What are the characteristics of external cylindrical grinding?
External cylindrical grinding process parameters
① Grinding wheel circumferential speed: When using a ceramic-bonded grinding wheel, the circumferential speed is generally less than 35 m/s; when using a resin-bonded grinding wheel, the circumferential speed is generally less than 50 m/s.
② Workpiece circumferential speed: The workpiece circumferential speed is generally 13–20 m/min. When grinding hardened steel, the circumferential speed is generally 20–26 m/min. When the workpiece has a large length-to-diameter ratio and poor rigidity, the workpiece speed should be reduced.
③ Grinding depth: The grinding depth is generally 0.02–0.05 mm for rough grinding and 0.005–0.015 mm for finish grinding. When the workpiece surface roughness is small and high precision requirements are high, several pass grindings without feed are necessary after finish grinding.
④ Longitudinal feed rate: For rough grinding, the feed rate per step is 0.5 to 0.8 times the wheel width; for fine grinding, the feed rate per step is 0.2 to 0.3 times the wheel width.
workpiece clamping
① Workpieces with a large length-to-diameter ratio are generally ground using a front and rear center clamping method. For hardened materials, the center hole of the center must be accurately ground, using carbide centers and appropriate clamping force.
② Workpieces with a small length-to-diameter ratio are generally clamped using a self-centering or single-action chuck. Workpieces clamped by a chuck are generally clamped using a process chuck to grind the stepped outer diameter sections in a single clamping operation.
③ Longer workpieces are generally clamped using a chuck and center combination. Slender, small-sized shaft workpieces with a large length-to-diameter ratio generally use a double-center clamping method.
④ Sleeve-type workpieces requiring coaxial inner and outer diameters are generally clamped using a mandrel. The mandrel locating surface is generally ground to match the workpiece hole diameter with a taper of 1/7000 to 1/5000.
Top center hole
Before turning and grinding the outer cylindrical surface, a center hole must be machined to provide a reliable positioning datum for subsequent processes. If the center hole has a large coaxiality error, it will prevent proper contact between the center hole and the center, affecting machining accuracy. Especially when the center hole has a roundness error, it will directly affect the workpiece, causing roundness errors as well. After heat treatment, the center hole of the part being ground needs to be corrected to eliminate any deformation or other defects that may have occurred during heat treatment, ensuring accurate positioning during grinding of the outer cylindrical surface and guaranteeing the required shape accuracy. Center hole correction can be performed using methods such as grinding, lathe work, and extrusion, and can be done on lathes, drilling machines, or special-purpose machine tools.
For center holes with less stringent accuracy requirements, multi-faceted centers are typically used for correction. Figure 3-6 shows a carbide multi-hair center for extruding a center hole. During extrusion, a multi-faceted center is installed inside the tapered hole of the lathe spindle. The operation is similar to that of center hole correction. The lathe's tailstock center presses the workpiece against the multi-faceted center, and the extrusion action of the center corrects the geometric errors of the center hole. This method has extremely high productivity (only a few seconds), but the quality is slightly inferior, and it is generally used to correct center holes with low precision requirements.
For center holes requiring high precision, grinding is generally used for correction.
Figure 3-7 shows the grinding method for correcting center holes on a lathe. A small amount of kerosene or machine oil is added to the center hole to be ground, and the workpiece is held by hand or the grinding wheel is moved using the tail tip. This method is efficient and produces good quality; however, the grinding wheel wears quickly and requires frequent dressing.
