What is the Turning Process? Comprehensive Analysis

In the world of machining, the Turning Process is like a “universal craftsman” – from watch gears to aero engine shafts, almost all rotationally symmetrical parts are machined without it. Whether it’s traditional manufacturing or high-end precision manufacturing, the stability, precision, and efficiency of the turning process directly determine the core performance of the product. This article will take you from basic concepts to practical skills, from equipment selection to quality control, to fully grasp this key technology and solve more than 90% of the problems you may encounter in practical applications.

1. Basic concept of turning process: understand the core logic of “rotary cutting”

1.1 Turning Process Definition: What exactly is turning?

In simple terms, the turning process is defined as a machining method that removes excess material from the surface of the workpiece by coordinating the rotational motion of the workpiece with the linear/curved feed motion of the tool, and obtaining a machining method that meets the dimensional accuracy, shape requirements, and surface quality. Its core feature is “workpiece rotation”, which is also the most essential difference from milling (tool rotation).

1.2 Principle of rotary machining: the mechanical logic behind it

The principle of rotary machining in turning can be broken down into three key movements:

• Main motion: the rotational movement of the workpiece (providing cutting power and determining the cutting speed);
• Feed movement: movement of the tool along the axial / radial direction of the workpiece (control of machining allowance and surface roughness);
• Auxiliary movements: such as the rapid advance and retreat of the tool, the clamping and release of the workpiece, etc. (improve the processing efficiency).

From a mechanical point of view, the cutting edge of the tool needs to overcome the elastic deformation, plastic deformation and fracture resistance of the material during turning, forming chips and detaching from the workpiece. The cutting resistance of different materials (such as steel, aluminum, cast iron) varies significantly, which is also the core basis for the selection of subsequent process parameters.

1.3 Workpiece and tool movement: How to work together for precise machining?

The coordination of workpiece and tool movement directly affects the machining quality. Take ordinary cylindrical turning as an example:

• The workpiece is fixed on the lathe spindle by a chuck, which is driven by a motor to rotate, and the speed can be adjusted by a gearbox or frequency conversion system;
• The tool is mounted on the tool holder, driven by the feed box to move along the bed guide, and the feed volume can be accurately controlled (e.g. 0.1mm/r means that the tool moves 0.1mm for each rotation of the workpiece);
• When processing conical or curved surfaces, it is necessary to control the compound movement of transverse and longitudinal feeds through the handwheel, or to achieve automatic trajectory control with the help of CNC system programming.

1.4 Material Removal Mechanism: How Are Chips Formed?

There are three main types of material removal mechanisms, corresponding to different processing scenarios:

• Plastic materials (such as mild steel, copper): the material undergoes plastic flow during cutting, forming continuous ribbon-like chips, which need to be broken through chip breaking grooves or adjusting cutting parameters (such as increasing feed rate) to avoid winding the workpiece;
• Brittle materials (such as cast iron, ceramics): the material breaks directly during cutting, forming chips, and attention should be paid to protect it from chip splashing and injury;
• Difficult-to-machine materials (e.g., stainless steel, superalloys): Edges are prone to build-up during cutting, and chip formation needs to be improved by increasing cutting speed and using specialized tool materials (e.g., PCD, CBN).

1.5 Traditional Lathes vs. Modern Lathes: What Does Technology Iteration Bring?

Real case: Before 2018, a mechanical processing plant used a traditional lathe to process automobile half axles, with a single processing time of about 45 minutes and a scrap rate of about 3%; After the introduction of CNC lathes in 2019, the processing time per piece was reduced to 18 minutes, the scrap rate was reduced to 0.5%, and the dependence on senior mechanics was reduced, and ordinary workers could start their jobs after 1 month of training.

