Expert Tips: 15 Essential Insights from a CNC Lathe Specialist
1. Get a small amount of depth by using trigonometric functions
In the precision machining industry, we frequently work with components that have inner and outer circles requiring second-level precision. However, factors such as cutting heat and friction between the workpiece and the tool can lead to tool wear. Additionally, the repeat positioning accuracy of the square tool holder can affect the quality of the finished product.
To address the challenge of precise micro-deepening, we can leverage the relationship between the opposite side and the hypotenuse of a right triangle during the turning process. By adjusting the angle of the longitudinal tool holder as needed, we can effectively achieve fine control over the horizontal depth of the turning tool. This method not only saves time and effort but also enhances product quality and improves overall work efficiency.
For instance, the scale value of the tool rest on a C620 lathe is 0.05 mm per grid. To achieve a lateral depth of 0.005 mm, we can refer to the sine trigonometric function. The calculation is as follows: sinα = 0.005/0.05 = 0.1, which means α = 5º44′. Therefore, by setting the tool rest to 5º44′, any movement of the longitudinal engraving disk by one grid will result in a lateral adjustment of 0.005 mm for the turning tool.
2. Three Examples of Reverse Turning Technology Applications
Long-term production practice has demonstrated that reverse-cutting technology can yield excellent results in specific turning processes.
(1) The reverse cutting thread material is martensitic stainless steel
When machining internal and external threaded workpieces with pitches of 1.25 and 1.75 mm, the resulting values are indivisible due to the subtraction of the lathe screw pitch from the workpiece pitch. If the thread is machined by lifting the mating nut handle to withdraw the tool, it often leads to inconsistent threading. Ordinary lathes generally lack random threading discs, and creating such a set can be quite time-consuming.
As a result, a commonly employed method for machining threads of this pitch is low-speed forward turning. High-speed threading does not allow sufficient time to withdraw the tool, which leads to low production efficiency and an increased risk of tool gnashing during the turning process. This issue significantly affects surface roughness, particularly when machining martensitic stainless steel materials like 1Cr13 and 2Cr13 at low speeds due to pronounced tool gnashing.
To address these challenges, the “three-reverse” cutting method has been developed through practical processing experience. This method involves reverse tool loading, reverse cutting, and feeding the tool in the opposite direction. It effectively achieves good overall cutting performance and allows for high-speed thread cutting, as the tool moves from left to right to exit the workpiece. Consequently, this method eliminates issues with tool withdrawal during high-speed threading. The specific method is as follows:
Before beginning the processing, slightly tighten the reverse friction plate spindle to ensure optimal speed when starting in reverse. Align the thread cutter and secure it by tightening the opening and closing nut. Start the forward rotation at a low speed until the cutter groove is empty, then insert the thread-turning tool to the appropriate cutting depth and reverse the direction. At this point, the turning tool should move from left to right at high speed. After making several cuts in this manner, you will achieve a thread with good surface roughness and high precision.
(2) Reverse knurling
In the traditional forward knurling process, iron filings and debris can easily get trapped between the workpiece and the knurling tool. This situation can lead to excessive force being applied to the workpiece, resulting in issues such as misalignment of the patterns, crushing of the patterns, or ghosting. However, by using a new method of reverse knurling with the lathe spindle rotating horizontally, many of the disadvantages associated with the forward operation can be effectively avoided, leading to a better overall outcome.
(3) Reverse turning of internal and external taper pipe threads
When turning various internal and external taper pipe threads with low precision requirements and small production batches, you can use a new method called reverse cutting without the need for a die-cutting device. While cutting, you can apply a horizontal force to the tool with your hand. For external taper pipe threads, this means moving the tool from left to right. This lateral force helps control the cutting depth more effectively as you progress from the larger diameter to the smaller diameter. The reason this method works effectively is due to the pre-pressure applied when striking the tool. The application of this reverse operation technology in turning processing is becoming increasingly widespread and can be adapted flexibly to suit various specific situations.
