Three parts of turning and seven parts of cutting tools. If you want to do a good job on a lathe, you must first ensure that the "weapon" is handy, and then practice more, practice the combination of outer circle, inner hole, length, taper, and thread, and then practice turning complex parts, such as: inner and outer trapezoidal threads, worms, slender shafts, thin-walled sleeves, etc., and learn to use the center frame and the tool holder.
1. Turning slender shafts "turners are afraid of turning rods." This sentence reflects the difficulty of turning slender rods. Due to the characteristics and technical requirements of slender shafts, when turning at high speed, it is easy to produce defects such as vibration, multi-edges, bamboo joints, poor cylindricality and bending. To turn it smoothly, you must pay full attention to the problems in the process.
1) The line connecting the center lines of the lathe spindle and the tailstock must be parallel to the lathe large guide rail up and down, left and right, and the tolerance should be less than 0.02mm.
2) When installing the workpiece, try not to over-position it. When clamping one end with a chuck, do not exceed 10mm.
3) The tool uses a 75°～90° offset tool. Note that the secondary back angle α′0≤4°～6°, which should not be too large. When installing the tool, it should be slightly higher than the center.
4) The tool rest must be trimmed after installation. The trimming method can be grinding, reaming, boring, etc., so that the arc surface R of the tool rest claw in contact with the workpiece ≥ the workpiece radius, and must not be less than the workpiece radius to prevent the generation of multiple edges. When adjusting the tool rest claw, just make the claw contact with the workpiece, do not use force to prevent bamboo knots.
5) When the aspect ratio of the auxiliary support workpiece is greater than 40, auxiliary support should be added during the turning process to prevent the workpiece from vibrating or bending due to centrifugal force. Pay attention to the adjustment of the top during cutting. It is better to just top the workpiece, not tight, and adjust it at any time to prevent the workpiece from thermal expansion, deformation and bending.
2. Reverse tool turning of slender rods There are many methods for turning slender rods, generally using the tool rest for forward or reverse tool turning. However, compared with forward turning, reverse turning has many advantages and is mostly adopted.
Two problems are prone to occur in turning. One is multi-faceted shape, which is mainly caused by the large back angle of the tool and the R of the claw of the tool holder does not match the diameter of the workpiece turned; the other is the bamboo problem, which is caused by the cutting depth from extremely small to suddenly increasing after the tool holder is well aligned at the mouth of the frame, when the tool is aligned and the tool is moved to the cutting surface, the cutting force changes, the workpiece moves outward, and the diameter suddenly increases. When the tool holder moves to a large diameter, the turned diameter becomes smaller again, and the cycle is repeated, making the processed workpiece bamboo-shaped.
In order to prevent the formation of bamboo-shaped shapes, when the mouth of the frame is turned, follow the tool holder carefully, reverse the tool after aligning the tool, use the middle drag handle, and then eat deep (0.04~0.08) mm, but it should be flexibly controlled according to the size of the cutting depth.
3. Rolling straightening method In mechanical processing, rolling processing is often used to improve the surface hardness, fatigue resistance and wear resistance of the workpiece, reduce the surface roughness of the workpiece, and extend the service life of the workpiece. At the same time, the metal can also be plastically deformed under the action of external force during the rolling process to change the internal stress to straighten the shaft and rod workpieces with good rigidity.
In the process of rolling the workpiece, the rolled workpiece is bent due to the uneven hardness of the surface layer under the action of external force. The high point of the bending rotation center bears a large rolling force, and the plastic deformation is also large, which makes the bending degree of the workpiece greater. This phenomenon is particularly prominent when using rigid rolling tools.
The method of rolling straightening is to check the radial runout of the workpiece after the first rolling of the workpiece, mark the concave part, and use a four-jaw chuck to adjust the concave part of the workpiece to the height of the machine tool rotation center, which is proportional to the size of the workpiece bending, and then roll it for the second time, and then use a dial indicator and adjust the jaws of the four-jaw chuck to correct the workpiece. Use a dial indicator to check the bending. If it is still bent, use the above method to adjust the workpiece and roll it for the third time until the required straightness of the workpiece is achieved. The length of the cut after the second time should be determined according to the specific situation. It is not necessary to go through the entire process, and the reverse cut should be used.