2. Types and classifications of turning processes: which one should be chosen for different scenarios?

2.1 Classification according to the processing surface: outer circle, inner circle, end face turning

• Cylindrical turning: the outer cylindrical, conical or curved surface of the machined workpiece is the most common type of turning, which is used for the outer surface processing of shafts and sleeve parts, such as motor shafts and drive shafts;
• Inner turning (boring): the inner cylindrical and conical surfaces of the machined workpiece, which are suitable for the inner hole processing of sleeve parts (such as bearing sleeves and hydraulic cylinder barrels), and their processing difficulty is higher than that of external turning, because the tool has poor rigidity and is difficult to remove chips;
• Face turning: The end face of the machined workpiece (the plane perpendicular to the axis) is often used for end face leveling, chamfering or step machining of parts, such as flange face processing.

2.2 Classification according to processing function: thread turning and special turning

• Thread turning: Through the precise movement of the tool and the workpiece, the processing of internal/external threads (such as ordinary threads, trapezoidal threads, pipe threads) is the core processing process of mechanical connection. CNC thread turning can be realized by G32, G92 and other instructions, and the accuracy can reach 6H/6g level;
• Precision turning: For the processing of high-precision parts (such as precision bearings and optical instrument parts), the dimensional tolerance can be controlled within ±0.001mm, and the surface roughness Ra≤0.8μm, which needs to be matched with high-precision lathes, diamond tools and constant temperature machining environments.
• Multi-axis turning: Using 2-axis, 3-axis or composite turning centers, it can complete multiple processes such as turning, milling, drilling, and tapping at the same time, and is suitable for the integrated processing of complex parts (such as engine crankshafts and planetary gear shafts), which can greatly reduce the number of clamping times and processing time.

2.3 Classification by control mode: the core advantages of CNC turning

CNC turning is the mainstream of modern turning processes, and its core advantages are reflected in:

• Accuracy and stability: through program control, avoid human operation errors, and have good batch processing consistency;
• Complex machining capabilities: It can realize non-circular curves (such as ellipses, parabolas), variable pitch threads, and other machining that cannot be completed by traditional lathes;
• Efficiency improvement: support automatic tool change, automatic measurement, automatic compensation, and shorten the processing time of a single piece by 30%-70%;
• Flexible production: When replacing processed parts, only need to modify the program without readjusting the tooling to adapt to the needs of small batches and multiple varieties of production.

Industry data: According to statistics from the China Machine Tool Industry Association, domestic CNC lathe production accounted for 78% of the total lathe output in 2023, and the penetration rate in automotive, aerospace and other industries has reached more than 90%, becoming the core equipment of intelligent manufacturing.

3. Equipment and tools for the turning process: choose the right “weapon” to get twice the result with half the effort

3.1 Lathe type: Different needs correspond to different equipment

• Ordinary lathe: simple structure, low price, suitable for single-piece small batch production, such as small machinery factories processing simple shaft bushing parts;
• CNC lathe: divided into horizontal CNC lathes (accounting for more than 80%) and vertical CNC lathes (suitable for large disc parts), which are the main equipment for mass production;
• Turning center: integrated milling, drilling and other functions, such as the Y-axis turning center for lateral feed machining, and the C-axis function for end milling;
• Special lathes: such as instrument lathes (processing small precision parts), pipe lathes (processing long pipe parts), crankshaft lathes (special for processing crankshafts).

3.2 Tool material: different materials correspond to different “blades”

The choice of tool material directly affects cutting efficiency and tool life, common types and application scenarios:

Practical skills: When machining stainless steel 304, if ordinary carbide tools are used, the tool life is only 20-30 minutes; When switching to carbide tools with TiAlN coating, the service life can be extended to more than 120 minutes, and the cutting speed can be increased from 80 m/min to 120 m/min.

3.3 Tool Holder and Fixture: Stable clamping is the foundation of precision

• Tool holder: used to connect tools and tool holders, to ensure rigidity and precision, commonly used types include BT tool holders, CAT tool holders, and HSK tool holders (special for high-speed machining);
• Fixtures: divided into general fixtures and special fixtures:
• General fixtures: three-jaw chuck (automatic centering, suitable for round workpieces), four-jaw chuck (manual centering, suitable for irregular workpieces), top (supporting long shaft parts);
• Special fixtures: such as clamp fixtures (processing thin-walled parts to avoid deformation), accompanying fixtures (mass production automatic loading and unloading).

