In the study of the cutting edge CNC sharpening theory, when designing the drill tip CNC sharpening machine tool, in addition to considering the correct shape of the drill tip from the motion relationship, attention should also be paid to the influence of various error factors on the cutting edge sharpening result. Sharpening equipment with low sharpening quality not only does not meet the requirements of the drill bit, but also wastes manpower, financial resources and raw materials. However, it is neither reasonable nor realistic to improve the accuracy of the sharpening equipment in a targeted manner. In the drill tip CNC sharpening process system, the impact of each link error on the final sharpening quality is different. This paper analyzes the various error factors affecting the sharpening accuracy of the system, especially the main influencing factors, and provides a reference for reasonable improvement of the sharpening precision of the drill tip.
1 Factors Affecting Sharpening Accuracy The machining process system of the drill tip CNC sharpening is composed of hardware and software. The hardware includes a control drive part and a mechanical part: the control drive part refers to a microcomputer and a control interface circuit, a stepping motor and a position detecting element, etc.; the mechanical part includes a machine tool, a grinding wheel, a chuck and a drill bit. The software includes process plans, calculation programs, and control programs. Errors in the process system that may affect the accuracy of the sharpening and its source.
The control pulse signal of the machine tool movement comes from the microcomputer and the control interface circuit. As long as the interface circuit is designed reasonably, the possibility of generating the pulse signal error is relatively small. Once it appears, it is faulty and destructive, it affects the stability of the processing and is not a problem of sharpening accuracy.
The stepping motor is the power source of the machine tool movement. Its step accuracy and out-of-step in operation directly affect the motion accuracy and positioning accuracy of each CNC axis and must be controlled. It is possible to select a high-precision high-power stepping motor or a stepping step, or to use closed-loop and semi-closed loop control.
The position detecting component is mainly used for initial zero positioning in sharpening, and its detection error directly affects the zero positioning accuracy of the coordinate axis. As the main body of sharpening, the machine tool is the main source of machining error. Zero positioning error, positioning error and motion accumulation error of each coordinate axis, shape and position errors during machine tool manufacturing, thermal deformation and rigidity during machine tool processing all affect the sharpening accuracy. Since the grinding wheel is in direct contact with the drill tip, the wear of the grinding wheel affects the grinding accuracy in a ratio of 1:1.
The chuck is an important positioning component of the workpiece, and its influence on the sharpening precision is mainly: manufacturing error of the chuck itself, positioning error of the workpiece in the chuck, positioning error of the chuck on the workpiece head frame, and the like. The manufacturing error of the chuck itself can be reduced by the processing precision; the key is the positioning error. The numerical control series sharpening machine adopts the elastic automatic centering chuck. The positioning error of the workpiece on the chuck is a common problem. analysis.
The positioning form of the chuck on the workpiece head frame is as shown in the figure. The matching gap will cause the radial deviation of the drill tip position, that is, = 2 1 + lslt(1) where ls?? drill bit extension length lt?? head frame guide After the length is ground, the tip surface is geometrically eccentric. Such a drill bit will generate radial runout and increase the aperture during use, so the gap and the drill extension length ls should be appropriately reduced to increase the guide length lt.
The original error and rigidity of the drill itself also affects the sharpening accuracy. If the drill has been bent, the deviation of the symmetry after sharpening is very large. Therefore, the drill must be screened before the sharpening to eliminate the unqualified drill. When clamping the drill bit, the length of the drill bit should be reduced as much as possible to ensure sufficient rigidity of the drill bit during sharpening.
The drill tip CNC sharpening is a one-time installation, and the process plan is mainly embodied in the software control program. The calculation error generated by the calculation process and the interpolation error in the control have an impact on the sharpening accuracy. Therefore, it is necessary to reduce the approximation calculation and select an appropriate interpolation method to reduce the resulting principle error. In summary, among the many error factors, especially the machine tool error has the greatest impact on the sharpening accuracy.
2 Influence of CNC axis positioning error on sharpening accuracy The influence of the positioning error of each CNC axis on the accuracy of the grinding tip structure parameters will be different due to the sharpening principle and machine structure. Due to the error-sensitive direction, the influence of different coordinate axes of the same machine tool is also different. In order to fully understand the sharpening principle of the CNC series sharpening machine and the rationality of the machine tool structure from the angle of error and precision, and to guide the future design of the sharpening machine, it is necessary to analyze the transmission law of the machine positioning error. .
Take the drill tip sharpener with six degrees of freedom (six CNC axes) as an example. The six-degree-of-freedom tip sharpening machine has six axes (numerical axis). When sharpening the taper, the C1 axis adjusts the angular position and the sharpening index. The C2 axis is used for the taper forming motion. The C3 axis is used to adjust the half cone angle 0 and the cone axis inclination angle # 0. The X and Y axes are used to adjust the position of the drill point relative to the grinding wheel in the horizontal plane and complete the feed motion. The Z axis is used to compare the drill point in the vertical plane. Adjustment of the position of the grinding wheel. The sharpening process is actually a synthesis of the motion of the six axes. For comparison, the machine is used to sharpen the taper drill tip, and the influence of the positioning error of each coordinate axis on the main blade front angle and the circumferential back angle fc of the post-grinding structural parameters is taken as an example for analysis.