Rolling straightening is generally completed during the rolling process of the workpiece. It will not only not damage the surface of the workpiece, but also make the outer surface of the workpiece roll more evenly, without dead bends, and is easy to operate.
4. The screw extrusion straightening method is very effective for screws with large diameters and lengths and several bends.
1) Working principle The straightening tool is used to squeeze the bottom surface of the screw teeth under the action of external force, so that the surface is plastically deformed and extends axially, changing the internal stress of the screw to make it straight.
2) Straightening method First, measure the position and direction of the screw bend on a lathe or platform, then turn the concave part of the bend upward and the convex part downward to contact the metal pad, and use a flat shovel and a hand hammer to hit the screw tooth bottom to deform the metal of the screw small diameter to achieve the purpose of straightening. During the entire straightening process, check the bending situation, hit the flat shovel and squeeze alternately until the screw is straightened. This method is simple and easy to use. It is not only suitable for large and small screws, but also for straightening shaft blanks. It is not easy to restore after straightening.
3) Issues that should be noted: The size R of the special flat shovel used for straightening should be greater than half of the screw tooth bottom diameter, b is less than the tooth bottom width, and α is less than the tooth angle; the R section in contact with the workpiece should be ground into a circular arc; after straightening, the squeezed tooth bottom should be flattened with a file.
5. Processing of rubber threads Because the hardness of rubber is very low, the elastic modulus is only 2.35N, which is equivalent to 1/85000 of carbon steel. Under the action of external force, it is very easy to deform and difficult to cut. Especially cutting some special-shaped threads is more difficult.
In order to solve the processing of rubber threads, a grinding head that can adjust the helix angle arbitrarily is installed on the lathe, or a pneumatic grinding head can be used instead when the thread precision requirement is not high. The grinding wheel uses a white corundum grinding wheel with a diameter of Φ60mm~Φ80mm and a particle size of 60#~100#. After the grinding wheel is installed, the shape of the grinding wheel is trimmed with a diamond pen. The shape of the grinding wheel is the normal section shape of the thread.
The thread lead is small, and the lathe nameplate has it, which can be obtained by directly turning the lathe handle. When there is no nameplate on the lathe, the required hanging wheel must be calculated. Generally, the manual can be checked, or the calculation method can be used to find and manufacture the required hanging wheel.
Generally, when the thread lead is greater than 300mm, the spindle speed must be reduced to avoid affecting the thread grinding quality due to the high spindle speed, and also causing tension in the operation or damaging the parts of the feed box. The methods of deceleration are: changing the diameter of the active and passive pulleys; adding a reduction box outside the lathe.
The method of splitting is the same as the method of turning multiple threads.
Grinding rubber threads on a lathe is a high-efficiency, high-quality processing technology. The grinding method is used successively to process single-head and multiple-head rubber threads with a lead of (1.5~1280) mm, and the quality meets the requirements.
6. Step deep hole turning method When turning holes with a length-to-diameter ratio greater than 4 on a lathe, due to the poor rigidity of the tool bar, vibration during cutting affects the cutting efficiency and the quality of the processed surface, which brings difficulties to turning. Especially when the hole diameter is large and the hole is deep, and there is a step, the processing is more difficult due to the influence of the tool bar and the rigidity of the machine tool.
First, install the workpiece on the lathe with the chuck and center frame, and use the inner hole cutter to process the short holes at both ends of the workpiece, and each is equipped with a sleeve and a special tool bar. When turning the middle long hole, first insert the left end support sleeve into the workpiece hole, then install the workpiece on the lathe, adjust the extension length of the cutter head on the tool bar, and install it together with the left end support sleeve into the inner hole of the workpiece, adjust the height of the tool bar with the tool pad, and fix the tool bar on the square tool table of the lathe so that the tool bar can slide freely in the sleeve, then the workpiece can be rotated and the cutting can be started until the longitudinal depth of the workpiece is reached.
When the workpiece is turned, move the large slide in the opposite direction, and withdraw it from the workpiece together with the right end support sleeve and tool bar, and then remove the workpiece. When processing the second piece, first install the left end support sleeve, clamp the workpiece, and then extend the tool bar into the left end support sleeve of the workpiece, install the right end support sleeve, and then start turning the second workpiece.