Real case: When a medical device factory processes a stainless steel slender shaft with a diameter of 5mm and a length of 50mm, it is clamped with an ordinary three-jaw chuck, and the workpiece is easy to bend and deform, and the straightness error exceeds 0.1mm; Instead, the clamping method of “one end chuck + one end live top” is adopted, and with the support of the center frame, the straightness error is controlled within 0.02mm to meet the requirements of medical parts.

3.4 Cooling system: cooling, chip removal, and lubrication are indispensable

The core functions of the cooling system are:

• Cooling: Take away the heat in the cutting zone (the temperature of the cutting zone can reach 800-1000°C during turning) to avoid overheating and deformation of the workpiece and tool;
• Chip evacuation: flushing chips away from the cutting area to prevent chips from scratching the surface of the workpiece or hindering cutting;
• Lubrication: Reduce the friction between the tool and the workpiece and chips, reduce the surface roughness, and extend the tool life.

Common Cutting Fluid Types:

• Emulsion: suitable for steel, cast iron and other materials, cooling and lubrication, the most widely used;
• Cutting oil: good lubricity, suitable for low-speed and heavy-duty processing (such as thread turning), but poor cooling effect;
• Aqueous solution: excellent cooling effect, suitable for high-speed cutting of non-ferrous metals, but poor lubricity and easy to rust.

3.5 Automation equipment: towards unmanned production

The automation trend of modern turning processes is obvious, and the core automation equipment includes:

• Automatic loading and unloading device: such as robotic arm and truss manipulator, to realize automatic clamping and removal of workpieces;
• Tool Magazine and Automatic Tool Change System: The tool magazine equipped with CNC turning center can store 8-24 tools, which can be automatically changed according to the program without manual intervention;
• Online testing equipment: such as contact probes, which can measure the size of the workpiece in real time, automatically compensate for tool wear, and ensure machining accuracy;
• MES system: real-time monitoring and traceability of production planning, equipment status, and quality data.

4. Process parameters and optimization of turning process: how to balance efficiency and quality?

4.1 Core process parameters: cutting speed, feed rate, cutting depth

These three parameters, known as the “turning trifecta”, directly determine machining efficiency, tool life, and machining quality:

• Cutting speed (vc): the linear speed of the cutting edge of the tool and the contact point of the workpiece (unit: m/min), calculation formula: vc=π×D×n/1000 (D is the diameter of the workpiece, n is the spindle speed). The higher the cutting speed, the higher the machining efficiency, but the faster the tool wear;
• Feed rate (f): divided into feed per revolution (mm/r) and feed per minute (mm/min), the larger the feed rate, the worse the surface roughness, but the higher the processing efficiency;
• Depth of cutting (ap): The depth of the tool into the workpiece (unit: mm), which determines the machining allowance of each tool, the larger the cutting depth, the shorter the machining time, but the greater the cutting force, which can easily lead to workpiece deformation or tool damage.

4.2 Process parameter optimization strategy: analysis of practical cases

Optimization principle: Maximize machining efficiency while ensuring machining quality and tool life. Here are examples of optimization parameters for different materials:

Real case: An auto parts factory processes a No. 45 steel crankshaft, the original parameters are vc=120m/min, f=0.15mm/r, ap=2mm, the single piece processing time is 25 minutes, and the tool life is 80 pieces; Through optimization, the VC was increased to 180m/min, the F was adjusted to 0.18mm/r, and the AP was maintained at 2mm, reducing the machining time per piece to 16 minutes, while maintaining the tool life at 75 pieces, and the production efficiency was increased by 36%.

