Lathe machining processes and specifications
Chapter 1 An Overview of Turning Operations
Section 1: The Role of Turning in the Machinery Manufacturing Industry
Mechanical manufacturing is a complex systems engineering. The birth of any machine cannot be achieved without the collaborative work of numerous types of work. These types of work, such as casting, forging, cutting (including turning, milling, planing, grinding), fitter work, etc., are interdependent and none of them can be dispensed with. Together, they form a complete chain of mechanical manufacturing. Lathe operators, as one of the core types of work in cutting processing, play an indispensable role in it.
The essence of turning is to drive the workpiece to rotate (main motion) through the lathe while making the cutting tool perform a feed motion. By utilizing the relative cutting motion between the cutting tool and the workpiece, the size, shape, and surface quality of the blank are changed to obtain mechanical parts that meet the design requirements. The capabilities of turning are extremely extensive. It can efficiently machine various rotating surfaces, such as internal and external cylindrical surfaces, conical surfaces, end faces, grooves, and threads. Moreover, some complex non-rotationally symmetric shapes can even be machined with specific tooling fixtures. It can be said that the machining process of the vast majority of mechanical parts is inseparable from the turning process, and the precision machining of many key parts relies on high-level turning technology. Therefore, turning occupies a fundamental and pillar position in the machinery manufacturing industry, and its machining quality and efficiency directly affect the performance of the whole machine and the production progress.
Section 2: Significance and Core Content of Learning the Basic Theory of Lathe Turning
The basic theory of turning is a knowledge system formed by humans through continuous exploration, summarization, and systematization in long - term production practices. It embodies the wisdom and experience of the working people. For a qualified lathe operator, it is crucial to learn and master solid basic theories. The value of theory lies in guiding practice. It can help lathe operators understand the machining principle, foresee possible problems in the machining process, and find solutions to these problems. In turn, practical experience can verify the correctness of the theory, enrich and develop it. Only by closely combining theory with practice can blind operations be avoided, and efficient, high - quality, and safe production be achieved, thus forming a complete set of professional skills.
As a lathe operator, one must master the following core basic knowledge:
1. Lathe structure and principle: Thoroughly understand the overall structure of the lathe in use, including the functions, working principles and transmission relationships between the core components such as the headstock, feed box, apron, tool post, tailstock, and bed. Be familiar with the functions of each control handle (such as the speed change handle and the feed rate adjustment handle) and be able to operate them correctly. At the same time, understand the identification methods of common lathe faults and basic maintenance knowledge, and be familiar with the locations of each lubrication point on the lathe and the lubrication requirements.
2. Use and maintenance of tools, fixtures, cutting tools, and measuring tools: Master the installation, adjustment, and correct use of commonly used lathe accessories (such as three-jaw chucks, four-jaw chucks, centers, faceplates, steady rests, follower rests, etc.). Be proficient in the types, structures, material selection, geometric angle grinding of various lathe tools (such as external turning tools, facing tools, boring tools, cutting-off tools, threading tools, etc.) and their matching with the processing materials and requirements. Be able to correctly select and use measuring tools (such as vernier calipers, micrometers, dial indicators, protractors, templates, etc.), understand their measurement principles, master their accuracy grades and maintenance methods to ensure the accuracy of measurement results.
3. Ability to read drawings and execute processes: Have proficient drawing-reading skills and be able to accurately understand the technical requirements such as dimensions, tolerances, geometric tolerances, and surface roughness on the part drawings. Be familiar with the process flow of the parts to be processed, clarify the processing content, technical parameters, and quality standards of each process, and be able to operate strictly in accordance with the requirements of the drawings and process documents.
4. Turning process calculation: Master the calculations related to turning operations, such as the selection and calculation of cutting parameters like cutting speed, feed rate, and back engagement of the cutting edge (cutting depth) to ensure machining efficiency and surface quality; master the pitch calculation and gear train configuration during thread machining; master the dimensional control calculations during the machining of formed surfaces, etc.
5. Knowledge of metal materials and heat treatment: Understand the mechanical properties (strength, hardness, plasticity, toughness), processing properties and applicable occasions of commonly used metal materials (such as carbon steel, alloy steel, cast iron, non - ferrous metals, etc.). Be familiar with the purposes, methods of common heat treatment processes (such as annealing, normalizing, quenching, tempering, surface quenching, carburizing, etc.) and their influence on material properties and processing technology, so as to reasonably select cutting parameters and cutting tools.
