Informationen - Genel Bilgiler

INFORMATION

DIAMOND AND CBN GRINDING WHEELS

THE SUPER GRINDING AGENTS

Diamond and cubic boron nitrit (CBN) are the hardest known abrasives available today. For this reason they are suitable for materials that could be machined with conventional abrasives (silicon carbide,corundum) only with great difficulty.

Both abrasives have the same crystal structure. Diamond consist of pure carbon; CBN however, is composed of boron and nitrogen.

Despite their physical relationtionship, their chemical behavior  differs. Diamond ; because of it's  hardness and wear resistance,  is an important abrasive for grinding hard and brittle materials like tungsten carbide, metallized alloys and oxide ceramics. Cubic boron nitride is used for grinding hard and tough materials like hardened steels, high speed steels and chilled cast iron.

-High speed steels(HSS)

-Alloyed steels(HRc³58)

-Special alloys

-Stellite

When a grinding problem needs to be solved, the question of which abrasive is best suitable for a given material is always of prime importance. Frequently it is advisable to run basic trials.

Another difference between these two abrasives is their thermal stability. In the presence of oxygen, diamond burns  at  temperatures above 700 °C, while CBN remains stable up to 1400 °C. The lifetime of grinding tools can be much longer by using pure oil or emulsion of oil.

Diamond and CBN types of various shapes, strengths and sizes are available, depending on the bond, the material and grinding process.

GRIT SIZES

Grit   size   as  well as quality of abrasive determine to a wide range the performance of the wheel, the material removal rate,the efficiency of the grinding process and the quality of the obtained surfaces.

In order to quarantee perfect cutting qualities at  prescribed surface finishes, accurate grit calibration is essential. This is achieved by sieving and precision sedimentation.                      

Through continuous research and development of our products we are always able to use optimal types with respect to the bond.

With coarser grains, the grinding performance is greater. On the other hand, finer grains increase the finish quality, but decrease the stock removal capability. 

MicroDiam grinding tools are manufactured and marked in accordance with FEPA standards.

BONDS

The bond and the diamond grit form the impregnated diamond section of the wheel. Apart from the diamond grit size and concentration, the bond itself decides the grinding performance of the wheel.

Generally, harder bonds give longer wheel life.

Material removal rate and wheel life are contradictory. As MRR increases, the wheel life decreases.

A grinding operation without wheel wear is impossible.

Depending on the grinding process and the application requirements, MicroDiam grinding tools are offered with various types of bonds.

Resin bonds & Metal bonds

  RESIN BONDS

Resin bonds are known for their high cutting performance.Resin bonds are recommended for medium, fine and super-fine grinding. They are especially suitable for material removal and can be used for dry and wet grinding.

METAL BONDS

Metal bonds without cooling are recommended only in exceptional cases for small contact surfaces.

SB   --> Wet grinding

SYB --> Wet grinding, high material removal rate

SKB --> Dry grinding, very hard bond

CSP --> Very hard bond, high requirement for profil and edge stability

CONCENTRATION

Concentration is the ratio between the weight of diamond or CBN in carats(0.2g) and the weight of a cubic centimeter; of bond. In accordance with FEPA standards, concentration "100" relates to a diamond content of 4,4 carats in a bond volume of 1 cubic centimeter; all other concentrations are proportional.

Normal concentrations for diamond and CBN wheels:   

WE GUARANTEE TO CONSIDER FOR OUR PRODUCTS THE VOLUMES OF DIAMOND AND CBN CORRESPONDING TO THESE STANDARDS.

THE MECHANICS OF THE GRINDING PROCESS

Grinding is a process in which chips are removed from the surface of the workpiece with the aid of abrasive materials. In contrast to other metal cutting processes such as turning, milling, etc., the material is removed by cutting edges of undefined geometry.

Basically a distinction must be made whether the grinding agent is bonded in a grinding tool or whether it exists in the form of loose grits such as grinding paste for polishing.

The selection of the optimum grinding wheel depends on the machine (grinding process, adjustment facilities, stability)and the material (hard and brittle materials such as tungsten carbide, ceramics or hard and tough materials such as HSS, chilled cast iron,etc.)

The specifications are influenced by the following parameters:

Grit size:Required surface finish, grinding process

Grinding agent:Material, e.g. diamond for tungsten carbide and CBN for HSS

Concentration:Contact surface grit size, grinding process

Bond:Grinding agent, grit size

SELF-SHARPENING

When a wheel works under optimum conditions, the surface largely sharpens itself by splintering worn grits and exposing new ones. If the grinding forces become too high because of an excessive feed rate, low speed of the wheel or high table speed, the abrasive grain can drop prematurely out of the bond.

With slow feed rates, high speeds or slow table speeds, the grinding forces are lower. This can result in flattening of the abrasive grains and unfavorable results.

In both cases the self-sharpening effect can be improved through corresponding modification of the machining parameters.  

