23 Essential Machining Lessons Learned from Decades at the Lathe

To achieve good results on a lathe, it’s important to understand three key aspects of turning and seven essential types of cutting tools. First, ensure that your equipment is well-prepared, or as some might say, that your “weapon” is ready. Then, practice consistently. Focus on mastering the basic operations: external turning, internal boring, length adjustments, tapering, and threading. Once you are comfortable with these fundamentals, move on to more complex parts, such as internal and external trapezoidal threads, worms, slender shafts, and thin-walled sleeves. Additionally, learn how to effectively use the tailstock and tool holder to enhance your skills.

 

1. Turning slender shafts
“Turning workers are often concerned about machining slender rods.” This statement highlights the challenges associated with turning thin shafts. Due to their specific characteristics and technical requirements, high-speed turning of slender rods can lead to defects such as vibrations, uneven edges, bamboo knots, poor cylindricality, and bending. To achieve smooth turning, it is essential to carefully address these issues throughout the process.

 

1) Machine tool adjustment

The alignment of the lathe spindle and tailstock must be parallel to the large guide rail of the lathe both vertically and horizontally, with a tolerance of less than 0.02mm.

 

2) Workpiece installation

When installing, try not to over-position, and when clamping one end with a chuck, do not exceed 10mm.

 

3) Tool

Use a Kr=75° to 90° offset tool. Ensure that the secondary back angle α’ is between 4° and 6°, as it should not be too large. When installing the tool, position it slightly above the center.

 

4) The tool rest must be trimmed after installation

The trimming method can involve grinding, reaming, boring, or other techniques. It is important that the radius of the arc surface (R) on the tool rest claw that comes into contact with the workpiece is greater than the workpiece radius. It should never be less than the workpiece radius to avoid the creation of multiple edges. When adjusting the tool rest claw, ensure that it only makes contact with the workpiece without applying excessive force, as this may cause bamboo knots.

 

5) Auxiliary support

When the aspect ratio of the workpiece exceeds 40, it is important to add auxiliary support during the turning process. This helps prevent the workpiece from vibrating or bending due to centrifugal force. Be attentive to the adjustment of the tailstock during cutting. It is advisable to lightly support the workpiece without applying excessive pressure, and to make adjustments as needed. This practice will help prevent issues related to thermal expansion, deformation, and bending of the workpiece.

 

2. Reverse tool turning of slender rods

There are several methods for turning slender rods, with forward and reverse tool turning being the most common. However, reverse tool turning has many advantages over forward tool turning and is generally preferred.

When turning, two common problems can arise. The first is the polygonal shape, which typically results from the tool’s large back angle and the radius of the tool rest not matching the diameter of the workpiece. The second issue is known as the “bamboo problem.” This occurs when the tool rest does not align properly at the mouth of the rack. As the tool is positioned and moved to the cutting surface, the cutting depth can suddenly increase from a very small value. This abrupt change in cutting force may cause the workpiece to yield outward, resulting in a sudden increase in diameter. As the tool rest moves to a larger diameter, the turned diameter may decrease again, creating a cycle that gives the processed workpiece a bamboo-like shape.

To prevent the formation of bamboo joints while turning, it is important to carefully align the tool with the tool holder at the frame mouth. After aligning, reverse the tool and use the middle drag plate handle to deepen the cut by 0.04 to 0.08 mm. Adjust this depth flexibly based on the specific cutting conditions.

 

3. Rolling straightening method

In mechanical processing, rolling is commonly used to enhance the surface hardness, fatigue resistance, and wear resistance of workpieces. It also helps to reduce the surface roughness and extend the service life of these components. Additionally, during the rolling process, metals can undergo plastic deformation under external forces, which alters the internal stress and straightens shafts and rods with good rigidity.

However, during the rolling of a workpiece, it may bend due to the uneven hardness of its surface layer when subjected to external forces. The highest point of the bend experiences a significant rolling force, resulting in substantial plastic deformation and an increased degree of bending. This issue is particularly pronounced when using rigid rolling tools.

The rolling straightening process involves checking the radial runout of the workpiece after its initial rolling. The concave areas are marked, and a four-jaw chuck is employed to adjust the concave section to the height of the machine tool’s rotation center. This adjustment is proportional to the extent of the workpiece’s bending. The workpiece is then rolled again. A dial indicator is used to calibrate the workpiece, and its bending is monitored. If it remains bent, the process is repeated, adjusting the workpiece and rolling it a third time until the desired straightness is achieved. The length of the cut after the second pass should depend on the specific circumstances; a complete pass may not be necessary, and a reverse cut can be utilized.