6. Work environment planning and labor discipline: Be able to scientifically and reasonably plan the personal work area, including the fixed - position stacking of workpieces (blanks, semi - finished products, finished products) (such as separating rough and fine workpieces and arranging them in order), the orderly placement of tools and measuring instruments, and keep the work environment clean and orderly. Strictly abide by all rules, regulations and labor disciplines of the factory and the workshop.
7. Production efficiency and cost awareness: Establish a sense of thrift. In the production process, strive to save raw materials (e.g., make rational use of blanks and reduce machining allowance), energy (e.g., avoid idling of machine tools), and auxiliary materials. Improve labor productivity by optimizing processing methods, enhancing operational proficiency, and reducing auxiliary time. On the premise of ensuring product quality, strive to reduce production costs.
8. Ability to consult technical materials: Be able to proficiently consult various technical materials such as lathe operator manuals, process manuals, tool samples, and material manuals to obtain necessary information on process parameters, material properties, standards and specifications to guide actual production.
Section 3: Planning of the Work Environment and Labor Discipline
The planning of the lathe operator's working environment, that is, the reasonable arrangement and organization of the personal work area (including the machine tools, auxiliary equipment, tools, fixtures, measuring tools, blanks, finished products and other accessories used), has a direct and significant impact on production efficiency, physical consumption, product quality and even work safety. Even if advanced technologies and equipment are adopted, if the work area is in a mess, the operator may waste a lot of time in frequent ineffective walking and searching for tools in a day, and at the same time increase unnecessary physical consumption, leading to fatigue and even causing production chaos or potential safety hazards. Therefore, the work position must be highly valued and scientifically organized.
The following principles should be considered when correctly organizing the work location:
1. Material concentration and freedom of movement: Items required for the operation should be placed as centrally as possible in an area around the operator that is easily accessible, but should not impede the operator's normal activities and the operation of the machine tool.
2. Zoning according to the access frequency of items: Frequently used items (such as commonly used cutting tools and measuring tools) should be placed in the nearest and most convenient positions; infrequently used items can be placed in relatively farther places but still easily accessible.
3. Ergonomic: The placement of items should conform to people's natural movement habits. For example, items commonly used by the left hand should be placed on the left side, and those commonly used by the right hand should be placed on the right side to reduce unnecessary actions such as turning around and bending over.
4. Safety and importance zoning: Precision measuring tools, valuable tools, or dangerous goods that need to be used with caution should be placed in a safe position that is high and not easily collided with; general tools or blanks can be placed on lower material racks or on the ground (ensure they are stable).
5. Information accessibility: Documents such as drawings, process cards, and work instructions should be placed within the operator's line of sight, in a location that is easy to access and not easily contaminated (e.g., a dedicated document rack).
6. Fixed placement of blanks and semi-finished products: Blanks, semi-finished products, and finished products should be stored in separate areas and neatly arranged according to the processing sequence or specifications to avoid mixed storage, which facilitates access and management and reduces errors.
7. Reduce bending movements: The placement height of items should conform to the human operation habits as much as possible. Try to avoid frequent bending or tiptoeing when taking them to reduce fatigue.
8. Neat and orderly environment: The area around the work area should always be kept tidy and clean, with no oil stains or debris on the ground and unobstructed passageways.
I. Before starting work, the following points must be met:
1. Equipment inspection and trial run: First, check whether the appearance of the lathe and all its mechanisms are in good condition. For example, check if the position of the speed change lever is correct, if the protective devices are complete and effective, and if there is any looseness at each connection part. After confirming that there are no abnormalities, conduct a low - speed idle run for 1 - 2 minutes (this is especially important in winter, which is beneficial for pre - heating and lubricating the engine oil). Carefully observe and listen to whether the running sound and smoothness are normal. If there is any abnormal sound or vibration, stop the machine immediately and report to the leader or maintenance personnel. It is strictly prohibited to operate the equipment with faults.
2. Lubrication and maintenance: According to the requirements of the lathe lubrication chart, check the oil levels in all oil filler holes, oil cups, and oil sumps. Add lubricating oil or grease to each lubrication point to ensure that all moving parts are fully lubricated.
