Common problems in machining and turning of precision parts with internal threads above three lines


When machining internal threads of precision parts, sometimes although the thread depth has been turned to the required size, it still cannot be screwed in when using a thread plug gauge or checking the external thread; sometimes although it can be screwed in, it may be screwed in a few more times. The fit is too loose, or one end is just screwed in but the other end cannot be screwed in; sometimes only a few teeth are screwed in at the threaded entrance.

  1. Cause

(1) The cutting edges on both sides of the turning tool are not straight, so that the two sides of the thread profile of the turning are correspondingly not straight, which reduces the thread accuracy.

(2) The top width of the turning tool is too narrow. Although the thread has been turned to the specified depth, the alveolar width at the pitch diameter of the internal thread does not reach the required size, so that it is smaller than the tooth mark at the pitch diameter of the external thread.

(3) Due to the inaccurate sharpening of the turning tool, the excessively large rake angle, and the high or low installation of the tool, the thread angle of the screw has a large error, which reduces the thread accuracy. Or because the tool is skewed, a large thread half-angle error is generated, and the thread profile is correspondingly skewed. During inspection, there will be a phenomenon that one end is screwed in, the other end is not screwed in or the fit is too loose.

(4) The bottom diameter of the internal thread has to be sized (the outer diameter is also correspondingly small), so it cannot be screwed in during inspection.

(5) The shank of the internal thread turning tool cannot be made too thick and short due to the limitation of the aperture size and length, so the rigidity is poor, and a small amount of bending deformation will be generated when the precision parts are processed by the cutting force. , There is a phenomenon commonly known as “knife letting”, which causes tooth profile errors in the internal thread. Therefore, only a few teeth can be screwed in at the entrance during inspection. Different methods are adopted to solve different problems when turning internal threads. At the same time, “prevention first” is advocated. First, the turning tool should be sharpened and installed correctly. When using the straight cutting method, it must be noted that the width of the nose of the thread turning tool should meet the requirements, so that after the internal thread is turned to the specified depth, the width of the tooth profile at the middle diameter basically reaches the specified size. The width of the tip of a pipe thread turning tool is generally 0.Ip (p is the pitch). The top width of the trapezoidal internal thread turning tool needs to be reduced by 0.05-0.1mm according to the calculated top width. Secondly, when machining the inner threaded bottom diameter hole of precision parts, it is necessary to ensure that the specified size is reached.