Rolling straightening is typically completed during the rolling process itself. This method not only preserves the workpiece’s surface but also ensures that its outer surface is rolled uniformly, eliminating dead bends and facilitating ease of operation.

 

4. Screw extrusion straightening method

For screws with large diameters, long lengths, and several bends, extrusion straightening is very effective.

 

1) Working principle

Use a straightening tool to compress the bottom surface of the screw teeth with an external force. This action induces plastic deformation, extends the screw axially, alters the internal stress, and straightens it.

 

2) Straightening method

First, measure the position and angle of the screw bend using a lathe or platform. Place the concave side of the bend facing upward and the convex surface downward, ensuring it is in contact with a metal pad. Using a flat shovel and a hand hammer, strike the bottom of the screw teeth to deform the metal at the smaller diameter, which will help achieve straightening. Throughout the straightening process, monitor the bending condition, alternating between hitting and squeezing with the flat shovel until the screw is straightened. This method is straightforward and effective, suitable for both large and small screws as well as for straightening shaft blanks. However, note that the screw may not easily revert to its original shape after the straightening process.

 

3) Issues that should be noted

The size R of the special flat shovel used for straightening should be greater than half of the bottom diameter of the screw thread. The dimension b should be less than the width at the bottom of the thread, while the angle α should be smaller than the thread angle. The section R that comes into contact with the workpiece should be shaped into a circular arc. After straightening, the deformed bottom of the thread should be flattened using a file.

 

5. Processing of rubber threads

The hardness of rubber is quite low, with an elastic modulus of only 2.35 N, which is equivalent to 1/85000 that of carbon steel. This low hardness makes rubber easy to deform under external forces, but cutting it can be challenging, especially when dealing with intricate thread shapes.

To address the challenges of processing rubber threads, one can use a lathe equipped with a grinding head that allows for arbitrary adjustments of the helix angle. Alternatively, a pneumatic grinding head may be utilized when thread precision requirements are not stringent. It is recommended to use a white corundum grinding wheel with a diameter ranging from 60 mm to 80 mm and a grit size between 60# and 100#. Once the grinding wheel is attached, its shape should be refined using a diamond pen to match the normal cross-section of the thread.

The thread lead is typically indicated on the lathe nameplate and can also be determined by turning the lathe handle. If it is not available on the nameplate, the required hanging wheel must be calculated. This can generally be done by consulting the manual or using a calculation method to identify and manufacture the needed hanging wheel.

When the thread lead exceeds 300 mm, it is essential to reduce the spindle speed. This helps prevent quality issues in thread grinding caused by excessive spindle speed and minimizes the risk of operational stress which can damage components of the feed box. Methods for reducing speed include changing the diameters of the active and passive pulleys or adding an external reduction box to the lathe.

Grinding rubber threads on a lathe is an effective method for achieving high efficiency and quality in processing. This grinding technique can be applied to produce both single-thread and multi-thread rubber components with leads ranging from 1.5 mm to 1280 mm, all while meeting the required quality standards.

6. Stepped deep hole turning method

When turning holes with a length-to-diameter ratio greater than 4 on a lathe, the poor rigidity of the tool bar can lead to vibrations during cutting. These vibrations negatively impact cutting efficiency and the quality of the machined surface, making the turning process more challenging. This issue is particularly pronounced when dealing with large-diameter and deep holes, especially when steps are involved, due to the limitations of the tool bar and the machine tool rigidity.

To begin, secure the workpiece on the lathe using a chuck and a center frame. Use an internal hole cutter to process short holes at both ends of the workpiece, equipping each with a sleeve and a special tool bar. When turning the long hole in the middle, first insert the support sleeve on the left end into the workpiece hole. Next, attach the workpiece to the lathe and adjust the tool head’s length on the tool bar. Install the tool bar into the inner hole of the workpiece, along with the support sleeve on the left end. Adjust the height of the tool bar using a tool pad and secure it on the square tool table of the lathe. This setup allows the tool bar to slide freely within the sleeve. You can then rotate the workpiece and commence cutting until the desired longitudinal depth is achieved.

As the workpiece is being turned, move the large slide in the opposite direction to withdraw both the support sleeve and the tool bar together from the workpiece. After that, remove the workpiece. When processing the second piece, first install the support sleeve at the left end and clamp the workpiece. Then, extend the tool bar into the support sleeve at the left end of the workpiece and install the support sleeve at the right end. You can then start turning the second workpiece.