3. Preparation of technical documents: Carefully read and familiarize yourself with the drawings and process documents of the parts to be processed, and place them on the designated document rack to keep them clean. If any doubts or errors are found in the drawings or processes, report them to the relevant technical personnel in a timely manner. Do not make any unauthorized changes.
4. Inspection of tools, fixtures and measuring tools: Check whether the required tools, fixtures, cutting tools and measuring tools are complete and in good condition, and whether their performance is normal. Whether the cutting tools are sharp, and whether the measuring tools are within the validity period of verification and have accurate readings.
5. Rough blank inspection: Check whether the material, specification, and quantity of the rough blank match the requirements of the drawing. Inspect the surface of the rough blank for defects such as cracks, sand holes, and shrinkage cavities, and ensure that the machining allowance is sufficient and uniform. Report any problems immediately.
II. During working hours, the following points must be adhered to:
1. Machine tool care: It is strictly forbidden to strike objects or place tools and measuring tools on the machining surfaces such as the machine tool bed and guide rails to avoid damaging the accuracy. The machine tool should be kept clean and lubricated regularly, but waste of lubricating materials should be avoided.
2. Energy conservation and consumption reduction: When working, the work processes should be reasonably arranged to avoid the long - term idling of the machine tool. When temporarily leaving the machine tool or finishing the work, the power supply should be turned off promptly to save electricity.
3. Safe operation: When it is necessary to change the spindle speed or feed rate during work, the machine must be stopped first, and the operation can be carried out after the inertia disappears. When the machine tool is running or idling, the operator shall not leave the post without permission to prevent accidents.
4. Tool management: All kinds of tools, cutting tools, and measuring tools should be placed in special toolboxes, tool holders, or measuring tool boxes, and stored in a fixed position. They must not be placed randomly, thrown around, or stepped on, so as to prevent damage or loss.
5. Tool maintenance: When a turning tool becomes dull, it should be sharpened or replaced in a timely manner to keep it sharp. A dull tool will increase the cutting force and cutting heat, which not only affects the machining quality and efficiency but also exacerbates the wear of the machine tool. It may even cause the tool to crack or the workpiece to be scrapped.
6. Measuring tool protection: Measuring tools are important tools to ensure machining accuracy. When using them, they should be handled with care to avoid being impacted, scratched or contaminated with oil. After use, they should be cleaned in time and properly stored.
7. First-piece inspection: After the first part is finish-turned, it must be submitted to the inspector for first-piece inspection. Only after it passes the inspection and is confirmed can mass production be carried out to ensure the quality of the entire batch of products.
III. After the work is completed, the following points should be adhered to:
1. Workpiece handover: Deliver the qualified parts processed on the same day to the quality inspection department or the next process as required. Unfinished work and relevant situations should be clearly handed over to the oncoming shift personnel in person and records should be kept.
2. Return tools and measuring tools: After wiping clean the tools, measuring tools, and fixtures that are no longer needed, return them to the tool room as required or put them back into the personal toolbox for proper storage.
3. Item arrangement: Wipe the used auxiliary tools, fixtures, accessories, etc. clean and put them back in their original places.
4. Machine tool cleaning and maintenance: Thoroughly clean the chips inside and outside the machine tool (use special hooks or brushes, and it is strictly prohibited to grab them directly by hand), wipe off the oil stains and iron chips on each surface of the machine tool, and add lubricating oil to key moving parts such as guide rails and lead screws as required to prevent rust.
5. Task preparation: After receiving the next production task, one should familiarize themselves with the drawings and process requirements of the new parts in advance and prepare the necessary tools, measuring tools, cutting tools, and materials.
Section 4. Lathe Operator Safety Technology
In our country, it is an important responsibility of the state and enterprises to safeguard the health and safety of workers. For the potential risk factors in the production process, there are strict safety protection measures and management systems. Mechanical equipment is usually equipped with necessary safety devices, and special personnel are responsible for inspection and maintenance, which provides a basic guarantee for safe production.
However, even with perfect safety devices and systems, if one is careless and operates in violation of regulations during work, it may still lead to personal injuries or equipment accidents. Therefore, every lathe operator must enhance their safety awareness and strictly abide by all safety operating procedures formulated by the factory and the workshop.