4.3 Balance between surface roughness and tool life

• Surface Roughness (Ra): Surface roughness for turning operations typically ranges from Ra0.8-6.3μm, with precision turning reaching Ra0.2-0.4μm. Influencing factors include: feed rate (the most important, the smaller the feed, the better the roughness), the arc radius of the tool tip (the larger the radius, the better the roughness), cutting speed (the higher the speed, the better the roughness);
• Tool life: Typically based on tool wear of 0.3mm (rear face wear). Ways to extend tool life: select appropriate tool materials and coatings, optimize cutting parameters (avoid excessive cutting speeds), improve cooling lubrication conditions, and reduce tool impact.

4.4 Improvement of processing efficiency: starting from multiple dimensions of equipment, process and management

• Equipment level: high-speed CNC lathe (spindle speed ≥3000r/min), equipped with high rigidity tool holder and high-power motor;
• Process level: adopt the processing process of “rough turning – semi-finishing turning – fine turning”, reasonably allocate the machining allowance (rough turning removes more than 80% of the margin, and finish turning ensures accuracy), and use compound tools (such as forming knives, multi-edged knives) to reduce the number of tool passes;
• Management level: Implement lean production to reduce equipment changeover time, optimize production schedules, and avoid material waiting.

5. Application fields of turning process: These industries cannot do without it

5.1 Auto parts processing: the core process of mass production

The application of the turning process in the automotive industry accounts for more than 40%, mainly processing:

• Engine parts: crankshaft, camshaft, piston pin, valve guide;
• Chassis parts: drive shaft, half shaft, steering knuckle, wheel hub;
• Transmission parts: gear shaft, spline shaft, shift shaft.

Industry data: A family car has about 150-200 turning and processing parts, and the CNC lathe production line of an automobile engine factory has a daily production capacity of more than 5,000 pieces per line, and the scrap rate is controlled within 0.3%.

5.2 Aerospace manufacturing: high precision and high reliability requirements

Aerospace parts have extremely high requirements for turning processes, and the main processing:

• Engine parts: turbine shaft, blades, receiver;
• fuselage structural parts: landing gear shaft, connecting sleeve, fasteners;
• Aerospace parts: rocket engine nozzles, satellite mounts.

Technical requirements: dimensional tolerance within ±0.005mm, surface roughness Ra0.4μm or less, need to withstand high temperature, high pressure, high load, machining materials are mostly titanium alloy, superalloy and other difficult-to-machine materials.

5.3 Medical device production: the dual standards of precision and cleanliness

The turning of medical device parts needs to meet:

• Accuracy requirements: such as the dimensional accuracy of surgical instruments (such as scalpel handles, implantable parts (such as artificial joint shafts) ±0.002mm;
• Material requirements: mostly medical stainless steel, titanium alloy, cobalt-chromium alloy, which must be biocompatible;
• Cleanliness requirements: The processing environment needs to reach 10,000 levels of cleanliness to avoid impurity pollution.

5.4 Mold manufacturing and general machining

• Mold manufacturing: Turning and processing mold cores, cavities, guide columns, guide sleeves and other parts require high precision and high surface quality to ensure the molding accuracy of the mold;
• General machining: pump valve parts (such as pump shafts, valve stems), motor parts (such as motor shafts, rotors), machine tool accessories (such as chucks, tops), etc., are the basic processing processes of the general machinery industry.

6. Quality control and testing of turning process: make every product qualified

6.1 Core quality indicators: dimensional accuracy, shape and position tolerance, surface quality

• Dimensional accuracy: including diameter tolerance, length tolerance, etc., commonly used tolerance levels are IT5-IT10, precision turning can reach IT3-IT4 levels;
• Shape and position tolerance: including roundness, cylindricity, straightness, coaxiality, etc., such as the roundness tolerance of shaft parts is usually ≤ 0.002mm, and the coaxiality tolerance is ≤0.01mm;
• Surface quality: In addition to surface roughness, it also includes surface hardness, residual stress, microstructure, etc., such as the surface hardness of hardened steel parts after turning needs to reach HRC55 or above.

