1. Basic Introduction to External Cylindrical Grinding Machines

　　Grinding machines are the most diverse category among various metal cutting machine tools, with main types including external cylindrical grinding machines, internal cylindrical grinding machines, surface grinding machines, centerless grinding machines, and tool grinding machines.

　　External cylindrical grinding machines are divided into ordinary external cylindrical grinding machines and universal external cylindrical grinding machines. Ordinary external cylindrical grinding machines can grind the outer cylindrical and conical surfaces of workpieces, while universal external cylindrical grinding machines can also grind internal cylindrical surfaces, internal conical surfaces, and end faces. The main parameter of external cylindrical grinding machines is the maximum grinding diameter.

　　External cylindrical grinding machines are centered on two centers, using a grinding wheel as a tool to grind cylindrical steel parts to achieve precise concentricity (also known as center grinding machines or cylindrical grinding machines).

　　Structure of External Cylindrical Grinding Machines

　　The main machine consists of the bed, headstock, tailstock, grinding head, and transmission dust collection device. The headstock and grinding head can rotate at an angle and are equipped with special fixtures for grinding center pins and chamfering rubber rollers. The dynamic balancing frame and rubber roller detector are components ordered by the customer.

　　Uses of External Cylindrical Grinding Machines

　　External cylindrical grinding machines have a wide range of applications and can process various cylindrical and conical outer surfaces as well as shoulder end faces. Universal external cylindrical grinding machines also come with internal grinding attachments, allowing for the grinding of internal holes and large conical internal and external surfaces. However, the automation level of external cylindrical grinding machines is relatively low, making them suitable only for small to medium batch production and repair work.

　　External cylindrical grinding machines are mainly used for the precision grinding of end faces, outer circles, and conical surfaces of batch-produced shaft parts, serving as key equipment in industries such as automotive engines. They are also suitable for military, aerospace, and general precision machining workshops that require high precision for small batch production of shaft parts.

　　⑴ Used in the textile spinning industry for processing rubber rollers on coarse and fine yarn machines, drawing machines, combing machines, and elastic machines.

　　⑵ Used in the production of office communication equipment, processing rubber rollers on fax machines, copiers, printers, and engraving machines.

　　⑶ Suitable for printing, food, and pharmaceutical industries for processing rubber rollers on automatic conveying devices and plastic processing.

　　Factors Affecting the Processing Precision of External Cylindrical Grinding Machines

　　1: The impact of the equal height of the grinding head, headstock, and tailstock on the dimensional accuracy of the workpiece.

　　The height error of the grinding head, headstock, and tailstock will cause the centerline of the headstock and tailstock to deviate from the spindle axis of the grinding wheel in space, resulting in a hyperbolic surface on the ground workpiece.

　　2: The impact of the parallelism error of the centerline of the headstock and tailstock to the spindle axis of the grinding head in the horizontal plane on the dimensional accuracy of the workpiece.

　　When this error occurs, the shape of the workpiece ground by the external cylindrical grinding machine will be a cone, meaning the grinding wheel is angled, resulting in spiral grinding marks on the surface.

　　3: The impact of the verticality error of the grinding head movement relative to the machine tool guide rail on processing precision.

　　The final result of this error is that the spindle axis deviates from the centerline of the headstock and tailstock, causing a verticality error between the shoulder end face and the workpiece axis when grinding the shoulder end face. When grinding the outer circle, it will affect the surface roughness and produce spiral grinding marks.

　　Selection of Abrasives for External Cylindrical Grinding Machines

　　External cylindrical grinding machines are extremely common in mold factories and hardware processing plants, becoming a commonly used equipment for technical personnel in the machinery industry. The selection of abrasives is very important when operating external cylindrical grinding machines.

　　1. Selection of Abrasives for External Cylindrical Grinding Machines

　　For hardened or non-hardened steel rollers, corundum grinding wheels should be used; for cold hard cast iron rollers, rubber rollers, copper rollers, and granite rollers, silicon carbide grinding wheels should be used. Different materials of steel rollers should also select matching corundum grinding wheels to achieve higher grinding precision and surface quality. For example, alloy steel rollers should use chrome corundum (Pa) grinding wheels, heat-resistant alloy steel rollers should use zirconia corundum (Za) grinding wheels, and stainless steel rollers should use single crystal corundum (Sa) grinding wheels.

　　2. Selection of Grinding Wheel Grit for External Cylindrical Grinding Machines

　　For rough grinding, coarse grit (24~60) grinding wheels should be selected; for fine grinding, fine grit (60~100) grinding wheels should be selected; for precision grinding, 150 grit grinding wheels should be used; for ultra-precision grinding or mirror grinding, micro powder (W63~W14) grinding wheels are generally used.

　　3. Selection of Grinding Wheel Hardness for External Cylindrical Grinding Machines

　　The harder the grinding roller surface, the softer the grinding wheel hardness should be selected.

　　(1) If the grinding wheel wears out too quickly, it indicates that the selected grinding wheel for the specific processed roller is too soft. The following improvement measures can be taken:

　　a. Increase the linear speed of the grinding wheel;

　　B. Increase the longitudinal feed speed of the carriage, that is, the longitudinal feed amount of the workpiece per revolution should be increased to 2/3~3/4 of the grinding wheel width;

　　C. Reduce the speed of the roller.

　　If the above measures do not yield significant grinding effects, it indicates that the grinding wheel is too soft and should be replaced with a harder one.

　　(2) If the grinding wheel is obviously obstructed (struggling) or very dirty while grinding rollers, with dulled abrasive grains not easily falling off, and the grinding wheel easily sticking to grinding debris, resulting in burning or roughening of the roller surface, it indicates that the selected grinding wheel is too hard. The following measures can be taken to improve:

　　a. Reduce the linear speed of the grinding wheel;

　　B. Increase the speed of the roller.

　　If the above measures do not yield significant grinding effects, it indicates that the grinding wheel is too hard and should be replaced with a softer one.

