How to correctly choose a grinding wheel for tool grinding?

Introduction

There are many types of grinding wheels and various shapes and sizes, and each type of grinding wheel has a certain range of applicability according to the abrasive material of the grinding wheel, the bond material and the manufacturing process. If not properly selected will directly affect the processing accuracy, surface roughness and production efficiency. Therefore, the grinding process, must be based on the specific circumstances of the selection of the appropriate grinding wheel. Then how to choose the correct grinding wheel? For grinding in the commonly used grinding wheel to summarize, so that we understand the selection of grinding wheels.

Ⅰ. Selection of General Grinding Wheels

1、The choice of abrasive depends mainly on the material of the workpiece and the heat treatment method.

a. When grinding materials with high tensile strength, use abrasives with high toughness.

b. When grinding materials with low hardness and high elongation, use brittle abrasives.

c. When grinding hard materials, use harder abrasives.

d. Select abrasives that are not susceptible to biochemical reactions in the processed material.

The most commonly used abrasives are brown corundum (A) and white corundum (WA), followed by black silicon carbide (C) and green silicon carbide (GC), with chromium corundum (PA), monocrystalline corundum (SA), microcrystalline corundum (MA), and zirconium corundum (ZA).

Brown corundum grinding wheel: brown corundum’s high hardness, toughness, suitable for grinding metals with high tensile strength, such as carbon steel, alloy steel, forged cast iron, hard bronze, etc., the grinding performance of this abrasive material is good, wide adaptability, commonly used in the removal of large residual amount of rough grinding, inexpensive, can be widely used.

White Corundum Grinding Wheel: The hardness of white corundum is slightly higher than that of brown corundum, while the toughness is lower than that of brown corundum. When grinding, the abrasive grains are easy to break, therefore, the grinding heat is small, and it is suitable for manufacturing grinding wheels for precision grinding of hardened steel, high carbon steel, high speed steel, and grinding of thin-walled parts, and the cost is higher than that of brown corundum.

Black Silicon Carbide Grinding Wheel: Black Silicon Carbide is brittle and sharp, with higher hardness than white corundum, suitable for grinding materials with lower mechanical strength, such as cast iron, brass, aluminum and refractory materials.

Green Silicon Carbide Grinding Wheel: Green Silicon Carbide has higher hardness and brittleness than Black Silicon Carbide, with sharp grits and good heat conductivity, suitable for grinding hard and brittle materials such as cemented carbide, optical glass and ceramics.

Chrome corundum grinding wheel: suitable for grinding tools, gauges, instruments, threads and other workpieces with high quality surface finish.

Monocrystalline Corundum Grinding Wheel: Suitable for grinding stainless steel, high vanadium high-speed steel and other tough and high hardness materials and workpieces prone to deformation and burns.

Microcrystalline corundum grinding wheels: suitable for grinding stainless steel, bearing steel and special ductile iron, etc., used for profile grinding, plunge-cut grinding, mirror grinding.

Zirconia grinding wheels: suitable for grinding austenitic stainless steel, titanium alloys, heat-resistant alloys, especially suitable for heavy-duty grinding.

2、The choice of grain size depends mainly on the surface roughness of the workpiece to be ground and the grinding efficiency.

Grain size refers to the size of the abrasive particles, and its size is expressed by the grain size number. When grinding with coarse grit grinding wheels, the productivity is high, but the surface of the workpiece is rougher; when grinding with fine grit grinding wheels, the surface of the workpiece is rougher and the productivity is lower. Under the premise of meeting the requirements of roughness, the grinding wheel with coarse grit should be selected as much as possible to ensure higher grinding efficiency. Generally, coarse grit grinding wheels are used for rough grinding and fine grit grinding wheels are used for fine grinding.

When the contact area between the grinding wheel and the workpiece is larger, a coarser grit grinding wheel should be used. For example, when grinding the same flat surface, the end face of the grinding wheel is coarser than the periphery of the grinding wheel.

3、The choice of hardness depends mainly on the material of the workpiece to be ground, the grinding efficiency and the quality of the machined surface.

Hardness refers to the ease with which abrasive grains can be dislodged from a grinding wheel under the action of an external force. In order to meet the requirements of grinding of different workpiece materials, grinding wheels are classified into different hardness grades when they are manufactured.

Wheel selection is too hard, grinding blunt abrasive particles are not easy to come off, the wheel is easy to clog, grinding heat increases, the workpiece is easy to burn, grinding efficiency is low, affecting the surface quality of the workpiece; wheel selection is too soft, the abrasive particles are still sharp on the shedding, increasing the wear and tear of the grinding wheel, easy to lose the correct geometry, affecting the accuracy of the workpiece. Therefore, the choice of grinding wheel hardness should be appropriate, but also according to the size of the contact area between the grinding wheel and the workpiece, the shape of the workpiece, the way of grinding, the cooling method, the type of grinding wheel compound and other factors for comprehensive consideration.

