1. External Cylindrical Grinding
The processing of external cylindrical surfaces of rotating parts such as guide pillars and bushings is completed on external cylindrical grinding machines using grinding wheels. The processing method involves grinding the workpiece with a high-speed rotating grinding wheel while the workpiece rotates at a low speed and makes longitudinal reciprocating movements relative to the grinding wheel. After external cylindrical grinding, the dimensional accuracy can reach IT5~IT6, and the surface roughness Ra is 0.8~0.2μm. If a high-finish grinding process is adopted, the surface roughness Ra can reach 0.025μm. This level of precision is often required for components that interface with injection molded parts in high-precision assemblies.
External Grinding Operations
The process involves precise coordination between grinding wheel speed, workpiece rotation, and longitudinal feed to achieve desired surface finishes, even on components that will eventually be mated with injection molded parts in complex assemblies.
Finished Cylindrical Components
Ground components exhibit exceptional roundness and surface quality, making them ideal for applications requiring tight tolerances, including those that interface with precision injection molded parts.
1.1 External Grinding Process Parameters
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Grinding Wheel Circumferential Speed
When using a ceramic bond grinding wheel, its circumferential speed is generally less than 35m/s. When using a resin bond grinding wheel, its circumferential speed is generally less than 50m/s. This parameter is crucial for achieving consistent results, whether working with raw stock or finishing components that will be assembled with injection molded parts.
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Workpiece Circumferential Speed
The circumferential speed of the workpiece is generally 13~20m/min. When grinding hardened steel, the circumferential speed is generally 20-26m/min. When the workpiece has a large length-to-diameter ratio and poor rigidity, the workpiece rotation speed should be reduced. This is particularly important for maintaining dimensional stability in parts that will be paired with injection molded parts in high-precision applications.
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Grinding Depth
For rough grinding, the grinding depth is generally 0.02~0.05mm, and for finish grinding, it is generally 0.005~0.015mm. When the workpiece surface roughness requirement is low and the precision requirement is high, several non-feeding light grinding passes are required after finish grinding. This level of precision ensures proper mating with injection molded parts that have been designed with tight tolerances.
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Longitudinal Feed Rate
For rough grinding, each feed rate is 0.5~0.8 times the width of the grinding wheel. For finish grinding, each feed rate is 0.2~0.3 times the width of the grinding wheel. This parameter must be carefully controlled to ensure the final surface finish will properly interface with any mating injection molded parts in the assembly.
1.2 Workpiece Clamping
Long Workpieces
Workpieces with a large length-to-diameter ratio are generally clamped using front and rear centers. For hardened steel workpieces, the center holes must be accurately ground, and carbide centers and appropriate顶紧力 should be used.
Proper clamping ensures that the geometric accuracy is maintained throughout the grinding process, which is essential when these components will later be assembled with injection molded parts requiring precise fits.
Short Workpieces
Workpieces with a small length-to-diameter ratio are generally clamped using self-centering or independent chucks. Workpieces clamped with chucks usually use process clamps to grind various stepped outer circles in one clamping, which is particularly useful for parts that will mate with injection molded parts in assembly.
Longer Workpieces
Longer workpieces generally use a combination of chuck and center for clamping. Slender small-diameter shaft workpieces with large length-to-diameter ratios generally use a double center clamping method. This method provides the stability needed for achieving the precision required when these components will be paired with injection molded parts in high-precision applications.
Sleeve-type Workpieces
Sleeve-type workpieces requiring coaxiality between inner and outer circles are generally clamped using a mandrel. The mandrel positioning surface is generally matched and ground according to the workpiece hole diameter with a taper of 1/7000 to 1/5000. This ensures the necessary precision for proper assembly with injection molded parts that have tight tolerance requirements.
1.3 Center Holes
Before turning and grinding the outer cylindrical surface, the center holes must be processed to provide a reliable positioning reference for subsequent processes. If the center hole has a large coaxiality error, it will prevent good contact between the center hole and the center, affecting processing accuracy.
In particular, when the center hole has a roundness error, it will be directly reflected on the workpiece, causing the workpiece to also produce roundness error. This is especially critical for parts that will mate with injection molded parts requiring precise rotational alignment.
The ground parts need to have their center holes corrected after heat treatment to eliminate possible deformation and other defects in the center holes during heat treatment, enabling accurate positioning during outer cylindrical grinding to ensure the shape accuracy requirements of the outer cylindrical surface.