3. New operation method and tool innovation for drilling small holes
When drilling holes smaller than 0.6 mm, the small diameter of the drill bit, combined with poor rigidity and low cutting speed, can result in significant cutting resistance, especially when working with heat-resistant alloys and stainless steel. As a result, using mechanical transmission feeding in these cases can easily lead to drill bit breakage.
To address this issue, a simple and effective tool and manual feeding method can be employed. First, modify the original drill chuck into a straight shank floating type. When in use, securely clamp the small drill bit into the floating drill chuck, allowing for smooth drilling. The straight shank of the drill bit fits snugly in the pull sleeve, enabling it to move freely.
When drilling small holes, you can gently hold the drill chuck with your hand to achieve manual micro-feeding. This technique allows for quick drilling of small holes while ensuring both quality and efficiency, thus prolonging the service life of the drill bit. The modified multi-purpose drill chuck can also be utilized to tap small-diameter internal threads, reaming holes, and more. If a larger hole needs to be drilled, a limit pin can be inserted between the pull sleeve and the straight shank (see Figure 3).
4. Anti-vibration of deep hole processing
In deep hole processing, the small diameter of the hole and the slender design of the boring tool make it inevitable for vibrations to occur when turning deep hole parts with a diameter of Φ30-50mm and a depth of approximately 1000mm. To minimize this vibration of the tool, one of the simplest and most effective methods is to attach two supports made from materials like cloth-reinforced bakelite to the tool body. These supports should be the same diameter as the hole. During the cutting process, the cloth-reinforced bakelite supports provide positioning and stability, which helps prevent the tool from vibrating, resulting in high-quality deep hole parts.
5. Anti-breaking of small center drills
In turning processing, when drilling a center hole smaller than 1.5 mm (Φ1.5 mm), the center drill is prone to breaking. A simple and effective method to prevent breakage is to avoid locking the tailstock while drilling the center hole. Instead, allow the tailstock’s weight to create friction against the surface of the machine tool bed as the hole is drilled. If the cutting resistance becomes excessive, the tailstock will automatically move backward, providing protection for the center drill.
6. Processing technology of “O” type rubber mold
When using the “O” type rubber mold, misalignment between the male and female molds is a common issue. This misalignment can distort the shape of the pressed “O” type rubber ring, as illustrated in Figure 4, leading to significant material waste.
After many tests, the following method can basically produce an “O”-shaped mold that meets the technical requirements.
(1) Male mold processing technology
① Fine Fine-turn the dimensions of each part and the 45° bevel according to the drawing.
② Install the R forming knife, move the small knife holder to 45°, and the knife alignment method is shown in Figure 5.
According to the diagram, when the R tool is in position A, the tool contacts the outer circle D with the contact point C. Move the large slide a distance in the direction of arrow one and then move the horizontal tool holder X in the direction of arrow 2. X is calculated as follows:
X=(D-d)/2+(R-Rsin45°)
=(D-d)/2+(R-0.7071R)
=(D-d)/2+0.2929R
(i.e. 2X=D—d+0.2929Φ).
Then, move the large slide in the direction of arrow three so that the R tool contacts the 45° slope. At this time, the tool is in the center position (i.e., the R tool is in position B).
③ Move the small tool holder in the direction of arrow 4 to carve cavity R, and the feed depth is Φ/2.
Note ① When the R tool is in position B:
∵OC=R, OD=Rsin45°=0.7071R
∴CD=OC-OD=R-0.7071R=0.2929R,
④ The X dimension can be controlled by a block gauge, and the R dimension can be controlled by a dial indicator to control the depth.
(2) Processing technology of negative mold
① Process the dimensions of each part according to the requirements of Figure 6 (the cavity dimensions are not processed).
② Grind the 45° bevel and end surface.
③ Install the R forming tool and adjust the small tool holder to an angle of 45° (make one adjustment to process both the positive and negative molds). When the R tool is positioned at A′, as shown in Figure 6, ensure that the tool contacts the outer circle D at the contact point C. Next, move the large slide in the direction of arrow 1 to detach the tool from outer circle D, and then shift the horizontal tool holder in the direction of arrow 2. The distance X is calculated as follows:
X=d+(D-d)/2+CD
=d+(D-d)/2+(R-0.7071R)
=d+(D-d)/2+0.2929R
(i.e. 2X=D+d+0.2929Φ)
Then, move the large slide in the direction of arrow three until the R tool contacts the 45° bevel. At this time, the tool is in the center position (i.e., position B′ in Figure 6).
