Abstract:
The grinding apparatus permits precision machining of a valve seat or a sealing surface, and includes a grinding spindle having a holding and guiding part extending into a finished workpiece bore with clearance. The holding and guiding part is smaller in diameter than the grinding spindle and includes a pin which carries on its free end a grinding tool. The other end of the pin passes into the grinding spindle which has a substantially larger diameter. The grinding spindle is located outside the bore of the workpiece and may thus be made very stable. The pin is held by the grinding spindle so that the workpiece is held and guided securely even in the case of very small workpiece diameters.

Description:
BACKGROUND OF THE INVENTION 
     In a known grinding apparatus of this type (DE-OS No. 29 12 814) the holding and guiding part is a cylindrical spindle sleeve, surrounding the grinding sleeve in the area of a center hole of the workpiece. The grinding spindle carries on one of its ends the grinding tool, adapted to the inclination of a valve seat surface. This known grinding apparatus no longer satisfies the high accuracy requirements for workpieces with a relatively small center hole, the diameter of which is approximately 6 mm. or less, for example. The reason for this is the lack of stability of the arrangement. There is an increasing need to produce workpieces with very small center bores, for example, for fuel injection pump nozzle bodies. 
     SUMMARY OF THF INVENTION 
     The object of the invention is to provide a grinding apparatus of the above-mentioned type whereby even in the case of workpiece holes with very small diameters high machining accuracies may be obtained. 
     As the grinding spindle is arranged according to the invention outside the workpiece, its diameter may be significantly larger than the bore of the workpiece and therefore it may be extremely heavy and stable. The shaft like pin is thereby held in the grinding spindle so that the workpiece is guided and held securely even in the case of very small workpiece diameters. As the result of the configuration according to the invention, conventional, high rpm and highly accurate grinding spindles with, for example, hydrostatic or aerostatic bearing supports and a rotating velocity of 60,000 to 100,000 rpm may also be used. The rigid, high strength configuration of the spindle ensures a very high machining accuracy, satisfying the highest requirements. 
     Further characteristics of the invention will become apparent from the following description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a part of the apparatus according to the invention in an axial section; 
     FIG. 2 is a top view of a part of a second form of embodiment of the apparatus of the invention; 
     FIG. 3 is a further form of embodiment of the apparatus according to the invention, in a view similar to FIG. 1, and 
     FIG. 4 is a section along the line IV--IV in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the embodiment of FIG. 1, there is shown therein a grinding apparatus with a workpiece 1 arranged therein. The workpiece has a honed center bore 2 and a valve seat surface 3 concentric with said bore 2, with said valve seat to be ground by means of the apparatus. For the machining of the valve seat surface 3, the apparatus has a shaft-like pin 5, carrying at its front end 38 a grinding tool 6 having a grinding surface adapted to the inclination of the valve seat surface 3. The grinding pin 5 is rigidly joined to a grinding spindle 4 of the apparatus. It is clamped in a simple manner, with one end 26 outside the bore 2 in a corresponding bore 27 of the spindle 4. To improve its strength, the grinding pin 5 has a massive configuration and consists preferably entirely of a hard alloy for example, carbide, and has a circular cross section constant over its entire length. Its diameter is slightly less than the diameter of the center bore 2 and substantially smaller than the diameter of the spindle 4. The diameter of the latter is a multiple of the pin diameter, in the FIG. 1 embodiment being approximately three to four times the diameter of said pin diameter. As the result of this large diameter the spindle 4 is especially strong, so that the pin 5 itself is held securely in the spindle even during the machining of very small valve seat surfaces. This permits the valve seat surface to be processed with an extremely high accuracy. 
     The workpiece 1 having the valve seat surface 3 to be machined is pushed onto the grinding pin 5 manually or preferably by a device (not shown) known in the art. The workpiece is then securely held and guided in the finish honed center bore, on the pin 5. 
     For guidance without play and the holding of the workpiece 1, a drive part 9 having the configuration of a roll is provided. It acts on an outer circumferential surface 8 of the workpiece 1, which is concentric to the center bore 2. The workpiece 1 is thereby pressed against the circumferential surface 7 of the pin 5, and driven in rotation. The drive roll 9 may be moved or displaced by means of a device well known, in the art (not shown) for example, a hydrostatic piston-cylinder arrangement, in the direction of the workpiece 1 (arrow 10) with a predetermined, preferably continuously adjustable force. The drive roll is driven by rotating drive means, for example, a hydraulic motor (not shown) in the direction of the arrow 11. Consequently, the workpiece 1 is rotating in a direction (arrow 13) opposite to the grinding spindle 4 (arrow 12), but with a lower rpm, for example at 200 to 3000 rpm. 
     In order to advance the workpiece 1 in the axial direction (arrow 25), the drive roll 9 is connected with an advance drive 14, also known in the art, whereby the grinding tool 6 may be applied with a fine adjustment to the valve seat surface 3 of the workpiece 1, and the grinding pressure regulated continuously. 
     As shown in FIG. 2 in a further embodiment of the invention, the axis 28 of a drive roll 9a is set at a predetermined acute angle 15, preferably approximately 2° to 5°, obliquely to the axis 29 of the workpiece, whereby an axial motion component is superposed on the rotation of the drive roll 9a in the direction of the workpiece 1 (arrow 24). By the appropriate setting and variation of the angle of inclination 15 the grinding pressure may be regulated. In place of the drive rolls 9 or 9a, a suitably designed belt drive may also be used. 
