Introduction of the fine grinding process in screw refurbishment


                        The outer diameter of the screw is easy to be rough and smooth, so one of the most common methods of refurbished screw is the fine grinding process. Fine grinding is an important process between 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 by polishing. Therefore, the quality is very important to the polishing effect. The method of fine grinding is divided into fine abrasive grinding and diamond grinding. The former is called traditional grinding, also known as free grinding; the latter is called high-speed fine grinding.


                        Ways of fine grinding methods

                        Fine grinding can generally be divided into bulk abrasive grinding and fixed abrasive grinding. The latter is divided into: molding surface method and Fan Cheng method. Grinding abrasive grinding is also called traditional method, which is that, the glass is grinded by silicon carbide in the middle of a metal mold (usually with brass). Each time the abrasive grain size is changed, the ball of curvature radius has to be replaced. For the processing of convex lenses, the abrasive grain size will be finer and finer, so the radius of curvature of the corresponding ball 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 grinding. The method of high-speed fine grinding by pellets is using a small circular disc made of a bronze matrix containing diamond particles to be glued into a spherical shape or a planar shape, and then milling glass. Fan Cheng method 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 requirement of the optical processing is generally pretty high, even the micron-scale precision. To achieve such high precision, it can only be achieved step by step using the fine grinding process. In traditional polishing, compared to polished finished parts, the surface geometry after fine grinding has difference from 4 to 8 Newton interference circles, which is about 2 microns. In modern high-speed polishing, the difference is only 2 Newtonian interference circles, about 0.5 microns. But in actual production, the surface after fine grinding should be a low aperture, in which case the radius of curvature should be larger for convex lens, but smaller for concave lens.

                        II. Surface roughness requirements

                        The surface roughness of optical glass after rough grinding process is large. That is to say, the surface roughness is very serious. The abrasive processing often uses silicon carbide to deal with the surface left after grinding. 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 degree of bump so that it can be polished. 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.