Lapping and polishing processes involve rubbing two surfaces together to produce the desired dimension, finish, or shape. Glass substrate lapping, for instance, consists of rubbing the glass against a surface, such as iron — or even another piece of glass — with an abrasive between them. Common abrasives used with this technique include aluminum oxide, optician’s rouge, and silicon carbide. This process causes microscopic conchoidal fractures to appear as the abrasive rolls between the two surfaces, removing material from both pieces.
Lapping and polishing finishing processes are used in countless industries and applications, including:
- microelectromechanical systems
- medical components
- tools and dies
Surface lapping involves the removal of a material to leave behind a smooth, flat, and unpolished surface. During the lapping process, subsurface damage caused by operations such as sawing or grinding is removed, and the desired thickness and flatness are achieved. Lapping is less damaging than grinding.
There are two types of lapping: free abrasive lapping and fixed abrasive lapping. During free abrasive lapping, an abrasive slurry is applied to a lapping plate. Free abrasive lapping is highly accurate, as the lapping surface can be made to suit a particular material. Fixed abrasive lapping, on the other hand, involves bonding an abrasive particle to a substrate, such as abrasive lapping films or silicon carbide (SiC) papers. Abrasive lapping films are capable of producing very flat surfaces, while SiC papers can create rounded edges. Lapping is essential for producing parts that require uniformly smooth, flat surfaces.
Double-sided lapping, in particular, is commonly used in the optical industry. This process makes use of planetary action, along with a top and bottom plate, to grind and polish precision, parallel optics. The optical pieces are held in geared or sprocketed carriers driven via planetary motion spanning the entire surface of the lapping plates and undergo a continually changing direction of rotation. The upper and lower plate can rotate in the same direction or opposite directions or can remain stationary. The specimen and plates slide over one another on a loosely applied slurry of abrasive particles in a liquid vehicle delivered through the top plate. The particles abrade the sample almost equally from both faces of the material.
Surface polishing, as opposed to surface lapping, involves the removal of material to produce a scratch-free surface using fine abrasive particles. Polishing is usually conducted at very low speeds employing polishing cloths, abrasive films, or specially designed lapping plates. Plate material and cloth material are critically important when polishing a sample, as the properties of the substrates directly affect the final polish quality.
There are two types of surface polishing: single-sided and double-sided. Single-sided polishing techniques are recommended for small components and are ideal for achieving precise parallelism and flatness on glass substrates. Double-sided polishing is typically used on precision optical parts such as wafers, mirrors, and windows. Polished surfaces are more aesthetically pleasing and allow for much easier cleaning.
The Finishing Touch
Lapping is an essential step in many manufacturing processes, allowing users to achieve full control over thickness identity, surface finish, uniformity, flatness, camber reduction, parallelism, and preparation for polish. Polishing, meanwhile, is typically one of the last — if not the last — tasks completed in a manufacturing process, and allows for a smooth, aesthetically pleasing finish. These techniques work in conjunction with one another to ensure optimal product aesthetics and function.
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