     The devices according to FIG. 1 and 2 further include a cooling medium system, of which only a nozzle 18 is shown (FIG. 1), whereby lubricating and cooling means may be introduced under a high pressure into the grinding zone 16 of the workpiece 1. The nozzle 18 is located aligned with and at a small distance from an orifice 17 of the workpiece, so that the jet of the cooling medium exiting from the nozzle 18 at a high pressure directly enters the grinding zone 16. 
     By means of the high pressure, the cooling and lubricating medium is further forced into the sickle-shaped gap 19 following the cutting zone 16 in the direction of advance 25, said gap remaining between the center bore 2 and the circumferential surface 7. In this gap 19, the cooling and lubricating medium forms a lubricating wedge between the pin 5 and the workpiece 1, resulting in a hydrostatically acting bearing between said parts. This bearing is highly accurate and assures only very slight friction between the shaft or pin 5 and the workpiece. The lubricating and cooling medium then exits at the frontal side 30 of the workpiece 1 facing the grinding spindle, and is conducted by way of a cover part 20 and a collecting device 21 to a return line 22. 
     The collecting device 21 is a flat box approximately circular in its cross section, with two coaxial passage orifices 31 and 32 in its opposed disk-shaped lateral walls. The workpiece 1 and the spindle 4 protrude, with their ends 33 and 34 facing each other, into the orifices 31 and 32. The cover part 20 has a circular configuration and is supported flat on the corresponding frontal surface 35 of the spindle end 34. The cover part has an edge 36 bent away from the spindle 4 and a diameter only slightly smaller than the collector device 21, so that the spindle 4 is satisfactorily sealed with respect to the cooling and lubricating medium. The return line 22 is connected to the cylindrical outer wall 37 of the collector device 21, and is preferably integrally formed therewith. 
     It is further possible to let the cooling and lubricating medium flow in the opposite direction. For this purpose, cooling means may be injected under a high pressure directly into the sickle-shaped gap 19, by means of a finger (not shown) protruding into an intermediate space 23 between the frontal side 35 of the grinding spindle 4 and the opposing frontal side 30 of the workpiece 1. The lubricant then passes into the grinding zone 16 and leaves the workpiece 1 through the orifice 17 of the workpiece. This path of the lubricant has the advantage that the abraded material does not arrive from the grinding zone 16 in the area of the bearing and guidance of the workpiece 1, but is transported directly through the workpiece bore 17 from the workpiece 1. 
     In the apparatus according to FIGS. 3 and 4, the axial advance movement is effected by the grinding spindle 4a (arrow 39). The workpiece 1 is guided and held in the axial direction by an axial bearing 40, which has the configuration of a hydrostatically acting thrust bearing and is arranged on the frontal side 41 of the workpiece 1 facing away from the grinding spindle 4a. An annular groove 42 concentric with the coolant and lubricant nozzle 18a is provided, which is supplied with liquid under pressure through a supply channel 43. 
     The thrust bearing 40 and the nozzle 18a are of a simple, single piece configuration, with the coolant and lubricant serving as the pressure liquid. The hydrostatic thrust bearing may have a different configuration, when, for example, only one line is provided for the supply of the coolant and lubricant. From this, a branch line is provided for carrying a partial flow of the lubricant to the annular groove. 
     The advance of the grinding spindle (arrow 39) and the control and setting of an optimum grinding pressure may be regulated simply by means of the bearing thrust occurring during processing. The bearing thrust is measured by a measuring instrument, for example a manometer, and passed to an evaluating and regulating device 45, connected with the advance control device 46 for the grinding spindle 4a. 
     It is particularly advantageous to run the grinding spindle 4a against a stationary stop (not shown). Thus, only the grinding pin 5a is run rapidly into the bore 2 of the workpiece, and the thrust bearing 40 is subsequently displaced axially during the working advance (arrow 47). In this case the evaluating unit 45 is actively connected with an advance device (not shown) for the thrust bearing 40. 
     As further shown in FIG. 3, the holding and guiding part having the configuration of the grinding pin 5a is provided on both ends with bearing surfaces 7a and 7b, adjacent partial sections 48 and 49, in which the outer diameter of the pin is reduced by turning. These bearing surfaces are resting against the terminal areas of the workpiece bore 2. The bearing surfaces 7a and 7b  have grooves 50 in the circumferential direction and uniformly spaced apart, extending in the axial direction of the grinding pin 5a and in an oblique or helical manner relative to the grinding spindle 4a, so that each groove 50 forms a section of an imaginary circumferential helix. As the result of this configuration, the bearing surfaces 7a and 7b possess several sliding surfaces, whereby the workpiece 1 is guided in an especially favorable manner. The direction of the grooves 50 is adapted to the direction of rotation of the grinding pin 5a, whereby a pumping effect supporting the flow (arrow 51) of the coolant and lubricant is generated. The grooves 50, viewed transversely to the spindle axis 56, are at an acute angle of approximately 15° with such axis. In addition, one groove of the bearing surface 7a is always aligned approximately with a groove of the other bearing surface 7b. 
     For the guidance without play and holding of the workpiece 1 on the grinding pin 5a, according to FIG. 4 two drive rolls 9b and 9c are provided, which are located on the circumferential surface 8 of the workpiece 1 axially following each other, so that, when viewed in the axial direction of the workpiece 1, they are partially superposed on each other. The pressure rolls 9b and 9c are further arranged so that their contact lines 52 and 53 with the circumferential surface 8 are symmetrical to a longitudinal center plane 55 of the workpiece 1 containing the contact line 54 of the grinding pin 5a with the workpiece bore 2. The imaginary tangential planes of the contact lines 52 and 53 form with the contact line 54 an acute angle of approximately 60°.