1. Error of oil pipe thread comb. It includes the manufacturing tolerance of the comber and the error after the comber is reground and worn. The tooth profile error caused by the wear of the carding tool is different according to the processing methods of roughing and finishing the thread with the carding tool directly or with the common thread turning tool first and then with the carding tool.
2. Installation error of carding knife. The internal and external comber of tubing thread are installed on the special comber bar respectively. The installation is not reliable or the alignment of the tool bar is not accurate, as well as the change of the position of the comber blade due to the cutting force, etc., which will cause the increase of the half angle error of the tooth profile.
3. Clamping error and processing method error of workpiece. These errors mainly affect the profile error of tubing thread, resulting in incomplete partial profile and large roundness error of cross section.