Than the large-diameter elongated screw thread and its length is 1:30 and above, it is called an elongated screw. Screw is a mechanical device transmitting motion member, is one of the rotary motion into linear motion parts, not only to pass a certain dynamic and accurate transmission of motion, and can be used for precision linear indexing element. Because of its larger aspect ratio, in the machining process, machine, tool and so the whole process system easily bending and vibration, can not obtain a satisfactory surface roughness and geometrical accuracy after processing, often due to the warp taper is too large, the drum roundness belly or reach other causes scrapped parts. In addition, since the difference between the elongated screw thermal performance, the cutting process of cutting heat to produce a large linear expansion, but also to work and bending deformation. Thus, not only turning the elongated screw production efficiency is low, but quality is not guaranteed. To this end we propose the following methods to resolve the problem of an elongated screw turning.
First, to increase the rigidity of the system
As the process system elongated screw machining process less rigid and affect productivity and quality, it is necessary for the machine tool, workpiece and tool for improvement. Here to make some major improvements from the aspect of the workpiece clamping, in order to achieve improved elongated screw machining cutting conditions, increase the rigidity of the workpiece.
After using the chuck clamping the workpiece is supported in the top, when the cantilever is formed not later than the top, the rigidity of the workpiece to improve a lot. When turning the elongated screw, use a center frame, so that the distance between the support has halved, to improve the rigidity of the workpiece. When using elongated screw with turret turning, cutting action to shorten the distance between the point and the support point, the rigidity of the workpiece has been greatly improved from the point of cutting action and the support point between about 5 ~ 10mm.
Second, the use with the knife
When lathe elongated screw, the one prone to vibration, is not conducive to cutting; and secondly, difficult to guarantee the quality of precision parts. Methods to solve this problem generally has two aspects: one is to improve the cutting angle of the cutting tool, choose a reasonable cutting parameters; the second is the addition of aids that fitted with the knife to eliminate vibration, to ensure that the parts the quality and accuracy. Speed ​​can also be a corresponding increase in the feed rate to be increased, vibration small, small car out of parts bent, improve productivity, but also improve the machining accuracy.
Improved Third, the method of clamping
When processing an elongated screw, a common problem is poor quality, low efficiency. It has been introduced to improve the rigidity of the method, but due to the impact of cutting heat, screw inevitably produce thermal elongation. At a time when the distance between the chuck and the top is fixed, there is no work axial stretching room, the screw bending deformation. To reduce or eliminate such deformation, the following methods can be used:
1. between the chuck jaws and the work of each pad into Φ4mm × 10mm steel wire sandwiched a length of 15 ~ 20mm. After the pad into the wire, so that a line contact between the workpiece and the jaw, so that between the workpiece and the jaw may be a slight relative motion. Avoid jaw member is stuck, play a role in regulating the direction of reducing the bending deformation of the workpiece.
2. The machine tailstock elastic band to the top of the spring. Elastic size is determined by the extent of the top tight top. When the workpiece by cutting heat generating expansion and elongation, compression spring for pushing back the top pivot shift, to avoid bending deformation of the workpiece, thus ensuring accuracy.
3. Using necking method. Screw chuck in a car out of a neck portion, the neck portion diameter d = D / 2 (D is the outer diameter of the screw blank). Because the diameter of the screw neck portion is reduced, to increase its flexibility, reduce and eliminate the bending of the screw itself forced clamping chuck axis skew under the influence also played a role as a universal joint .
Fourth, the school work straight
Elongated screw material bending, machining will have a huge impact, especially in the high-speed rotation, the centrifugal force due to inertia, exacerbated by bending the blank, and cause vibration, resulting in processing difficulties, reduced quality. Thus, the elongated screw alignment work in process is also an essential element. Straightening it normally cold hot school school two, depending on the process requirements and stock availability.
1. Thermal school. Usually hot straightening in both cases. First, after heat treatment (quenching and tempering treatment generally screw) to eliminate bending roughing and heat treatment arising. The method is heat treatment after the work piece when the work piece is cooled to a certain extent, to check the size of deformation of the workpiece, such as more than drawing the technical requirements, the need for alignment, usually in the hand pressure bed conducted within the school to process requirements. This alignment, the workpiece is not easy to rebound, to ensure the quality of the work after finishing.
Another method is to heat the school in half after finishing, the method is the school after the workpiece half after finishing straight, at a temperature of oil pool soaking the workpiece straightening process to get rid of stress, the internal organization of the work stable, and then changed back after finishing difficult (to return to the state before straightening), so that the accuracy of the workpiece to be finished after the long-term. This method is generally used for high precision ball screws.