④ Move the small tool holder in the direction of arrow 4 to cut cavity R, and the feed depth is Φ/2.
Note: ①∵DC=R, OD=Rsin45°=0.7071R
∴CD=0.2929R,
⑤The X dimension can be controlled by a block gauge, and the R dimension can be controlled by a dial indicator to control the depth.
7. Anti-vibration when turning thin-walled workpieces
During the turning process of thin-walled casting parts, vibrations often arise due to their poor rigidity. This issue is particularly pronounced when machining stainless steel and heat-resistant alloys, leading to extremely poor surface roughness and a shortened tool lifespan. Below are several straightforward anti-vibration methods that can be employed in production.
1. Turning the Outer Circle of Stainless Steel Hollow Slender Tubes**: To reduce vibrations, fill the hollow section of the workpiece with sawdust and seal it tightly. Additionally, use cloth-reinforced bakelite plugs to seal both ends of the workpiece. Replace the support claws on the tool rest with support melons made of cloth-reinforced bakelite. After aligning the required arc, you can proceed to turn the hollow slender rod. This method effectively minimizes vibration and deformation during cutting.
2. Turning the Inner Hole of Heat-Resistant (High Nickel-Chromium) Alloy Thin-Walled Workpieces**: Due to the poor rigidity of these workpieces combined with the slender toolbar, severe resonance can occur during cutting, risking tool damage and producing waste. Wrapping the outer circle of the workpiece with shock-absorbing materials, such as rubber strips or sponges, can significantly reduce vibrations and protect the tool.
3. Turning the Outer Circle of Heat-Resistant Alloy Thin-Walled Sleeve Workpieces**: The high cutting resistance of heat-resistant alloys can lead to vibration and deformation during the cutting process. To combat this, fill the workpiece hole with materials such as rubber or cotton thread, and securely clamp both end faces. This approach effectively prevents vibrations and deformations, allowing for the production of high-quality thin-walled sleeve workpieces.
8. Clamping tool for disc-shaped discs
The disc-shaped component is a thin-walled part featuring double bevels. During the second turning process, it is essential to ensure that the shape and position tolerances are met and to prevent any deformation of the workpiece during clamping and cutting. To achieve this, you can create a simple set of clamping tools yourself.
These tools utilize the bevel from the previous processing step for positioning. The disc-shaped part is secured in this simple tool using a nut on the outer bevel, allowing for the turning of the arc radius (R) on the end face, hole, and outer bevel, as illustrated in the accompanying Figure 7.
9. Precision boring large diameter soft jaw limiter
When turning and clamping precision workpieces with large diameters, it’s essential to prevent the three jaws from shifting due to gaps. To achieve this, a bar that matches the diameter of the workpiece must be pre-clamped behind the three jaws before any adjustments are made to the soft jaws.
Our custom-built precision boring large diameter soft jaw limiter has unique features (see Figure 8). Specifically, the three screws in part No. 1 can be adjusted within the fixed plate to expand the diameter, allowing us to replace bars of various sizes as needed.
10. Simple precision additional soft claw
In turning processing, we frequently work with medium and small precision workpieces. These components often feature complex inner and outer shapes with strict shape and position tolerance requirements. To address this, we have designed a set of custom three-jaw chucks for lathes, such as C1616. The precision soft jaws ensure that the workpieces meet various shape and position tolerance standards, preventing any pinching or deformation during multiple clamping operations.
The manufacturing process for these precision soft jaws is straightforward. They are made from aluminum alloy rods and drilled to specifications. A base hole is created on the outer circle, with M8 threads tapped into it. After milling both sides, the soft jaws can be mounted onto the original hard jaws of the three-jaw chuck. M8 hexagon socket screws are used to secure the three jaws in place. Following this, we drill positioning holes as needed for precise clamping of the workpiece in the aluminum soft jaws before cutting.