Center holes can be corrected using grinding, lapping, and extrusion methods, which can be performed on lathes, drilling machines, or special machines. For injection molded parts that require metal inserts with precise center holes, this correction process is essential to ensure proper assembly and functionality.
Precision center hole preparation ensures accurate positioning during grinding operations, which is vital for components that will interface with injection molded parts.
For center holes with low precision requirements, multi-edge centers are usually used for correction. Figure 3-6 shows a carbide multi-edge center for extruding center holes. During extrusion, the multi-edge center is installed in the taper hole of the lathe spindle, and its operation is similar to that of a center hole. The workpiece is pressed against the multi-edge center using the tail center of the lathe, and the geometric error of the center hole is corrected through the extrusion action of the multi-edge center. This method has extremely high productivity (only a few seconds) but slightly lower quality, and is generally used for correcting center holes with low precision requirements, such as those in less critical components that may interface with injection molded parts in non-precision applications.
2. External Cylindrical Lapping
When the surface roughness and dimensional accuracy requirements for external cylindrical surfaces are high, lapping processing of the external cylindrical surface is required. In mass production, lapping is generally performed on special lapping machines. For single-piece or small-batch production, lapping tools can be used for manual lapping. Lapping accuracy can reach IT3~IT5, and surface roughness Ra can reach 0.1~0.008μm. This level of surface finish is often required for components that must form a precise seal or bearing surface with injection molded parts.
2.1 Lapping Mechanism
Lapping is a precision machining method that uses lapping tools and free abrasives for micro-machining of the processed surface. Free abrasives and lubricants are placed between the processed surface and the lapping tool to generate relative motion between the processed surface and the lapping tool and apply a certain pressure. The abrasives produce cutting, extrusion, and other effects, thereby removing the凸起 on the workpiece surface, improving the accuracy of the processed surface, and reducing surface roughness. This process is particularly effective for preparing metal components that will be assembled with injection molded parts requiring exceptional surface quality.
Complex physical and chemical changes occur on the processed surface during lapping. The main functions of lapping are as follows:
Micro-cutting Action
When the lapping tool and the processed surface move relative to each other, the abrasive grains cut the processed surface slightly under pressure, as shown in Figure 3-8. The micro-cutting method varies under different processing conditions. When the lapping tool has low hardness and the lapping pressure is high, the abrasive grains can be embedded into the lapping tool to produce a scraping effect, which has higher lapping efficiency. When the lapping tool has high hardness, the abrasive grains roll between the lapping tool and the processed surface for micro-cutting. This action creates the ultra-smooth surfaces needed for optimal performance when mated with high-precision injection molded parts.
Extrusion Plastic Deformation
Blunt abrasive grains extrude the rough peaks on the processed surface under lapping pressure, causing micro-extrusion plastic deformation on the processed surface, making the peaks on the part surface tend to be flat and smooth. This process is essential for creating the proper surface finish on components that will form sliding interfaces with injection molded parts, reducing wear and improving longevity.
Chemical Action
When using abrasives such as chromium oxide and stearic acid, the abrasive reacts chemically with the processed surface to form a very thin oxide film, which is easily worn away without damaging the material matrix. During lapping, the oxide film is continuously and rapidly formed and quickly worn away, improving lapping efficiency. This chemical action helps achieve the mirror-like finishes required for certain high-precision applications involving injection molded parts.
Figure 3-8: Micro-cutting action of abrasive particles during lapping, which produces the ultra-fine surface finishes required for components that must interface precisely with injection molded parts.
2.2 Lapping and Polishing Process
2.2.1 Lapping and Polishing Allowance
Excessive lapping and polishing allowance will prolong processing time, increase tool and material consumption, and increase processing costs. Insufficient allowance will fail to achieve the required surface roughness and accuracy after processing. In principle, the lapping and polishing allowance only needs to be able to remove surface processing marks and metamorphic layers.
When the dimensional tolerance of the part is large, the allowance can be taken within the dimensional tolerance of the part. The lapping allowance for hardened external cylindrical surfaces is shown in Table 3-2. This careful control of material removal ensures that the final dimensions match precisely with any mating injection molded parts in the assembly.
| Part Diameter (mm) | Lapping Allowance (mm) | Typical Applications |
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| Up to 10 | 0.005 - 0.01 | Small precision shafts, often mated with injection molded parts in micro-mechanisms |
| 10 - 50 | 0.01 - 0.02 | Medium-sized components, including those used with injection molded parts in automotive applications |
| 50 - 100 | 0.02 - 0.03 | Larger shafts and cylinders, sometimes requiring配合 with injection molded parts in industrial equipment |
| Over 100 | 0.03 - 0.05 | Large precision components, occasionally paired with injection molded parts in heavy machinery |
Table 3-2: Lapping allowances for hardened external cylindrical surfaces, showing appropriate material removal for different component sizes, including those that will be assembled with various injection molded parts.
