaceable cutting head ( 22 ) according to claim 1 , wherein each external thread root ( 58 ) merges tangentially with the respective external loaded surface ( 59 a ).
14. The replaceable cutting head ( 22 ) according to claim 1 , wherein each external thread root ( 58 ) merges with a respective external non-loaded surface ( 59 b ) at the second external root point (P 2 ).
15. The replaceable cutting head ( 22 ) according to claim 14 , wherein the external non-loaded surfaces ( 59 b ) are straight.
16. The replaceable cutting head ( 22 ) according to claim 15 , wherein each external thread root ( 58 ) merges tangentially with the respective external non-loaded surface ( 59 b ).
17. The replaceable cutting head ( 22 ) according to claim 1 , wherein the first and second external root points (P 1 , P 2 ) are spaced apart in an axial direction by a point distance d, the point distance d being greater than a third of the external thread pitch PE and less than half of the external thread pitch PE.
18. The replaceable cutting head ( 22 ) according to claim 1 , wherein:
the external loaded surfaces ( 59 a ) are inclined at an external flank angle α with respect to a head radial plane (RP 1 ) perpendicular to the external thread axis (B); and
the external flank angle α is in the range of 28°<α<34°.
19. The replaceable cutting head ( 22 ) according to claim 1 , wherein in said cross-sectional view taken in an axial plane containing the external thread axis (B):
the external top surface ( 50 ) forms a plurality of external thread crests ( 56 ) each comprising a radially outermost external crest surface ( 61 a ), the radially outermost external crest surfaces ( 61 a ) being parallel to the external thread axis (B) and co-linear with each other.
20. The replaceable cutting head ( 22 ) according to claim 1 , wherein in said cross-sectional view taken in an axial plane containing the external thread axis (B):
each straight external loaded surface ( 59 a ) has an external loaded surface height H S measured perpendicular to the external thread axis (B); and
the external loaded surface height H S is greater than a third of the external thread height H E and less than three-fifths of the external thread height H E .
21. A rotary cutting tool ( 20 ) having a longitudinal axis (L) and extending in a forward (D F ) to rearward direction (D R ), comprising:
a tool holder ( 24 ) having a holder longitudinal axis (C) and a replaceable cutting head ( 22 ) in accordance with claim 1 threadingly engaged to the tool holder ( 24 ).
22. The rotary cutting tool ( 20 ) according to claim 21 , wherein:
the replaceable cutting head ( 22 ) is made from a first material;
the tool holder ( 24 ) is made from a second material; and
the first material is harder than the second material.
23. The rotary cutting tool ( 20 ) according to claim 21 , wherein:
the tool holder ( 24 ) has a female coupling member ( 68 ) having an internal thread ( 72 ) extending rearwardly from a holder forward surface ( 70 ), the holder forward surface ( 70 ) extending transversely with respect to the holder longitudinal axis (C); and
the rotary cutting tool ( 20 ) is adjustable between:
a released position in which the internal and external threads ( 42 , 72 ) are not threadingly engaged to one another, and
a locked position in which the male coupling member ( 38 ) is removably retained in the female coupling member ( 68 ) with the internal and external threads ( 42 , 72 ) threadingly engaged to one another.
24. The rotary cutting tool ( 20 ) according to claim 23 , wherein the internal thread ( 72 ) of the female coupling member ( 68 ) is a straight thread defined by internal inner and outer cylinders (IC 1 , IC 2 ).
25. The rotary cutting tool ( 20 ) according to claim 23 , wherein:
the internal thread ( 72 ) comprises an internal thread ridge ( 74 ), extending helically about an internal thread axis (D), and comprising forward and rearward internal flank surfaces ( 76 , 78 ) and an internal top surface ( 80 ) extending therebetween;
the forward and rearward internal flank surfaces ( 76 , 78 ) generally face in opposite axial directions (D F , D R ) and delimit a helical internal thread groove ( 82 ) that comprises an internal bottom surface ( 84 ); and
the forward external flank surface ( 46 ) and the forward internal flank surface ( 76 ) face in the forward direction (D F );
the rearward external flank surface ( 48 ) and the rearward internal flank surface ( 78 ) face in the rearward direction (D R ); and
in the locked position, the rearward internal flank surface ( 78 ) abuts the forward external flank surface ( 46 ).
26. The rotary cutting tool ( 20 ) according to claim 25 , wherein the internal thread ( 72 ) has a constant internal thread pitch (PI) which is the same as the external thread pitch (PE).
27. The rotary cutting tool ( 20 ) according to claim 25 , wherein in the locked position:
the forward internal flank surface ( 76 ) is spaced apart from the rearward external flank surface ( 48 );
the internal top surface ( 80 ) is spaced apart from the external bottom surface ( 54 ); and
the internal bottom surface ( 84 ) is spaced apart from the external top surface ( 50 ).
28. The rotary cutting tool ( 20 ) according to claim 25 , wherein in a cross-sectional view taken in an axial plane containing the internal thread axis (D):
the forward and rearward internal flank surfaces ( 76 , 78 ) form a plurality of internal non-loaded surfaces ( 91 b ) and a plurality of internal loaded surfaces ( 91 a ) respectively, the internal loaded surfaces ( 91 a ) being straight.
29. The rotary cutting tool ( 20 ) according to claim 25 , wherein:
the internal top surface ( 80 ) forms a plurality of internal thread crests ( 88 ), each of the plurality of internal thread crests ( 88 ) comprising a radially innermost internal crest surface ( 96 a ) which is parallel to the internal thread axis (D), the radially innermost internal crest surfaces ( 96 a ) being co-linear with each other.
30. The rotary cutting tool ( 20 ) according to claim 29 , wherein:
each of the plurality of internal thread crests ( 88 ) comprises a relieved internal crest surface ( 98 ) extending between a respective radially innermost internal crest surface ( 96 a ) and a respective internal loaded surface ( 91 a ); and
each relieved internal crest surface ( 98 ) is oriented transversely with respect to the respective radially innermost internal crest surface ( 96 a ) and the respective internal loaded surface ( 91 a ).