10 practical guides of lathe processing summarized by an old turner

1. Method of correcting workpiece by copper rod when doing lathe processing

Workpiece calibration, also known as alignment, is a method of checking whether the installation of the workpiece is in the correct position before turning the workpiece. For the purpose of calibration, the rough turning is to ensure that the workpiece margin is basically the same; when semi-finishing and finishing, it is to ensure that the relative position of the surface to be processed and the processed surface meets the requirements. Correcting quickly and correctly is an important measure to ensure product quality and shorten auxiliary time.

The method of using copper rods to calibrate the workpiece is a quick correction method when the workpiece is installed after the outer circle and the end face are rough-turned. Clamp a copper rod or aluminum rod on the lathe's square tool bed, clamp the workpiece slightly on the three-jaw chuck, start the lathe to rotate at a speed of about 100r/min, so that the copper rod contacts the end face or outer circle of the workpiece, and shake it by hand After a certain pressure is applied by the carriage to bring the surface of the workpiece into full contact with the copper rod, slowly remove the copper rod from the workpiece, and then stop to clamp the workpiece, and the workpiece is corrected.

This correction method is fast and accurate, and can reach a certain accuracy. If the workpiece clamping is reasonable (less than 10mm), the surface of the workpiece is smooth, and the radial runout of the shaft and the end face of the disk type are generally not greater than 0.02mm.

2. The method of straightening the slender rod when doing lathe processing

The slender rod must be straightened before turning, otherwise it will cause uneven machining allowance and non-round turning, or increase the bending of the rod due to the large bending centrifuge, which cannot be turned. To straighten the slender rod on the lathe, the following methods can be used.

1) Use hammering method. First, clamp one end of the slender rod with a three-jaw chuck for about 10mm, and support the center of the other end. Use a lower speed to rotate the work, draw a high point on the workpiece with chalk, and stop. Hold a concave iron block in the left hand, make the concave surface against the opposite side of the high point of the workpiece, and use the right hand hammer to hit the high point of the workpiece. The impact force is proportional to the bending of the workpiece. This is repeated several times, and the workpiece is straightened. This method is suitable for long and thin rods.

2) Use the lever to pry the pressure method. After the slender rod is installed on the lathe, the work piece is driven to rotate, and a wooden rod with a length of 300 mm is placed on the middle carriage and the square cutter table, and the middle carriage is shaken so that the wooden stick is pressed toward the curved part of the workpiece. Continue to move the middle carriage, keep up with the tailstock center to prevent the workpiece from coming out, wait for the workpiece to continue to rotate for a few seconds, then slowly withdraw the middle carriage, and properly loosen the tailstock center, depending on whether the workpiece is straight. If it still bends, continue with the above method until it is straightened. This method is suitable when the workpiece is short.

3) Use the counterattack method. When the slender rod is long and the diameter is relatively large, drill the center holes at both ends first, and use the spindle center and the lathe tailstock center to jack it up. Then, rotate the workpiece by hand to find the high point on the workpiece and draw a mark with chalk. At this time, use a piece of iron about 25mm thick and 40mm wide that is wider and longer than the large rail of the lathe or a relatively large wooden block. Place it horizontally on the large rail, and place a head with a V-shaped or not 60° pointed shape on it. The concave-arc-shaped thread jack is supported at the high point of the work piece's flexure and slightly supported. The left hand holds the work piece and the right hand hits the low point of the work piece with the round head of the hammer. The number of strikes, the strength and the length of the workpiece are proportional to the size of the bend. Workpieces straightened in this way are not easy to recover from bending.

In addition to the above-mentioned method of straightening the slender rod on the lathe, it can also be visually inspected outside the machine tool, and straightened by the above-mentioned method on the platform.

3. Turning tool with inner spherical surface in deep hole when doing lathe processing

When turning workpieces such as plastic, nylon and plexiglass, the connection point between the cylindrical surface of the inner hole and the inner spherical surface of the deep hole must be very smooth and no steps, which makes the processing difficult. For this reason, when turning the inner hole and the inner spherical surface, it must be done in one finishing turning.

