Thread turning is a very common and complex problem in machining. The requirements for thread turning are higher than those of other ordinary turning operations, and the cutting forces generated during turning are generally larger.
When turning threads, due to the influence of the thread rising angle, the position of the cutting plane and the base surface is changed, so that the values of the rake angle and the rear angle of the turning tool are different from the rake angle and the rake angle of the sharpening, which affects the normal turning. When turning threads, if there is a problem in one link, it will have unexpected consequences and affect the normal processing, which should be solved in time.
1. Gnawing knives and slapping knives
(1) One of the reasons for the failure: the turning tool is installed too high. The turning tool clamping is too high, during cutting, due to the action of the main cutting force generated between the workpiece and the turning tool, the tip of the threaded turning tool is moved downward by the pressure of the workpiece, so that the turning tool is pressed to the maximum outer circle of the workpiece, so that the turning tool that is not deep in the original turning is more and more into the depth of the workpiece, so that the turning depth becomes deeper, and the turning force between the workpiece and the turning tool is further increased, resulting in the phenomenon of gnawing or punching.
Solution: The height of the turning tool should be adjusted in time to make its tip equal to the axis of the workpiece (the tailstock center can be used to set the tool).
(2) The second cause of failure: the clamping strength of the workpiece is not enough. When turning threads, the main turning force generated by the workpiece and the turning tool is the backlash force (i.e., depth of cut resistance).
force), which is consistent with the diameter direction of the workpiece, and the workpiece is subjected to an outward force at any time, which makes the workpiece have a tendency to bend outward at any time. As a result, the workpiece is turned shallow on one side and deep on the other. When the turning is deep, the turning force increases, resulting in the phenomenon of gnawing or punching.
Solution: The workpiece should be clamped firmly, and the tailstock center can be used to increase the rigidity of the workpiece.
(3) The third cause of failure: the rear angle of the turning tool is small. In rough turning, the relief angle is small but the turning is deep, and the amount of each tool is not easy to show. However, when the turning reaches a certain depth, the amount of tools gathers together to produce a large turning depth, so that the turning force between the tool and the workpiece increases suddenly, so the phenomenon of gnawing the knife is produced. When finishing, because the left and right turning method is used to turn (i.e., turning with small slide plate feed), the amount of cutter is small, so the cutting force between the workpiece and the turning tool is small, and the cutting force generated by the workpiece and the turning tool can not resist the resistance between the flank face and the workpiece, so that the turning tool can not turn the workpiece although the first few knives are fed in the first few cuts, and a certain knife in the back will finish the measurement of the first few knives at once, so as to produce the phenomenon of gnawing the knife.
Solution: The rear angles on both sides of the thread turning tool should be grinded larger. For right-hand threads, the left relief angle is larger than the right one.
(4) Cause of failure: incorrect turning method. When turning with the straight method, when the turning tool enters the workpiece deeply, if no measures are taken, the thread turning tool will have the phenomenon of three edges turning at the same time, resulting in excessive turning force on the turning tool and gnawing the knife, or even punching.
Solution: The left and right turning method or oblique method should be used to turn to increase the width of the spiral groove and avoid the simultaneous turning of three edges when the thread turning tool is turned.
2. Random buckle
(1) One of the causes of failure: the ratio of the pitch of the lead screw of the lathe to the pitch of the workpiece is not an integer multiple. When threading, it takes several feeds to complete. If, on the second feed, the tip of the turning tool deviates from the spiral groove out of the previous feed car, there will be a tooth disorder (buckle). Because the ratio of the pitch of the lead screw and the pitch of the workpiece is not an integer multiple, when the first spiral groove is lifted after the first spiral groove is turned, when the second spiral groove is turned, the screw rod turns a turn, and the opening and closing nut is pressed again, and the workpiece does not turn the integer circle at this time, resulting in the turning tool can not enter the spiral groove that feeds the car out of the previous time, so there is a disorderly tooth (disorderly buckle) phenomenon.
