2019 How to Choose The Right Superabrasive Grinding ...

A grinding wheel applied superabrasive (diamond or cubic boron nitride) as the cutting material is called superabrasive grinding wheel. Because superabrasive have great advantages over conventional abrasives, superabrasives have good grinding performance compared with conventional abrasives and have been used more and more widely.

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1 Superabrasive grinding wheel features

1)Superabrasive itself has extremely high hardness and can process various high hardness materials, especially for materials that are difficult to process with conventional abrasives.

2)In the processing of superhard abrasives, the wear is small, and the service life is long, and the grinding ratio is high, can obtain better economic efficiency.

The shape retention is good, and should not replace and dress the grinding wheel frequently. Save the working hours, and improve the working conditions, and it is good for processing.

3)With sharp cutting and small grinding force, the superabrasive is good for improving the processing accuracy and surface roughness of the machined parts, reducing the power consumption of the machine tool.

Sharp abrasive grains make the grinding temperature of the grinding tool in the processing is lower than that of conventional abrasive tool, which can greatly improve the quality of the surface to be processed, reduce and avoid the crack, burn, metallographic reorganization, and other defects, thus improving the service life of the parts and greatly improving the overall economic indicators.

In view of the unparalleled superiority of superhard grinding, with the continuous emergence of new materials and new technologies, the technological advancement of processing industry, its application has been continuously expanded, and the usage has been increasing, which has strongly promoted economic development.

However, due to the high tool cost of the superabrasive grinding wheel and its own unique performance and regularity, improper use will only cause serious waste, but also fail to obtain satisfactory processing quality and excellent economic effects.

2. Choose the right features of the superabrasives grinding wheel

Superabrasives are different abrasive tools from conventional abrasive tools. Its structural forms are different, and they have unique features. The features of superabrasives refer to abrasives, grain size, bond, concentration, shape, and size.

1).

Selection the grain of Superabrasive grinding wheel

Superabrasives mainly include diamond and CBN (cubic boron nitride). The diamond used for superhard grinding wheels mainly refers to synthetic diamond, and natural diamond is basically not used for making abrasive tools.

Synthetic diamond and cubic boron nitride are both synthetically synthesized under high temperature and high pressure, features high hardness, good sharpness and wear resistance. Nature is still very different, so the application has a clear division of labor.

Diamond is the hardest material recognized by the human in nature. It has excellent hardness, strength, grinding ability, thermal conductivity, and thermal expansion coefficient.

It can process many hard materials such as tungsten carbide, ceramics, gemstones, glass, stone, building materials, concrete and semiconductor materials.

However, since diamond is an allotrope of carbon, it is easy to have a chemical reaction with iron group elements of steel at the higher temperature, causing strong chemical wear, which directly affects the grinding performance and loss of the abrasive tools, so it is not suitable to process the steel.

Although the hardness of cubic boron nitride is lower than that of a diamond, it is still much higher than that of conventional abrasives, and its thermal conductivity, thermal expansion coefficient, and grinding ability are also much better.

It is also an excellent grinding material. Its stability and chemical inertness are also much better than that of the diamond. Therefore, it is suitable for hard and tough metal materials that are difficult to process with conventional abrasives and not suitable for processing with diamond, such as tool steel, die steel, stainless steel, heat resistant alloy, and etc.

especially high vanadium high-speed steel and other metal materials that are sensitive to the grinding temperature.

There are many varieties of synthetic diamond and cubic boron nitride, and their applications are different. It is clearly stated in GB/T-94 standard (see table below), and some kinds of the synthetic diamond are only used for making tools, not for abrasives tool.