6.2 Detection method: from offline detection to online measurement

• Offline inspection: use calipers, micrometers, dial indicators to measure dimensions; use roundness and cylindrical meter to measure shape and position tolerance; Use a roughness meter to measure surface roughness;
• Online measurement: Through the contact probe equipped with the CNC lathe, the workpiece size is measured in real time during the processing process, and the tool wear is automatically compensated to avoid batch scrapping;
• Non-destructive testing (NDT): For critical parts, ultrasonic testing, magnetic particle testing, penetrant testing, and other methods are used to check for internal defects (such as cracks and porosity).

6.3 Quality control system: establish whole-process control

• Incoming material inspection: check the size, material, and surface quality of raw materials to avoid unqualified raw materials flowing into the processing process;
• Process inspection: set up key process inspection points, sample and inspect the parts after rough turning, semi-fine turning, and fine turning, and find processing abnormalities in time;
• Finished product inspection: 100% dimensional inspection and sampling shape and position tolerance and surface quality inspection of the final product, and can only be put into storage after qualification;
• Traceability system: Through product numbers, process records, equipment numbers, etc., the whole process of product quality is traced.

7. Yigu Technology’s view

As the core basic process of machinery manufacturing, the turning process is developing in the direction of “high precision, high efficiency, automation, and greening”. In the future, CNC and intelligence will become the mainstream, and the integration of 5G, industrial Internet, AI and other technologies will realize real-time monitoring, intelligent diagnosis and independent optimization of turning processing. For enterprises, they should pay attention to the combination of process optimization and equipment upgrades, and reduce production costs under the premise of ensuring quality by selecting high-efficiency tools, optimizing parameters, and introducing automation systems. At the same time, pay attention to the trend of green manufacturing, use environmentally friendly cutting fluids and energy-saving equipment to reduce energy consumption and pollution in the processing process and achieve sustainable development.

8. FAQ: FAQ

1. Q: What should I do if the workpiece vibrates during turning?

Answer: The main solutions are: (1) Improve the rigidity of workholding (such as adding support and using rigid fixtures); (2) Reduce the cutting speed and feed rate, and reduce the cutting force; (3) Increase the rigidity of the tool (choose a thick tool bar and shorten the extending length of the tool); (4) Check the accuracy of the lathe spindle and troubleshoot the equipment.

2. Q: How do I choose the tip arc radius of a turning tool?

Answer: According to the machining needs, choose (1) a small radius (0.4-0.8mm) for rough turning to reduce cutting force and vibration; (2) choose a large radius (1.2-2.0mm) for fine turning to improve surface roughness; (3) choose a small radius for machining slender shafts to avoid interference between the tool and the workpiece.

3. Q: Is there a big difference in cost between CNC turning and regular turning?

Answer: Initial investment: the price of CNC lathes is 3-5 times that of ordinary lathes; But in the long run, the production efficiency of CNC lathes is 2-4 times that of ordinary lathes, the scrap rate is lower, the labor cost is lower, and it is suitable for mass production; Small batches, simple parts processing, ordinary lathes have more cost advantages.

4. Q: How can I avoid too fast tool wear when turning stainless steel?

Answer: Key measures: (1) Select cobalt-containing cemented carbide or coated tools (such as TiAlN coating); (2) Reduce the cutting speed (80-120m/min) and appropriately increase the feed rate; (3) High-pressure cooling (cooling pressure ≥10MPa) is used to ensure that the cutting fluid reaches the cutting area; (4) Avoid long-term low-speed cutting to prevent the occurrence of edges.

5. Q: What should I do if the surface roughness of the turning process does not meet the requirements?

Answer: Optimization scheme: (1) Reduce the feed rate (e.g., from 0.2mm/r to 0.1mm/r); (2) Increase the radius of the tip arc (e.g. from 0.8mm to 1.2mm); (3) Increasing the cutting speed (such as carbide tools processing steel parts, the speed is increased from 150m/min to 200m/min); (4) Check whether the tool is worn and replace the tool in time; (5) Improved cooling lubrication and reduced friction.