The following principles of wheel hardness selection are for reference:

• For grinding soft materials, a harder grinding wheel should be selected, while for grinding hard materials, a softer grinding wheel should be chosen.
• When grinding soft, tough non-ferrous metals, the hardness should be lower.
• For materials with poor thermal conductivity, softer grinding wheels should be selected.
• For face grinding, compared to circumferential grinding, the grinding wheel should be softer.
• Under the same grinding conditions, a resin-bonded grinding wheel should have a hardness level 1-2 grades higher than a ceramic-bonded wheel.
• For high-speed grinding, the hardness of the wheel should be lowered by 1-2 grades.
• Grinding with coolant requires a grinding wheel hardness 1-2 grades higher than dry grinding.

4、The choice of bond should be considered according to the grinding method, speed of use and surface finish requirements.

The most commonly used wheel binders are ceramic binders (V) and resin binders (B).

Ceramic bond is a kind of inorganic bond, with stable chemical properties, heat resistance, good corrosion resistance, large porosity, high grinding efficiency of the grinding wheel made by this bond, small wear, better able to maintain the geometry of the grinding wheel, the widest range of applications. The grinding wheel is suitable for grinding ordinary carbon steel, alloy steel, stainless steel, cast iron, hard alloys, non-ferrous metals and so on. However, ceramic bond grinding wheels are more brittle and cannot be subjected to severe vibration. Generally, it can only be used at a speed of 35 meters per second or less.

Resin bond is a kind of organic bond, the grinding wheel made by this bond has high strength, a certain degree of elasticity, low heat resistance, good self-sharpness, easy to make, and short process cycle. It can produce grinding wheels with a working speed of more than 50 meters per second and very thin grinding wheels. Its application range is second only to ceramic bond, widely used in rough grinding, barren grinding, cutting and free grinding, such as grinding steel ingot, casting burring, and so on. It can be used to produce high speed, high finish grinding wheels, heavy load, slicing and all kinds of special requirement grinding wheels.

5、The selection of the organization mainly takes into consideration the pressure exerted on the workpiece, the grinding method, and the material of the workpiece.

Tissue refers to the percentage of the volume of abrasive grains in the wheel. The classification of wheel organization is 62% of the volume of abrasive grains as “0” organization, and for every 2% reduction in the volume of abrasive grains, the organization is increased by one number, and so on, with a total of 15 numbers. The larger the number, the looser the organization.

A tightly organized grinding wheel can produce a better surface of the workpiece, and a loosely organized grinding wheel with a large void can ensure that the grinding process can accommodate abrasive chips and avoid clogging of the wheel. Usually rough grinding and grinding of softer metals, the wheel is easy to clog, should be used loose organization of the wheel; molding grinding and precision grinding, in order to maintain the geometry of the wheel and get a better roughness, should be used tighter organization of the wheel; grinding machine guides and hard alloy tools, in order to reduce the thermal deformation of the workpiece, to avoid burns and cracks, it is appropriate to use the loose organization of the wheel; grinding of heat-sensitive materials, non-ferrous metals, For grinding heat-sensitive materials, non-ferrous metals and non-metallic materials, it is suitable to use grinding wheels with organization greater than 12#.

6、The selection of shape and size should be based on the conditions of the grinding machine and the shape of the workpiece.

Commonly used grinding wheel shapes include flat grinding wheel (P), single-sided concave grinding wheel (PDA), double-sided concave grinding wheel (PSA), thin-flake grinding wheel (PB), cylindrical grinding wheel (N), bowl-shaped grinding wheel (BW), disc-shaped No. 1 grinding wheel (D1) and so on.

There is a range of grinding wheel shapes and sizes that can be used on each type of grinder. Whenever possible, the outside diameter of the grinding wheel should be as large as possible to increase the line speed of the wheel and achieve higher productivity and surface quality of the workpiece, and the same effect can be achieved by increasing the width of the wheel.

The current national standard grinding wheel writing order: grinding wheel code, size (outer diameter × thickness × aperture), abrasive grit, grain size, hardness, organization, binding agent, maximum working line speed.

e.g. P400×150×203A60L5B35

Ⅱ. Selection of diamond grinding wheels

Diamond grinding wheels have sharper cutting edges, less wear, longer life, higher productivity, and better machining quality than grinding wheels made from general abrasive grains such as boron carbide, silicon carbide, and corundum, but are more expensive, making them suitable for precision grinding of hard alloys, ceramics, semiconductors, and other hard and brittle materials that are difficult to machine.