 

Type

         Application

Series

Code

Grit

Recommended application

Narrow scope

Wide scope

Synthetic   diamond

RVD

60/70&#;325/400

Resin bond, vitrified bond products

MBD

35/40&#;325/400

35/40&#;60/80

Metal bond abrasives, sawing, drilling, and electroplated products

SCD

60/70&#;325/400

Resin bond abrasives

SMD

16/18&#;60/70

16/20&#;60/80

Sawing, drilling, and dressing tools

DMD

16/18&#;60/70

16/20&#;40/50

Dressing tools

M-SD

36/54&#;0/0.5

Finishing, lapping, and polishing for hard and brittle materials

CBN

CBN

20/25&#;325/400

Resin, vitrified, metal bond products

M-CBN

36/54&#;0/0.5

Lapping and polishing for hard, brittle metal materials

There are diamond coatings and CBN coatings, that is, a layer of material is be plated on the surface of the abrasive grains. Usually, when dry grinding with resin bond abrasive, choose Cu coating abrasives. When wet grinding with resin bond abrasive, choose Ni coating abrasives. Usually, the weight gain of the coating should not over 50% of the abrasive weight, but the weight gain of profile grinding and intermittent grinding can reach 50-100%.

2).Selection the grit of superabrasive grinding wheel

Grit refers to the measure of grain size. The grit number refers to the classification mark of the abrasive grain size according to the standard. When marking the features of superabrasive tool, grit refers to the grain size number.

In the use of the superabrasive grinding wheel, the grit of grinding wheel is usually considered according to the surface roughness requirements of the workpiece to be processed, the grinding efficiency, the process requirements, the type of the bond. The following 2 tables can be used as a reference.

Abrasive Grit No.

The surface roughness Ra(μm)

Rein bond

Metal bond

80/100&#;100/120

&#;&#;

0.32&#;2.5

100/120&#;170/200

0.16&#;0.63

0.16&#;1.25

170/200&#;270/325

0.08&#;0.32

0.16&#;0.63

325/400&#;M10/20

0.04&#;0.16

&#;&#;

M8/12&#;M4/8

0.02&#;0.08

&#;&#;

M4/8&#;M1.5/3

0.01&#;0.04

&#;&#;

 

 

Grinding Process

Grit

Rough grinding

80/100&#;120/140

Medium finish grinding

120/140&#;200/230

Finish grinding&#;Fine grinding

200/230&#;M36/54

Lapping, Polishing

M22/36&#;M0.5/1.5

In the grinding process, the operator usually has the experience of using conventional abrasives.

The principle of particle size selection is basically the same, but considering the difference of the two types of abrasive properties, it is required to achieve the same surface roughness due to superabrasive grain sharpness and non-abrasiveness, the grain size number is 1~2 grain size smaller than conventional abrasive.

When choosing the grain size of the superhabrasive, also should consider the bond. Because all kinds of bond have their optimum grain size range, and too rough or too fine is not good for the grinding process. Usually, 100/120# is used for resin bond abrasive, and 100/120-170/200# is used for vitrified bond abrasive, and 70/80-230/270# is used for metal bond abrasive.

Of course, it should consider processing efficiency and surface roughness requirements.

When high processing efficiency is required, should choose rough grain size. On the contrary, choose a finer-grained number.

3). Select the bond of superabrasive grinding wheel

The bond of superabrasive is indispensable for making abrasive tools. Its function is to bond the abrasive grains into a certain geometric shape and hold the abrasive grains to bear the grinding force during processing which plays the cutting role.

There is resin bond (B), vitrified bond(V) and metal bond(M) for superabrasive bond. How to choose the bond mainly depends on its own performance and the requirements for abrasives.

The resin bond has good self-sharpening, not block easily, rarely dressed, high grinding efficiency, low grinding temperature, certain elasticity and can be used for polishing, so it is widely used in superabrasives.

Resin bond diamond abrasives are often used in tungsten carbide workpieces, steel-based tungsten carbide workpieces, and semi-finishing and fine grinding of some non-metallic materials.

