INTRODUCTION When the ordinary lathe is turning a large pitch thread, sometimes the saddle vibration occurs, and the light causes ripples on the machined surface, and severely breaks the knife. When cut off, students often have a knife or a broken knife. There are many reasons for the above problems. Now we mainly discuss the phenomenon and solutions by analyzing the force conditions of the tools.
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1 The cause of the problem and the reasons We know: When turning a thread with a small pitch, the straight knife cutting method is usually used (straight feed in the direction perpendicular to the workpiece axis); when turning a thread with a large pitch, the cutting force is reduced. It often uses the left and right knife cutting method (by moving the small slide to allow the thread turning tools to cut with the left and right cutting edges, respectively). When turning the thread, the movement of the saddle is achieved by the rotation of the long screw to drive the movement of the opening and closing nut. There is an axial clearance at the bearing of the long screw, and there is also an axial play between the long screw and the opening nut. When the left and right knife cutting method is used to cut the right main cutting edge of the right-handed worm, the tool bears the force P given by the work piece (ignoring the friction between the chip and the rake face, as shown in Fig. 1), and the force P is decomposed into Axial component force Px and radial component force, where the axial component force Px is the same as the feed direction of the tool, the tool transmits this axial component force Px to the saddle, which promotes the saddle to the gapped side. Doing quick and swift motions back and forth, the result is to move the tool back and forth, causing ripples in the machining surface, and even broken blades. However, when cutting with the left main cutting edge, there is no such phenomenon. When cutting with the left main cutting edge, the axial component force Px of the tool is opposite to the feed direction, and moves in the direction of eliminating clearance. At this time, the saddle moves at a uniform speed. . When cutting off, the movement of the middle sliding plate is achieved by the rotation of the middle sliding platen screw which drives the nut. There is an axial clearance at the screw shaft bearing and an axial clearance between the screw shaft and the nut. When cutting on a lathe, the rake face (with rake angle) of the tool is subjected to the force P given by the workpiece (ignoring the friction between the chip and the rake face, as shown in Figure 2), and the force P is split into the force Pz and The radial component is exquisite, and the radial component is the same as the cutting direction of the cutting tool. It points to the workpiece and pushes the tool towards the workpiece. It will pull the middle slide to move in the direction of the gap, so that the cutting blade will suddenly pull the artificial. Pieces, causing the knife or workpiece to bend.
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2 Solution When the turning pitch is larger and the thread of the left and right borrow cutting method is used, besides adjusting the relevant parameters of the lathe, the adjustment gap between the saddle and the bed rail should also be adjusted to make it slightly tight to increase Friction during movement reduces the possibility of the saddle to move, but this gap can't be adjusted too tightly enough to shake the saddle steadily. Adjust the clearance of the middle skateboard to minimize the gap; adjust the tightness of the small skateboard to make it slightly tighter to prevent the shift of the turning tool when turning. The length of the workpiece and the shank stick should be shortened as much as possible. The left main blade should be used to cut as much as possible. When cutting with the right main blade, the amount of the back knife should be reduced. When the front rake of the right main blade is increased, the edge of the knife should be straight and sharp. , in order to reduce the axial component of the tool Px. In theory, the larger the rake angle of the right main blade, the better. When cutting or grooving, first adjust the tightness of the small slide to make it slightly tighter to prevent the turning tool from turning. Adjust the clearance of the middle slide board to reduce the axial clearance between the screw bearing and the axial clearance between the screw and nut. Sharpen the cutting knife; it is not appropriate to take the large rake angle, because the rake angle increases, the radial force of the tool is increased, which increases the possibility of cutting or cutting. Knife flute should not be too deep, generally 0.75 ~ 1.5mm depth is appropriate, (Figure 3) the angle of the front corner is too large to reduce the blade head heat dissipation area to reduce the blade tip strength, tool life is reduced. It is not advisable to take a large feed. If it is a manual feed, it must be steadily and uniformly fed. When the current angle is 0 degrees, the radial force component of the tool is Py=O, and the tool is durable but not sharp. 3 Conclusions The emergence of the above problems has a common feature, that is, when turning the thread, the axial force component Px of the tool and the radial force component Py of the tool when cutting are the same as the direction of the tool, and there is a gap ( The axial clearance at the bearing and the axial clearance between the screw and the nut, etc.) move in the direction, so that the two forces accelerate the abnormal movement of the tool, so when the tool is subjected to the force and the tool is moved When the directions are the same, we have to be especially careful to avoid unnecessary economic losses. Pay attention to the sharpening and clamping of the tool, carefully adjust the gap between the various parts of the machine tool, and ensure that the machine tool can move freely and minimize the gap.
Shackle
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