What are the non-clamping factors affecting the bending deformation of slender carbon steel seamless pipe shafts during turning

Slender carbon steel seamless pipe shafts are prone to bending deformation after turning due to their large length-to-diameter ratio (usually L/D > 20) and poor rigidity. Besides improper clamping methods, the root causes of deformation can be traced back to four core dimensions: release of internal material stress, uneven distribution of cutting load, imbalance of process parameters, and insufficient rigidity of the tooling system. Specific influencing factors and mechanisms are as follows:

First, the inherent material properties and pretreatment defects of carbon steel seamless pipes.

The mechanical properties and internal state of the carbon steel seamless pipe material itself are the fundamental causes of deformation, mainly reflected in the following three aspects:

1. Residual internal stress in the billet of carbon steel seamless pipes.

During the rolling and cold drawing process, the metal lattice undergoes plastic deformation, forming an internal stress gradient (such as surface compressive stress and core tensile stress). After the surface metal is removed during turning, the internal stress balance is broken, and the shaft parts will re-establish stress balance through bending deformation.

2. Inhomogeneous Material Composition and Metallographic Structure of Carbon Steel Seamless Pipes.

If carbon steel seamless pipes exhibit compositional segregation (e.g., uneven carbon element distribution) or metallographic structure differences (e.g., local imbalance in the ratio of pearlite to ferrite), regional fluctuations in material hardness and elastic modulus will occur. During turning, the cutting resistance of the tool to different areas can vary by 15%-20%, making slender shafts prone to bending under unbalanced loads.

3. Defects in the Heat Treatment Process of Carbon Steel Seamless Pipes.

Improper heat treatment during the pretreatment stage of carbon steel seamless pipes will directly exacerbate the risk of deformation:

(1) Incomplete annealing of carbon steel seamless pipes: If the stress-relief annealing temperature is lower than Ac1 or the holding time is insufficient, the internal stress relief rate will be lower than 60%, and residual stress will be slowly released after turning, leading to deformation.

(2) Inherited quenching deformation of carbon steel seamless pipes: If the carbon steel seamless pipe has undergone quenching treatment in the early stage without correction, the internal structural stress will be activated during turning, causing "secondary bending" of the slender shaft.

Second, the coupling effect of cutting force and cutting heat in seamless carbon steel pipes.

The poor rigidity of slender shafts means that the coupling effect of cutting force fluctuations and cutting heat accumulation during cutting is the direct cause of bending deformation. The influence of uneven cutting force distribution: During turning, the cutting force can be decomposed into the main cutting force (Fz), radial cutting force (Fy), and axial cutting force (Fx). Among these, the radial cutting force (Fy) has the greatest impact on the bending of slender shafts, because its direction is perpendicular to the shaft's axis, which easily causes lateral deflection.

Specific contributing factors include:

1. Inappropriate tool geometry: For example, a small principal cutting edge angle (<60°) increases radial cutting force. When the principal cutting edge angle increases from 45° to 90°, the radial cutting force can be reduced by 40%-50%.

2. Fluctuations in cutting parameters: Uneven feed rates or uneven depth of cut can lead to a difference in peak cutting force exceeding 20%, making slender shafts prone to plastic deformation under alternating loads.

3. Tool wear: When the flank wear exceeds 0.3mm, the cutting force increases sharply, and uneven wear can lead to radial force imbalance, causing shaft bending.

Thermal deformation caused by cutting heat: During the turning of seamless carbon steel pipes, approximately 70%-80% of the cutting heat is transferred to the workpiece. The linear expansion coefficient of carbon steel is approximately 11.5×10⁻⁶/℃. When the workpiece temperature rises by 30-50℃, the axial elongation can reach 0.3-0.5mm. If heat dissipation is poor, heat will accumulate locally on the shaft, leading to uneven temperature gradients: the surface temperature is higher than the core temperature, the surface metal's thermal expansion is hindered, generating compressive stress, while the core generates tensile stress, ultimately causing axial bending; during intermittent cutting, the workpiece temperature repeatedly rises and falls, and the alternating thermal stress exacerbates the accumulation of plastic deformation.

Third, improper design of process parameters and machining paths for carbon steel seamless pipes.

The rationality of the process plan directly determines the deformation control effect. Common problems include:

1. Imbalance in the matching of cutting parameters for carbon steel seamless pipes. Turning slender shafts requires adherence to the principle of "low cutting force and low heat input." Improper parameter selection can amplify deformation risks:

(1) Excessive cutting speed: For example, when turning 45# steel at a speed exceeding 120 m/min, the cutting temperature will rise sharply, and the thermal deformation will increase by more than 60% compared to 80-100 m/min;

(2) Excessive depth of cut: When the depth of cut exceeds 5 mm in a single pass, the radial cutting force will exceed the yield limit of the slender shaft, leading to permanent bending; Inappropriate feed rate: Too small a feed rate (<0.1 mm/r) will increase the friction between the tool and the workpiece, resulting in "tool deflection." Too large a feed rate will increase the cutting force. It should be controlled within a reasonable range of 0.15-0.3 mm/r.

2. Errors in the machining path and allowance allocation of seamless carbon steel pipes.

(1) Failure to separate roughing and finishing: If finishing is performed directly after roughing, the internal stress and thermal deformation generated during roughing will be transferred to the finishing process, resulting in bending of the final part;

(2) Uneven allowance allocation: If the allowance difference on one side exceeds 2mm, the cutting force will be unbalanced in the circumferential direction. During machining, slender shafts will shift towards the side with the smaller allowance, resulting in bending.

(3) Inappropriate tool feed direction: When climb milling, the cutting force direction is consistent with the table feed direction, which easily causes "tool pulling," leading to increased radial deflection of the shaft. While conventional milling provides a stable cutting force, the surface quality is poor, requiring selection based on material hardness (e.g., climb milling for soft steel, conventional milling for hard steel).

Fourth, Defects in Tooling and Auxiliary Support Systems for Seamless Carbon Steel Pipes.

Besides the clamping method, insufficient rigidity of the tooling system or failure of auxiliary supports can also exacerbate deformation:

1. Insufficient Rigidity of the Tool System: Too small a tool shank diameter, poor tool holder rigidity, or excessive clearance between the tool holder and the spindle can cause "chatter" under cutting forces. If the chatter frequency couples with the natural frequency of the slender shaft, it can trigger resonance, amplifying the shaft's bending deformation by 3-5 times.

2. Inappropriate Auxiliary Support Design: Turning slender shafts typically requires the use of a follow post or center rest, but improper support can exacerbate deformation: If the follow post support block is made of excessively hard material or lacks lubrication, it can cause sliding friction with the workpiece surface, leading to localized overheating and wear, resulting in shaft bending; improper adjustment of the center rest support point can generate additional radial clamping force on the workpiece, causing plastic deformation of the shaft; misalignment between the auxiliary support and the spindle can generate additional bending moments during machining, leading to tapered bending of the slender shaft.