Lapping, Polishing and Electropolishing
WS Associates now has all three of these surface preparation and metal finishing processes available to meet our customer’s needs.
WS Associates’ partners have long been experts in electropolishing for the manufacture of high purity components, particularly for stainless steel gas fittings for use in the Semiconductor, Biotech, Medical, Pharmaceutical, Industrial, and Aerospace Industries.
Recently lapping and polishing have been added. This article describes all three of these processes as well as some details of the new lapping capability.
Lapping
Lapping involves two surfaces that are rubbed together with an abrasive material between them. One of them is the product that is being finished; the other is a high density, cast iron plate known as a lap plate. Lapping can be a manual operation, however in a commercial setting the lapping is performed by a lapping machine, such as the Lapmaster 15, which will ensure more reproducible results with higher volume.
There are two types of lapping. In the first type of lapping, abrasive material is in the form of abrasive slurry consisting of a material such as aluminum oxide, optician’s rouge, emery, silicon carbide or diamond suspended in a liquid medium, usually either water or oil. The lapping action causes microscopic conchoidal fractures, which are breaks in the material that do not follow any natural planes of separation. As the abrasive rolls about between the two surfaces it removes material from both.
The other form of lapping involves a softer material such as pitch or a ceramic, which is “charged” with the abrasive. The lap is then used to cut a harder material. The abrasive embeds within the softer material which holds it and permits it to score across and cut the harder material.
Lapmaster 15
The Lapmaster 15 is a bench top lapping and polishing machine which features a rigid, lightweight, cast aluminum base and uses lapping rings to achieve high standard of flatness and surface finish on a continuous basis through specific positioning of the conditioning rings.
The conditioning rings ensure that the lapping plate flatness is maintained and thus ensure that the lapping of the finished product is also flat.
Some of the features of the Lapmaster 15 include three conditioning rings with roller yoke and bearing assemblies, an abrasive feed system to ensure that the slurry is continuously fed to the lapping surface, an automatic digital cycle timer and a high torque drive unit.
Polishing
Polishing and buffing are finishing processes for smoothing a surface using an abrasive and a work wheel. Technically polishing refers to processes that use an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process while buffing is less harsh, which leads to a smoother, brighter finish.
Polishing is often used to enhance the looks of an item, prevent contamination of instruments, remove oxidation, create a reflective surface, or prevent corrosion in pipes. In metallography and metallurgy, polishing is used to create a flat, defect-free surface for examination of a metal’s microstructure under a microscope. Silicon-based polishing pads or a diamond solution can be used in the polishing process.
In a production setting, polishing is often performed on the same type of machinery as the lapping process. When used for polishing, a different plate is used with a polishing paste or powder. When used with the proper polishing powder, consistent surface finishes of 1-2 microinches are obtainable.
Electropolishing
Electropolishing is also known as electrochemical polishing or electrolytic polishing. Electropolishing is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and remove burrs from metal parts. It is often described as the reverse of electroplating. It may be used in lieu of abrasive fine polishing in microstructural preparation.
Typically, the workpiece is immersed in a temperature controlled bath of electrolyte and serves as the anode as it is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode. A current passes from the anode, where metal on the surface is oxidized and dissolved in the electrolyte, to the cathode. At the cathode, a reduction reaction occurs. The electrolytes used are acid solutions of a high viscosity.
To achieve electropolishing of a rough surface, the protruding parts of a surface profile must dissolve faster than the recesses. Electropolishing works because the peaks in the rough surface generate stronger electric fields and faster replenishment of the electrolytes; since the removal of material is dependent on these two factors, electropolishing will preferentially remove the peaks and not the valleys in the material.
Electropolishing has many applications in the metal finishing industry. Because of its simplicity, it can be applied to objects of complex shape. In the semiconductor, medical and other industries, high purity stainless steel gas connectors are required to have a highly polished surface. Electropolishing is the method to achieve this. Electropolishing is also commonly applied to the preparation of thin metal samples for transmission electron microscopy. Ultra high vacuum (UHV) components are typically electropolished in order to have a smoother surface for improved vacuum pressures, out-gassing rates, and pumping speed.
Summary
WS Associates merges Lapping, Polishing and Electropolishing along with state-of-the-art manufacturing processes already available to our customers. Since 1970, WS Associates has served the precision manufacturing needs for a variety of industries. Today, with developing technologies rapidly changing the machining industry, we keep up-to-date with the latest improvements so that we meet our customers’ exacting expectations.