After entering the work site, generally the following should be done (including but not limited to):
1. Personal protection: When working, one must wear well - fitting and close - fitting work clothes, and the cuffs should be tightened; do not wear accessories such as scarves and ties that are easily entangled by rotating parts.
2. Head protection: Lathe operators must wear work caps. Female workers must tuck their long hair or braids completely into the work caps to prevent their hair from being caught in rotating parts.
3. Eye protection: During turning operations, chips fly. Protective glasses must be worn to prevent chips, coolant, etc. from harming the eyes.
4. Do not touch rotating parts: It is strictly prohibited for hands and any part of the body to approach or touch the rotating parts of the lathe (such as chucks, workpieces, tool rests, lead screws, etc.). It is even more not allowed to play around the rotating parts.
5. Securely clamp the workpiece and the tool: The workpiece must be firmly clamped with a chuck or other fixtures, and the tool must be tightly fixed on the tool post to ensure reliable clamping. Check again before starting the machine tool to prevent the workpiece or tool from flying out and causing injury due to loosening.
6. Safety in heavy object handling: When components such as workpieces or chucks are too heavy to be loaded and unloaded by a single person, it is strictly prohibited to force the operation. Instead, one should seek assistance from others or use hoisting equipment.
7. Do not measure or touch during rotation: When the lathe is running, it is strictly prohibited to touch the workpiece surface with your hands or directly measure the workpiece dimensions with measuring tools to prevent hand injuries, damage to measuring tools, and impacts on measurement accuracy.
8. Correctly remove chips: Special hooks or brushes must be used to remove chips. It is strictly forbidden to directly grab or pull the chips by hand to avoid being scratched by the sharp chips.
9. Prohibit braking rotating parts: It is strictly forbidden to use hands or other tools to forcibly brake the rotating chuck or workpiece. Wait for it to stop naturally or use the braking device (if equipped).
10. Do not operate rotating parts while wearing gloves: When the lathe spindle is rotating (e.g., during turning operations), do not wear gloves to prevent the gloves from being caught in the rotating parts and causing hand injuries. (When loading and unloading workpieces or performing non - cutting operations, wear protective gloves as required according to regulations).
11. Equipment failure report: If a fault is found in an electrical device or machine tool, the machine should be stopped immediately and reported to the supervisor or maintenance personnel. It is strictly prohibited to disassemble or repair it privately to avoid expanding the fault or causing dangers such as electric shock.
12. Stay at the post: When the lathe is running, the operator shall not leave the machine tool without permission or walk to other places to prevent accidents.
13. Safety in tool sharpening: When sharpening a turning tool on a grinding wheel, the operating rules of the grinding wheel should be followed. Wear safety goggles. Stand on the side of the grinding wheel. Hold the workpiece firmly and bring it into contact with the grinding wheel slowly. Also, make sure to turn off the power of the grinding wheel before leaving.
Section 5 Lathe Lubrication
The lubrication of a lathe is a key link to ensure the normal operation of the machine tool, reduce friction and wear, extend the service life, and maintain the machining accuracy. Operators must fully recognize the importance of lubrication and strictly follow the requirements of the machine tool manual to regularly and correctly lubricate all friction parts.
Requirements for greases and key points for lubrication:
1. Selection of lubricating oils:
* Usually, parts such as the sliding bearings, guide rail surfaces, lead screw nuts of a lathe, as well as the parts marked with "oil holes", are generally lubricated with No. 40 engine oil (or engine oil with the viscosity grade recommended in the machine tool manual).
* For rolling bearings, gear meshing parts in gearboxes, and parts marked "apply grease" (such as some oil cups), generally use No. 3 industrial grease (butter) for lubrication.
2. Gearbox lubrication: There must be sufficient lubricating oil in the gearboxes such as the headstock, feed box, and change gear box of the lathe. The oil level should generally be added to 1/2 to 2/3 of the height of the oil level gauge window to ensure that the gears can bring the lubricating oil to each meshing point and bearing through the splash lubrication method or by the reciprocating oil pump.
3. Regular oil change and cleaning: The lubricating oil in closed boxes such as the headstock and feed box of the lathe should be changed regularly, usually once every three months (the specific cycle refers to the machine tool manual and can be appropriately adjusted according to the workload and oil pollution situation). When changing the oil, the internal oil stains in the box should be cleaned up before adding new lubricating oil. The change gear box, feed box, carriage box, etc. also need to be maintained according to this principle.
