Processing technology of bracket sleeve
Bracket sleeves are key components in mechanical structures that provide support, guidance, or connection. They are widely used in machine tool guideways, automotive chassis, and construction machinery. Their machining requires guaranteed inner and outer bore coaxiality (≤0.02mm), end face perpendicularity to the axis (≤0.01mm/100mm), and inner bore dimensional accuracy (typically H7-H8). Bracket sleeves are typically made of gray cast iron (HT200), ductile iron (QT450), or 45 steel. Some high-strength applications utilize 20CrMnTi carburized and quenched steel (hardness 58-62HRC). The key to machining technology lies in controlling deformation and cumulative error through a rational process arrangement. For example, a machine tool bracket sleeve requires an inner bore of φ80H7, an outer diameter of φ120H6, and coaxiality of 0.015mm. This is achieved through a “rough turning – semi-finishing turning – heat treatment – finish turning – grinding” process. Therefore, developing a scientific bracket sleeve machining process is crucial to ensuring performance.
Blank selection and pretreatment are fundamental to ensuring machining quality. The blank type should be selected based on the production batch size: open-die forgings or round steel (45 steel) for single-piece, small-batch production; centrifugal casting (cast iron) or die forgings (alloy steel) for mass production. Blank dimensional allowances should be uniform (3-5mm per side) to avoid deformation caused by uneven allowances during machining. Cast iron sleeves require aging (natural aging for 2-3 months or artificial aging at 550-600°C for 4 hours) to eliminate casting stresses and reduce internal stress to ≤50 MPa, preventing post-machining deformation (≤0.03mm). Steel sleeves require quenching and tempering (220-250 HB) after rough turning to increase material hardness and cutting performance, reducing tool wear during finish turning. Blanks should be visually inspected for defects such as cracks and shrinkage cavities. Important sleeves require magnetic particle inspection or ultrasonic testing to ensure internal quality. After artificial aging of the QT450 bracket sleeve, a certain engineering machinery factory reduced the processing deformation from 0.1mm to 0.02mm, and the dimensional stability was significantly improved.
Rough machining requires rapid stock removal and lays the foundation for semi-finishing. When rough turning the external diameter and end faces, a three-jaw chuck is used. The end face is first turned to ensure flatness (flatness ≤ 0.1mm), followed by drilling a center hole as a reference for subsequent machining. Drilling (roughing internal holes) uses a high-speed steel drill (diameter 5-10mm smaller than the drawing), with a feed rate of 0.2-0.3mm/r and a cutting speed of 20-30m/min. After drilling to depth, a reamer is used to expand the hole to a 1-2mm allowance for the final dimension. When rough turning the external diameter, a cutting speed of 80-100m/min, a feed rate of 0.3-0.5mm/r, a back cut of 2-3mm, and a 1-2mm allowance for semi-finishing. For bracket sleeves with flanges, the flange end faces and external diameter must be rough turned first to ensure a uniform flange thickness allowance (±0.5mm). After rough machining, burrs must be removed, chips cleaned, and the roundness of the inner and outer holes (≤0.1mm) must be checked to avoid impacting subsequent processes. By optimizing rough machining parameters, a machine tool manufacturer increased the machining efficiency of bracket sleeves by 30% while ensuring stock uniformity.
Semi-finishing and heat treatment processes require precision control and performance improvement. Semi-finish turning is performed using the inner bore as the reference (using a tire clamping system). The outer diameter is turned to a surface roughness of Ra ≤ 3.2μm, with an outer diameter cylindricity ≤ 0.03mm. Semi-finish turning of the inner bore is performed with a boring tool at a cutting speed of 100-120m/min and a feed rate of 0.15-0.2mm/r. A finishing grinding allowance of 0.3-0.5mm is left, and the inner bore roundness is ≤ 0.02mm. For steel sleeves requiring quenching, semi-finish turning is followed by carburizing or induction hardening: the carburized layer depth is 1-1.5mm (20CrMnTi), and the post-quenching hardness is 58-62HRC. After quenching, low-temperature tempering (180-200°C for 2 hours) is required to eliminate quenching stresses. Deformation after heat treatment must be controlled within 0.05mm. If it exceeds this limit, straightening is required (pressure straightening, deformation ≤ 0.03mm). An automotive parts factory induction hardened a 20CrMnTi bracket sleeve, controlling the inner hole deformation to 0.03mm, allowing for finishing without secondary straightening.