Key features of this tooling method include:
1. The tool bar is supported by sleeves at both ends, significantly increasing its rigidity and eliminating cutting vibrations, which ensures a smooth processed surface.
2. This support arrangement guarantees position accuracy between holes.
3. It is user-friendly, with an efficiency that is over five times greater than traditional hole expansion methods.

 

7. Methods for adjusting the center stand

When turning the inner hole and end face of a hollow workpiece with a relatively large length and diameter, it is essential to use a center stand. If the center stand is not properly adjusted, the axis of the workpiece may not align with the center line of the machine tool’s main axis. This misalignment can lead to issues such as end face depressions, bulges, and taper errors in the hole during processing. In severe cases, the workpiece could fall out of the chuck, posing a safety hazard.

To install such workpieces, a three-jaw chuck or a four-jaw chuck is used on one end, while the other end is supported by the center stand. A piece of wood is securely inserted into the hole of the workpiece, or a piece of paper is glued to the end face of the workpiece with butter. The tip of the tailstock is then positioned against the wood or paper. It is advisable to select a lower spindle speed to rotate the workpiece for one or two minutes. During this time, the tip will trace a circle on the wood or paper.

Next, the three supports of the center stand should be adjusted so that the center of the traced circle aligns with the tip of the tailstock. This adjustment ensures that the center line of the workpiece is closely aligned with the axis of the machine tool’s main axis. After semi-finishing, if the flatness of the end face and the cylindricity of the hole are not within tolerance, slight adjustments to the three supports of the center stand can help correct these discrepancies.

 

8. How to remove the center drill tip in the hole

When drilling the center hole, inconsistencies may arise between the center of the lathe tailstock and the rotation center of the workpiece. Additionally, excessive force, high plasticity of the workpiece material, and chip blockages can cause the center drill to break in the center hole, making it difficult to remove.

If you attempt to enlarge the center hole to extract the broken drill, it may result in a change in size that does not meet quality requirements. In such cases, use a sharpened steel wire and insert the tip into the chip groove of the broken drill in the center hole. Rotate the wire a few times; as soon as the drill tip becomes loose, use a magnet or magnetic table to retrieve it. This method allows for the safe removal of the broken center drill from the hole.

 

9. Methods for eliminating defects when turning slender shafts

1) Bulbous shape

After turning, the workpiece has a smaller diameter at both ends and a larger diameter in the middle. This defect occurs because the rigidity of the slender shaft is low, the support claws of the tool rest do not make contact with the surface of the workpiece, and wear creates gaps. When machining the middle section, the radial force acts on the turning tool, causing it to push the rotation center of the workpiece to the right of the spindle’s rotation center. This leads to a reduction in cutting depth in the middle, while the rigidity at both ends of the workpiece remains good, keeping the cutting depth relatively unchanged. As a result, the slender shaft bulges in the middle due to the cutting action.

To eliminate this issue, ensure that the tool rest claws make solid contact with the workpiece surface without any gaps. It is also advisable to select the main rake angle of the turning tool between 75° and 90° to decrease the radial force. Additionally, the tool rest claws should be made of cast iron to ensure good wear resistance.

 

2) Bamboo joint shape

The shape of the workpiece resembles that of a bamboo joint. Its pitch is approximately equal to the distance between the tool rest support claws and the tip of the turning tool, creating a cyclic pattern. This defect occurs because the gap between the lathe’s large and middle carriages is too wide. As the blank is bent and rotated, centrifugal force causes what is referred to as “giving up the tool” at the reference connection point of the tool rest support. This results in the diameter of the turned section being slightly larger than that of the reference section.

When the turning continues, the tool rest support claw makes contact with the section of the workpiece that has the larger diameter. Consequently, the rotation center of the workpiece is pushed towards the turning tool, which reduces the diameter of the turned workpiece. This causes the tool rest to be supported on different diameters of the workpiece in a cycle, leading to the characteristic bamboo joint shape.

Additionally, when the tool follows the tool rest claw during feeding, excessive force can press the rotation center of the workpiece towards the turning tool, resulting in a further reduction of the diameter. This continued tool feeding contributes to the cyclic formation of bamboo joints.

**Elimination Method:** To remedy this issue, adjust the gaps between the various components of the machine to enhance its rigidity. Ensure that the claw surface of the tool rest is in contact with the workpiece, but avoid applying excessive force. When cutting at the tool joint, make deeper cuts of approximately 0.05 to 0.1 mm to prevent the “tool letting go” phenomenon during movement. The cutting depth should be controlled flexibly according to the specifications of the machine.