Diamond wheel characteristics include abrasive type, grit, hardness, concentration, bond, wheel shape and size.

Abrasives: Widely used synthetic diamonds (JR), according to its crystalline shape and grain strength, divided into various models, according to its specific purpose to select the model.

Grain size: It should be considered in terms of workpiece roughness, grinding productivity and diamond consumption.

Hardness: Only resin-bonded diamond wheels are characterized by “hardness”. Generally S (Y1) grade or higher is used.

Binding agent: There are four kinds of binding agents in common use, and their binding ability and abrasion resistance are in the order of resin, ceramics, bronze and electroplated metal, which are gradually strengthened in the following order. Resin-binding agent diamond grinding wheel has high grinding efficiency, good roughness of processed workpiece, wide range of application, good self-sharpness, not easy to clog, small heat generation, easy dressing, and is mainly used in fine grinding process. Ceramic bonded diamond grinding wheels are mainly used for grinding various non-metallic hard and brittle materials, hard alloys and super-hard materials.

Concentration: The choice of concentration depends on the requirements of the grinding wheel’s grain size, bond, shape, processing method, productivity and wheel life. Highly concentrated diamond grinding wheels have a strong ability to maintain the shape of the grinding wheel, while low concentration grinding wheels tend to consume less diamond when grinding, and should be selected according to the needs of discretion.

Shape and size: Selected according to the shape and size of the workpiece and the conditions of the tooling machine.

Ⅲ. Cubic boron nitride (CBN) grinding wheel selection

Cubic boron nitride (CBN) grinding wheel of cubic boron nitride particles sticking to the surface of the ordinary grinding wheel is only a very thin layer, its abrasive toughness, hardness, durability is corundum type of grinding wheels 100 times, the most suitable for processing high hardness, viscosity, high temperature strength, low heat conduction rate of hard-to-grind steels and high-speed or ultra-high-speed grinding. Its scope of application is complementary to that of synthetic diamonds. Diamond grinding wheels have unique effects in grinding hard alloys and non-metallic materials, but the effects are not obvious in grinding steel, especially in grinding specialty steels. Cubic boron nitride grinding wheels are nearly 100 times more efficient than corundum grinding wheels and five times more efficient than diamond grinding wheels in grinding steel, but less effective than diamond in grinding brittle materials.

Cubic boron nitride grinding wheels are selected similarly to diamond grinding wheels. However, in the choice of bonding agent, most of the resin bonding agent, followed by electroplating, metal bonding agent. Ceramic bond CBN grinding wheel is mainly used for titanium alloy, high-speed steel, forging cast iron and other difficult to process ferrous metal grinding, resin bond CBN grinding wheel for grinding ferromagnetic materials, is the ideal choice for processing steel. CBN grinding wheel concentration is generally in the 100%~150% between the selection of a more economical and reasonable, it can not be used with ordinary cutting fluid, the need for special cutting fluid.

Ⅳ. Selection of large air hole grinding wheel

Large air hole grinding wheels have the advantages of being less likely to be clogged when grinding, high durability and strong cutting ability. It is suitable for coarse and fine grinding of non-metallic materials such as soft metals and plastics, rubber and leather. At the same time, it is characterized by fast heat dissipation, so it has a good effect in grinding some heat-sensitive materials, thin-walled workpieces and dry grinding processes (such as sharpening hard alloy tools and machine guides, etc.).

A macroporous wheel is manufactured in the same way as a normal ceramic bonded wheel. The difference is that a certain amount of pore-enhancing agent is added to the dosage, which is completely volatilized or incinerated before the wheel is sintered, thus creating large pores.

The production range of large air hole grinding wheels is as follows: abrasives generally choose carbide and corundum, such as commonly used black silicon carbide (C), green silicon carbide (GC) and white corundum (WA) and so on, the hardness of these abrasives is high, brittle and sharp, with good thermal and electrical conductivity; abrasive grit size (36#~180#); binder (ceramic binder); hardness (G~M levels); shape (flat, cup, bowl or disc, etc.); air hole size (about 0.7~1.4 mm). (flat, cup, bowl or disk, etc.); pore size (about 0.7~1.4mm).

Generally speaking, the grinding wheel factory will be marked with a line of values, the general order is shape code, size, abrasive, grit number, hardness, organization number, combining agent and the maximum permissible line speed. For example, “P400×40×127WA60L5V35” is analyzed as follows:

“P” indicates that the grinding wheel shape is parallel.