Features of the tooling: The tool bar is supported by support sleeves at both ends, which greatly increases the rigidity of the tool bar, makes cutting vibration-free, and ensures the roughness of the processed surface; the tool bar is supported by support sleeves at both ends for turning, which ensures the position accuracy between holes; it is easy to operate and the efficiency is more than 5 times higher than the traditional hole expansion method.
7. Method of adjusting the center frame When turning the inner hole and end face of a hollow workpiece with a relatively large length and diameter, a center frame is required. If the center frame is not adjusted well, the axis of the workpiece and the main axis of the machine tool do not coincide, and the end face depression and bulge and the taper error of the hole will occur during processing. In severe cases, the workpiece will fall out of the chuck, causing an accident.
When installing this type of workpiece, a three-jaw chuck or a four-jaw chuck is used on one end of the workpiece, and the other end is placed on the center frame. Then, a wooden board is inserted into the hole of the workpiece or a piece of paper is pasted on the end face of the workpiece with butter, and the tip of the tailstock tip is placed against the wooden board or paper. A lower spindle speed is selected to rotate the workpiece for one or two weeks. At this time, a circle is drawn on the wooden board or paper by the tip. Then adjust the three supports of the center frame so that the center of the circle is aligned with the tip of the tip. In this way, the center line of the workpiece is basically coincident with the axis line of the machine tool spindle. After semi-finishing, if the end face flatness and hole cylindricality are out of tolerance, the three supports of the center frame are slightly adjusted to eliminate them.
8. Cleverly remove the center drill tip in the hole When drilling the center hole, the center drill often breaks in the center hole due to the inconsistency between the center of the lathe tailstock and the rotation center of the workpiece, or excessive force, high plasticity of the workpiece material, and chip blockage. It is not easy to remove.
If the method of enlarging the center hole is used to remove it, the center hole will change its original size and fail to meet the quality requirements. At this time, just use a sharpened steel wire, insert the tip into the chip groove of the drill tip in the center hole, turn it a few times, and once the drill tip moves, use a magnet or magnetic table to suck it, and the center drill tip broken in the center hole can be taken out.
9. Methods for eliminating defects when turning slender shafts 1) Belly shape, that is, after turning, the diameter of the workpiece is small at both ends and large in the middle. The reason for this defect is that the rigidity of the slender shaft is poor, the support claws of the tool holder are not in contact with the surface of the workpiece, and wear produces a gap. When turning to the middle part, due to the action of radial force, the turning tool presses the rotation center of the workpiece to the right side of the rotation center of the spindle, reducing the cutting depth, while the rigidity of the two ends of the workpiece is good, and the cutting depth is basically unchanged. The slender shaft is bulged due to the "cutting off" in the middle. Elimination method: When following the claws of the tool holder, be careful to make the claw surface contact with the surface of the workpiece without gaps. The main deflection angle of the turning tool should be selected as 75°~90° to reduce radial force. The tool rest claws should be made of cast iron with good wear resistance.
2) The bamboo-shaped shape is like a bamboo joint, and its pitch is approximately equal to the distance between the tool rest support claw and the tip of the turning tool, and it appears cyclically. The reason for this defect is that the gap between the lathe's large and middle slides is too large, the blank is bent and rotated, causing centrifugal force and "letting go" at the tool rest support reference connection point, making the diameter of the turned section slightly larger than the reference section. Continue to turn, the tool rest support claw contacts the section with a larger diameter of the workpiece, so that the rotation center of the workpiece is pressed toward the turning tool, and the diameter of the turned workpiece is reduced. In this way, the tool rest is cyclically supported on different diameters of the workpiece, so that the workpiece leaves and approaches the turning tool, forming a regular bamboo joint shape. In addition, during the tool feeding, the tool rest claws are too strong, so that the rotation center of the workpiece is pressed toward the turning tool, causing the diameter of the turned section to become smaller, and the tool feeding continues, and the cycle also forms bamboo joints.