Solution: When turning, the thread is turned by the method of opening and reversing the turning, that is, at the end of a stroke, the opening and closing nut is not mentioned, and after the turning tool is withdrawn, the spindle is reversed, so that the thread turning tool is returned along the longitudinal direction, and then the second turning is carried out.
(2) Cause 2: After reloading the tool in the middle of the process, the tooth is messy when turning. When turning threads, when the rough turning is completed, the tool has been severely worn, and the next step of fine turning cannot be carried out, and a new thread turning tool needs to be installed. The reloaded tool cannot be installed in the same position as the original turning tool, so it needs to be realigned. If the tool is not driven forward when the tool is set, but the knife is shaken into the spiral groove that has been turned in the parking state, when the turning will occur, the tooth will be messy.
Solution: After the cutter is installed, press down the opening and closing nut, and the lathe rotates, so that the turning tool stops along the outer surface of the workpiece into the spiral groove that has been turned, and the cutter is shaken into the spiral groove, if the cutter does not enter the spiral groove, then rotate the small slide plate to make the thread turning tool enter the spiral groove that has been roughly turned, and the lathe can not be reversed when the tool is set. If the chuck is slightly reversed and stopped when stopping, the tool setting can not be carried out at this time, the lathe should be restarted to rotate, so that the thread turning tool continues to go forward, stop, and then shake the middle slide plate and the small slide plate to make the thread turning tool enter the spiral groove, so that the tool setting is correct.
3. The pitch is incorrect
(1) One of the causes of failure: the thread is incorrect on the entire workpiece. The reason is that when selecting the handle of the feed box, the correct position of the handle should be selected by mistake, so that when the handle of the feed box is wrenched, the position of the other pitch handle is wrongly selected, and the pitch of the car becomes other pitch.
Solution: Carefully observe the data marked on the nameplate of the feed box of the lathe, and correctly change the position of the handle of the feed box. And when turning, a very shallow helix is turned on the workpiece, and the pitch is measured with a caliper or thread template to observe whether the pitch is correct.
(2) Cause of failure 2: local incorrectness. The reason may be that the axial movement of the lead screw and spindle is too large. The handwheel of the slip box rotates unbalancedly. The gap between the opening and closing nuts is too large. The opening and closing nuts are automatically lifted during turning.
Solution: Adjust the axial channeling momentum of the spindle and the lead screw, pull out the handwheel of the slide box to disconnect it from the transmission shaft, make the saddle move evenly, and hang a heavy object on the opening and closing nut to prevent lifting halfway.
4. The middle diameter is incorrect
(1) One of the causes of failure: the turning depth is incorrect, and the correct tooth height is not turned.
Solution: According to the formula for calculating the tooth height, calculate the number of grids that the middle slide plate should enter.
(2) the second cause of the failure: the wrong tool installation, so that the half angle of the two threads out of the car is not equal, resulting in a skewed tooth type (commonly known as inverted teeth), when the ring gauge is tested and turned, the ring gauge is not used to repair the tooth side, until the middle diameter of the car is wrong, and the gauge can be screwed in when the gauge can be screwed in.
Solution: Load the knife with the tool setting template, so that the angle bisector of the thread cutter is perpendicular to the workpiece axis.
5. The thread surface is rough
Cause of failure: the cutting edge of the turning tool is not polished, the cutting speed is fast, the turning tool angle is small, and the cutting process produces vibration.
Solution: Use oil stone to refine the cutting edge of the turning tool to eliminate the burr generated when grinding; select a slower cutting speed and add sufficient cutting fluid when using high-speed steel turning tool; adjust the lathe saddle pressing plate and the middle and middle The inlay of the dovetail guide rail of the small slide plate ensures the accuracy of the clearance of each guide rail and prevents the vibration during cutting; the rear angle of the threaded fine turning tool should be sharpened, which not only makes the turning tool sharp, but also reduces the turning tool caused by the small rear angle during turning, so that the turning is smooth, reduces the friction between the turning tool and the workpiece, and makes the tooth surface quality better guaranteed.
In short, there are various faults when operating thread turning on ordinary lathes, and the faults should be analyzed according to the specific situation and solved by corresponding methods.