　　4. Selection of Grinding Wheel Bonding Agents for External Cylindrical Grinding Machines

　　Except for mirror grinding of roller surfaces, resin-bonded grinding wheels should be used for other grinding types, while ceramic grinding wheels are preferred.

　　The external cylindrical grinding method for workpieces on an external cylindrical grinder.

　　The external cylindrical surface of the workpiece is generally ground on a conventional external cylindrical grinder or a universal external cylindrical grinder. External cylindrical grinding generally has three methods: longitudinal grinding, transverse grinding, and deep grinding.

　　(1) Longitudinal grinding method

　　In the longitudinal grinding method, the high-speed rotation of the grinding wheel is the main motion, while the workpiece performs a circumferential feeding motion and also moves longitudinally back and forth with the worktable, achieving axial feeding along the workpiece. At the end of each stroke or each reciprocating stroke, the grinding wheel moves periodically in the transverse direction, achieving radial feeding along the workpiece, thereby gradually grinding away all the radial allowance of the workpiece. After reaching the size, a finishing process without transverse feeding is carried out until the sparks disappear. Due to the small radial feeding amount in the longitudinal grinding method, the grinding force is small, and the heat dissipation conditions are good, which significantly improves the grinding accuracy and surface quality of the workpiece, meeting higher processing quality requirements, but the grinding efficiency is relatively low. The longitudinal grinding method is suitable for grinding larger workpieces and is a common method for single-piece and small-batch production.

　　(2) Transverse grinding method

　　When using the transverse grinding method to grind the external surface, the width of the grinding wheel is larger than the grinding width of the workpiece, and the workpiece does not need to perform longitudinal (axial) feeding motion. The grinding wheel continuously or intermittently moves in the transverse direction at a slow speed, achieving radial feeding of the workpiece until the grinding reaches the size requirement. Its characteristics are: it fully utilizes the cutting ability of the grinding wheel, has high grinding efficiency, and is also suitable for form grinding. However, during the grinding process, the contact area between the grinding wheel and the workpiece is large, which increases the grinding force, making the workpiece prone to deformation and burning. In addition, the shape error of the grinding wheel directly affects the geometric shape accuracy of the workpiece, resulting in lower grinding accuracy and larger surface roughness values. Therefore, it is necessary to use a grinder with high power and good rigidity, and sufficient cutting fluid must be provided during grinding to achieve cooling. Using the transverse grinding method requires good rigidity of the process system, and the workpiece should be short rather than long. The fine grinding process of short stepped shaft journals usually adopts this grinding method.

　　(3) Deep grinding method

　　The deep grinding method is a relatively advanced method with high productivity, and the grinding allowance is generally 0.1 to 0.35 mm. This method can grind the entire allowance in one pass. During grinding, the feeding amount is small, generally taking the longitudinal feeding amount as 1 to 2 mm/r, which is about 15% of the 'longitudinal grinding method', and the processing time is about 30 to 75% of the longitudinal grinding method.

　　(4) Mixed grinding method

　　Grinding end faces: On a universal external cylindrical grinder, the end face of the grinding wheel can be used to grind the shoulder and end face of the workpiece. Before grinding begins, the end face of the grinding wheel should slowly approach the end face of the workpiece to be ground. During the grinding process, the axial feeding amount of the workpiece should also be very small. This is because the rigidity of the end face of the grinding wheel is very poor and cannot withstand large axial forces. Therefore, the best method is to use the outer conical surface of the grinding wheel to grind the end face of the workpiece, at which point the worktable should be tilted at a larger angle.

　　Grinding inner circles: The inner circle grinding tool of the external cylindrical grinder can grind the inner circle of the workpiece. When grinding the inner circle, the workpiece is mostly positioned using the outer circle or end face as a reference and clamped on the chuck for grinding. When grinding the inner conical surface, it is only necessary to deflect the inner circle grinding tool by one circumferential angle.

　　The importance of center holes in CNC external cylindrical grinding.

　　Center holes play a very important role in CNC external cylindrical grinding, and their quality directly affects the processing quality. This article introduces several requirements that center holes should meet to ensure the quality of external cylindrical grinding and prevent the production of defective products.

　　Center holes play a very important role in CNC external cylindrical grinding, and their quality directly affects the processing quality. When the workpiece is positioned and clamped on the front and rear centers of the grinder for external cylindrical grinding, the center hole serves as the positioning reference for the workpiece. The collective shape errors of the 60° conical holes, such as elliptical polygons and poor taper, will all reflect on the ground external cylindrical surface. To ensure the quality of external cylindrical grinding and prevent the production of defective products, the center holes should meet the following requirements.

　　1. The roundness of the 60° conical hole should be good. Otherwise, defects such as non-roundness will occur on the external cylindrical surface of the grinder.

　　2. The surface roughness value of the 60° conical hole should be small, and there should be no burrs, scratches, or other defects. Due to the rough surface of the center hole, there is significant friction during grinding, generating a lot of heat, which can easily damage the center hole and the centers, even leading to workpiece scrapping. For workpieces that have undergone heat treatment, their center holes must be ground and should not have any salt or oxide scale left.

　　3. The angle of the 60° taper should be accurate, and the center holes at both ends should be on the same axis; otherwise, it will cause poor contact between the center hole and the centers, resulting in the workpiece shaking during grinding, leading to roundness errors and coaxiality errors of the external cylindrical surface.

　　CNC external cylindrical grinders pay great attention to the concentricity of the workpiece during grinding, so that larger errors do not occur during the grinding process.

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