Implementing this solution can yield significant economic benefits, as illustrated in Figure 9.
11. Additional anti-vibration tools
Due to the low rigidity of slender shaft workpieces, vibration can easily occur during multi-groove cutting. This results in poor surface finish on the workpiece and can cause damage to the cutting tool. However, a set of custom-made anti-vibration tools can effectively address the vibration issues associated with slender parts during grooving (see Figure 10).
Before starting work, install the self-made anti-vibration tool in an appropriate position on the square tool holder. Next, attach the required groove turning tool to the square tool holder and adjust the spring’s distance and compression. Once everything is set up, you can begin operating. When the turning tool makes contact with the workpiece, the anti-vibration tool will simultaneously press against the surface of the workpiece, effectively reducing vibrations.
12. Additional live center cap
When machining small shafts with various shapes, it is essential to use a live center to hold the workpiece securely during cutting. Since the ends of the prototype CNC milling workpieces often have different shapes and small diameters, standard live centers are not suitable. To address this issue, I created custom live pre-point caps in different shapes during my production practice. I then installed these caps on standard live pre-points, allowing them to be effectively used. The structure is shown in Figure 11.
13. Honing finishing for difficult-to-machine materials
When machining challenging materials like high-temperature alloys and hardened steel, it is essential to achieve a surface roughness of Ra 0.20 to 0.05 μm and maintain high dimensional accuracy. Typically, the final finishing process is carried out using a grinder.
To improve economic efficiency, consider creating a set of simple honing tools and honing wheels. By using honing instead of finishing grinding on the lathe, you can achieve better results.
Honing wheel
Manufacturing of honing wheel
① Ingredients
Binder: 100g epoxy resin
Abrasive: 250-300g corundum (single crystal corundum for difficult-to-process high-temperature nickel-chromium materials). Use No. 80 for Ra0.80μm, No. 120-150 for Ra0.20μm, and No. 200-300 for Ra0.05μm.
Hardener: 7-8g ethylenediamine.
Plasticizer: 10-15g dibutyl phthalate.
Mold material: HT15-33 shape.
② Casting method
Mold release agent: Heat the epoxy resin to 70-80℃, add 5% polystyrene, 95% toluene solution, and dibutyl phthalate and stir evenly, then add corundum (or single crystal corundum) and stir evenly, then heat to 70-80℃, add ethylenediamine when cooled to 30°-38℃, stir evenly (2-5 minutes), then pour into the mold, and keep it at 40℃ for 24 hours before demolding.
③ The linear speed \( V \) is given by the formula \( V = V_1 \cos \alpha \). Here, \( V \) represents the relative speed to the workpiece, specifically the grinding speed when the honing wheel is not making a longitudinal feed. During the honing process, in addition to rotational movement, the workpiece is also advanced with a feed amount \( S \), allowing for reciprocating motion.
V1=80～120m/min
t=0.05～0.10mm
Residue<0.1mm
④ Cooling: 70% kerosene mixed with 30% No. 20 engine oil, and the honing wheel is corrected before honing (pre-honing).
The structure of the honing tool is shown in Figure 13.
14. Quick loading and unloading spindle
In turning processing, various types of bearing sets are often used to fine-tune outer circles and inverted guide taper angles. Given the large batch sizes, the loading and unloading processes during production can result in auxiliary times that exceed the actual cutting time, leading to lower overall production efficiency. However, by using a quick-loading and unloading spindle along with a single-blade, multi-edge carbide turning tool, we can reduce auxiliary time during the processing of various bearing sleeve parts while maintaining product quality.
To create a simple, small taper spindle, start by incorporating a slight 0.02mm taper at the rear of the spindle. After installing the bearing set, the component will be secured onto the spindle through friction. Next, utilize a single-blade multi-edge turning tool. Begin by turning the outer circle, and then apply a 15° taper angle. Once you’ve completed this step, stop the machine and use a wrench to swiftly and effectively eject the part, as illustrated in Figure 14.
