The outer diameter of the screw is easy to rough and smooth, so one of the most common methods of refurbished screw is the fine grinding process. Fine grinding is also called fine grinding. It is an important process between the two processes of rough grinding and polishing. The purpose of fine grinding is to ensure that the workpiece meets the surface accuracy, dimensional accuracy and surface roughness required before polishing. Therefore, the quality of the finish is very important for the polishing effect. The method of fine grinding is divided into fine abrasive grinding and diamond grinding. The former is called classical grinding, also known as free grinding; the latter is called high-speed fine grinding.

Classification of fine grinding methods

Fine grinding can generally be divided into a bulk abrasive grinding method and a fixed abrasive method. The latter is divided into: the molding surface tool and the Fan Cheng method. Grinding abrasive grinding is also called classical method, which is a metal forming mold (usually with brass), and the diamond is gradually grinded in the middle with diamond. Each time the abrasive grain size is changed, a ball of curvature radius is replaced. For the processing of convex lenses, the abrasive grain size becomes finer and finer, and the radius of curvature of the corresponding earth mold will become smaller and smaller. The fixed abrasive grinding method is also called diamond high-speed fine grinding, which is also called fine powder fine grinding. The so-called molding method of high-speed fine grinding of pellets is to use a small circular disc made of a bronze matrix containing diamond particles to be glued into a spherical shape or a planar shape. This method is most commonly used for milling glass. Fan Chengfa's high-speed fine grinding is exactly the same as spherical milling. The only difference is that the grinding wheel has a finer diamond grain size.

Technical requirements for fine grinding

I. Geometrical surface accuracy requirements

The geometrical finish of the optical finish is generally very high, often in micron-scale precision. To achieve such high precision, it can only be stepped up by the fine grinding process to prepare for the final polishing process. In classical polishing, the surface geometry after fine grinding is 4 to 8 Newton interference circles, about 2 microns, compared to polished finished parts. In modern high-speed polishing, only 2 Newtonian interference circles can be distinguished, about 0.5 microns. . It should be noted here that in actual production, the surface after fine grinding should be a low aperture, in which case the radius of curvature should be larger for the convex lens, and the radius of curvature should be smaller for the concave lens.

II. Surface roughness requirements

The surface roughness of the rough-finished optical glass is relatively large, that is, the surface roughness is very serious. The abrasive abrasive processing often uses the surface left after grinding of the corundum. The surface damage layer of the glass is about 30 micrometers and the surface roughness RZ is less than 6 micrometers. The surface of the fixed abrasive is often processed by a diamond grinding wheel, and the surface damage layer is about 50 micrometers. The surface roughness RZ is about 0.9 microns or less.

One of the purposes of fine grinding is to make the surface of the optical glass less concave and convex to the extent that it can be polished and thrown away. At present, it is considered that the fracture layer after processing with diamond abrasive is below 12 microns and the roughness is below 0.4 microns. When the abrasive is processed, the fracture layer after processing with W10 diamond pellet is below 8 microns, surface roughness Rz is below 0.35 microns.