Resin bond CBN abrasives are mainly used for sharpening of high vanadium high-speed steel and semi-finishing and fine grinding of tool steel, die steel, stainless steel, and heat resistant alloy workpieces. However, the resin bond has poor heat resistance, and the wear resistance is poor at high grinding temperature, and the wear of the abrasive tool is large, which is particularly obvious at the time of large load grinding, and is often improved by using metal-coated abrasive.

Vitrified bond is a kind of bond which is widely used at present. The abrasive with this bond has good chip space on the working surface, sharp cutting, high grinding efficiency, not heat and block easily,

which is good for stable grinding, and small thermal expansion, and is easy to control the processing accuracy.

Also, this abrasive tool is relatively easy to truing and dressing, and it very good for facilitating the cutting ability of the working surface of the abrasive.

Usually, it is used for rough grinding, semi-finishing, and large-scale forming grinding. Due to the poor heat resistance of diamond, the vitrified bond should be sintered at low temperature and it is very difficult. However, in recent years, making vitrified bond diamond grinding wheel has become more and more mature, mainly used for grinding PCD, PCBN, tungsten carbide tools, PDC drill bits, and etc.

Vitrified bond CBN abrasive is the most widely used bond method for CBN. It is widely used in the automotive industry to grind steel parts such as crankshafts and convex bearings.

The metal bond has two types of bronze bond and electroplated bond.

Bronze bond abrasives are fully mixed with copper powder and tin powder as the main material and other materials improved their performance, and add the abrasive to mixture, and then place in the mold for compression molding( usually cold pressed- pressed at room temperature and hot pressed-pressed under heating condition), and then sintered it.

This bond and the abrasive grain have high bonding strength, good wear resistance, small wear, long service life, good shape retention, and can withstand large loads.

However, it has poor self-sharpening, easy to block the surface, large heat, and it is also very difficult to dress. It is mainly used for rough grinding, finish grinding non-metallic materials such as glass, ceramic, stone, building materials, concrete, semiconductor materials, and cutting process.

Some few are used for tungsten carbide, composite superhard materials grinding, and profile grinding, honing, electrolytic grinding and etc.

Bronze bond CBN abrasives are currently rarely used except for honing of alloy steel workpiece.

The electroplated bonding agent is one type with higher bonding strength. Usually, plate the single or multi-layer abrasive grains on the metal substrate by an electroplating method.

The number of abrasive grains per unit area on the working surface of the bond is higher than other bonds, and the abrasive grains are exposed to the surface of the bond so that the cutting is sharp and the grinding efficiency is high.

However, due to the thickness of limitation of the coating, the total service life of the grinding wheel cannot be too high, and it is usually used for a special-purpose processor such as abrasive tools for forming grinding, electroplating reamer, boring tools, and etc.

Recent years, the application of CBN metal-bond abrasives has been expanding, especially in the processing of small holes, cavities and steel saw blades of various steel parts, which are more unique and have high grinding efficiency, good economy, and good shape accuracy.

4). Select the concentration of superabrasive grinding wheel

The superabrasive concentration refers to the quality of superabrasives per unit volume in the working layer of the abrasive tool, which is unique to superabrasives.

Internationally, when it is agreed to contain 0.88 g of super-hard abrasive per cubic centimeter of volume, the concentration is called 100%, code 100, and the commonly used concentrations are as follows:

The superabrasive concentration refers to the quality of superabrasives per unit volume in the working layer of the abrasive tool, which is unique to superabrasives.

International, when it is agreed to contain 0.88g of superabrasive per cubic centimeter of volume, the concentration is called 100%, the code is 100, and the commonly used concentration are as follows:

Concentration

Superabrasive contain in working layer     &#;g/cm3&#;

Code

25&#;

0.22

25

50&#;

0.44

50

75&#;

0.66

75

100&#;

0.88

100

150&#;

1.32

150

It is not good for normal use of superabrasives when the concentration is too high or too low, and directly affects grinding efficiency, processing costs, and processing quality.

The type of abrasive bond should be considered first. The various bond has its optimal concentration range due to the different bonding ability. The concentration of synthetic diamond abrasives is shown in the following table.

