1. Introduction

   Thread is a continuous protrusion and groove formed on the surface of a cylindrical workpiece along a spiral line with the same cross section. In various machine and mechanical manufacturing industries, parts with threaded structures are widely used, especially trapezoidal threads. The commonly used method of thread processing is turning, and four standard threads, namely, metric, British, module, and diameter pitch, can be turned on a common horizontal lathe. The skill of turning threads is a basic skill that ordinary lathe operators and CNC lathe operators must master proficiently. It is one of the main contents of the basic skills training for ordinary turning. It almost includes the basic content and most of the operation requirements of the previous chapters. It requires students to have a solid theoretical foundation, but more importantly, it requires students to have quick reaction ability and skilled hands-on skills. The comprehensive quality requirements for students are relatively high. Students are also very interested in this skill training. However, in actual thread turning, due to various reasons, the motion from the spindle to the tool has a problem at a certain link, resulting in thread turning failure, causing the thread accuracy to not meet the requirements of the part drawing, affecting normal production, and may also occur unsafe accidents. At this time, it should be solved in time. Below, we summarize and summarize the common fault phenomena, thread quality problems and solutions that have occurred in the thread turning skills training internship for many years:

Two, during the process of turning issues

〈one〉,tools tip impact or tooth biting:the tool is installed too high or too low, the workpiece is not firmly clamped or the tool is worn too much, which can easily cause the phenomenon of tool tip impact or tooth biting.

1.The tool is installed too high or too low: if too high, it causes the tool's working back angle to become smaller, and when the tool cuts to a certain depth, the tool's back surface touches the workpiece, increasing friction. Under the action of the radial force, it even bends the workpiece, resulting in the phenomenon of biting the tool, and it cannot cut normally; if too low, the chips are not easy to discharge. The direction of the tool's radial force is towards the center of the workpiece. If the clearance between the middle slide plate's transverse lead screw and the nut is too large, it causes the cutting depth to automatically deepen, which is equivalent to pulling the tool into the workpiece, thus lifting the workpiece and causing vibration and tool jamming. At this time, the tool height should be adjusted in time to make its tip the same height as the workpiece's axis(the tailstock centering can be used to adjust the tool, etc.). When rough cutting and semi-fine cutting, the tip position of the tool is about1% ×Dhigher than the center of the workpiece(Dis the diameter of the workpiece to be machined), which is equivalent to a smaller main back angle, giving the workpiece a support, increasing the rigidity of the workpiece, and thus reducing vibration and avoiding tool jamming; while in fine machining, the tool must be the same height as the workpiece center.                                                                        

2.Workpiece clamping is not firm: The rigidity of the workpiece itself cannot withstand the cutting force during turning, resulting in excessive deflection, changing the height between the tool and the workpiece center(the workpiece is lifted), forming a sudden increase in cutting depth, and the tool gets stuck. At this time, the workpiece should be clamped firmly. For longer workpieces, a tailstock center can be used for clamping, to increase the rigidity of the workpiece.

3.The tool is worn out too much; when the tool has been used for a period of time, the tool becomes blunt, the cutting edge is worn seriously, the back angle becomes smaller or even negative, increasing the friction force, causing the cutting resistance and friction resistance to increase, bending the workpiece and causing the tool to benibbed. At this time, the tool edge should be ground or replaced.

4.Due to insufficient proficiency in the method of oblique cutting and left and right cutting, especially in turning, three cutting edges participate in the cutting work at the same time, if the depth of cut is too large, the tool contact area is large, the chip removal is difficult, and the tool will jam or chip.

〈II〉、Misalignment of Teeth

    When the lead screw revolves one turn, if the workpiece does not revolve an integer number of turns, the cutting tool will the original spiral groove, resulting in a  phenomenon. Regardless of which type of thread is being cut, it is required that there must be a strict transmission relationship between the spindle of the lathe and the tool: that is, for each revolution of the spindle(that is, the workpiece revolves one turn), the tool should uniformly move a(workpiece of)the lead distance. Their transmission relationship is ensured in this way: the spindle revolves the workpiece together, the motion of the spindle is transmitted to the feed box through the change gear box, and then from the feed box, after speed change, it is transmitted to the lead screw, and then the lead screw and the nut on the slide box cooperate to drive the tool rest on the bed to move linearly in the longitudinal direction. In this way, the revolution of the workpiece and the movement of the tool are both realized through the drive of the spindle, thus ensuring that there is a strict transmission relationship between the workpiece and the tool.

1. When the lead of the lathe lead screw is not an integer multiple of the lead of the workpiece; if, when retreating the tool, the tool carriage is opened, the lead screw is turned to the starting position, and the tool carriage is closed, the tool tip will not be in the spiral groove cut by the previous cut, resulting in a mess of threads. The solution is to use the forward and reverse cutting method to retreat the tool, that is, at the end of the first cutting stroke, the tool carriage is not lifted, the tool is quickly retreated radially, and then the main shaft is reversed to return the tool along the longitudinal direction, and then the second cutting stroke is performed, so that in the reciprocating cutting process, the transmission between the main shaft, the lead screw, and the tool carriage has not been separated, and the transmission relationship has not changed, and the tool is always in the original spiral groove, so that a mess of threads will not occur.

