Analysis of common problems in turning (Figure)

introduction
When the ordinary lathe is powerfully turning large pitch threads, the saddle vibration sometimes occurs, and the light causes the machined surface to corrugate, and the heavy one breaks the knife. When cutting off, students often have a knife or a broken knife. There are many reasons for the above problems. Now we mainly discuss this phenomenon and the solution by analyzing the force condition of the tool.

figure 1

figure 2
1 The cause of the problem and its causes
We know that when turning a thread with a small pitch, the straight-cut cutting method is generally used (linear feeding is performed perpendicular to the axis of the workpiece); when the thread with a large pitch is turned, in order to reduce the cutting force, the left and right borrowing tools are often used. Cutting method (by moving the small slide to make the thread cutter cut with the left and right cutting edges).
When the thread is turned, the movement of the saddle is achieved by the rotation of the long screw to drive the opening and closing nut. There is an axial gap at the bearing of the long screw, and there is also an axial gap between the long screw and the opening and closing nut. When using the right and left knife cutting method to strongly turn the right-handed worm with the right main cutting edge, the tool bears the force P of the workpiece, (ignoring the friction between the chip and the rake face, as shown in Figure 1), and decomposing the force P into The axial component force P _x and the radial component force, wherein the axial component force P _x is the same as the tool feeding direction, the tool transmits the axial component force P _x to the bed saddle, thereby pushing the saddle to the bed The side of the gap makes a quick and violent back and forth movement, and the result is that the tool is moved back and forth, and the machined surface is corrugated or even broken. However, there is no such phenomenon when cutting with the left main cutting edge. When cutting with the left main cutting edge, the axial component force Px of the tool is opposite to the feeding direction, and moves in the direction of eliminating the gap, at which time the saddle is moved at a uniform speed. .
When cutting, the movement of the middle slide plate is realized by the movement of the nut of the middle slide screw, the axial position of the screw bearing, and the axial gap between the lead screw and the nut. When cutting on the lathe, the tool rake face (with the front angle) withstands the force P given to the workpiece (ignoring the friction between the chip and the rake face, as shown in Figure 2), breaking the force P into the force P _z And the radial component is fine, wherein the radial component is the same as the cutting direction of the cutting tool, pointing to the workpiece, pushing the tool toward the workpiece, thereby pulling the middle sliding plate to move in the direction of the gap, so that the cutting knife is suddenly tied A human workpiece that causes a tie (broken) knife or workpiece to bend.

image 3
2 Solution
When the turning pitch is large, the thread of the left and right knife cutting method is used. In addition to adjusting the parameters of the lathe, the matching clearance between the bed saddle and the bed rail should be adjusted to make it slightly tight to increase the movement. Friction, reduce the possibility of the saddle of the bed, but this gap can not be adjusted too tightly, so that the saddle can be shaken smoothly.
Adjust the gap of the middle slide, try to minimize the gap; adjust the tightness of the small slide to make it slightly tighter to prevent the shifting of the turning tool during turning.
The length of the workpiece and the shank should be shortened as much as possible, and the left main cutting edge should be used as much as possible. When cutting with the right main cutting edge, reduce the amount of back knives; increase the front angle of the right main cutting edge, the cutting edge should be straight, sharp To reduce the axial component force P _{x that the} tool is subjected to. In theory, the larger the front angle of the right main blade, the better.
When cutting or grooving, first adjust the tightness of the small skateboard to make it slightly tighter to prevent the turning of the turning tool during turning. Adjust the clearance of the middle slide plate to reduce the axial clearance at the screw bearing and the axial clearance between the lead screw and the nut.
Grind the cutting knives; it is not advisable to take the large rake angle. As the rake angle increases, the radial component of the tool increases, which increases the possibility of knives or broken knives.
The chip flute should not be too deep, generally 0.75~1.5mm deep. (Fig. 3) The front corner is too large. The wedge angle reduces the heat dissipation area of â€‹â€‹the cutter head, which reduces the tool tip strength and reduces the tool life.
It is not advisable to take a large feed. If it is a manual feed, it must be fed smoothly and evenly.
When the current angle is 0 degrees, the tool receives a radial component force P _y =O, and the tool is durable, but not sharp.
3 Conclusion
The above-mentioned problems have a common feature, that is, the axial component force P _{x of the} tool when turning the thread and the radial component force P _{y 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., are tilted, so that these two forces accelerate the abnormal movement of the tool, so when the tool is subjected to the force and the tool When the direction is the same, we have to be 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 try to make the gap as small as possible while ensuring that the various parts of the machine tool can move freely.
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