The numbers “400×40×127” represent the size of the grinding wheel (outer diameter × thickness × bore diameter). “WA” indicates that the abrasive material of the grinding wheel is white alumina. “60” represents the grain size of the grinding wheel, with 60# being finer (suitable for finish grinding). “L” indicates medium-soft hardness. “5” represents the wheel’s structure, which is dense. “V” indicates the bonding agent is ceramic, which is relatively brittle. “35” represents the grinding wheel’s speed, with 35 m/s being moderate.

Principles for selecting grinding wheels:

• For grinding steel, use alumina-based wheels. For grinding hard cast iron, hard alloys, and non-ferrous metals, use silicon carbide wheels.
• For soft and hard materials, use hard and soft wheels, respectively.
• When grinding soft, tough materials, use coarse abrasives (such as 12-36#); for hard, brittle materials, use finer abrasives (such as 46-100#).
• When a low surface roughness is required, use fine-grained abrasives; for high metal removal rates, use coarse abrasives.
• When high surface quality is required, choose resin or rubber-bonded grinding wheels; for maximum metal removal rates, choose ceramic-bonded wheels.

There are two ways of describing the hardness of grinding tools:

I. Abrasive (work) hardness refers to the resistance of the bond to the detachment of abrasive grains from the surface of the abrasive (work) under the action of an external force, or the ease of detachment of abrasive grains from the surface of the abrasive (work).

II, the hardness of the abrasive (work) means the bond and abrasive grains in the work of resistance to external forces from the surface of the grinding wheel rupture when the integrated strength

Ⅴ. Selecting the hardness of the grinding wheel

The hardness of the grinding tool reflects the bonding agent’s ability to hold the abrasive grains, not the hardness of the abrasive itself. The basic principle when selecting the hardness of a grinding tool is to ensure the tool maintains proper self-sharpening during grinding, avoids excessive wear, and prevents high grinding temperatures.

As mentioned before, the hardness of the grinding (work) and the number of binders, the higher the hardness of the grinding (work), the number of binders should be more, the more thick and strong the binders bridge, the binders on the abrasive holding force is greater, so that the abrasive grains can withstand a larger grinding force and not broken or fall off. On the contrary, when the hardness of the grinding (work) tool is low, the holding power of the binding agent on the abrasive grain is small, and the abrasive grain is easy to break or fall off. Therefore, if the hardness of the abrasive tool is too high, it will not only make the passivated abrasive grains not easy to break or fall off and lose the cutting ability, but also increase the friction between the abrasive tool and the workpiece, and the surface of the workpiece will be easily heated up and burns will occur. In order to remove the dull abrasive grains in time, it is necessary to frequently dress the grinding tool, resulting in a large amount of abrasion of the grinding tool. If the hardness of the abrasive tool is chosen to be soft, the abrasive grains will fall off when they are still sharp, resulting in unnecessary wear. Meanwhile, if the abrasive tool wears too fast, the working surface will be unevenly abraded, which will affect the machining accuracy of the workpiece.

In summary, only the correct choice of grinding (work) tool hardness, in order to maintain its normal grinding state, to meet the needs of processing. Especially when sharpening the Magic Puppet Writing Tool, even if the hardness of the grinding (work) tool deviates by a small degree, it will affect the quality of sharpening, so it can be seen that the impact of the hardness of the grinding (work) tool is very important.

When selecting the hardness of the abrasive tool, the most basic method is: if the hardness of the workpiece is high, the hardness of the abrasive tool should be low; if the hardness of the workpiece is low, the hardness of the abrasive tool should be high. Because the workpiece hardness is low, the abrasive (work) tool on the abrasive grains cut into the workpiece to withstand the pressure is correspondingly smaller, the abrasive grains are not easy to blunt, in order to make the abrasive grains will not be blunt in the abrasive before the production of crushing or shedding, so the selection of higher hardness of some of the abrasive (work) tool is more appropriate; on the other hand, workpiece hardness is high, the abrasive grains cut into the workpiece withstand a greater pressure correspondingly blunt, the selection of softer hardness of the abrasive (work) tool can be produced in time to produce self-sharpening, and the workpiece hardness is low, the abrasive (work) tool is high. The use of softer abrasives can produce self-sharpening in time and maintain the grinding performance of the abrasives. However, when the workpiece material is softer and tougher (e.g., soft bronze, brass, etc.), the cut metal will easily block the abrasive tool, so it is appropriate to use a softer abrasive tool with a coarser grit to process the workpiece.