In order to process the inner hole of this workpiece, first make an inner hole turning tool. The material of the blade 2 is tool steel or alloy tool steel, and the quenching is HRC (60-62). Production method: first turn a knife blank on the lathe, heat-treat and quench, grind both end faces, install with the inner hole of the blade and the mandrel, grind the outer circle and the back angle to the requirements in the cylindrical grinding or tool grinding, and then remove the excess according to the shape of the blade Partly worn away to prevent the back side of the hole from being obstructed and unable to be turned. Then fix the blade to the arbor with screws, so that the rake face of the blade is close to the center of the arbor, so as not to hinder the lower part of the arbor, so that the cross-section of the arbor can be larger, which is helpful to improve the rigidity of the arbor.

When turning the inner hole, first drill the hole with a drill, and use the inner hole cutter to roughen the inner hole. When finishing, install the tool on the square tool bed of the lathe, and make the cutting edge and the center of rotation of the workpiece equal. First use this knife to semi-finish the inner hole of the car, and the hole depth basically meets the requirements. After finishing the cylindrical part of the inner hole of the fine turning, the inner spherical surface in the depth of the hole is also turned in the same pass. At this time, all the inner holes were finished. This kind of tool and operation method make the inner hole and the inner spherical surface free of tool marks, which is very smooth.

4. Turning flat thread when doing lathe processing

The so-called plane thread is the thread processed on the end face of the cylinder or disc. The trajectory of the turning tool relative to the workpiece is an Archimedes spiral, which is different from the commonly processed cylindrical thread.

When turning flat threads on an ordinary lathe, a light bar drive is generally used to rotate the lead screw of the middle carriage and drive the middle carriage of the lathe to move laterally to move the cutter for turning. This requires that each time the workpiece turns, the middle carriage moves the screw on the workpiece laterally.

When the thread pitch of the workpiece is not strict, the pitch of the plane thread of the workpiece can be divided by the multiple of the increase of the thread pitch of the lathe (such as C620-1 lathe can be increased by 2, 8, 32 times), using the obtained quotient, select the lathe nameplate to approximate The amount of lateral feed, and pull the tool box handle as required, then turn the increased pitch handle on the spindle box to the increased pitch position, and the variable speed handle on the spindle box to the required position, install the tool , You can turn flat threads.

When the thread pitch of the work piece is strict, it is necessary to replace the change gear of the change gear box. Before calculating the change-over wheel, select an approximate amount of horizontal knife as described above, and pull up the knife box, increase the screw pitch and the shifting handle to carry out the horizontal knife. Then use the integer of the main shaft (more than 5 revolutions) to divide the distance moved by the horizontal carriage, and the resulting quotient is the actual pitch of the lathe. Under normal circumstances, it will not be equal to the required pitch of the workpiece, which must be calculated and replaced by the change gear of the change gear box.

Example: The thread pitch P of the workpiece is 8mm, the measured pitch P is 8.24mm, and the number of teeth of the lathe hanging wheel is determined.

Solution 1 Use the calculation method:

Answer: The number of teeth of driving wheel is 100, and the number of teeth of passive wheel is 103. This changer is a single changer, and the gear with any number of teeth in the middle can be used as the intermediate wheel.

Solution 2 Use the table lookup method:

Check the "Metal Cutting Manual" published by Shanghai Science and Technology Press or the "Mechanical Worker Cutting Manual" published by Machinery Industry Press to get the corresponding number of teeth of the hanging wheel.

Answer: Z1=100, Z2=80, Z3=70, Z4=90.

When turning, it is best to use an elastic cutter bar. The geometric parameters of the cutter head are the same as those of the cylindrical thread. However, the auxiliary clearance angle on the inner circle side of the cutter head must be ground with a double clearance angle to prevent this part from hindering the turning. Use the lathe spindle to turn the tool forward and backward and return the tool. There are two ways to eat the knife: one is to use the lathe small knife holder to eat the knife and return the knife, and the small dial is counted; the second is to install the magnetic table frame and dial indicator on the large rail in front of the large carriage to control the large The position of the carriage and the amount of knife eating, and use the large carriage to eat and withdraw the knife.

In the process of turning flat threads, in addition to the square thread, turning other thread threads also needs to be “cuttered” like turning cylindrical threads to refine both sides of the thread.

