Reaming tool with a guide shank

[0002] The invention relates to a reaming tool for reaming a plurality of coaxial bores of a workpiece that are arranged at an axial distance from one another. The reaming tool includes a guide shank that can be clamped in place in a machine spindle and supported by external supporting elements in prepared bores of the workpiece to be machined and/or in a guide bush. The reaming tool includes a reaming head coaxially adjoining the front end of the guide shank and provided with cutting edges, and if need be, additional supporting elements.

[0003] To finish-ream camshaft or crankshaft bearing bores, a reaming tool with a long guide shank, which is supported in the pre-machined bores, is required in order to achieve the requisite alignment. In known reaming tools of this type, the reaming head is connected in one piece with the guide shank. A disadvantage there is that the rework of the cutting edges is relatively expensive. If the cutting edges are worn, the entire tool on the machine has to be exchanged.

[0004] Starting herefrom, the object of the invention is to develop a reaming tool having a guide shank of the type mentioned at the beginning, which reaming tool permits a simple change in the event of wear.

[0005] The features specified in claim 1 are proposed in order to achieve this object. Advantageous configurations and developments of the invention follow from the dependent claims.

[0006] According to a second embodiment variant of the invention, it is proposed, for the purpose of a kinematic reversal relative to the first embodiment variant, that the self-centering clamping mechanism have a tapered receptacle which is arranged concentrically in the reaming head and is defined by an annular flat face, a tapered spigot which projects coaxially forward beyond the guide shank, is defined by an annular flat face and can be inserted into the tapered receptacle, and a clamping screw which can be screwed from the reaming head into a coaxial internal thread of the guide shank, the reaming head and the guide shank, in the region of their flat faces, in the state free of stress, limiting a defined gage clearance dimension and, in the clamped state, bearing against one another with the tapered receptacle and the supporting elements arranged on the outside in this region being expanded elastically to a guide dimension. The rotary driving between the guide shank on the one hand and the reaming head on the other hand is expediently effected in this case by the reaming head having an internal hexagon arranged between tapered receptacle and annular flat face, while the guide shank has an external hexagon arranged between the tapered spigot and the annular flat face and complementary to the internal hexagon.

[0007] The clamping screw is expediently designed as a differential screw screwed into opposing internal threads of the reaming head and of the guide shank.

[0008] Tests have shown that the optimum gage clearance dimension lies between 0.01 and 0.06 mm, preferably 0.02 and 0.04 mm, while the taper angle of the tapered spigot and of the tapered receptacle should be 8° to 16°. In order to avoid distortions in the region of the plane-parallel contact, it is important that the clamping screw, in the clamping state, is fastened with a tightening torque of D=X+d, in which case X denotes the tightening torque at the instant when the flat faces run against one another and d denotes an additional torque for producing a frictional connection in the order of magnitude of 0.4 to 4 Nm.

[0009] The supporting elements are advantageously designed as guide strips. A preferred configuration of the invention provides for the reaming head to have several, preferably four to six, cutting edges distributed over the circumference preferably at unequal spacing, and for the guide shank to have a corresponding number of guide strips distributed over the circumference and running in an axially parallel manner, the guide strips being arranged in the circumferential direction such as to be offset in each case from the spatially associated cutting edges. With these measures, a high surface quality in the bores to be machined can be achieved. A further improvement in this respect is achieved in that the flat face on the reaming-head side and the flat face on the rod side are continuous in the circumferential direction, that in other words no transverse grooves or transverse pins are arranged in this region.

[0010] A preferred method of producing a guide shank for the reaming tool according to the invention in accordance with the first embodiment variant provides for the pre-machined guide shank, provided with supporting elements or guide strips and with a front-end tapered receptacle, to be slipped onto a grinding arbor provided with a tapered spigot and a flat face defining the tapered spigot in an annular manner and to be clamped in place on this grinding arbor by means of a central clamping mechanism with the rod-side supporting elements or guide strips being partly expanded radially, and to then be ground on the outside diameter, with a predetermined diameter guide dimension being set, and to be removed again from the grinding arbor. The guide shank to be ground on the outside diameter, with a predetermined gage clearance dimension of preferably 0.01 to 0.06 mm being maintained, is put onto the tapered spigot of the grinding arbor and is tightened up to the stop with a defined clamping force against its flat face. To this end, the guide shank is advantageously clamped in place on the tapered spigot of the grinding arbor with a defined tightening torque by means of a differential screw.

[0011] Alternatively, to produce a reaming head for the reaming tool according to the invention in accordance with the second embodiment variant, it is proposed that the pre-machined reaming head, provided with supporting elements or guide strips and with a rear tapered receptacle, is slipped onto a grinding arbor provided with a tapered spigot and a flat face defining the tapered spigot in an annular manner and is clamped in place on this grinding arbor by means of a central clamping mechanism with the reaming-head-side supporting elements or guide strips being partly expanded radially, and is then ground on the outside diameter, with a predetermined diameter guide dimension being set, and is removed again from the grinding arbor. In this case, the reaming head to be ground on the outside diameter, with a predetermined gage clearance dimension of preferably 0.01 to 0.06 mm expediently being maintained, is put onto the tapered spigot of the grinding arbor and is tightened up to the stop with a defined clamping force against its fiat face. To this end, the reaming head is advantageously clamped in place on the tapered spigot of the grinding arbor with a defined tightening torque by means of a differential screw. The best repetitive accuracy is achieved if the differential screw, during the clamping operation, is fastened with a tightening torque D=X+d, in which case X denotes the tightening torque at the instant when the flat faces run against one another and d denotes an additional torque of 0.4 to 4 Nm for producing a frictional connection.