The lapping process requires careful control of pressure, speed, and abrasive particle size to achieve the desired results. Operators must monitor the process to ensure that the correct amount of material is removed, leaving a surface that meets both dimensional and finish requirements. This level of control is especially important for components that will interface with injection molded parts, as any deviation from specifications can result in poor fit, increased wear, or premature failure.
For manual lapping operations, skilled technicians use specialized tools and their experience to achieve the required precision. The process involves moving the lapping tool in a figure-eight pattern to ensure uniform material removal across the entire surface. This technique is particularly useful for small batch production or repair work where specialized machinery is not available, and is often employed for creating precision fits between metal components and injection molded parts in custom assemblies.
3. Internal Grinding
Internal cylindrical surfaces of mold parts requiring high precision are generally finish-machined using internal grinders. Internal grinding can be performed on internal grinders or universal external cylindrical grinders. The dimensional accuracy of hole grinding on internal grinders can reach IT6~IT7, and the surface roughness Ra is 0.8~0.2μm. If high-precision grinding technology is adopted, the dimensional accuracy can be controlled within 0.005mm, and the surface roughness Ra is 0.1~0.025μm. These precise internal surfaces are often designed to accept precision shafts or form tight fits with injection molded parts in high-performance assemblies.
Precision Internal Grinding
Internal grinding requires specialized techniques to achieve the same level of precision as external grinding, due to the challenges of accessing the internal surface and maintaining tool stability.
This process is critical for creating bearing surfaces, hydraulic cylinders, and other precision bores that must function reliably with mating components, including injection molded parts designed for precise fits.
3.1 Grinding Wheel Selection
The grinding wheel diameter is generally 0.5~0.9 times the workpiece hole diameter. A larger ratio is used for small workpiece hole diameters, and a smaller ratio is used for larger ones. The grinding wheel width is generally 0.8 times the hole depth. For grinding non-hardened steel, brown corundum ZR~Z (ZR and Z respectively indicate that the grinding wheel hardness is medium-soft and medium, and the subscript 2 represents the细分等级 under this hardness level) is generally selected, with 46#~60# grinding wheels. For grinding hardened steel, brown corundum, white corundum, and single crystal corundum ZR₁~ZR₂ are generally selected, with 46#~80# grinding wheels. The proper wheel selection ensures optimal material removal and surface finish, which is essential when these internal surfaces will interface with injection molded parts or other precision components.
3.2 Grinding Parameter Selection
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Grinding wheel circumferential speed
Generally 20~25m/s. This speed is carefully chosen to balance material removal rate with surface finish quality, which is particularly important for internal surfaces that will mate with injection molded parts requiring precise fits.
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Workpiece circumferential speed
Generally 20~25m/min. When higher surface quality is required for the workpiece, a lower workpiece circumferential speed is generally used. This is especially relevant for internal surfaces that will form seals or bearing surfaces with injection molded parts.
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Grinding depth
Refers to the depth that the grinding wheel radially cuts into the workpiece during one reciprocation of the worktable. For rough grinding of hardened steel, it is generally 0.005~0.2mm. For finish grinding of hardened steel, it is generally 0.002~0.01mm. This precise control of material removal ensures that the final dimensions match the design specifications, which is crucial when these internal surfaces will interface with injection molded parts or other precision components.
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Longitudinal feed rate
For rough grinding, it is generally 1.5~2.5m/min. For finish grinding, it is 0.5~1.5m/min. The feed rate directly affects both productivity and surface quality, and must be optimized for the specific application, especially when the internal surface will be mated with injection molded parts requiring precise dimensional control.
3.3 Workpiece Clamping
Proper clamping is essential for achieving accurate internal grinding results, as any movement or vibration during the process can compromise dimensional accuracy and surface finish. The clamping method must be selected based on the workpiece geometry and material, as well as the specific grinding requirements.
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For short workpieces like guide bushes, self-centering chucks are generally used for clamping.
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For grinding type holes on small rectangular templates, independent chucks are generally used to clamp the template.
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For grinding type holes, guide pillar holes, and guide bush holes on large templates, workpieces are generally clamped on flange plates using pressure plates.
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For grinding longer shaft holes, chucks and steady rests are generally used to clamp the workpiece.
