A must-read for beginners! Getting started with bar turning steel parts: process, precautions and core skills

Beginners who have just started machining are often confused by the complex process of bar turning: Why do you need to control the extension length of the workpiece when clamping? How to operate the tool setting on a CNC lathe? What are the requirements for choosing cutting fluid? This article takes a zero-based perspective and disassembles the entire process of bar turning steel parts in easy-to-understand language, focusing on the mistakes that novices often make (such as improper tool installation height and excessive feed rate), and extracts the twelve-character core skills of "steady clamping, accurate tool setting, and flexible parameters" to help you get started quickly.

1. Definition of bar turning steel parts

Bar turning steel parts refer to mechanical parts that are made of metal bars (such as carbon steel, alloy steel, stainless steel, etc.) as raw materials and processed into specific shapes and sizes through turning processes. Turning is the process of removing excess material and forming high-precision geometric shapes through the cutting movement of rotating workpieces and fixed tools. Such steel parts are widely used in automotive, aerospace, petrochemical and other fields, such as flanges, shaft parts, etc.

2. Process of  bar turning steel parts

(1) Preparation before processing

1) Equipment and tool inspection:

Confirm that the lathe (such as ordinary lathe, CNC lathe) is operating normally, the position of each handle is correct, and the lubrication system is in good condition.

Select suitable tools (such as external cylindrical turning tools, end turning tools, cutting tools, etc.), check whether the tool is firmly installed and whether the blade is sharp, and avoid chipping or wear that affects the processing quality.

2) Billet preparation:

Check the bar specifications (diameter, length), material (such as 45 steel, Q235, stainless steel, etc.), remove burrs and oxide scale on the billet surface, and ensure reliable clamping.

3) Fixture selection and clamping:

Three-jaw chuck: suitable for round and regular triangular bars, automatic centering, and high clamping efficiency.

Four-jaw chuck: suitable for square, eccentric or irregular bars, manual adjustment and centering are required.

Center clamping: For shaft parts with large aspect ratio, double centring is used with chicken heart chuck to ensure coaxiality.

Clamping points: moderate clamping force to avoid workpiece deformation; keep the extension length as short as possible to improve rigidity.

(2) Tool setting and coordinate system setting (applicable to CNC lathes)

Manual tool setting: test cut the outer circle and end face of the blank, measure the actual size and input it into the machine tool system, and set the workpiece coordinate system (such as G54).

Tool compensation setting: input tool radius compensation (G41/G42) and length compensation (H code) to ensure that the programmed trajectory is consistent with the actual cutting.

(3) Turning

1) Rough turning:

Purpose: quickly remove most of the machining allowance and leave 0.5~2mm fine turning allowance.

Parameter selection: higher back cutting amount (3~5mm) and feed rate (0.2~0.5mm/r), lower spindle speed (adjusted according to material hardness, such as 500~800r/min for rough turning of 45 steel).

Processing content: turning the outer circle, end face, step surface, and processing chamfers or tool back grooves when necessary.

2) Finish turning:

Purpose: To ensure dimensional accuracy (such as IT6~IT8 level) and surface roughness (Ra1.6~3.2μm).

Parameter selection: small back cutting depth (0.1~0.5mm), small feed rate (0.05~0.2mm/r), high spindle speed (800~1500r/min).

Processing content: Finish turning the outer circle and end face to the drawing size, processing precision chamfers, grooves or threads, etc.

3) Cutting or grooving (if necessary):

Use a cutting tool to cut from the end of the workpiece, pay attention to the moderate feed rate to avoid tool breakage; multiple feeds can be used when the cutting depth is large.

(4) Process detection and adjustment

Use calipers, micrometers, dial indicators and other tools to measure the dimensions during processing. If deviations are found, adjust the tool position or modify the program in time.

Observe the cutting status: chip color (e.g. normal steel chips are silvery white, turn blue when overheated), shape (ribbon chips need chip breaking treatment), judge tool wear or whether parameters are reasonable.

(5) Post-processing

Deburring: Use files and emery cloth to remove burrs on the edge of the workpiece to ensure safety and assembly accuracy.

Cleaning and rust prevention: Clean the cutting fluid and chips on the surface of the workpiece, and apply anti-rust oil (if long-term storage is required).

