Abrasive grains are available in a wide range of grit sizes and bonding agents. Determining which wheel is right for the job requires considering seven key factors:
The first thing to consider when selecting a grinding wheel specification is the workpiece material type and hardness. Is the material easy or difficult to grind? The relative ease of grinding is a major predictor of the appropriate abrasive type, grain attribute, grit size and bond type for the application.
By convention, aluminum oxide grains are used for grinding ferrous metals, and silicon carbide grains for non-metals and non-ferrous metals. Ceramic and superabrasive grains are compatible with all three types of materials, but are typically meant for specific circumstances where aluminum oxide and silicon carbide perform poorly.
With the grain type is established, material grindability determines many of the other necessary attributes for the grinding wheel. If the material is easy to grind, use a tough, durable grain. Since the material is easy to grind, grains shouldn’t break down too soon or too easily, so the whole grain can be used to maximize wheel life. A coarser grit is best for these materials, as the grains can easily penetrate the material and maximize stock removal. A harder grade (that is, a harder bond between the grains and the wheel) also corresponds to easier grinding, as the bond will prevent the wheel from releasing the grains before they are consumed.
For hard-to-grind materials, reverse these recommendations. Mild, friable grades perform better on these materials, as they fracture more easily and stay sharper. Finer grit sizes improve the ability of the particles to penetrate hard materials and form chips. Because the grits will dull and cause metallurgical damage such as burning if held for too long, soft grades are necessary to release dulled grains and expose the material to sharper ones.
Users should also consider the grinding pressure, or force per grain. The higher the pressure, the more severe the operation, and the better ceramic and superabrasive grains are likely to perform. The severity of the operation also helps determine the attributes of the abrasive grain.
Tough, durable grains are able to tolerate higher levels of pressure and not break down prematurely. Coarser grits also assist the grains in holding up to grinding pressure. There may be times where distributing the pressure over more cutting points is best, but even that situation requires balance to prevent the pressure from turning the finer grains into dust. Heavy pressure also requires harder grades so grains can stay on the wheel long enough to perform the required grinding work.
In contrast, mild, friable grains perform better in light-pressure operations, since durable grains will only rub and dull. Finer grit sizes ensure that the grains can still fracture properly and self-sharpen, and softer grades release dulling grains before they begin to rub and burn the material undergoing grinding.
Grinding wheels achieved their ubiquity due to their speed, form repeatability and ability to achieve desired finishes. When selecting a wheel, it is important to determine whether the application requires rapid stock removal or a fine finish. Equally important is whether the part will be simple and flat, or if there is a form to hold.
The required surface finish, dimensional tolerances, form holding requirements and stock removal rates factor into the appropriate grit size, grade and bond type.
For low-Ra finishes or close geometric tolerances, finer grits are helpful because they provide more points of contact between the work and wheel. This helps with precision finishes, which have a shallower scratch pattern and a lower micro-inch finish. Finer grains also aid in achieving and holding small-radius and complex forms. Coarser grits, by contrast, improve stock removal rates. Finding the optimal balance in grit size will decrease cutting cycle times.
Close geometric accuracy and form holding requires a harder grade. Harder grades enable the wheel to hold its profile longer and ensure the grains are held long enough to achieve the desired results.
This next recommendation may seem contradictory, but softer bonds are optimal for both finer finishes and higher stock removal. A wheel with a softer bond will easily release dull grains and keep newer, sharper grains in contact with the material. Sharper grains increase stock removal and improve finish by preventing dull abrasives from rubbing and burning the part during stock removal operations. Although the actual finish and stock removal rates from this operation are primarily dependent on grit size, keeping sharp grains in the grind zone benefits both.
Part requirements also determine the bond type. Vitrified wheels perform best for close tolerances and form holding, while organic and resin bonds are best for reflective and other fine finishes. Organic bonds, unlike vitrified bonds, have a little give to them. Some of the grinding forces go into the bond, reducing the chip size. Another benefit for fine finish grinding is the way organic bonds break down from the heat of the grind. They tend to hold grains a little longer, allowing them to run and dull. Planned plowing and sliding interactions that take place under these circumstances improve upon the initial scratch pattern formed during stock removal to generate a finer finish.
Area of contact is partly related to severity of operation in that it considers the area of contact between the work and the wheel. When a wheel is applied to the work, the force applied is distributed over all the cutting points in the grind zone. The larger the area of contact, the lower the force per grain. Conversely, the smaller the area, the higher the force per grain.
