Method for generating and eliminating medium diameter error during tap grinding

Tap diameter error is a problem often encountered in tap production, which has a serious impact on quality. Since the contact surface of the thread is mainly on the one-side tooth surface and the median diameter of the thread, the error of the medium diameter will cause the change of the thread tooth thickness and the weakening of the joint strength, which will accelerate the tooth surface wear and affect the adjacent and partial taps. Accumulated error of pitch, resulting in uneven rotation torque, affecting transmission and displacement accuracy. It can be seen that the consequences caused by the error in the diameter of the tap are very serious.

How to meet the precision requirements of the tap, ensure the quality of the product, eliminate the medium diameter error, and eliminate the failure of the thread grinder is a difficult problem often encountered in the tap thread grinding process.

The accuracy of the thread diameter can be divided into: medium diameter dimension tolerance; medium diameter roundness; medium diameter taper. The main reason for the medium-diameter size error is that the re-positioning accuracy of the grinding wheel frame is not good, and the repeated positioning accuracy of the cross-feeding hand wheel is unstable. This question is a good judgment, and it is relatively easy to overhaul. The taper error of the middle diameter is mainly for the error caused by the long thread such as the processing screw, and it can be ignored for the production of the tap. Here we mainly discuss the problem of the roundness of the tap diameter. The roundness of the medium diameter is extremely poor, that is, the "live flat" often said. There are two Inch 7901 thread grinders in our factory. For many years, there have been "live flats" that have not been solved. The following is a brief introduction to the influencing factors that may cause the roundness of the medium diameter and the solutions to be solved in the repair through the experience in theory and practical work.

1 The head frame spindle rotation accuracy is out of order. This is due to the excessive clearance between the headstock main shaft and the bearing. This accuracy tolerance is directly reflected in the 1:1 diameter of the processed tap. For the headstock spindle of our factory's thread grinder, there is a type of headstock that does not turn at the top, commonly known as "dead top." Therefore, there is no such problem. However, another type of thread grinder is a live-top type in which the head of the headstock rotates together with the main shaft. The radial runout of the headstock of such a machine head should not exceed 0.01 mm. If it exceeds 0.01 mm, the rotation accuracy is excessive. For the rolling bearing type, it is necessary to replace the bearing and the grinding spacer; for the sliding bearing structure, it is necessary to adjust the bearing bush and re-grind to achieve the accuracy requirement of the headstock spindle.

2 The top cone's beating is out of tolerance. This means that the front and rear tips are not round. It includes both the 60° cone of the top tap and the Morse cone mounted on the workpiece spindle and the tailstock sleeve. This accuracy requires that the taper beating should be within 0.005mm, otherwise it needs to be replaced and repaired.

For a thread grinder with a head-mounted spindle with a “live top” structure, the impact of the top jump of the headstock on the tap is greater than the impact of the tail tip jump on the tap. In order to eliminate the combined error caused by the top rotation of the headstock, this type of thread grinder has a head-mounted self-grinding device. For example, Y7520WMM582, Soviet-made MM5K822B type, etc., after the self-grinding top jump can reach 0.003mm. The British 79 series thread grinders often have problems with roundness in the middle diameter because they do not have a self-grinding attachment. Therefore, if the 7901 is equipped with a self-grinding top device, it is estimated that the problem of “live flat” will be greatly improved.

3 The taper of the center hole of the tap is not round. This is because the front center hole is not well, which is a problem that often occurs. The solution is to grind the center hole and keep the contact area with the tip at around 80%.

4 The headstock dial is installed with a deviation. If the screw on the lever clamps the taper end of the tap too tightly, the error on the dial will be transmitted to 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 taper tail.

5 The table is not moving smoothly. Check that the roller frame ensures that each roller can smoothly roll. In addition, local wear of the bed rail causes loss of geometric accuracy and will also cause the table to move unsteadily. The following analysis of the impact of the three errors of the guide rail on the precision of the tap processing:

figure 1

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