[0012] The invention is explained in more detail below with reference to the exemplary embodiments shown schematically in the drawing, in which:

[0013] FIG. 1 shows an arrangement for boring a camshaft alley bore with a reaming tool having a long guide shank which can be supported in the pre-machined bores;

[0014] FIG. 2 shows, in a longitudinal section, a detail of the reaming tool in the region of the coupling between reaming head and guide shank;

[0015] FIG. 3 shows, in a longitudinal section, a detail of the reaming tool in the region of the coupling, modified compared with FIG. 2, between reaming head and guide shank.

[0016] The reaming tool 10 shown in FIG. 1 is intended for finish-reaming camshaft bearing bores 14 arranged at an axial distance from one another on a cylinder head 12 and in alignment with one another. To achieve the required alignment, the reaming tool 10 has a long guide shank 16 which, by means of guide strips 18 oriented in an axially parallel manner and distributed over the circumference of the guide shank 16, are supported in the respectively pre-machined bores 14 and if need be in a guide bush 20 arranged in a fixed position in the machine. At its machine-side end, the guide shank 16 is provided with a clamping shank 22 for connection to a machine spindle (not shown), which in the exemplary embodiment shown is designed as a hollow shank taper. Also located there is a compensating holder 24, with which alignment differences can be compensated for.

[0017] At its front end, the guide shank 16 carries a reaming head 26 which is fitted with a plurality of cutting edges 28 which are at a distance from one another in the circumferential direction. The cutting edges 28 perform a guidance function at their end facing the shank. In principle, it is possible for the cutting edges 28 to merge into guide strips at their shank-side end in the region of the cutting head. The reaming head 26 is designed as a component which is separate from the guide shank 16. As can be seen from FIGS. 2 and 3, the reaming head 26 can be connected to the guide shank 16 by means of a self-centering clamping mechanism. Since a prestress which leads to an elastic deformation is produced during the clamping operation in the region of the clamping mechanism, it is important on account of the stringent accuracy requirement that the supporting elements positioned in the vicinity of the clamping mechanism can be radially expanded during the clamping operation at least partly from a diameter undersize to a predetermined diameter guide dimension.

[0018] In the exemplary embodiment shown in FIG. 2, the self-centering clamping mechanism 30 has a tapered receptacle 34 arranged concentrically in the guide shank 16 and defined by an annular flat face 32, a tapered spigot 38 that projects coaxially rearward beyond the reaming head 26 and is defined by an annular flat face 36 for insertion into the tapered receptacle 34, and a clamping screw 40 designed as a differential screw for screwing into opposing internal threads 42, 44 of the reaming head 26 and the guide shank 16. For rotary driving, the guide shank 16 has an internal hexagon 46 arranged between the tapered receptacle 34 and the annular flat face 32, while the reaming head 26 has an external hexagon 48 arranged between the tapered spigot 38 and the annular flat face 36 that is complementary to the internal hexagon 46.

[0019] The clamping mechanism shown in FIG. 3 differs from the clamping mechanism according to FIG. 2 merely owing to the fact that the tapered receptacle 34′ is arranged at the rear on the reaming head 26 and the tapered spigot 38′ is arranged so as to project at the front end on the guide shank 16. Accordingly, the internal hexagon 46′ is here arranged on the reaming head and the external hexagon 48′ is arranged on the guide shank 16. In both cases, during the clamping operation, the clamping screw 40 is actuated through its bore 50 from the front end of the reaming head 26.

[0020] The reaming head 26 and the guide shank 16, in the region of their flat faces 32, 36, in the state free of stress, limit a defined gage clearance dimension a of preferably 0.01 to 0.06 mm, whereas in the clamped state they bear against one another with the tapered receptacle 34, 34′ and the guide strips 18 arranged on the outside in this region being expanded elastically to a predetermined guide dimension. The taper angle of the tapered spigot 38, 38′ and of the tapered receptacle 34, 34′ is about 12° in the exemplary embodiment shown. In order to ensure high repetitive accuracy, it has proved to be especially important that the clamping screw 40 is always tightened with the same tightening torque. The tightening torque in this case depends on the friction behavior of the connecting partners and on the actual gage clearance dimension a. To overcome the gage clearance dimension up to the stop between the flat faces, a tightening torque X is required. There is also an additional torque d for producing a frictional connection on account of internal elastic deformation. The tightening torque D is accordingly obtained as X+d. In this case, d=0.4 to 4 Nm, whereas the tightening torque X, depending on the gage clearance dimension a, may be 3 to 16 Nm.

[0021] In order to always obtain the same guide diameter, it is of particular importance that, in the case of the exemplary embodiment according to FIG. 2, the guide shank is clamped in place on a grinding arbor with a tapered spigot and is ground there on the outside diameter in the clamped state with a defined diameter guide dimension being set. Alternatively, in the exemplary embodiment according to FIG. 3, corresponding machining of the reaming head 26 drawn onto the grinding arbor is to be carried out. In order to obtain reproducible results, the clamping conditions are the same as when mounting the reaming head 26 on the guide shank 16.

[0022] In summary, the following may be stated: the invention relates to a reaming tool for reaming a plurality of bores 14 of a workpiece 12 which are arranged at an axial distance from one another. The reaming tool has a guide shank 16, which can be clamped in place in a machine spindle and can be supported by means of external supporting elements 18 in prepared bores 14 of the workpiece 12 to be machined, and a reaming head 26 coaxially adjoining the front end of the guide shank 16. In order to permit a simple tool change in the event of wear, it is proposed according to the invention that the reaming head 26 can be connected to the guide shank 16 by means of a self-centering clamping mechanism 30 and that the supporting elements 18 positioned in the vicinity of the clamping mechanism 30 can be radially expanded during the clamping operation at least partly from a diameter undersize to a predetermined diameter guide dimension.