There are also the following two methods for "catch the knife": one is to use a large carriage to eat and withdraw the knife, rotate the small knife holder 90° counterclockwise and fix it, and shake the handle of the small knife holder when "catch the knife"; the second is to use The large carriage or the small knife holder eats and retracts the knife. When you want to "catch the knife", place the cutter head outside the work piece, and stop the spindle during the pass, but there must be no reversal. At this time, the falling worm handle will fall , Rotate the handle of the middle carriage to the value required to "catch the knife", and then lift off the worm handle. To "catch the knife" with this method, the clearance of the drive chain must be eliminated, which is the direction to "catch the knife", and the middle carriage must move the knife in the same direction. After "catch the knife", the cutter head is gradually cut into the workpiece.

5. What is the knack for sharpening knives, how to sharpen high speed steel trapezoidal thread turning tools?

Thread turning is mostly more hands-on, and learn more from the teacher, so that progress can be made fast.

The thread is a continuous protrusion with a prescribed tooth shape formed along the spiral line on the surface of a cylinder or cone. The thread is widely used in various machines. For example, four screws are used to clamp the lathe on the lathe square tool holder, and the thread is used to transmit power between the lathe screw and the opening and closing nut.

There are many methods for processing threads, and the threading method (one of the basic skills of lathe workers) is usually used in general machining. When processing threads on a horizontal lathe, the movement relationship between the workpiece and the tool must be ensured, that is, the tool moves evenly one pitch (or lead) for each revolution of the spindle (workpiece revolution).

Their movement relationship is ensured as follows: the spindle drives the workpieces to rotate together, and the movement of the spindle is transmitted to the feed box through the hanger box, and then to the lead screw after the gear box is changed in speed. The opening and closing nut cooperates to drive the tool holder and the turning tool to move linearly. In this way, the rotation of the workpiece and the movement of the tool are achieved by the driving of the main shaft, thereby ensuring the strict movement relationship between the workpiece and the tool. When actually turning the thread, due to various reasons, the movement between the spindle and the tool causes a problem in a certain link, which causes a problem when turning the thread and affects normal production. It should be resolved in time.

6. Incorrect tooth angle when doing lathe processing

1) The tip angle is incorrect

The angle of the tool tip is incorrect when sharpening the turning tool, that is, the angle between the projections of the two cutting edges of the turning tool on the base surface and the tooth profile angle of the processed thread is inconsistent, resulting in an incorrect thread angle. Solution: When sharpening the turning tool, you must use an angle ruler or model to detect it to get the correct tooth angle. The method is: parallel the model or angle ruler to the front of the turning tool, and then check it with the light transmission method. Commonly used metric thread profile angles: triangular thread 60°, trapezoidal thread 30°, worm 40°.

2) The radial rake angle is not corrected

In order to smooth the chip removal of the turning tool, reduce the surface roughness and reduce the phenomenon of built-up edge, the radial rake angle is often worn, which causes the cutting on both sides of the turning tool to not coincide with the axial direction of the workpiece, so that the thread of the workpiece is turned out. The profile angle is larger than the tip angle of the turning tool. The larger the radial rake angle, the greater the error of the tooth profile angle. At the same time, the turned thread profile is not a straight line in the axial section, but a curve, which affects the quality of the thread pair. Solution: When sharpening the thread of a turning tool with a large radial rake angle, the tip angle must be corrected by the angle between the two edges of the turning tool. Especially for threads with high machining accuracy, the correction calculation method is:

tanεr=cosrp·tanα

In the formula, εr is the angle between the two edges of the turning tool; rp is the radial rake angle; α is the tooth shape angle.

3) Tooth shape angle is too large when cutting high-speed steel

During high-speed thread cutting, due to the squeezing deformation of the turning tool against the workpiece, the processed tooth shape will expand and the workpiece will expand at the same time, so when sharpening the turning tool, the angle between the two edges should be appropriately reduced Small 30′. In addition, the major diameter of the workpiece before turning the external thread is generally smaller than the nominal size (about 0.13p).

4) Incorrect installation of turning tool

If the turning tool is installed incorrectly, the center line of symmetry of the two cutting edges of the turning tool is not perpendicular to the axis of the workpiece, causing the angle of the processed tooth to be inclined (commonly known as inverted tooth). Solution: Use an angle ruler or template to install the turning tool so that the center of symmetry is perpendicular to the axis of the workpiece, and the tip of the tool is the same height as the center of the workpiece.

