Producing and eliminating method of diameter error during tapping

Tap diameter error is a problem frequently encountered in tap production, and it has a serious impact on quality. Because the contact surface of the thread is mainly on the unilateral tooth surface and the medium diameter dimension of the thread, an error in the middle diameter will cause the change in the thread thickness and the weakening of the bond strength, which will accelerate the tooth surface wear and affect the adjacent and local parts of the tap. Accumulated pitch error causes uneven rotation torque, affecting transmission and displacement accuracy. It can be seen that the consequences of errors in the diameter of the tap are very serious. How to meet the accuracy requirement of the tap, ensure the product quality, eliminate the middle diameter error and eliminate the failure of the thread grinder are a problem often encountered in tap thread grinding process. The medium-diameter precision of the thread can be divided into: medium-diameter size tolerance; medium-diameter roundness; medium-diameter taper. The main reason for the size error of the medium diameter is that the accuracy of repeated positioning of the wheel frame is not good, and the repeated positioning accuracy of the transverse knife hand wheel is not stable. This question is good to judge, maintenance is relatively easy. The taper error of the medium diameter is mainly caused by the error caused by machining longer screws such as screw, and it does not need to be considered for the production of taps. This is mainly to discuss the problem of the roundness of the tap diameter in the tap. The roundness of the middle diameter is very poor, which is often referred to as "living." There are two British Type 7901 thread grinders in our factory. For many years, there existed "live flats" to varying degrees that have not been resolved. The following is a brief introduction to the factors that may cause the roundness of the medium-diameter roundness to be exceeded and the methods to be solved in the repair through theoretical and practical experience. Headstock Spindle Rotation Accuracy This is due to the large clearance between the headstock spindle and the bearing. The overshoot of this accuracy will be directly reflected to the tap diameter of the processed 1:1. For the headstock spindle of the thread grinder of our factory, there is one type of headstock that does not turn at the top, commonly known as "dead top." Therefore, this problem does not exist. However, another type of thread grinder is a live tip type in which the top of the head frame rotates together with the spindle. The radial runout of the headstock of such a machine head should not exceed 0.01 mm. If it exceeds 0.01 mm, then the rotation accuracy is poor. For the form of rolling bearings, bearings and septums need to be replaced. For the structure of sliding bearings, the bearings must be adjusted and reground to achieve the accuracy of the headstock spindle. The top cone's trespassing is poor. This means that the front and rear cones are not round. Including the top cone tap 60 ° cone, but also included in the workpiece spindle and tailstock sleeve Morse taper. This accuracy requires that the cone beating should be within 0.005mm, otherwise it needs to be replaced and repaired. For a thread grinder whose headstock spindle is a "live-tip" structure, the effect of the top-beat of the headstock on the tap is greater than that of the tailstock tip-bend. In order to eliminate the overall error generated in the rotation of the headstock, this type of thread grinder has headgear top self-grinding devices. For example Y7520WMM582, Soviet-made MM5K822B type, etc., after the top run after self-grinding can reach within 0.003mm. Because the British 79 series thread grinders do not have self-grinding accessories, the problem of roundness in the roundness of the middle diameter often occurs. Therefore, if the 7901 is equipped with a self-grinding device, it is estimated that it will greatly improve the "live flat" problem.

figure 1

The taper of the center hole of the tap is not round This is due to the fact that the center hole of the pre-order is not properly shot, which is also a problem that often occurs. The solution is to grind the center hole and maintain the contact area with the tip of about 80%. Head Mount Dial Installation Deviation If the screw on the lever tightens the tap square clip too tightly, it will cause the error on the dial to thread into the thread. The solution is to correct the deviation of the dial so that the screw on the lever has a slight gap with the square tail of the tap. Table motion is not smooth Check whether the roller frame can ensure that each roller can roll smoothly. In addition, the local wear of the bed rails leads to a loss of geometrical accuracy and will also cause the table to move unevenly. The impact of the three errors of the guide rail on the tapping accuracy is analyzed as follows: The influence of the linearity of the guide rail in the vertical plane on the size of the tap diameter can be seen from Fig.1. Since the straightness error of the guide rail in the vertical plane causes the tap center Lower than the center of the wheel, the tap radius increment ∆r can be calculated as follows: ∆2=(R+r+∆r)2-(R+r)2≈2(R+r)∆r ∴∆r = ∆2 2(R+r) Let wheel radius R=200mm, center of tap is lower than center of wheel ∆=0.05mm, tap radius r=10mm Then: ∆r= 0.052 = 0.000 005 9mm 2 (200+10) Visible, The straightness of the guideway in the vertical plane has little effect on the accuracy of the tap diameter. The influence of the straightness of the longitudinal rail in the horizontal plane on the tapping accuracy This error has a direct influence on the tapping accuracy. If the center of the V-shaped guide rail is concave relative to the grinding wheel, the tapped diameter of the tap is drum-shaped; otherwise, it is ground into a saddle shape.

figure 2

The influence of the parallelism of the two guide rails on the accuracy of the diameter of the tap has no error in the V guide rail, and the flat guide rail has an inclination in the longitudinal direction. As shown in Figure 2, the table moves along the bed rails to a certain position, the inclination angle is a, and the center of the tap moves from point O to point O1, resulting in a radius error of d≈H·tga=H·∆ · Center setting H= 300mm, l center = 300mm, ∆ = 0.05mm then d = 300 x (0.05/300) = 0.05mm. It can be seen that the parallelism of the two guide rails has a considerable influence on the precision of the tap diameter. At the same time, the reason why the Type 7901 is more severe than the Type 79 “live flat” is precisely due to the structure of the headboard design. Type 7901 has been improved by the metric system. The height H of the guide surface to the top center increases by about 20mm. It can be seen from the d=H·∆/l center that H and d are proportional to each other. Therefore, from the aspect of rail wear, the 7901 is more likely to have a rounding error in the diameter of the tap than the 79. Impact of Tailstock After long-term use of the tailstock, the fit clearance between the sleeve and the tailstock shell will increase due to wear, causing the knife to be abraded during the grinding process, resulting in an intermediate diameter error. The gap between the repaired sleeve and the tailstock housing should be within 0.008mm. In addition, the tolerance of parallelism between the tailstock sleeve axis and the table must be 0.02/100mm. The coaxiality of the worktable lead screw and the nut and the parallelism with the direction of the table movement are different because of the different axis of the screw and the nut, which causes the transmission to be fluctuating. Because it is not parallel to the longitudinal guide rails, the resistance of the guide rails is large or small, and the table is slightly changed during the movement, causing periodic errors. Impact of the drive chain error The movement errors caused by the wear of gear parts, universal joints, etc. in the headstock headstock and the eccentricity error during installation will cause the angular velocity of the tap to rotate unevenly, affecting the middle of the tap. Diameter accuracy. In actual work, errors often occur as a result of several factors. For example, the solution to the problem of 7901 "live and flat" is like this. Although there are many factors influencing, it is only necessary to look carefully one by one in the repairs and eliminate them one by one, which will eventually be resolved.

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