 

10. Reverse knurling

In the traditional forward knurling process, chips can easily become trapped between the workpiece and the knurling tool. This can subject the workpiece to excessive force, leading to chaotic patterns and double images. However, if the spindle is reversed, these issues can be effectively avoided, resulting in a clear and precise pattern being rolled out.

 

11. Methods to prevent the center drill from breaking

When drilling a center hole with a diameter of less than 1.5 mm on a lathe, it’s common for the center drill to break easily. To prevent this, it’s important to remove chips frequently and to be cautious during the drilling process. Additionally, avoid locking the tailstock while drilling; instead, allow it to move freely. This way, the tailstock can utilize its own weight for friction against the machine tool guide rails. If resistance becomes too high during drilling, the tailstock will automatically retreat, providing protection for the center drill.

12. Sleeves for turning small eccentric workpieces

Using sleeves to clamp the workpiece results in an eccentricity that is 6 to 8 times higher than that achieved with 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 using the formula φ1 = 2e + φ2. When machining the inner diameter (φ1) of the fixture sleeve, it is crucial to ensure the accuracy of the inner hole to avoid negatively impacting the dimensional accuracy of the workpiece’s eccentricity.

 

13. Spindle method

The screw conveying mechanism is commonly used in factories to transport granular materials. During the manufacturing of the screw shaft in this mechanism, the spiral blades are welded onto steel plates. The teeth of the spiral plates are designed to be high, with a small bottom diameter, and the outer diameter must align coaxially with the shaft neck. To achieve this, the outer diameter of the screw shaft needs to be turned on a lathe.

This shaft is generally long. When processing the outer diameter, challenges arise due to the large pitch, deep and thin teeth, and poor rigidity, which can lead to intermittent cutting. As a result, the teeth experience impact from the cutting process, causing vibrations that hinder proper cutting and can even damage the cutting tool. To address these issues, it becomes necessary to reduce the cutting speed, cutting depth, and feed rate, which significantly decreases work efficiency.

To improve both work efficiency and quality, a straightforward method for turning threads is employed. A hanging wheel is suspended according to the pitch of the screw shaft, and the large lead screw drives the large slide plate to execute the turning. After the first cut, the position of the middle slide should be noted. Once the large slide returns, the small tool holder is advanced by 0.5 to 0.7 mm before starting the second cut. This process continues until the outer circle is fully turned.

The method used ensures that the top of the spiral shaft is flat, effectively minimizing intermittent cutting and enhancing processing efficiency by nearly ten times.

 

14. Processing of threads other than those on the lathe nameplate

Among various mechanical transmissions, the pitch and lead of multi-start worms, multi-start screws, multi-start helical splines, variable lead worms, double-pitch variable tooth thickness worms, and helical gear meshing worms are often not listed on the lathe nameplate. This can create challenges during processing. Here, we offer a solution for cases where the necessary pitch (or lead) is not available on the lathe nameplate, eliminating the need to create a hanging wheel.

For instance, in an imported milling machine, the worm that meshes with the helical gear has a normal module of 3.175 and a circumferential module of 3.184. As this 3.184 module is not present on the lathe, a hanging wheel must be calculated and created for processing. After performing necessary calculations and analysis, we can convert the module pitch into metric pitch: 3.184 × 3.1416 = 10.003 mm. Thus, processing can be done according to a pitch of 10 mm.

In equipment overhaul and maintenance, thread pitches are often measured in the metric system, which may lead to non-standard pitches. In reality, threads can be categorized as ordinary, inch, module, diametral pitch, and non-standard threads, and their pitches can be converted between these categories. For example, 9.4248 mm, 12.5664 mm, 12.7 mm, 25.4 mm, and 7.9756 mm can all be processed as different thread types. Specifically, 9.4248 mm corresponds to a module 3, and 12.5664 mm corresponds to a module 4.

Additionally, 12.7 mm and 25.4 mm are imperial thread measurements, representing 2 teeth per inch and 1 tooth per inch, respectively. Meanwhile, 7.9756 mm corresponds to a diametral pitch thread with a DP of 10.

 

15. Tooling for boring long inner tapered holes

When machining large diameter and long inner tapered holes on a lathe, using the general turning method can lead to challenges such as poor tool bar rigidity and vibrations. These issues often result in minimal or no cutting being achieved. However, large inner holes or inner tapered holes that meet specified requirements have been successfully machined multiple times.