15. Turning of hardened steel parts
(1) One of the key examples of turning hardened steel parts
- Remanufacturing and regeneration of high-speed steel W18Cr4V hardened broaches (repair after fracture)
- Self-made non-standard thread plug gauges (hardened hardware)
- Turning of hardened hardware and sprayed parts
- Turning of hardened hardware smooth plug gauges
- Thread polishing taps modified with high-speed steel tools
To effectively handle the hardened hardware and various challenging CNC machining parts encountered in the production process, it is essential to select the appropriate tool materials, cutting parameters, tool geometry angles, and operating methods in order to achieve favorable economic results. For instance, when a square broach fractures and requires regeneration, the remanufacturing process can be lengthy and costly. Instead, we can use carbide YM052 and other cutting tools at the root of the original broach fracture. By grinding the blade head to a negative rake angle of -6° to -8°, we can enhance its performance. The cutting edge can be refined with an oilstone, using a cutting speed of 10 to 15 m/min.
After turning the outer circle, we proceed to cut the slot and finally shape the thread, diviTurninge process into Turningnd fine turning. Following rough turning, the tool must be re-sharpened and ground before we can proceed with fine turning the outer thread. Additionally, a section of the inner thread of the connecting rod must be prepared, and the tool should be adjusted after the connection is made. Ultimately, the broken and scrapped square broach can be repaired through turning, successfully restoring it to its original form.
(2) Selection of tool materials for turning hardened parts
① New carbide blades such as YM052, YM053, and YT05 generally have a cutting speed below 18m/min, and the surface roughness of the workpiece can reach Ra1.6~0.80μm.
② The cubic boron nitride tool, model FD, is capable of processing various hardened steels and sprayed turned components at cutting speeds of up to 100 m/min, achieving a surface roughness of Ra 0.80 to 0.20 μm. Additionally, the composite cubic boron nitride tool, DCS-F, which is produced by the State-owned Capital Machinery Factory and Guizhou Sixth Grinding Wheel Factory, exhibits similar performance.
However, the processing effectiveness of these tools is inferior to that of cemented carbide. While the strength of cubic boron nitride tools is lower than that of cemented carbide, they offer a smaller depth of engagement and are more expensive. Moreover, the tool head can be easily damaged if used improperly.
⑨ Ceramic tools, cutting speed is 40-60m/min, poor strength.
The above tools have their own characteristics in turning quenched parts and should be selected according to the specific conditions of turning different materials and different hardness.
(3) Types of quenched steel parts of different materials and selection of tool performance
Quenched steel parts of different materials have completely different requirements for tool performance at the same hardness, which can be roughly divided into the following three categories;
① High alloy steel refers to tool steel and die steel (mainly various high-speed steels) with a total alloying element content of more than 10%.
② Alloy steel refers to tool steel and dies steel with an alloying element content of 2-9%, such as 9SiCr, CrWMn, and high-strength alloy structural steel.
③ Carbon steel: including various carbon tool sheets of steel and carburizing steels such as T8, T10, 15 steel, or 20 steel carburizing steel, etc.
For carbon steel, the microstructure after quenching consists of tempered martensite and a small amount of carbide, resulting in a hardness range of HV800-1000. This is considerably lower than the hardness of tungsten carbide (WC), titanium carbide (TiC) in cemented carbide, and A12D3 in ceramic tools. Additionally, the hot hardness of carbon steel is less than that of martensite without alloying elements, typically not exceeding 200°C.
As the content of alloying elements in steel increases, the carbide content in the microstructure after quenching and tempering also rises, leading to a more complex variety of carbides. For instance, in high-speed steel, the carbide content can reach 10-15% (by volume) after quenching and tempering, including types such as MC, M2C, M6, M3, and 2C. Among these, vanadium carbide (VC) possesses a high hardness that surpasses that of the hard phase in general tool materials.
Furthermore, the presence of multiple alloying elements enhances the hot hardness of martensite, allowing it to reach about 600°C. Consequently, the machinability of hardened steels with similar macrohardness can vary significantly. Before turning hardened steel parts, it is essential to identify their category, understand their characteristics, and select suitable tool materials, cutting parameters, and tool geometry to effectively complete the turning process.
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