5) Tool wear

After the cutter is worn, it is not sharpened in time, resulting in a curved or "rotten tooth" on both sides of the processed tooth angle instead of a straight line. Solution: Reasonably choose the cutting amount, and sharpen in time when the turning tool is worn.

7. Large surface roughness value when doing lathe processing

The reason for the large surface roughness value: one is that the tool tip generates a built-up edge; the second is that the tool shank is not rigid enough to generate vibration during cutting; the third is that the radial rake angle of the turning tool is too large, and the middle slide screw nut gap is too large to cause tie Knife; Fourth, when the high-speed steel is cutting the thread, the cutting thickness is too small or the chips are discharged in the oblique direction, and the surface of the flank has been processed; the fifth is the poor rigidity of the workpiece and the cutting amount is too large; the sixth is the rough surface of the turning tool.

Solution:

1) If it is caused by built-up edge, the cutting speed should be adjusted appropriately to avoid the range of built-up edge (5～80m/min); when cutting with a high-speed steel turning tool, the cutting speed should be properly reduced and the cutting fluid should be selected correctly ; When using carbide thread, the cutting speed should be increased appropriately.

2) Increase the cross-sectional area of the shank and reduce the extended length of the shank to increase the rigidity of the turning tool and avoid vibration.

3) Reduce the radial rake angle of the turning tool and adjust the screw nut of the middle slide plate to make the gap as small as possible.

4) When high-speed steel cuts the thread, the chip thickness of the last knife is generally greater than 0.1mm, and the chip is discharged along the vertical axis to prevent the chip from contacting the processed surface.

5) Choose a reasonable amount of cutting.

Sixth, the surface roughness of the cutting edge of the tool is 2 to 3 grades lower than the roughness of the threaded surface. The grinding wheel must be ground with oil stone after the turning tool is ground.

8. Messy teeth when doing lathe processing

The reason for messy teeth is that when the screw rotates one revolution, the workpiece does not rotate through the integer multiple of the screw rotation, that is, the workpiece rotation is not an integer multiple of the screw rotation.

Commonly used methods to prevent tooth turmoil are to turn the car backwards at the end, that is, at the end of a stroke, without lifting the opening and closing nut, after the knife is withdrawn in the radial direction, the spindle is reversed, the turning tool is retracted in the longitudinal direction, and then the second The second stroke, in this reciprocating process, because the transmission between the main shaft, the screw and the tool holder has not been separated, the turning tool is always in the original spiral groove, and there will be no messy teeth. Secondly, when the longitudinal stroke of the feed is completed, lift the opening and closing nut to disengage from the transmission chain, and the position of the tip of the knife will be displaced.

9. Incorrect diameter when doing lathe processing

The reason for the incorrect diameter is that the cutting depth of the turning tool is incorrect. The cutting depth is controlled based on the top diameter, ignoring the effect of the top diameter error; improper use of the dial; not measured in time when turning.

Solution: During finishing, check whether the dial is loose and use it correctly. The remaining amount of finishing should be appropriate. The middle diameter should be measured in time, the influence of the top diameter should be considered, and the cutting depth should be adjusted.

10. Piercing or bending work pieces when doing lathe processing

Reasons for sticking or bending the workpiece: the tip of the turning tool is lower than the center of the workpiece (machine tool); the front angle of the turning tool is too large, the clearance of the lead screw of the middle slide is large; the rigidity of the workpiece is poor, and the cutting amount is too large.

Solution:

1) When installing the turning tool, the tool tip should be aligned with the center of the workpiece, or slightly higher.

2) Reduce the front angle of the turning tool, reduce the radial force, and adjust the clearance of the lead screw in the middle slide.

3) Choose a reasonable amount of cutting according to the rigidity of the workpiece; increase the rigidity of the workpiece and increase the rigidity of the turning tool.

In short, the types of failures generated when turning threads are diverse, including equipment reasons, tools, measurements, operations and other reasons. When troubleshooting, you need to analyze the specific circumstances and find out the specifics through various detection methods and diagnostic methods. Effective factors and adopt effective and reasonable solutions.