During the machining process, one end of the workpiece is secured in a chuck, while the other end is supported by a center frame. A counter-center is placed in the spindle hole of the lathe, and the tool bar is positioned at one end with a steel ball. The other end of the tool bar is fixed to the tailstock sleeve of the lathe using a connecting sleeve and a fastening screw. This setup ensures that as the workpiece rotates, the tool bar remains stationary. A key mechanism allows the cutter disc to slide axially on the tool bar.

One end of a wire is attached to the cutter disc, while the other end is secured to the large slide of the lathe. As the large slide moves longitudinally, it pulls the wire, causing the cutter disc to move axially, completing the feed motion and cutting.

Before installing the tool bar, it is essential to position the lathe tailstock in front of the large slide for optimal movement. The feed amount can be adjusted using the handle of the feed box. When machining a tapered hole, the tailstock can be offset so that the axis of the tool bar is at an angle to the axis of the workpiece in a horizontal direction. After cutting, the cutter disc can be returned by simply pushing it by hand.

This tooling method is convenient for machining large internal holes on a lathe. Its structure is simple, and it offers good rigidity for the tool bar.

 

16. Change the number of teeth

Increase the number of teeth on the driving wheel of the C620-1 lathe wheel box from 32 to 48. Additionally, the modulus thread not indicated on the nameplate can also be processed. If the 32-tooth driving wheel is replaced with a 64-tooth wheel, the worm gear can operate without being limited by the spindle speed ratio. This configuration allows for low-speed finishing, which can enhance the surface roughness of the thread.

 

17. Methods for reducing the surface roughness of slender shafts

There are two methods for reducing the surface roughness of slender shafts (rods) on lathes: single-wheel honing and rolling. These methods are effective for addressing low roughness requirements when grinding is not possible on a lathe, utilizing simple tools and processes.

After finishing the slender shaft (rod) on a lathe, if the surface roughness does not meet the specifications in the drawing, the single-wheel honing method can be used to reprocess the workpiece surface. This method can reduce the surface roughness from Ra 6.3 μm to between 1.6 μm and 0.2 μm. The angle between the honing wheel axis and the lathe spindle axis is typically between 28° and 30°. A larger angle results in higher efficiency but also higher roughness, while a smaller angle yields lower efficiency but lower roughness.

The honing wheel speed is generally set between 30 and 60 m/min, with a feed rate of 0.5 to 2 mm/r. The larger value is typically chosen for rough honing. The pressure applied by the honing wheel on the workpiece ranges from 150 to 200 N. For workpieces with poor rigidity, a tool rest should be used. The particle size of the honing wheel usually falls between 100# and 180#. If the target roughness is Ra 0.2, then a honing wheel particle size of W40 to W280 is recommended. The lubricant for honing should be kerosene or diesel mixed with 5% to 10% oleic acid. If these conditions are not met, an ordinary emulsion can also be used for cleaning and lubrication during the honing process.

The rolling process for slender shafts (rods) can effectively reduce surface roughness while also improving surface hardness and wear resistance. Due to the poor rigidity of the workpiece, a follower must be used during the rolling process. This follower should be positioned in front of the rolling tool to prevent its claws from scratching the workpiece surface. Both rigid and elastic rolling tools can be used for this process. The number of rolling passes is generally limited to two. The rolling speed should be maintained between 20 and 30 m/min, and the feed rate is set between 0.1 and 0.2 mm/r. Lubrication is performed using engine oil, although emulsion lubrication may also be applied.

 

18. Method of Correcting Workpieces with Copper Rods

The correction of workpieces, also known as alignment, is a method used to verify that a workpiece is properly positioned before turning. The purpose of this process is to ensure that the workpiece allowance remains consistent during rough turning. During semi-finishing and finishing, it is essential to ensure that the relative positions of the surfaces being processed and those that have already been processed meet specified standards. Rapid and accurate correction is crucial for maintaining product quality and minimizing auxiliary time.

One effective method for correcting workpieces is using copper rods, which provides a quick solution after the outer circle and end face of the workpiece have undergone rough turning. To use this method, clamp a copper rod or aluminum rod onto the square tool table of the lathe. Lightly secure the workpiece in the three-jaw chuck and start the lathe, setting it to a speed of about 100 revolutions per minute (r/min). As the lathe rotates, ensure that the copper rod makes contact with either the end face or the outer circle of the workpiece. By manually adjusting the slide plate, apply a certain amount of pressure until the workpiece is fully in contact with the copper rod. Afterward, slowly separate the copper rod from the workpiece, stop the machine, and properly clamp the workpiece; at this point, the workpiece is considered corrected.