Bond

Concentration&#;&#;&#;

Resin bond

50&#;75

Vitrified bond

75&#;100

Bronze bond

100&#;150

 

The concentration of CBN abrasives is usually 25-50% than that of diamond abrasives due to the different nature of the abrasive and processing object.

 

For processing with special requirements, it should adjust the concentration.

For example, in polishing and high precision grinding, the concentration is lower than that in general grinding.

For a high processing efficiency process, coarse-grained and high concentration abrasive tools can be used.

For forming grinding, end face grinding, groove grinding with good shape retention, should use higher concentration abrasives.

 5). Select the shape and size of superabrasive grinding wheel:

The shape and size of the superhard abrasive can be determined according to the technical conditions and requirements of the machine tool, as well as the requirements of the processing part of the workpiece to be processed.

The shape of the commonly used superhard abrasive and applications are as follows:

Series

Name

Code

Application

Plane   series

Plane grinding wheel

1A1

Mainly used for cylindrical grinding, surface grinding, sharpening, grinder grinding, and centerless grinding.

Plane chamfering wheel

1L1

Mainly used for grinding with an arc between the bottom diameter and the side

Plane reinforcing wheel

14A1

Mainly used in processing occasions where the rigidity of the abrasive is high.

Arc wheel

1FF1

Mainly used for arc, molding grinding

Plane dovetail wheel

1EE1V

Mainly used for edging or forming grinding

Double inner bevel wheel

1V9

Thin grinding wheel

1A1R

Mainly used for cutting and slow-feeding of non-metallic materials such as stone and concrete.

Cutting wheel

1A6Q

Mainly used for cutting non-metallic materials such as stone and concrete.

Plane small grinding wheel

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1A8

Mainly used for internal grinding

Double beveled wheel

1E6Q

 

 

Mainly used for the groove, thread and other grinding

14E6Q

14EE1

14E1

1DD1

Single bevel wheel

4B1

Mainly used for sharpening, profile grinding, etc.

Single-sided concave grinding wheel

6A2

Mainly used for surface grinding, face grinding, etc.

Double-sided concave grinding wheel

9A1

 Mainly used for grinders, grinding gauges, etc.

9A3

Cylinder  series

Cylinder 1 grinding wheel

2F2/1

 

Special optical glass milling

Cylinder 2 grinding wheel

2F2/2

Cylinder 3 grinding wheel

2F2/3

Cup     series

Cup wheel

6A9

Mainly used for surface grinding, face grinding, etc.

Bowl-shaped wheel

11A2

 Mainly used for sharpening, surface grinding, etc.

11V9

Dish     series

Dish grinding wheel

12A2/20°

 

 

Mainly used for sharpening, gear grinding, etc.

12A2/45°

12D1

12V9

12V2

Special process series

Edging wheel

1DD6Y

 Specially used for optical glass edging

2EEA1V

Grinding disc

1A2

 Mainly used for surface grinding

10×6A2T

Electroplated diamond inner circular cutting saw blade

GN

Used for cutting precious material

Electroplated mounted wheel

MY

For internal hole grinding and jade grinding, dental grinding, etc.

Diamond finish pellet 1#

1A8/1

 Mainly used for fine grinding and superfinishing of materials such as optical glass, ceramics, and gemstones

Diamond finish pellet 2#

1P8/2

Diamond finish pellet 3#

1A2/3

Diamond finish pellet 4#

1A2/4

Stone series

 

Straight stone with arc

HMA/1

 

 

Specially used for all kinds of honing

Straight stone

HMA/2

Arc stone

HMH

Straight stone with a groove

2HMA

Stone with base

HMA/S°

3. Reasonable grinding amount

The superhard abrasive should use reasonable grinding amount after selecting the appropriate features of abrasives due to its own features. It can obtain the best grinding effect, and obtain high efficiency, good processing quality, and low processing cost.