4. Lubrication cycle of key parts:
* The hand-pressed oil cup on the change gear box (used for bearing lubrication) should generally be filled with grease once per work shift (or once every five days, depending on the frequency of use).
* Usually, the worm, worm gear and related mechanisms in the drag box should be lubricated once per shift.
* Lubricate the oil grooves or oil holes on the feed box once per shift.
* The guide surfaces of the bed, the guide surfaces of the slide, the lead screw, the feed rod, etc. shall be wiped clean and lubricated before or during work (e.g., at the start of each shift or after a long stop in the middle).
5. Lubrication inspection: Before starting daily work, the oil level and lubrication conditions of all lubrication parts should be checked to ensure smooth oil passages and sufficient oil quantity. If oil leakage or oil shortage is found, it should be dealt with promptly.
I. Production Safety and Standardized Operations
(I) Functions and daily management of safety devices
All mechanical equipment at the production site needs to be equipped with safety devices. The core function of these devices is to reduce the risk of mechanical injuries through mechanisms such as physical isolation (e.g., protective covers), emergency braking (e.g., emergency stop buttons), and overload protection (e.g., torque limiters). The safety devices need to be inspected by a dedicated person before work every day: confirm that the protective covers are intact, the interlocking devices are sensitive and effective, and the warning signs are clear and complete. Functional tests should be carried out regularly (weekly) to ensure that the power source can be immediately cut off in case of an emergency. Such management measures are the first line of defense for ensuring safe production. However, it should be clear that safety devices are only "auxiliary protection," and standardized operation by personnel is the fundamental factor. Even if the devices are in good condition, accidents may still occur due to misoperation if the operating procedures are ignored.
(II) Core safety operation rules
After entering the work site, the following procedures must be strictly followed. Each rule corresponds to specific risk prevention and control objectives:
1. Dress code: You must wear form-fitting work clothes (with cuffs and hems tightened) or special work uniforms to avoid loose clothing getting caught in rotating parts (such as chucks and pulleys). Lathe operators need to wear work caps. Female workers must fully tuck their long hair and braids into the caps to prevent hair entanglement, which may cause scalp tearing or mechanical jams.
2. Head and eye protection: During processing, keep a safe distance of at least 30 cm between the head and the workpiece to prevent the direct impact of flying chips on the face. When processing brittle materials such as cast iron and cast steel (chips are prone to cracking) or performing high-speed cutting (linear speed
3. Isolate the body from rotating parts: It is strictly forbidden to touch the rotating chuck, workpiece or tool with hands or any part of the body. It is even more prohibited to lean on the machine tool body or play near the rotating parts when the machine tool is running. There was once a finger - amputation accident where a hand was caught in the chuck due to "joking pushing and shoving".
4. Workpiece and tool clamping: The workpiece needs to be firmly clamped by fixtures such as chucks and centers. After clamping, manually rotate the chuck to confirm that there is no looseness; The tool shank of the turning tool needs to be inserted into the tool post by at least 1/2 of its length, and the tool tip should be at the same height as the center of the workpiece. When tightening the screws of the tool post, apply force step - by - step in a diagonal order to prevent the tool from flying out and injuring people during the cutting process (especially under working conditions with large cutting forces such as cutting off and grooving).
5. Handling of heavy workpieces: When the weight of the workpiece/chuck exceeds 25 kg, single-person handling or loading and unloading is prohibited. Auxiliary tools such as overhead cranes and hydraulic lifting platforms must be used, and two people must operate in coordination (one person commands the positioning, and the other controls the hoisting) to avoid the workpiece falling and causing injuries or the chuck overturning and damaging the machine tool due to the shift of the center of gravity.
6. Prohibited actions during operation: When the machine tool spindle is rotating, it is strictly prohibited to conduct measurements (such as directly touching the workpiece with a caliper), touch the workpiece surface by hand (it is easy to cause scratches due to burrs on the workpiece surface or rotational inertia), pull the chips forcefully (special iron hooks should be used for removal to prevent the sharp edges of the chips from cutting fingers), or brake the chuck by hand (it is easy to cause wrist sprains or the chuck to lose its roundness); It is prohibited to wear gloves when operating rotating parts (the fibers of gloves are easy to entangle, causing the hands to be drawn in).