Operations with a small area of contact should use tough, durable grains that will not fracture too early or suffer premature wear under higher force per grain. Ceramic or superabrasive grains may even be necessary in these operations. Finer grit sizes are optimal for small areas of contact, because in addition to providing more abrasive points at the area of contact, the relative pressure or grinding forces will be split among many grains. The high forces of operations with smaller areas of contact also call for harder-grade wheels, as these will hold their shape against premature wheel wear.
When the area of contact increases and becomes larger, such for a Blanchard segment, milder grains are more appropriate. Due to the increased number of grains in contact with the work in the grind zone, the force per grain is lower and the grains will fracture and self-sharpen more easily. Coarse grits spread the pressure into fewer grains to ensure they will continue to penetrate the work. As the risk of burning from dulled grains is higher in these operations, softer wheels grades should be used so grains release before doing damage to the part.
Operations’ wheel surface speeds can narrow down the bond type and wheel grade necessary to complete them. To calculate surface speed, use these equations:
Surface Speed (SFPM) = (π × Diameter (inch) × RPM) / 12
Surface Speed (m/s) = (π × Diameter (mm) × RPM) /
Wheel speed determines what bond type is best for the required speed, or if a special high-speed bond might be required.
As a general rule:
Wheels will also act differently based on their speed. For every 1,000 SFPM (5.08 m/s) the surface speed changes, the wheel will act one grade harder or softer. Slower wheel speeds equal softer performance, as the higher force per abrasive particle causes the grains and bond to break down quicker. Faster wheel speeds lead to harder performance, with the lower force per abrasive particle making the grains and bond act more durable.
Coolant in a grinding system affects vitrified and organic (resin) bonded wheels differently, and plans for its use must be considered when determining the wheel's grade or hardness.
If coolant is used:
If no coolant is used:
The horsepower of the grinding machine can play a role in determining the grade of the bond or the hardness of the wheel.
Due to how many factors are involved in determining the starting specification for a grinding wheel, there will be situations when factors point in opposite directions. In such cases, look at where the majority of the factors are pointing, or place precedence on the most important factors for the application. For simple comparison, review the chart below:
11、What are the precision dressing techniques for grinding wheels in high-speed grinding?
A: The current application of more mature grinding wheel dressing technology are:
(1) ELID online electrolytic dressing technology;
(2) EDM wheel dressing technology;
(3) cup-shaped grinding wheel dressing technology;
(4) electrolytic - mechanical composite shaping technology
12、What is precision grinding? Try to briefly describe the ordinary grinding wheel precision grinding in the grinding wheel selection principles.
A: Precision grinding refers to the precision grinding machine, select fine-grained grinding wheel, and through the fine dressing of the wheel, so that the abrasive grains have micro-edged and isometric, after grinding, so that the grinding traces left on the surface to be grinded is extremely fine, the residual height of a very small, coupled with the role of the non-sparking grinding stage, to obtain the machining accuracy of 1 ~ 0.1mm and the surface roughness of Ra for the surface of the 0.2 ~ 0.025mm Grinding method.
Principles for selecting grinding wheels in ordinary grinding wheel precision grinding:
(1) grinding wheel abrasive precision grinding when the abrasive used in the grinding wheel in order to easily produce and maintain the micro-edge and its isotropy as a principle.
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(2) Grinding wheel grain size? Considered solely from the geometric factors, the finer the grinding wheel grain size, the smaller the surface roughness value of grinding. However, when the abrasive grain is too fine, not only is the grinding wheel easy to be clogged by abrasive debris, if the thermal conductivity is not good, on the contrary, it will produce burns and other phenomena on the machined surface, so that the surface roughness value increases, therefore, the grinding wheel grit size is often taken to be 46 # ~ 60 #.
(3) grinding wheel binding agent? Wheel binding agent has resin, metal, ceramics, etc., to the resin class is widely used. For coarse-grained grinding wheel, available ceramic bond. Metal, ceramic bond is currently an important aspect of research in the field of precision grinding.
13、What are the characteristics of precision grinding with superabrasive wheels? How to choose its grinding dosage?
A: The main features of superabrasive grinding wheel grinding are:
(1) can be used to process a variety of high hardness, high brittleness metal and non-metallic materials.
(2) Strong grinding capacity, good wear resistance, high durability, can be longer to maintain the grinding performance, less dressing, easy to maintain the particle size; easy to control the processing size and realize processing automation.
(3) The grinding force is small, the grinding temperature is low, which can reduce the internal stress, no burns, cracks and other defects, and the processing surface quality is good. Diamond grinding wheel grinding carbide, its grinding force is only green silicon carbide 1/4 ~ 1/5.