This correction method is both rapid and accurate, achieving a commendable level of precision. When the workpiece is clamped correctly (with a tolerance of less than 10 mm) and has a smooth surface, the radial runout of shafts and the end face runout of disc workpieces usually do not exceed 0.02 mm.

 

19. Methods for straightening slender rods on a lathe

Slender rods must be straightened before machining to ensure even material removal. If they are not straightened, uneven machining allowances can occur, preventing the rods from being turned round. Additionally, excessive bending due to the centrifugal force during turning can complicate the process. To achieve the best results, the following methods can be used to straighten slender rods on a lathe.

 

1) Hammering method
First, clamp one end of the slender rod with a three-jaw chuck about 10mm, and support the top of one end. Rotate the workpiece at a lower speed, draw the high point on the workpiece with chalk, and then stop. Hold a concave iron block in your left hand so that the concave surface is against the opposite side of the high point of the workpiece, and use a hammer in your right hand to hit the high point of the workpiece. The magnitude of the striking force is proportional to the bending of the workpiece. Repeat this several times, and the workpiece will be straightened. This method is suitable for thin and long rods.

 

2) Use lever prying method
Once the slender rod is installed on the lathe, start the machine to rotate the workpiece. Place a 300 mm long wooden stick on the middle slide and the square tool table. Adjust the middle slide to press the wooden stick against the bent part of the workpiece. Continue moving the middle slide while keeping it close to the tailstock top to prevent the workpiece from falling out.

After a few seconds of rotation, slowly withdraw the middle slide and gently loosen the tailstock top to check if the workpiece has straightened. If it is still bent, repeat the process until the workpiece is straightened. This method is suitable for short workpieces.

 

3) Use counterattack method
When working with a slender rod that is long and has a relatively large diameter, start by drilling center holes at both ends. Then, use the spindle top and the lathe tailstock top to support it. Manually rotate the workpiece to identify the high point, and mark it with chalk.

Next, take an iron block or a wooden block that is approximately 25 mm thick and 40 mm wide. This block should be longer and wider than the lathe’s large guide rail. Place it horizontally on the large guide rail. Use a threaded jack with a V-shaped or concave arc-shaped head (not a 60° pointed one) to support the high point of the workpiece’s bend, applying a little force.

With your left hand, hold the workpiece steady. Use the round head of a hammer in your right hand to strike the low point of the bend. The number of strikes, the force applied, and the duration of each strike should be proportional to the severity of the bend. If done correctly, this method will help straighten the workpiece effectively without easily restoring the bend.

In addition to the straightening method on the lathe, you can also visually inspect the rod outside the machine tool and apply the same techniques to straighten it on a platform.

 

20. Turning tool for turning inner spherical surface in deep hole

When machining workpieces made of materials such as nylon and plexiglass, it is essential to ensure that the connection between the inner cylindrical surface and the inner spherical surface within deep holes is very smooth and free of steps. This requirement poses challenges during processing. To address this, both the inner hole and the inner spherical surface must be completed in a single fine CNC turning process.

To effectively process the inner hole of the workpiece, an inner hole turning tool is first crafted. The blade material should be either tool steel or alloy tool steel, and it should be hardened to a Rockwell hardness of HRC 60-62. The production method involves several steps: begin by turning a blade blank on a lathe. Afterward, subject it to heat treatment and quenching, then grind both end faces. Next, install the blade onto the mandrel for grinding the outer circle and back angle, achieving the required specifications using an outer circle grinder or tool grinder. Carefully grind away any excess material according to the blade shape specified in the drawing, ensuring that no part of the blade obstructs the turning process inside the hole.

Once the blade is prepared, secure it onto the tool bar with screws, ensuring that the front cutting surface of the blade is aligned with the center of the tool bar. This configuration prevents the lower part of the tool bar from interfering during operation and increases the tool bar’s cross-sectional area, thereby enhancing its rigidity.

When turning the inner hole, begin by drilling a pilot hole, followed by rough turning the inner hole with the inner hole cutter. For finishing, position the tool on the square tool table of the lathe, aligning the blade with the rotation center of the workpiece. Use the tool first to semi-finish the inner hole, achieving a depth that meets the specifications. After completing the cylindrical portion of the inner hole, the inner spherical surface located deep within the hole is also turned in this same pass. This tool and operational approach ensures that both the inner hole and the inner spherical surface are free of contact marks and exhibit a very smooth finish.