7. Equipment fault handling: In case of electrical faults (such as abnormal noises from the motor or electric leakage) or mechanical faults (such as jamming of the guide rail or abnormal noises from the gearbox), the equipment should be stopped immediately, and a "No operation" sign should be hung. The faults should be handled by professional maintenance personnel. It is strictly prohibited to disassemble the equipment privately. Once, an operator blindly disassembled the motor, causing a short - circuit of the internal coils and resulting in a fire.
8. Process continuity: Do not leave the machine tool without permission during the turning process (such as fetching materials or chatting midway). It is necessary to monitor the cutting state at all times (such as the chip flow direction and tool wear) to prevent the workpiece from flying out after loosening. After the machining is completed, the spindle power must be turned off first. After it has completely stopped rotating, the workpiece can be loaded, unloaded or cleaned.
9. Safety during tool grinding: When grinding a turning tool, turn off the machine tool motor (cut off the main power supply) in advance and follow the principle of "turn off the machine when leaving" - to prevent accidental rotation of the grinding wheel caused by accidentally touching the start button, which may lead to tool cracking and injuring people.
II. Lubrication and Maintenance of Lathes
(I) The core function of lubrication
During high-speed friction, various kinematic pairs of the lathe (such as guide rails, gears, and lead screws) will experience wear and generate heat. The core objectives of lubrication are as follows: form an oil film with grease to reduce direct metal contact (decrease the wear rate by over 50%); carry away heat through the flow of grease (for example, splash lubrication in the gearbox can keep the oil temperature below 60°C); prevent corrosion (especially for exposed parts like guide rails and lead screws). Operators need to master the principle of "lubrication on demand": different types of friction (sliding friction, rolling friction), loads (heavy-duty gears vs. light-duty guide rails), and rotational speeds (high-speed spindles vs. low-speed lead screws) correspond to different types of grease and filling cycles.
(II) Specifications for the selection and filling of lubricating oils and greases
1. Types of oils and fats and their suitable application areas
No. 3 industrial grease (butter): It is used for low-speed, heavy-load, and intermittent motion components, such as the bearing oil cup of the change gear box (to lubricate rolling bearings) and the worm disengaging mechanism of the carriage box (to lubricate sliding friction surfaces). It has a high consistency (NLGI Grade 3) and can form a long-lasting oil film on vertical surfaces, making it less likely to run off.
No. 40 engine oil (machine oil): It is used for high-speed, light-load and continuously moving parts, such as the gear meshing surfaces of the headstock (lubricated by splash oil), the bed guide rails (supplied with oil through the oil grooves), and the screw-nut pairs (lubricated by manual oiling). It has a moderate viscosity (kinematic viscosity of 37 - 43 mm²/s at 40°C) and good fluidity, and can quickly penetrate into the gaps.
2. Lubrication details of key parts
Gearbox (headstock, feed box, change gear box): It is necessary to fill No. 40 engine oil to 1/2 of the height of the oil level gauge window (about 50mm) to ensure that when the gears rotate, the engine oil is splashed onto the meshing surface by the "oil splashing method", and at the same time, supply oil to the reciprocating oil pump (the oil suction port of the pump core needs to be completely immersed). Change the oil completely every 3 months: First, drain the old oil, flush the inner wall of the box with kerosene (to remove iron filings and sludge), add new oil after drying. The oil change cycle needs to be adjusted according to the processing load (for example, if it runs continuously for more than 8 hours a day, it can be shortened to 2 months).
Bearing oil cup of the change gear box: Use a grease gun to fill it with No. 3 grease every 5 days. Each time, press until the piston of the oil cup rises by 1/3 (excessive filling may easily cause the bearing to overheat).
Apron and feed box: The worm dropping mechanism of the apron should be filled with No. 3 grease once per shift (8 hours) (the dosage is about 0.5 g); The oil groove on the feed box should be filled with No. 40 machine oil until it overflows per shift (to ensure uniform lubrication of the guide rail surface).
Bed guide rails and lead screws: Add No. 40 machine oil once before work (before starting the machine) and once after work (after stopping the machine) every day. Use a brush to evenly apply the oil on the guide rail surface (especially the sliding contact area) and in the thread grooves of the lead screws to prevent "scuffing" of the guide rails or jamming of the lead screws caused by dry friction during processing.