Elimination method: Adjust the gaps between the various parts of the machine tool to enhance the rigidity of the machine tool. When following the tool holder claw, make sure that the claw surface is in contact with the workpiece without applying too much force. Cut deeper (0.05-0.1) mm at the joint to eliminate the "letting go" phenomenon during tool feeding. The depth of cut should be controlled flexibly according to the rules of the machine tool.
10. Reverse knurling In the traditional forward knurling, the chips are easy to enter between the workpiece and the knurling during the rolling process, causing the workpiece to be subjected to excessive force, resulting in chaotic patterns and ghosting. If the spindle is reversed, the above-mentioned drawbacks can be effectively prevented, and a clear pattern can be rolled out.
11. How to prevent the center drill from breaking When drilling a center hole with a diameter of less than 1.5 mm on a lathe, the center drill is very easy to break. In addition to being careful and frequently removing chips when drilling, do not lock the tailstock when drilling, and let the friction between the tailstock's own weight and the machine tool guide rails to drill. When the resistance to drilling is too large, the tailstock will retreat automatically, protecting the center drill.
12. The sleeve for turning small eccentric workpieces. The sleeve is used to clamp the workpiece to turn the eccentricity. Its clamping efficiency is 6 to 8 times higher than that of a four-jaw chuck. If the eccentricity e and the outer diameter Φ2 of the workpiece are known, the inner diameter Φ1 of the fixture sleeve can be calculated, Φ1 = 2e + Φ2. When machining the inner diameter Φ1 of the fixture sleeve, it is important to pay attention to the inner hole accuracy to avoid affecting the eccentricity dimension accuracy of the workpiece.
13. The method of rotating the shaft. The screw conveying mechanism is widely used in factories that convey granular materials. When the screw shaft in this mechanism is manufactured, its spiral blades are welded with steel plates. This kind of spiral plate has a high tooth shape, a small bottom diameter, and the outer diameter must be coaxial with the shaft neck. To achieve this requirement, the outer diameter of the screw shaft must be turned on a lathe.
This kind of shaft is generally long. When machining the outer diameter, due to the large pitch, deep teeth, thin teeth, poor rigidity, and intermittent cutting, the teeth are subjected to cutting impact and vibration, which makes it unable to cut normally and also damages the tool. In order to solve this problem, the cutting speed, cutting depth and feed rate have to be reduced, which greatly reduces the work efficiency.
In order to improve work efficiency and quality, a simple and easy method of turning threads is adopted. The hanging wheel is hung according to the pitch of the spiral shaft, and the large lead screw is used to drive the large slide plate to turn. After the first cut, remember the scale of the middle slide plate. After the large slide plate returns, move the small tool holder forward (0.5-0.7) mm, and then start the second cut, and continue until the outer circle is turned.
The top of the spiral shaft turned by this method is flat, which basically eliminates intermittent cutting, and the processing efficiency is nearly 10 times higher than before.
14. Processing of threads outside the lathe nameplate In many mechanical transmissions, the pitch and lead of multi-start worms, multi-start screws, multi-start helical splines, variable lead worms, double-lead variable tooth thickness worms, helical gear meshing worms, etc. cannot be found on the lathe nameplate, which brings difficulties to processing. Here is a solution for the problem that the required pitch (or lead) cannot be found on the lathe nameplate, which can save the trouble of making a gear.
For example, the worm that meshes with the helical gear on the imported milling machine has a normal module of 3.175 and a circumferential module of 3.184. The 3.184 module cannot be found on the lathe, so the gear must be calculated and made to process it. After calculation and analysis, the module pitch is converted into a metric pitch, that is, 3.184×3.1416=10.003mm, so it can be processed according to a pitch of 10mm.
In equipment overhaul and maintenance, the pitch of the thread is mostly measured in metric, which will result in non-standard pitch. In fact, threads are divided into ordinary, inch, module, diametral pitch and non-standard threads, and their pitches can be converted to each other. For example, 9.4248mm, 12.5664mm, 12.7mm, 25.4mm and 7.9756mm can all be processed as other types of threads, and the results are P=9.4248mm and P=12.5664mm, which are module 3 and module 4 respectively.
For example, 12.7mm and 25.4mm are imperial threads with 2 threads/inch and 1 thread/inch respectively. P=7.9756mm is a diametral pitch thread with DP=10.
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