 

21. Turning flat threads

Flat threads are machined on the end faces of cylinders or disks. The turning tool follows an Archimedean spiral trajectory, which is different from the cylindrical threads typically machined.

When turning flat threads on standard lathes, a light rod transmission is often used to rotate the middle carriage screw. This mechanism drives the middle carriage of the lathe to move the tool horizontally for turning. It is essential for the middle carriage to move horizontally by one pitch for each complete rotation of the workpiece.

If the pitch requirements for the workpiece are not strict, the pitch of the flat thread can be divided by a multiple of the lathe’s increased pitch (for instance, the C620-1 lathe can increase it by factors of 2, 8, or 32). After calculating the quotient, choose a lateral feed amount that corresponds with the lathe’s specifications. Then, adjust the feed box handle, set the pitch increase handle on the spindle box to the appropriate increased pitch position, and adjust the speed change handle to the desired setting. Once the tool is installed, the flat thread can be turned.

When strict pitch accuracy is required, it is necessary to replace both the wheel box and the wheel. Before doing the calculations for the wheel, select an approximate horizontal cutting amount using the previous method. Then, adjust the feed box, increase the pitch, and change the speed handle to perform the horizontal cutting. Using the spindle’s integer (more than five full rotations), divide the distance moved by the horizontal slide, and the quotient obtained will be the actual pitch of the lathe. It is common for this to differ from the required pitch of the workpiece, necessitating the calculation for the replacement of the wheel box and wheel.

During turning, using an elastic toolbar is recommended. The geometric parameters of the tool head should be the same as those used for turning cylindrical threads. However, the secondary back angle of the tool head’s inner circle must be ground to a double back angle to avoid interference during turning. Use the lathe spindle to move the tool forward and backward.

There are two methods for cutting: the first is to use the lathe’s small tool holder to cut and retract, while the small millimeter hoop is utilized for counting; the second method involves installing a magnetic meter stand and a dial indicator on the large guide rail in front of the large slide to control the position and cutting amount of the large slide, allowing it to cut and retract.

In the process of turning plane threads (including square threads), it is important to also “drive the tool,” similar to turning cylindrical threads, to finely shape the two sides of the tooth type.

There are two methods for “driving the tool”: one is to use the large slide to cut and retract the tool, rotate the small tool holder counterclockwise by 90 degrees, and secure it in place while manipulating the handle of the small tool holder; the other method involves using either the large slide or the small tool holder to cut and retract the tool. When “driving the tool,” position the tool head outside the workpiece, ensuring to stop the spindle during the tool’s movement without any reversal. At this point, lower the handle of the shedding worm, rotate the handle of the middle slide to the required value for “driving the tool,” and then raise the handle of the shedding worm. During this process, it is essential to eliminate any gaps in the transmission chain; this means the middle slide must move in the same direction as the tool’s intended path.

After “driving the tool,” the tool head should gradually cut into the workpiece.

 

22. How to sharpen a trapezoidal thread turning tool?

For thread turning, hands-on practice and learning from an expert are essential for making rapid progress. Threads are continuous protrusions with a specific tooth shape formed along a spiral line on the surface of a cylinder or cone. They are commonly used in various machines; for example, four screws on the square tool holder of a lathe clamp the turning tool, and threads transmit power between the lathe’s lead screw and the opening and closing nut.

There are several methods to process threads, but the thread turning method is one of the fundamental skills for a lathe operator in general mechanical processing. When threading on a horizontal lathe, it is crucial to maintain the correct motion relationship between the workpiece and the tool. Specifically, the tool must move a distance equal to one pitch (or lead) for each full rotation of the spindle (or the workpiece).

This relationship is maintained as follows: the spindle rotates the workpiece, and the motion is transmitted to the feed box through the pulley system. The feed box then sends this motion to the lead screw after altering the speed. The opening and closing nut on the lead screw, in conjunction with the slide box, drives the tool holder and the turning tool to move linearly. This coordinated action ensures that both the workpiece’s rotation and the tool’s movement remain synchronized, thereby preserving the precise motion relationship necessary for successful thread turning.

However, in actual thread turning, various factors can disrupt the movement between the spindle and the tool at different stages, leading to failures in the thread turning process and impacting normal production. It is important to address these issues promptly.

23. Incorrect tooth profile angle

1) Incorrect tool tip angle

When sharpening a turning tool, it is important to ensure that the tool tip angle is correct. The angle formed by the projections of the two cutting edges of the turning tool on the base surface should match the tooth profile angle of the thread being machined. If these angles are inconsistent, it can lead to an incorrect thread angle.