THE G RATIO

The wear characteristic of a grinding wheel is expressed by the "G" ratio.

                (cm3)

G=Removed material vol.

     Volume of rim wear

                (cm3)

When the volume of the grinding wheel and the wheelprice are known, the cost-effectiveness of a grinding operation can be determined  with    the aid   of the G-value, taking into account also personnel and machine costs.

  RECOMMENDED PERIPHERAL SPEED

Dry grinding should only be applied on materials with good thermal conductivity. In the connection with diamond, heat developement should be avoid by an adequate cooling.

Generally, CBN should be used  with a higher peripheral speed. A condition for that is an efficient coolant and a stable machine. The wheel's life and the surface quality may be improved.

SURFACE QUALITY  

The achievable surface depends on the grit size, the grinding operation and consequently also on the material. The table below relates to general tool grinding (cross grinding) with diamond and CBN in resin bonded wheels on the tungsten carbide grade K20 or HSS

In order to achieve an equivalent surface quality with CBN, the grit size has to be smaller by two or three fractions.

RECIPROCAL GRINDING AND DEEP GRINDING

In the reciprocal grinding process,table speeds of ³6m/min and feed rates of £0.05mm are commonly used. Because the grinding rim recontacts the workpiece with each run-out, it is subjected to numerous contact shocks, the severity of which depends on the total material to be removed and the selected feed rate.

In deep grinding operations the table speed is considerably reduced and in turn the feed rate is selected as high as possible, i.e. the grinding body is subjected to very few contact shocks.

The advantage of deep grinding is not necessarily the shorter grinding time or the higher stock removal capacity. Important is the fact that the grinding rim contacts the material only twice with very slow table movement. This significantly reduces the wear on the grinding rim and correspondingly increases its economy.

A prerequisite for deep grinding is a stable machine with sufficient spindle capacity and efficient cooling.

TRUING AND DRESSING OF DIAMOND AND CBN GRINDING WHEELS

The purpose of truing diamond or CBN wheels is to guarantee the geometry of the wheel and to eliminate true or plane running errors. Wheels that have become deformed due to uneven wear or which do not run true, can be reshaped or trued again.For this purpose the surface of the grinding rim is brought to the desired shape by means of silicon carbide or corundum wheels. In the case of CBN wheels a multigrit diamond dressing tool can also be used.

Particularly for CBN wheels these preparations do not meet the requirements. Due to the difference in the chip formation when grinding highspeed steels or similar materials, the CBN grain must penetrate deeper into the material and the chip requires a larger chip space for removal.

Dressing(sharpening) of the grinding rim is, therefore, an absolute necessity. The wheels should prefefably be dressed directly on the machine by pressing a sharpening stone against the grinding  rim   of   the    rotating wheel. Minimal coolant flow is beneficial, in any case the stone should be at least moistened. Optimum grinding performance is achieved if the wheel is subsequently used in the same sence of rotation.The same applies to diamond grinding wheels.

RECOMMENDATIONS FOR PROPER USE OF DIAMOND AND CBN WHEELS

Diamand and CBN tools, being expensive, require more attention and care in handling.

-The use of COOLANT is generally for faster cutting, better surface finish and longer wheel life. Wet grinding should be used wherever possible although resin bond wheels can be used dry. Alkaline liquids for resin bonds sould be avoided.

When CBN wheels are used wet, optimum performance is achieved with pure oil. If an emulsion has to be used, 20% oil will give good results in many applications.

Pure water or water with rust inhibitor will damage  the CBN grit, and should be avoided.

Use a high horsepower and vibration-free machine.

The wheels are supplied balanced, but it is recommended to recheck the balance.

-Check the spindle and the back plate of the grinding machine  to  ensure their precise runout.

-Check the runout of the wheel, before and after closing the mounting screws. With cup wheels, use shims to make the face run true. Straight wheels are to be shifted on the spindle, tapping them lightly, until the wheels run out.

-Corner grinding is generally not recommended; the corner will round off fast and might cause chipping.

-If you remove the wheel from the machine to month again, you must mark the wheel and the flange. When you want to month the wheel again, you have to press until the marks are  across from eachother.

-Avoid excessive pressure, excessive infeed and excessive table movement.

HOW TO SPECIFY YOUR ORDERS  

Please give the following details:

1-Type of abrasive(diamond or CBN) and type of wheel

2-Dimensions:

a-Wheel diameter

b-Width and thickness of the abrasive layer

c-Hole size

3-Required surface finish

4-CBN or diamond concentration

5-Bond type

6-Wet and dry grinding  (for wet grinding , please specify type of coolant)

7-Materials to be ground

8-Details regarding the machine,e.g.power

9-Type of work-off-hand or mechanical grinding

10-Desired cutting speed, in- feed and table speed

THE ECONOMICAL WORKING SPEEDS OF DIAMOND AND CBN