2. 1 Conical surface sharpening parameter analysis As shown, the coordinate system O 1 XYZ is established, the coordinate origin O 1 is on the core tip; the vector coordinate system Oij k is selected, and the coordinate origin O is the bit axis and the cone axis. The intersection of the projections on Oik. The Z axis and the k axis of the two coordinate systems coincide with the bit axis. O 2 is the intersection of the cone axis and the j axis. The cone apex is P.0, which is the half angle of the cone apex, which determines the size of the cone. CX and CY represent the absolute values ​​of the i and j coordinate values ​​of the cone apex P, which determine the position of the drill tip on the cone surface. 0 is the intersection angle of the bit axis and the grinding wheel surface on the Oik surface, which determines the position of the bit axis relative to the cone axis. 0, CC, CY, 0 and other four sharpening parameters determine the shape of the flank of the sharpened point. The edge of the ordinary twist drill is a spiral surface that is formed when the drill tip is rolled or lapped, and is not affected by the sharpening of the flank face of the drill tip.
However, part of it intersects the flank to form the main cutting edge, which is the rake face. As shown, the vector coordinate system Oij k is selected, the coordinate origin is selected from the drill point p 0 , and the AB segment is the main cutting edge formed by the intersection of the rake face and the flank face. When the p 0 value is changed, the initial position of the rake face in the coordinate system Oij k is affected, resulting in a change in the position of the main cutting edge. It can be seen that the rake face affects the shape of the main cutting edge. Let %= B- p 0, so determine % as another sharpening parameter. At this point, the five sharpening parameters of the drill tip are obtained, namely: CX, CY, 0, 0, %.2. Positioning error of the 2C1 axis C1 The C1 axis acts to adjust the angular position or the indexing effect. The resulting C1 is indirectly affected by the influence of the sharpening parameter % and fc. The relationship between the error % of C1 and % is % = r 0 tanC1( 2) where r 0?? The core radius represents the relationship between the positioning error C1 and the parameters of the drill tip structure, fc, and fc (the sharpening parameter is : CX = 50, CY = 3, 0 = 14°, 0 = 60°, % = 0).
It can be seen that with the increase of C1 (counterclockwise from the tip of the drill tip), the ratio of the main blade front angle and the circumferential back angle error increases in a negative direction. The back angle error is greater than the main edge angle error. Their relationship is = - 1 150C1f c = - 1 80C1 (3) It can be seen from equation (3) that the error transfer coefficient is much smaller than 1, so the C1 axis direction is, and fc is an error-insensitive direction. 2. Positioning error of the 3C2 axis The C2 axis is used for the taper forming motion, and the error C2 caused by it does not affect the structural parameters of the tapered tip. 2. Positioning error of 4C3 axis C3 The C3 axis is used to adjust the half cone angle 0 and the cone axis inclination angle # 0. The positioning error generated by the C3 axis affects four of the five sharpening parameters, namely: CX, 0, 0, and %, which cause C XC3 to have a relationship with the tip structure parameter, fc error, and fc. 5°。 When the C3 is increased, the error and the fc ratio are increased, and the C3 has a large influence on the back angle error. When C3 = 0. 5°, the fc is 2. 5°. Their relationship is = 1. 67 C3f c = 5 C3 ( 5) The same reason, when the C3 axis is used to adjust the taper axis tilt angle # 0, the relationship between C3 and the drill tip structure parameter, fc error, fc is = 1 33 C3f c = 4. 5 C3 (6) Their error transfer coefficients are all greater than 1, especially for fc, the error magnification is up to 5 times. Therefore, factors that cause errors in the C3 axis should be strictly controlled.
2. Positioning error of 5X axis The positioning error caused by the XX axis causes the error CX and % of the sharpening parameter CX, % as shown. X is related to them as CX = cos(0 - 0)sin 0 sin 0 - 1 X = X tan 0( 7) is the relationship between X and the tip structure parameter, fc error, and fc. It can be seen from the figure that as the X increases, the error and fc increase proportionally. Their relationship is that their error transfer coefficients are smaller than those of the C2 and C3 axes, both of which are less than 1, but much larger than C1. .
2. Positioning error of 6Y axis The positioning error caused by the Y axis causes the error CX and % of the sharpening parameters CX and %. As shown, Y and their relationship are CX = sin(0 - 0)sin 0 sin 0Y = Y(9)fc error, fc relationship curve, as can be seen from the figure, as Y increases, fc increases proportionally, but the proportion of fc increases is small, their relationship is = 0. 8 Yf c = 0. 25 Y(10) has the opposite effect on the positioning error of the X-axis, and the Y-pair has a large influence and the error transfer coefficient is also large.
2. Position Z of the 7Z axis As shown, the deviation of Z in the Z-axis direction causes the variation pr of the radial tip of the grinding wheel relative to the position of the grinding wheel. The relationship between Z and pr is pr = Z sinZ RS(11) where RS?? Grinding wheel radius Due to the large radius of the grinding wheel (RS = 150 mm), sinZ RS is small, so pr is small. Therefore, the error generated along the axial direction has little effect on the precision of the taper sharpening. This direction is the direction of error insensitivity.
3 Conclusion From the error transmission law when sharpening the taper point, the order of the influence of the positioning error of the six CNC axes on the sharpening accuracy is: C3 axis, Y axis, X axis, C1 axis, Z axis and C2 axis. The C3 axis is the weakest link in the sharpening accuracy of the machine. When designing the machine tool, its positioning error transfer coefficient should be minimized to ensure the sharpening accuracy of the drill tip.
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