Final inspection: According to the requirements of the drawings, the dimensions, geometric tolerances (such as cylindricity, coaxiality) and surface quality are fully inspected. Unqualified products are reworked or scrapped.

Equipment maintenance: Clean the chips on the lathe guide rails and tool holders, add lubricating oil, and turn off the power.

3. Precautions for machining of bar turning steel parts

(1) Safe operation

Wear protective equipment: Wear goggles, non-slip gloves, and work clothes. It is forbidden to operate rotating parts with gloves.

Avoid dangerous behaviors: It is forbidden to clean chips directly with your hands during processing, and it is forbidden to measure or adjust the workpiece when the machine tool is running.

Emergency handling: If you hear abnormal noise, vibration or smoke, press the emergency stop button immediately to cut off the power supply.

(2) Clamping and tool safety

After clamping, make sure that the workpiece and tool are firm. Before starting the machine tool, run it at low speed to check if there is any jumping or looseness.

The tool installation height must be the same as the center of the lathe spindle, otherwise it will cause the tool tip to break or the processing size to deviate.

(3) Cutting parameter optimization

Adjust the parameters according to the hardness of the material: hard materials (such as alloy steel) need to reduce the speed and back cutting amount; soft materials (such as low carbon steel) can appropriately improve efficiency.

Avoid excessive tool wear: Check the blade regularly during continuous processing, and replace it in time if wear is found (such as back tool wear > 0.3mm).

(4) Cooling and chip removal

When turning steel parts with bars, use cutting fluid (such as emulsion, cutting oil) to reduce cutting temperature, extend tool life, and improve surface quality.

Clean up chips in time to avoid wrapping around the tool or guide rail, which will affect processing accuracy and equipment operation.

4. Core points for machining of bar turning steel parts

(1) Clamping and positioning accuracy

Ensure that the axis of the workpiece is coaxial with the axis of the lathe spindle. Long-axis parts use top support to reduce the overhang length, improve rigidity, and avoid the phenomenon of "cutting off" (dimensional deviation caused by deformation of the workpiece due to force).

(2) Tool selection and wear control

Tool material: High-speed steel tools are suitable for low-speed finishing; carbide tools are suitable for high-speed roughing or hard materials.

Tool angle: The main rake angle of the external turning tool (such as 90°, 75°) affects the radial cutting force, and the secondary rake angle affects the surface roughness; the rake angle and back angle need to be adjusted according to the plasticity of the material (plastic materials have a large rake angle and brittle materials have a small rake angle).

Wear monitoring: The cutting sound (abnormal sharp sound) and the deterioration of surface roughness can be used to determine whether the tool needs to be replaced.

(3) Dimensional accuracy control

Tool setting accuracy: The manual tool setting error must be controlled within ±0.02mm. CNC machining can use an automatic tool setting instrument to improve efficiency.

Machining allowance allocation: leave uniform allowance for rough turning, and form in one step for fine turning to avoid cumulative errors from multiple passes.

Thermal deformation control: When machining long shaft parts or high-precision machining, it is necessary to consider the expansion of the workpiece caused by cutting heat. The impact can be reduced by cooling or staged machining (such as air cooling after rough turning and then fine turning).

(4) Balance between machining efficiency and surface quality

Rough turning prioritizes efficiency and adopts "high back cutting depth + high feed rate + medium and low speed"; fine turning prioritizes quality and adopts "low back cutting depth + low feed rate + high speed".

Chip breaking control: By changing the shape of the tool chip breaker groove, feed rate or using a chip breaker, the strip chips are converted into C-shaped or fragments to avoid the risk of winding.

(5) First piece inspection and process monitoring

After machining the first piece, conduct a comprehensive inspection to confirm that the program, tool and parameters are correct before mass production.

Regular spot checks are carried out during mass processing to prevent dimensional deviations due to tool wear or machine vibration.

5. Summary

Bar turning steel parts is one of the most basic processes in mechanical processing. The core lies in balancing efficiency and precision through reasonable clamping, tool selection and parameter setting. The key process must strictly follow the logic of "preparation → clamping → tool setting → roughing and finishing → inspection", while paying attention to safe operation and equipment maintenance, in order to stably produce qualified parts.