(4) High grinding efficiency. In the processing of cemented carbide and non-metallic hard and brittle materials, the metal removal rate of diamond grinding wheel is better than that of cubic boron nitride grinding wheel; however, in the processing of heat-resistant steel, titanium alloy, mold steel and other materials, the cubic boron nitride grinding wheel is much higher than that of diamond grinding wheel.
(5) Low processing cost. Diamond grinding wheel and cubic boron nitride grinding wheel is more expensive, but its long life, high processing efficiency, so the comprehensive cost is low.
Superabrasive grinding wheel grinding dosage selection:
(1) grinding speed non-metallic bond diamond grinding wheel grinding speed is generally 12 ~ 30m / s. Cubic boron nitride grinding wheel grinding speed can be much higher than the diamond grinding wheel, optional 45 ~ 60m / s, mainly due to the better thermal stability of cubic boron nitride abrasive.
(2) The depth of grinding is generally 0.001～0.01mm, which can be selected according to the specific conditions of grinding method, abrasive grain size, bonding agent and cooling condition.
(3) The workpiece speed is generally 10～20m/min.
(4) Longitudinal feed speed? Generally 0.45～1.5m/min.
14、What is ultra-precision grinding? Try to briefly describe its mechanism, characteristics and applications.
A: Ultra-precision grinding refers to the processing accuracy of 0.1mm or less, the surface roughness is lower than Ra0.025mm wheel grinding method, is a sub-micron level processing methods, and is developing to the nanometer level, suitable for steel, iron materials and ceramics, glass and other hard and brittle materials processing.
Ultra-precision grinding mechanism:
(1) The abrasive grain can be regarded as an elastomer with elastic support and a large negative cutting edge, the elastic support for the bonding agent, although the abrasive grain has a considerable hardness, the deformation of its own force is very small, in fact, still belongs to the elastomer.
(2) The depth of penetration of the cutting edge of the abrasive grain starts to increase gradually from zero, and then gradually decreases to zero after reaching the maximum value.
(3) The whole contact process between the abrasive grain and the workpiece is elastic zone, plastic zone, cutting zone, plastic zone and elastic zone.
(4) In ultra-precision grinding, micro-cutting action, plastic flow, elastic destructive action and sliding rubbing action appear sequentially according to the change of cutting conditions. When the cutting edge is sharp and there is a certain grinding depth, the micro-cutting effect is stronger; if the cutting edge is not sharp enough, or the grinding depth is too shallow, plastic flow, elastic damage and slip rubbing will occur.
The characteristics of ultra-precision grinding:
(1) Ultra-precision grinding is a systematic project.
(2) Ultra-hard abrasive grinding wheel is the main tool of ultra-precision grinding.
(3) Ultra-precision grinding is a kind of ultra-micro removal processing.
Application of ultra-precision grinding:
(1) Grinding of metal materials such as steel and its alloys especially hardened steel that has been quenched and other treatments.
(2) It can be used to grind non-metallic hard and brittle materials? such as ceramics, glass, quartz, semiconductor materials, stone and so on.
(3) At present, there are mainly ultra-precision grinding machines such as cylindrical grinding machines, surface grinding machines, internal grinding machines, coordinate grinding machines, etc., which are used for ultra-precision grinding of cylindrical, surface, holes and hole systems.
(4) Ultra-precision grinding and ultra-precision free abrasive processing are complementary.
15、Try to briefly describe the principle and characteristics of ELID mirror grinding.
A: ELID mirror grinding principle: in the grinding process between the grinding wheel and tool electrode pouring electrolytic grinding fluid and DC pulse current, so that the anode as the anode of the grinding wheel metal bond to produce an anodic dissolution effect is gradually removed, so that the electrolysis does not affect the abrasive grains protruding wheel surface, with the electrolysis process, gradually formed on the surface of the wheel with an insulating layer of oxidized film, to prevent electrolysis process. As the electrolysis process proceeds, an insulating oxide film is gradually formed on the surface of the grinding wheel, preventing the electrolysis process from continuing. When the abrasive grains of the grinding wheel are worn out, the passivated film is removed by scraping of the workpiece, and the electrolysis process continues to be carried out, and the process continues to be carried out, and the cycle begins again and again.
Characteristics of ELID grinding:
(1) The grinding process has good stability;
(2) The dressing method keeps the diamond grinding wheel from wearing out too quickly and improves the utilization rate of precious abrasives;
(3) The ELID dressing method makes the grinding process have good controllability;
(4) With the ELID grinding method, it is easy to realize mirror grinding, and it can greatly reduce the residual cracks of the super-hard material parts being ground.