The hardness of the grinding tool is also an important factor affecting the temperature of the grinding zone. Grinding low thermal conductivity of the workpiece (such as alloy steel), because the surface temperature of the workpiece is relatively high, so it is often easy to produce burns, cracks, then you should choose a lower hardness, looser organization of the grinding wheel, while also strengthening the cooling, so as to effectively avoid burns on the workpiece. Similarly, when grinding thin workpieces, it is also necessary to use a wheel with a looser organization and lower hardness. When grinding the outer circle of thin-walled hollow workpieces, the hardness of the grinding wheel is lower than when grinding solid workpieces, which is also to prevent the workpiece from deformation caused by the increase in grinding temperature.

When selecting the hardness of an abrasive tool, the following conditions should be considered in general

• When the contact area between the grinding tool and the workpiece is large, the hardness of the grinding tool should be lower to avoid excessive heat generation in the workpiece, which may affect the grinding quality. For example, the hardness of grinding tools used for vertical shaft flat grinding is lower; the hardness of grinding wheels used for flat grinding and internal soft grinding is lower than that of grinding wheels used for cylindrical grinding. However, when grinding small and long internal holes, due to the low speed of the grinding wheel, the grinding wheel is easy to wear the workpiece taper (flare), so the hardness of the grinding wheel should be higher than the general internal grinding. Similarly, a harder grinding wheel should be used for grinding small diameter workpieces, while a softer wheel should be used for grinding large diameter workpieces.
• When grinding broken surfaces and burring castings, hard or super-hard grinding wheels should be used; when grinding billets under heavy loads, hard or super-hard grinding wheels should also be selected to prevent the wheels from wearing out too quickly.
• The diamond grinding tools (grinding wheels or oil stones) used for dressing are often of super-hard grade due to the higher pressure and hardness required for dressing.
• Heavy grinders and grinders with good rigidity can be used with lower hardness grinding wheels because they vibrate less during grinding and the abrasive grains are not easily damaged.
• In plunge-cut grinding, the hardness of the grinding wheel should be lower than that of axial feed in order to avoid burns on the workpiece.
• Grinders with automatic tool setting can use softer grinding wheels than grinders with manual tool setting.
• The smaller the roughness value required for the processed surface, the more precise the size of the workpiece is required, the lower the hardness of the grinding wheel should be selected to avoid excessive grinding heat, and the surface organization of the workpiece is deteriorated. For example, with ultra-soft mirror grinding resin bond grinding wheel, can be ground roughness Rz for 0.05μm surface, but for general fine grinding wheel, the hardness should be higher, otherwise it will be due to the working surface of the wheel to produce uneven wear and affect the processing accuracy of the workpiece.
• Scratches on the surface of the workpiece are often associated with improper selection of the hardness of the abrasive tool. When the hardness of the abrasive (tool) is too low, the abrasive grain is easy to fall off, so due to the action of extrusion or friction, the abrasive grain will scratch the surface of the workpiece, so this time to appropriately improve the hardness of the abrasive (tool).
• When dry grinding, the workpiece is easy to heat up, so you should use a softer grinding wheel than when wet grinding.
• Higher production efficiency requirements, you can choose a softer grinding wheel, in order to facilitate the self-sharpening of the grinding wheel, reduce the number of dressing. However, the grinding wheel wear will increase accordingly, and thus in the technical and economic indicators to carry out a comprehensive analysis and comparison.
• High-speed grinding, when the feed rate is unchanged, the chip cut by the abrasive grain becomes thin, the cutting force borne by the abrasive grain is reduced accordingly, and the wear of the grinding wheel is also slower, in order to improve the self-sharpening of the grinding wheel, its hardness should be softer than that of the general grinding when grinding 1~2 small grades, which is the case of high-speed fine grinding. Similarly, for some unbalanced workpieces (e.g. crankshafts, etc.), since the speed of the workpiece during grinding cannot be too high, the hardness of the grinding wheel should also be chosen to be lower to avoid burning the workpiece. In order to improve the cutting efficiency for the main purpose of high-speed grinding, the cutting feed should be increased, and the grinding force on the abrasive grains should be increased at this time. In order to ensure that the abrasive grain is not prematurely dislodged, the hardness of the grinding wheel should be higher than the ordinary grinding 1 ~ 2 small level.
• When grinding steel balls, the grinding wheel should be of super-hard grade; for general cutting, the hardness of the grinding wheel should be of medium to medium-hard grade.
• When sharpening carbide and HSS tools, choose a grinding wheel with J~G hardness.
• In profile grinding, in order to maintain the correct geometry of the workpiece, the wear of the grinding wheel should not be too great, so the hardness of the grinding wheel should be higher.