Definition of Grinding
What is grinding? Grinding can be described as “an operation that reduces the surface roughness of a workpiece as much as possible without damaging its shape.” Grinding is a machining action carried out using a grinding machine. In Chinese, grinding machines are referred to as “grinders,” known as “ken sakuban” in Japan. Some also call them “grinding disks.” In optics, grinding is mainly divided into lapping and polishing. Some optical texts refer to lapping as sanding, but other texts consider sanding to be a broader definition of polishing. To reduce the surface roughness of a formed workpiece, tools such as files, whetstones, sandpaper, buffing wheels, and disk grinders can be used.
Grinding Methods
Grinding operations, ranging from deburring to polishing, employ various methods. About 30 years ago, ultra-precision mechanical processing became practical, used to create multifaceted mirrors or aspherical surfaces. Products cut with diamond tools have extremely high shape accuracy, and their surface roughness is minimal, eliminating the need for grinding.
Grinding Methods Include:
(1) Deburring using heating, tumbling, magnetic grinding, sandblasting, polishing, and buffing wheels.
(2) Coarse grinding with disk sanders and belt sanders.
(3) Grinding using abrasives as the medium.
(4) Fine grinding using abrasive belts.
(5) Whetstone polishing, including manual and automatic polishing with rotating or flat grinders.
(6) Polishing machines.
(7) Manual grinding.
Applications of Grinding
The purpose of grinding varies from workpiece to workpiece:
(1) Grinding injection molds and stamping molds improves the optical function and gloss of the molded product. It reduces resin flow resistance and demolding resistance, increases mold rigidity, prevents rust, and extends the mold’s lifespan.
(2) Grinding the inside of CD stampers ensures more uniform thickness, reduces internal roughness, and minimizes signal errors on the molded disk surface.
(3) Lenses and reflectors are ground to give the product optical functions, or in some cases, for material research to observe structures under a microscope and determine the depth of the processing-affected layer.
Five Effects of Grinding
Grinding improves the surface finish of the workpiece to meet production and application requirements.
(1) Cutting (effect of many small abrasives).
(2) Crumbling (polishing tools cause internal deformation or fiber layer sliding near the surface).
(3) Melting (in some cases, localized high temperatures occur in the metal).
(4) Rebinding (particularly significant in metal grinding, where small atoms or molecules bind strongly).
(5) Formation of a processing-affected layer (plastic flow forms a ground surface).
Proper Grinding Technique
The abrasive must be non-continuous. Grinding with alumina or diamond abrasives involves small, hard particles that contact the workpiece intermittently. The abrasive should be friable, meaning that under excessive force, it breaks off inside the polishing tool, preventing large scratches on the workpiece.
Grinding Overheating: Frictional overheating during grinding can lead to melting or shape distortion of the workpiece. It is necessary to use water-based or oil-based grinding fluids for lubrication, which cool the process, disperse the abrasive, remove grinding swarf, and cushion the grinding action.
What is an Abrasive?
An abrasive is a material used for grinding, made of powders such as alumina, silicon carbide, diamond, cerium oxide, rouge, and CBN. These include alumina powder, diamond powder, and grinding wheels. Abrasives break down during the grinding process or wear into smaller particles. This property is crucial for grinding a workpiece to a mirror finish. The ideal abrasive should be hard yet friable, small, and sharp, with uniform size and shape, and should not react with the workpiece.
Difference Between General and Precision Molds: The difference lies in accuracy requirements. General molds for stationery or electrical appliances have tolerances of ±0.1 to ±0.05 mm and rougher base surfaces, while precision molds for optical components, gears, or connectors have tolerances of ±0.05 to ±0.01 mm, with stricter surface roughness and shape precision requirements.
Characteristics of Different Machined Surfaces:
(1) Cut Surface: Inevitably has pick-feed marks with rough surface texture; grinding may result in waviness but minimal micro-cracking.
(2) Ground Surface: Has high processing precision, but micro-cracking is possible. When a glossy surface is required, further lapping of the ground surface is necessary.
(3) Electrical Discharge Machined Surface: Prone to work hardening or softening layers, and achieving strict shape accuracy is difficult.
(4) Lapped Surface: Surfaces such as flat, spherical, cylindrical, or tapered holes can be processed with high precision.
(5) Gold-Plated Surface: Sometimes directly ground; in other cases, ground after micro-cutting with a diamond tool.
Micro-Removal Processing
The portion of a material that deforms or deteriorates due to some external force is called a processing-affected layer. Cutting, grinding, and heat treatment can cause it. Defects such as pinholes or orange peel, which are not found during grinding, may appear during use due to the residual affected layer. If the affected layer is severe during cutting, milling, or turning marks may emerge from beneath the ground surface. To avoid this, the workpiece must be processed in stages, removing small amounts each time. This process is called micro-removal processing.