III. Turning process for shaft parts
(I) Functions and machining importance of shaft parts
The shaft is the core component of the mechanical transmission system. It supports parts such as gears and pulleys through the cylindrical surface (to ensure coaxiality) and transmits torque through keyways and splines (to achieve power transmission). Lathe turning is the most basic and widely used machining method for shaft parts, which can complete the forming of features such as outer circles, steps, grooves, and threads. Its machining quality directly affects the assembly accuracy of the whole machine (such as the bearing fit clearance) and the operating stability (such as the dynamic balance of the shaft).
(II) Types and Structural Characteristics
Shaft parts can be divided into three categories according to their structures, and their machining difficulties vary:
Smooth shaft: The outer diameter is uniform (or with only a slight taper). It is mainly used to support lightly loaded parts (such as guide shafts). The key points in machining are to ensure the dimensional accuracy of the diameter (IT8 - IT10 grade) and the surface roughness (Ra 3.2 - 1.6μm).
Stepped shaft: It consists of multiple cylindrical sections with different diameters (such as the motor spindle). It is necessary to ensure the perpendicularity between the end - face of each step and the axis (≤0.02mm/100mm) and the coaxiality of adjacent outer circles (≤0.01mm) to prevent the "runout" of parts after assembly.
Hollow shaft (such as the machine tool spindle): The inner hole needs to be coaxial with the outer circle (coaxiality ≤ 0.005mm), which is used for inserting the drawbar or reducing weight. During processing, the mutual position accuracy between the inner hole and the outer circle needs to be taken into account.
Functions of structural elements:
Groove (tool withdrawal groove): The width is 2 - 5 mm, and the depth is 0.5 - 1 mm. It is used for tool withdrawal (to avoid tool collision when machining to the root of the step). At the same time, it enables the shaft shoulder to fit tightly with the end face of the part during assembly (without clearance).
Chamfer: Generally C1~C3 (45° chamfer with a side length of 1~3mm). Its functions are to remove sharp edges (to prevent assembly workers from being scratched) and guide the assembly of parts (to avoid jamming during the fitting of holes and shafts). It is usually machined after finishing.
Arc (transition fillet): The radius is R0.5 - 5mm, which is used at the root of the shaft shoulder. It can reduce stress concentration (especially under alternating loads) and prevent the shaft from breaking. The arc of quenched parts needs to ensure the surface finish (Ra above 1.6μm) to avoid quenching cracks.
(III) Precision requirement indicators
The precision of shaft parts needs to be controlled from four aspects, and the specific indicators vary greatly depending on the usage scenarios (e.g., drive shafts vs. guide shafts).
1. Dimensional accuracy: The diameter tolerance is generally h6 - h9 (basic shaft system), and the length tolerance is ±0.1 - ±0.5mm; the diameter tolerance of precision shafts (such as the main shaft) needs to reach h5 (±0.005mm), which should be measured by a micrometer or a length measuring instrument.
2. Geometric shape accuracy: Ovality ≤ 0.01mm (high-speed shaft), taper ≤ 0.02mm/100mm (mating shaft), straightness ≤ 0.03mm/300mm (long shaft), measured by a dial indicator.
3. Mutual positional accuracy: Radial runout (journal relative to the axis) ≤ 0.01 mm, end face perpendicularity (step end face relative to the axis) ≤ 0.02 mm/100 mm. It needs to be detected on a runout tester.
4. Surface roughness: For non-mating surfaces, Ra is 6.3 - 12.5 μm (rough turning); for mating surfaces, Ra is 1.6 - 3.2 μm (finish turning); for sealing surfaces, Ra is 0.8 μm (grinding required).
(IV) Blanks and machining allowances
1. Selection of blanks
Preferentially select profiles (hot-rolled round steel or cold-drawn round steel):
Hot-rolled material: Suitable for parts with ordinary precision (below IT10 level). It has low cost, but there is scale on the surface (which needs to be removed by rough turning), and the diameter tolerance is ±0.5~±1mm.
Cold-drawn material: Suitable for parts with relatively high precision (Grade IT8~IT9), which do not require rough turning. The surface is smooth (Ra 3.2μm), the diameter tolerance is ±0.1~±0.3mm, and it can be directly semi-finish turned.