2. Cool school. Cold school there are two cases: one in front of the roughing screw straightening rough material, in order to ensure that after the roughing car circle; one after thread roughing, semi-finishing or before finishing the threads. Its role and thermal straightening the same, to ensure the smooth processing and improve geometric precision screw after processing.
Cold straightening of the two methods, the method is usually taken to be in hand pressure bed, straightening rough material is in the hand press table mat two high V-shaped iron supporting the workpiece. Semi-finished straightening the center hole of the supporting hand presses against the two top screw. This method is to use a dial indicator to find the highest point of the curved portion of the screw, with the press ram highest point of direct pressure (the pressure when the required intermediate semifinished mat board), plastic deformation of the workpiece, so that the curvature change control process the required range. This method of alignment of the workpiece, after finishing or heat treatment, the workpiece is likely to bounce back, the whole or partial recovery to the state before straightening, resulting in loss of accuracy of the workpiece after finishing, affect product quality.
Change V. cutting mode
In turning, the cutting direction is generally from tailstock to the direction of the bed, commonly known as positive towards pass. Turning elongated bars need to switch to a reverse pass, pass resistance to tensile stress direction of the workpiece. Reverse cutting the workpiece by the stretching action, to eliminate vibrations, so that the cutting steady, especially in turning and turning the screw coarse thread cutting step, the cutting power, but also need to use a reverse cutting, tailstock to be installed scalable live center.
It is worth mentioning that, when installing the tool, the tip should be slightly above the centerline of the workpiece 0.1 ~ 0.15mm, the cutting process of cutting tool rake angle increases, reduce cutting forces, thus reducing the cutting force of the compression artifacts . Meanwhile, in the cutting process, the tip also plays a role to hold up the work, with the knife to offset the reaction force of the workpiece supporting block, the fourth corresponds with the knife support block.
In order to reduce friction with the knife support block and the workpiece support block and cause serious wear and tear, reducing the temperature of the workpiece while cooling the blade, at any time to adjust with the knife while the tightness, the need for adequate cooling in the cutting process and lubrication, cutting smoothly, ensure that the surface roughness after roughing threads.

Sixth, a reasonable choice of tools geometry
When turning slender shaft, due to the poor rigidity of the workpiece, tool geometry of the workpiece is very sensitive to vibration. If the geometry of the inappropriate choice of tools, it is impossible to obtain good results. Mainly consider the following choices:
1. In order to reduce the cutting force, reduce the angle editor of the elongate shaft is bent, turning to take 75 ° ~ 93 °.
2. In order to reduce the cutting force, should choose a larger rake angle, take 15 ° ~ 30 °.
3. Tool wear front should have chipbreaker R1.5 ~ 3, so that the chip curler broken.
4. Select the negative blade angle, take -3 ° ~ -10 °, the chips flow surface to be processed. On the other hand, turning too easy to get into the workpiece, and reduce cutting forces.
5. edge roughness is higher, and to keep sharp.
6. In order to reduce the radial cutting force, tool nose radius should be selected to be small (R <0.3mm), chamfer width should be selected.

Seven double knife cutting tool holder
The use of force and the principle of deformation of the workpiece offset by using dual turret on, that does not require the use of the center frame, with the knife does not need to use, simply use the appropriate tool geometry of the pole "knife" cutting not only greatly reduces the bending deformation of the workpiece, but also with a large amount of feed to improve the cutting speed, simultaneous roughing, semi-finishing or finishing, shorten the processing time, the quality assurance process.
On the lathe saddle tops both before and after the carriage holder, screw in the carriage turned into the left and right hand threads drive. Before and after the two diametrically opposed installation tools, semi-finishing tools being installed, finishing tool reversed, pass along the same axial direction, rotating around the screw driven carriage in two turret while for radial infeed or retraction, so twice while cutting, cutting force reached offset purposes. To make cutting force balance, finishing tool requires the use of 0 ° negative or small angle after posterior horn, increase cutting force generated by the finishing tool, and make the semi-finishing (cutting a large margin) cutting force generated by the equilibrium . Radial cutting forces cancel each other, to clear the elongated screw cutting defects easily deformed. After finishing tool while the negative angle formed knife friction face of the workpiece, making it play a role in the calender, to improve the surface quality and improve labor productivity.
Because of its elongated screw larger aspect ratio, poor thermal performance, not only turning the elongated screw production efficiency is low, but quality is not guaranteed. Therefore, in order to improve the efficiency and quality of labor productivity, a method to solve the problem of turning an elongated screw. But it must be for specific circumstances and different requirements alone or mixed use, in order to achieve the desired results