1&#;Grinding wheel speed &#;vs&#;

The grinding wheel speed refers to the use speed of grinding wheel, the circumferential speed when working, that is, the tangential speed of grinding wheel measured at the maximum diameter on the circumference.

It is a vital indicator in the grinding parameters, and it has a great effect on the grinding. If the wheel speed is low, the grinding debris of the working surface has a large thickness, which causes the grinding wheel wear large, and the surface of the workpiece per unit time passes less abrasive grains, making the surface roughness poor.

As the speed of the grinding wheel increases, both of them are improved, but the increase of the grinding wheel speed is limited by the machine condition. When the grinding wheel speed is too high, it will vibrate and aggravate the wear of the grinding wheel, surface roughness and the increase in the grinding temperature, destruction of the surface quality of machined surface due to insufficient rigidity of the machine tool. These should be avoided.

Choosing the grinding wheel is based on the type of bond, grinding form and cooling conditions. Due to the poor thermal stability of the abrasive, diamond grinding wheel should be used at limited speed. It is often considered from the following two tables:

 

Bond

Cooling condition

Wheel speed&#;m/s&#;

Bronze

Dry grinding

12~18

Wet grinding

15~22

Resin

Dry grinding

15~20

Wet grinding

18~25

 

Grinding form

Wheel speed&#;m/s&#;

Surface grinding

20~30

Cylindrical grinding

20~30

Internal grinding

12~15

Tool grinding

12~20

 

CBN grinding wheel has much higher thermal stability than that of the diamond. The higher grinding temperature caused by the processing at higher speed does not cause the strong wear like the diamond grinding wheel, which causes the grinding wheel to wear seriously and the processing quality is lowered, and the processing cost is increased.

so CBN grinding wheel can use higher grinding speed, and higher grinding wheel speed can better exert the features of CBN abrasives, and obtain better processing effect. Usually, use the following parameters:

 

Grinding form

Wheel speed&#;m/s&#;

Remark

Dry grinding

Wet grinding

Surface grinding

25~30

28~35

Wet type

Cylindrical grinding

25~30

30~40

Wet type

Internal grinding

17~25

18~28

Wet type

Tool grinding

15~25

18~28

Wet type

With the development of grinding machine design and manufacturing technology, as well as the advancement of abrasive materials, more and more high speed grinding processes have been used, and achieve the better grinding results, especially CBN abrasives are widely used with high value. Electroplated CBN grinding wheel is 120-150m/s or even higher, and individually reaches 250m/s.

 

2). Selection of grinding depth (a):

Grinding depth refers to the distance along the radial direction of the grinding wheel when the workpiece reciprocates.

Its value directly affects the machining efficiency, machining quality, and processing cost. It is also a very important parameter for grinding.

As the grinding depth increase, the machining efficiency increases, and the wear of the abrasives increases, and the contact pressure between the grinding wheel and the workpiece increases accordingly, making the grinding force and grinding temperature increase.

When reaching the a certain amount, increase the wear of grinding wheel, and cause machine vibration, so that the surface quality is quickly deteriorated, and the grinding temperature is too high, it will also cause burns and cracks on the surface to be machined, and the life of the workpiece will be greatly reduced.

The selection of the grinding depth should be considered in accordance with the requirements of the processing efficiency, the grinding method, abrasive grain size, bond type, and the processing cost. Under the current machine conditions, the following parameters are recommended for diamond grinding wheels.

Abrasive grain size No.

Grinding depth&#;mm/stroke&#;

Vitrified, resin bond

Bronze

70/80~120/140

0.01~0.02

0.01~0.03

140/170~230/270

0.005~0.015

0.01~0.02

230/270 and finer grain size

0.002~0.005

 

Grinding form

Grinding depth&#;mm/stroke&#;

Surface grinding

0.005~0.02

Cylindrical grinding

0.005~0.015

Internal grinding

0.002~0.01

Sharpening

0.01~0.03

CBN grinding wheel has better self-sharpness and better physical properties such as thermal conductivity and thermal stability, and can well maintain the sharpness of the working surface of the grinding wheel, and the grinding depth can be larger than that of the diamond grinding wheel.