To achieve the correct tooth profile angle while sharpening the turning tool, you should use an angle ruler or template. The process is as follows: align the template or angle ruler parallel to the front of the turning tool, and check the alignment using the light transmission method.

Here are the commonly used metric thread tooth profile angles:
- Triangular thread: 60°
- Trapezoidal thread: 30°
- Worm thread: 40°

 

2) Uncorrected radial rake angle

To facilitate smooth chip removal during turning operations, minimize surface roughness, and reduce the occurrence of built-up edge, radial rake angles are often ground. This process causes the cutting edges on both sides of the turning tool to misalign with the axial direction of the workpiece. As a result, the thread profile angle of the workpiece becomes greater than the tool tip angle of the turning tool. A larger radial rake angle leads to a larger discrepancy in the tooth profile angle. Additionally, the thread profile of the turned thread is not a straight line in the axial section, but rather a curve, which can adversely affect the matching quality of the threaded pair.

To address this issue when grinding a thread turning tool with a large radial rake angle, it is essential to correct the tool tip angle based on the angle between the two edges of the turning tool, particularly for threads requiring high machining accuracy. The correction calculation can be expressed as follows: tan(εr) = cos(rp) · tan(α), where εr is the angle between the two edges of the turning tool, rp is the radial rake angle, and α is the tooth angle.

 

3) Too large tooth angle when cutting high-speed steel

When cutting threads at high speeds, the extrusion pressure of the turning tool on the workpiece can lead to deformation. This deformation causes the processed tooth profile to expand, which in turn enlarges the workpiece. To mitigate this effect, the angle between the two edges of the turning tool should be reduced by approximately 30 minutes during grinding. Additionally, the major diameter of the workpiece before turning the external thread is typically smaller than the nominal size by about 0.13 times the pitch (0.13p).

 

4) Improper installation of the turning tool

Improper installation of the turning tool occurs when the symmetrical center line of the two cutting edges is not perpendicular to the 5 axes machining of the workpiece. This results in an inclined tooth angle, often referred to as a reverse tooth.

To resolve this issue, use an angle ruler or template to ensure that the symmetry center line is perpendicular to the workpiece axis. Additionally, make sure that the tool tip is aligned at the same height as the center of the workpiece.

 

5) Tool wear

When the tool is not sharpened promptly after it becomes worn, the angles of the processed teeth may end up uneven, resembling curves or “rotten teeth” on both sides. To address this issue, it is important to select appropriate cutting parameters and to sharpen the turning tool in a timely manner after it shows signs of wear.

 

6) Incorrect pitch (or lead)
a) Incorrect thread length.
The incorrect thread length may result from an improperly calculated or assembled exchange gear, as well as the incorrect positioning of the feed box and slide box handles. To resolve this issue, you should recheck the position of the feed box handle and examine the hanging wheel.

 

b) Partial thread is incorrect.
The issue with the partial thread arises from several potential factors: excessive movement of the lathe screw and spindle, unbalanced rotation of the slide box handwheel, and a gap between the opening and closing nuts that is too large.

 

To address these issues, consider the following solutions:

1. Axial Movement of the Screw: If the problem is due to the axial movement of the screw, adjust the round nut located at the connection between the lathe screw and the feed box to eliminate the axial clearance of the thrust ball bearing at that connection.

2. Axial Movement of the Spindle: If the spindle is experiencing axial movement, adjust the nut located behind the spindle to eliminate the axial clearance of the thrust ball bearing.

3. Misalignment of the Opening and Closing Nut: If the opening and closing nut of the slide box is misaligned with the screw, trim the opening and closing nut and adjust its clearance.

4. Unbalanced Slide Box Rotation: If the slide box rotates unevenly, pull out the slide box handwheel to disengage it from the rotating shaft and then rotate it evenly.

These adjustments should help resolve the issues you’re experiencing.

 

c) The opening and closing nut is automatically lifted during the turning process, causing incorrect pitch.

Solution: Adjust the opening and closing nut strip to appropriately reduce the gap, control the opening and closing nut to lift during transmission, or hang a heavy object on the opening and closing nut handle to prevent it from lifting in the middle.

 

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com

The advantages of Anebon are lessened charges, a dynamic income team, specialized QC, sturdy factories, premium quality services for high pressure aluminum die casting parts making service, and CNC turning and milling parts making service.