2. For the thread whose pitch is an integer multiple of the pitch of the workpiece on the lathe; both the workpiece and the screw are rotating. When cutting the tail end of the thread (or the groove), lift the slide rest by hand, and move it to the starting position of the thread cutting. As long as the slide rest is re-closed, when the screw turns one revolution, the workpiece turns an integer multiple, and the cutting tool can enter the spiral groove cut by the previous cut, so that no disorderly threads will appear. In this way, the open and close slide rest can be used to manually retract the tool. This method of retracting the tool is fast, which is conducive to improving the productivity and maintaining the accuracy of the screw, and the cutting is also safer.

2. Quality issues with the threads themselves

(1)、 pitch incorrect

Thread pitch error is divided into the following four cases:

1. The pitch on the full length of the thread is incorrect due to improper gear matching or the incorrect position of the feed box handle. You can re-check the position of the feed box handle or recalculate the gear. The inspection method is to perform a trial cut before officially starting the threading, allowing the tool tip to gently draw a spiral mark on the workpiece surface, and measure whether the pitch is correct using a measuring tool.

2. The local lead is incorrect because of the wear of the lathe lead screw, which causes local error in the lead. The lead screw can be replaced or repaired locally. If the tool is not properly positioned in the spiral groove when changing the tool during fine cutting, it is necessary to carefully adjust the tool after changing the tool.

3. The pitch of the thread is uneven along the full length of the thread, and there are many reasons for this, mostly caused by the accuracy of the lathe itself: if it is caused by the axial play of the lead screw, the adjusting nut at the connection between the lathe lead screw and the feed box can be adjusted to eliminate the axial clearance of the thrust ball bearing at the connection. If it is caused by the axial play of the spindle, the adjusting nut at the rear of the spindle can be adjusted to eliminate the axial clearance of the rear thrust ball bearing. If it is caused by the poor meshing of the slide box's dead nut and the lead screw being out of alignment, the dead nut can be repaired and the dead nut clearance can be adjusted. If it is caused by the wear of the dovetail guide, the dovetail guide and the wedge block can be matched to meet the correct fit requirements. If it is caused by the large gap of the change gear, the gap of the change gear can be re-adjusted.

4.Appearance of bamboo joints; this is caused by periodic errors in the gear transmission between the spindle and the lead screw, such as the gears in the change gear box and the feed box, due to manufacturing errors or local wear, etc., which cause uneven rotation at the moment of rotation, thus causing periodic unevenness in the rotation of the lead screw, periodic unevenness in the movement of the tool, resulting in the appearance of bamboo joints, and the gears with errors or wear can be replaced. In addition, it is also possible that the position of the handle has been changed during the turning process, generally speaking, do not move the middle slide plate and the small slide plate handle during the turning process.

〈II〉、Minor diameter incorrect

If the cutting knife is too large or the scale disk is not accurate, or the bottom diameter of the thread is not controlled well, and not measured in time, it is easy to cause the middle diameter to be incorrect, and the middle diameter is an important parameter index to evaluate the accuracy of the thread. This requires careful inspection of whether the scale disk is loose, whether the machining allowance is appropriate, whether the calculated bottom diameter size is turned, whether the cutting tool edge is sharp, and timely measurement.

〈Three〉、Thread surface rough

During the finishing turning stage (especially for threads with high surface requirements), if the cutting tool edge is not ground smoothly and straight, the cutting fluid used is inappropriate, the cutting data is not properly selected, the finishing turning allowance is insufficient, the cutting tool rigidity is poor, and vibration is generated during the cutting process, etc., it can cause the surface roughness of the thread to not meet the requirements of the part drawing. This requires the use of a true wheel or an oil stone to finely grind the tool when finishing turning threads, select an appropriate cutting speed (especially when using high-speed steel tools, it is better to use low speed), appropriate cutting fluid and small cutting depth; adjust the bed of the lathe and the wedge guide of the middle and small slide plates, and try to eliminate the empty stroke of the handle, ensure the accuracy of the guide clearance, and the cutting tool extends too long to prevent vibration during cutting.

〈Four〉、Thread profile skew

    The thread profile is incorrect, the half-angle of the profile is asymmetric, resulting in a skew, somewhat like a sawtooth shape, the tooth surface is arc-shaped, this may be due to the shape of the cutting tool being ground incorrectly, or it may be due to the lack of correct tool setting with the tool setting block, or the cutting tool has been used for too long and is worn out, this requires careful grinding of the tool or correct tool setting before threading.

〈Five〉、Thread termination is not good

Some workpieces do not have a chamfer designed at the end of the thread, and the tool is manually retracted by sense when tapping the thread, resulting in inconsistent tool retraction points each time, causingthe thread to be poorly finished, causing the mating workpiece to not tighten, more seriously, it is easy to jam the tool or collide with the workpiece step, and it is also easy to break the tool and cause accidents. The solution is either to lightly slide a mark on the end of the thread with a lathe according to the drawing requirements, or to use a slow speed to tap to the end of the thread for the first cut, remember the scale line of the large slide plate, and as long as the scale line is reached in the subsequent tapping process, immediately retract the tool, so that the ending will be consistent.

To sum up, the fault forms and quality problems that occur during thread turning are diverse, with both machine tool and tool and operator reasons. When eliminating faults, specific situations should be analyzed specifically, and effective solutions should be adopted through various detection and diagnosis means to process products that meet the requirements.