3. Confirmation of workpiece speed &#;vw&#;

The workpiece speed refers to the relative instantaneous speed of the workpiece machined surface relative to the working point of the grinding wheel.

It has only a slight effect on the grinding effect and is much weaker than other machining parameters. Of course, the workpiece speed is too low, which affects the machining efficiency and the surface roughness to a certain extent.

If the workpiece speed is too high, the wear of the grinding wheel will increase slightly, and due to the limitation of the rigidity of the machine tool, the impact force will be aggravated and the noise will be increased.

Usually, when the superhard abrasive is ground, the workpiece speed is recommended to be within 10-20 m/min. For deep cutting grinding, the workpiece speed is very slow, as low as tens to hundreds of millimeters per minute.

4). Selection of axial feed rate &#;va&#;

The axial feed rate refers to the distance that the workpiece moves relative to the axis of rotation of the grinding wheel in a unit time. When the superabrasive is ground, the grinding efficiency increases as the axial feed rate increases, and the wear of the abrasive increase, and the surface roughness is reduced. For general processing, the parameters in the following are recommended:

 

Grinding form

Axial feed rate

Cylindrical,  internal grinding

0.5~1m/min

Surface grinding

0.5~2mm/stroke

When sharpening, the operator manually controls the table to perform the feed motion in many cases. It is often 1-2 m/min.

For the processing of some special requirements, it should be adjusted. If finishing grinding and polishing the low surface roughness, should use small axial feed rate. A large axial feed rate is used for high-efficiency grinding of the coarse-grained grinding wheel.

4 Suitable grinding fluid

During the use of superabrasive grinding wheel, the temperature of the grinding zone is very high due to the removal of the workpiece by the abrasive grains and the relative friction between the abrasive and the workpiece, and the superabrasive has high thermal conductivity and is quickly transmitted.

It causing the rapid increase of the temperature of abrasives and will affect the normal progress of the grinding process&#;increase the wear of the grinding wheel, reduce the surface roughness of the workpiece, and deteriorate the processing quality.

When the temperature is too high, the tensile stress of the workpiece surface may even be caused greatly improved, causing surface cracks.

The too-high grinding temperature of the resin bond abrasive will also cause thermal decomposition of the bond near the abrasive grains, destroying the bonding strength, aggravating the wear of the abrasive, greatly reducing the economics of grinding process.

Therefore, when using superhard abrasives for processing, the grinding fluid should be used as much as possible under the conditions, reducing the temperature of the grinding zone, enhancing the lubrication effect, improving the processing conditions, and fully reach the purpose of use of the superhard abrasives.

When choosing the grinding fluid, in addition to the general requirements of non-corrosive, pollution-free, environment-friendly, it should also have lubricity, washing and cooling effects.

When grinding tungsten carbide and various high alloys by the diamond grinding wheel, it is advisable to use light mineral oil, soda water( resin bond is not suitable) and various water-soluble grinding fluids, weak alkaline emulsions, especially kerosene. Light diesel oil and water-soluble grinding fluid are best.

When processing the non-metallic material by the diamond grinding wheel, water is used as the grinding fluid. When processing by CBN grinding wheel, due to the abrasive is easy to hydrolyze with water at high temperature, which greatly aggravates the wear of the abrasives. Usually, water-soluble grinding fluid is not used, and most of the light mineral oil( such as kerosene, light diesel oil, etc.) is used.

When the water-soluble grinding fluid should be used, add the extreme pressure additive to reduce the hydrolysis of the abrasive.

Conclusion

The reader has a macroscopic understanding of the superabrasive grinding wheel( diamond and CBN) by the explanation. In the actual grinding work, choose the suitable grinding wheel for the workpiece correctly.

 

 

 

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