Buying a grinding machine: the grinding process | Modern Machinery Workshop

Potential buyers of new grinding machines should understand the ins and outs of the abrasive process, how the abrasive bond works, and various forms of grinding wheel dressing.
This blog post is adapted from an article published by Barry Rogers in the November 2018 issue of the Machine/Shop supplement of Modern Machine Shop magazine.
In the last article on the topic of grinders, we discussed the basic appeal of grinders and how they are constructed. Now, we take a closer look at how the abrasive process works and what it means for shopkeepers of new machines on the market.
Grinding is an abrasive processing technology that uses a grinding wheel as a cutting tool. The grinding wheel consists of hard, sharp-edged particles. When the wheel rotates, each particle acts like a single-point cutting tool.
Grinding wheels are available in a variety of sizes, diameters, thicknesses, abrasive grain sizes and binders. Abrasives are measured in units of particle size or particle size, with particle sizes ranging from 8-24 (coarse), 30-60 (medium), 70-180 (fine) and 220-1,200 (very fine). The coarser grades are used where a relatively large amount of material must be removed. Generally, a finer grade is used after a coarser grade to produce a smoother surface finish.
The grinding wheel is made of a variety of abrasives, including silicon carbide (usually used for non-ferrous metals); alumina (used for high-strength iron alloys and wood; diamonds (used for ceramic grinding or final polishing); and cubic boron nitride (usually used for Steel alloy).
Abrasives can be further classified as bonded, coated or metal bonded. The fixed abrasive is mixed with abrasive grains and a binder, and then pressed into a wheel shape. They are fired at high temperatures to form a glass-like matrix, commonly known as vitrified abrasives. Coated abrasives are made of abrasive grains bonded to a flexible substrate (such as paper or fiber) with resin and/or glue. This method is most commonly used for belts, sheets, and petals. Metal bonded abrasives, especially diamonds, are fixed in the metal matrix in the form of precision grinding wheels. The metal matrix is ​​designed to wear to expose the grinding media.
The bonding material or medium fixes the abrasive in the grinding wheel and provides bulk strength. Voids or pores are intentionally left in the wheels to enhance coolant delivery and release chips. Depending on the application of the grinding wheel and the type of abrasive, other fillers can be included. Bonds are usually classified as organic, vitrified or metallic. Each type provides application-specific benefits.
Organic or resin adhesives can withstand harsh grinding conditions, such as vibration and high lateral forces. Organic binders are particularly suitable for increasing the amount of cutting in rough machining applications, such as steel dressing or abrasive cutting operations. These combinations are also conducive to the precision grinding of superhard materials (such as diamond or ceramics).
In the precision grinding of ferrous metal materials (such as hardened steel or nickel-based alloys), the ceramic bond can provide excellent dressing and free cutting performance. The ceramic bond is specifically designed to provide strong adhesion to cubic boron nitride (cBN) particles through a chemical reaction, resulting in an excellent ratio of cutting volume to wheel wear.
Metal keys have excellent wear resistance and shape retention. They can range from single-layer electroplated products to multilayer wheels that can be made very strong and dense. Metal bonded wheels may be too hard to wear effectively. However, a new type of grinding wheel with a brittle metal bond can be dressed in a manner similar to a ceramic grinding wheel and has the same beneficial free-cutting grinding behavior.
During the grinding process, the grinding wheel will wear out, become dull, lose its contour shape or “load” due to chips or chips sticking to the abrasive. Then, the grinding wheel starts to rub the workpiece instead of cutting. This situation generates heat and reduces the efficiency of the wheels. When the wheel load increases, chattering occurs, which affects the surface finish of the workpiece. The cycle time will increase. At this time, the grinding wheel must be “dressed” to sharpen the grinding wheel, thereby removing any material left on the surface of the grinding wheel and restoring the grinding wheel to its original shape, while bringing new abrasive particles to the surface.
Many types of grinding wheel dressers are used for grinding. The most common is a single-point, static, onboard diamond dresser, which is located in a block, usually on the headstock or tailstock of the machine. The surface of the grinding wheel passes through this single point diamond, and a small amount of the grinding wheel is removed to sharpen it. Two to three diamond blocks can be used to modify the surface, sides, and shape of the wheel.
Rotary trimming is now a popular method. The rotary dresser is coated with hundreds of diamonds. It is usually used for creep feed grinding applications. Many manufacturers find that for processes that require high part production and/or tight part tolerances, rotary trimming is better than single-point or cluster trimming. With the introduction of ceramic superabrasive wheels, rotary dressing has become a necessity.
Oscillating dresser is another type of dresser used for large grinding wheels that require deeper and longer dressing strokes.
The offline dresser is mainly used for grinding wheels away from the machine, while using an optical comparator to verify the shape profile. Some grinders use wire-cut electric discharge machines to dress metal bond wheels that are still installed on the grinder.
Learn more about purchasing new machine tools by visiting the “Machine Tool Buying Guide” in the Techspex Knowledge Center.
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Post time: Aug-02-2021

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