2. Principles for determining machining allowance
The machining allowance is the difference between the blank size and the part size, and both quality and efficiency need to be taken into account.
Minimum allowance principle: On the premise of ensuring the accuracy after machining (for example, when the surface finish Ra is 1.6μm, the allowance should be 0.3 - 0.5mm), the smaller the allowance, the better. This can shorten the cutting time (for example, when the allowance is reduced from 2mm to 1mm, the rough turning efficiency can be increased by 40%). However, insufficient allowance should be avoided (which may expose the defects of the blank, such as pores and cracks).
Size correlation: For large-diameter shafts (e.g., those with a diameter of over φ100mm), a larger machining allowance (3 - 5mm) is required because the workpiece deformation (elastic deformation) caused by the cutting force is more significant. For slender shafts (with a length-to-diameter ratio
Process adaptability: For shafts that require heat treatment (such as quenching and tempering), leave a machining allowance of 1.5 - 2.5 mm after rough turning (to compensate for heat treatment deformation); for shafts that require grinding, leave a machining allowance of 0.3 - 0.5 mm after semi-finish turning (grinding allowance).
(V) Principles for selecting turning steps
A reasonable processing sequence is the key to ensuring quality and efficiency. The core principles are as follows:
1. Process concentration vs. dispersion
Process concentration: It is suitable for single-piece and small-batch, high-precision parts (such as spindles), that is, "complete the machining of multiple surfaces in one clamping" (such as rough turning → semi-finish turning → finish turning), which can reduce clamping errors (the coaxiality is improved by 50%), but requires high machine tool accuracy (the spindle runout ≤ 0.005mm).
Process dispersion: Suitable for mass production (such as bolt shafts), that is, "multiple machine tools perform division-of-labor machining" (a special machine rough-turns the outer circle → a special machine cuts grooves → a special machine chamfers). Efficiency can be improved through an assembly line, and it is suitable for low-precision parts.
2. Processing in stages (rough turning → semi-finish turning → finish turning)
It must be carried out in stages. The core reason is:
Rough turning: With a large cutting volume (ap = 3 - 5 mm, f = 0.3 - 0.5 mm/r), it generates a large cutting force (workpieces are prone to clamping deformation) and heat (causing thermal expansion). It needs to be carried out separately. After completion, the workpiece is naturally cooled (for 1 - 2 hours) to release internal stress.
Semi-finish turning: Medium cutting amount (ap = 1~2 mm, f = 0.1~0.2 mm/r), correct the form errors (such as ovality) after rough turning, and leave a machining allowance (0.3~0.5 mm) for finish turning.
Finish turning: Small cutting parameters (ap = 0.1 - 0.3 mm, f = 0.05 - 0.1 mm/r), process on high-precision machine tools (guideway straightness ≤ 0.01 mm/1000 mm) to ensure dimensional accuracy and surface finish, and it is necessary to conduct inspection immediately after finish turning (to avoid dimensional deviation caused by the cooling shrinkage of the workpiece).
3. Typical processing sequence
Stepped shaft: Start machining from the large-diameter end. Since the large-diameter section has high stiffness (small deformation), it can serve as the positioning reference for subsequent machining.
Grooving: It is carried out after semi-finishing turning and before finishing turning. Since the cutting force of grooving is large (prone to vibration), if it is processed after finishing turning, it may cause scratches on the already-finished surface. Grooving can be done after finishing turning for shafts with low precision (the depth is easy to control).
Thread machining: It is carried out after semi-finishing turning. At this time, the shaft diameter is already close to the final size (leaving a margin of 0.1 mm). The thread cutting force is small, and the deformation is controllable. For slender shafts (length-to-diameter ratio
(VI) Introduction to knurling process
Shaft parts often need to be knurled (such as handle shafts), with the purpose of increasing friction (facilitating manual operation). Before knurling, the knurling part needs to be rough-turned to the size (the diameter is 0.1mm larger than the drawing to compensate for the rolling deformation). Select a straight or diamond knurling tool (module 0.5 - 1.5mm), the cutting speed ≤ 10m/min, the feed rate 0.2 - 0.3mm/r. Cutting fluid (for cooling + lubrication) needs to be added during rolling to ensure that the patterns are clear and burr-free.