Each clamping method is chosen to provide the necessary stability and alignment for the specific workpiece, ensuring that the final ground surface meets the required specifications for assembly with other components, including injection molded parts that may have tight tolerance requirements.
Clamping Considerations for Internal Grinding
Workpiece Rigidity
Thin-walled parts require special care to prevent deformation during clamping and grinding, which is especially important when these parts will interface with rigid injection molded parts.
Centering Accuracy
Precise centering ensures that the ground hole is concentric with other features, critical for proper assembly with shafts or injection molded parts.
Thermal Expansion
Clamping forces must account for potential thermal expansion during grinding to maintain dimensional accuracy, particularly important for components that will operate in temperature-varying environments with injection molded parts.
Accessibility
Clamping fixtures must allow proper access for the grinding wheel while maintaining stability, ensuring that complex internal geometries can be accurately ground for proper mating with injection molded parts.
Different clamping configurations for internal grinding applications, each designed to provide optimal stability for specific workpiece geometries, ensuring precise results that meet the requirements for assembly with injection molded parts and other components.
4. Internal Honing
To further improve the surface quality of holes, a honing process can be added. For single-piece and small-batch production, simple honing tools can be used, and honing can be performed on ordinary lathes. As shown in Figure 3-10, during honing, the guide bush is套在 the honing head and the workpiece is held by hand to make reciprocating movements in the axial direction. The lathe spindle drives the honing head to rotate, and the guide bush is held by hand to make reciprocating linear movements on the lapping tool. This process is often used to prepare precision bores that will mate with shafts or form precise fits with injection molded parts in high-performance assemblies.
Figure 3-10: Internal honing setup showing manual operation on a lathe, a common method for finishing precision bores that will interface with injection molded parts.
Figure 3-11: Structure of a honing head: 1-workpiece; 2-abrasive sticks; 3-honing frame; 4-adjusting shaft. This precision tooling creates the superior surface finishes required for optimal performance with injection molded parts.
The structure of the honing head is shown in Figure 3-11. The honing head is composed of several abrasive sticks mounted on a honing frame. The honing head equipped with several abrasive sticks is inserted into the processed hole, and the abrasive sticks are brought into contact with the hole wall with a certain pressure for honing. By adjusting the adjusting shaft on the honing head, the diameter of the honing head can be adjusted to control the amount of honing. This adjustability allows for precise control over the final dimensions, ensuring proper fits with mating components including injection molded parts.
Honing Process Characteristics
Typical Honing Allowance
Generally, the honing allowance is 0.015~0.02mm. This small material removal rate allows for precise control over the final dimensions while achieving the desired surface finish, which is ideal for preparing surfaces that will mate with injection molded parts requiring tight tolerances.
Dimensional Accuracy
Honing can achieve dimensional accuracy grades of IT4~IT5, making it suitable for the most demanding precision applications, including those involving critical fits with injection molded parts.
Surface Roughness
After honing, surface roughness Ra is 0.1~0.25μm. This exceptional surface finish reduces friction and wear in moving parts, which is particularly beneficial for components that will interface with injection molded parts in dynamic applications.
Form Accuracy
Roundness and cylindricity of 0.003-0.005mm can be achieved, but honing cannot improve the positional accuracy of holes. This makes honing ideal for refining existing features to precise dimensions for assembly with injection molded parts that have been manufactured to matching specifications.
Honing differs from grinding in that it uses a set of abrasive sticks that conform to the existing hole geometry, correcting any minor irregularities while improving surface finish. The combination of rotational and reciprocating motion creates a characteristic cross-hatch pattern on the surface, which is ideal for retaining lubrication in bearing applications. This surface texture is particularly beneficial when the honed surface will interact with injection molded parts, as it can help reduce friction and improve the longevity of the assembly.
In mass production, specialized honing machines are used to achieve consistent results across large quantities of parts. These machines can be automated for high-volume production while maintaining the precision required for critical applications. Automated honing systems are often used to process components that will be assembled with injection molded parts in automotive, aerospace, and medical device applications, where consistent quality and performance are essential.
The selection of abrasive material and grain size for honing depends on the workpiece material and the desired surface finish. For most steel components, silicon carbide or aluminum oxide abrasives are used, with finer grit sizes producing smoother surfaces. The honing process parameters, including pressure, speed, and stroke length, must be carefully controlled to achieve the desired results. This level of control ensures that the honed surfaces will meet the precise requirements for assembly with injection molded parts and other components in high-performance systems.