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Abrasive wheel selection    
     

Abrasive grain type

Selecting the right abrasive grain is vital in selecting the right wheel for the job. Using the correct abrasive grain will it enable it to cut the material accurately without producing too much heat which can burn or blue the material being ground and can also cause the wheel to break up and explode. The most common forms of abrasive are:

 

Conventional aluminium oxide abrasive.

These fall into three main types:

1.       Regular (A) fused aluminium oxide a tough abrasive used extensively on bench and pedestal grinders and in grinding and cutting discs used on portable grinders for ferrous materials including mild steel, carbon steel and wrought iron.

2.       Purer and more friable (ability to fracture) aluminium oxide abrasive. (23A) for cast iron, ductile iron, high carbon and high speed steels, (PA) Ruby with chrome oxide additive creating a stronger abrasive than type (WA) for the sharpening of high speed stainless steel and vanadium alloys. (WA) white abrasive is the abrasive of choice for most high speed steels and is used extensively for cylindrical, surface and internal grinding of tools, dies and gauges.

3.        Mixtures of the above including (48A) often referred to as semi-friable free cutting abrasive for grinding most steels in cylindrical, centre-less and angle approach applications.

 

Silicon Carbide abrasive

These fall into three main types:

1.     Standard silicon carbide used in bench or pedestal grinding wheels.

2.      Black silicon carbide used for grinding and cutting stone products in resinoid bonded wheels.

3.      (C) Crystolon highly pure silicon carbide abrasive for grinding cemented carbide cutting tools, fired ceramics and glass.

 

Ceramic Aluminium Oxide Abrasive. (SG3)

This abrasive has a unique microcrystalline structure. The abrasive is harder and sharper than conventional abrasive allowing cool cutting with rapid stock removal on medium to hard steels. More on this abrasive….

 

  
     

Grain size

 

The grain size will determine both surface finish and its ability to remove material. The size is designated by a number that increases as the grain size decreases. For example 10 grit has a median size of about 2.00mm and 60 grit 0.25mm.

Use coarse grit:

    1. For soft, ductile, stringy materials such as soft steel or aluminium.
    2. For rapid stock removal.
    3. Where finish is not important.
    4. For large areas of contact.

Use fine grit:

    1. For hard, brittle materials such as hard tool steels, cemented carbides and glass.
    2. For fine finishes.
    3. For small areas of contact.
    4. For holding small corner radii. 

Bond is the material that holds the abrasive grains together. There are several types of bond:

 

  1. Vitrified bond (V). This is the most common bond for precision grinding applications. The bond is fired at very high temperatures enabling it to withstand the heat generated during the grinding process. Vitrified wheels have a high modulas of elasticity. Consequently its rigidity makes it particularly suitable for form grinding. Using both conventional abrasives as well as CBN superabrasive.
  2. Organic bonds are cured at low temperatures between 150°-200° and are available in Reinoid (B), Rubber(R) and Shellac(E) bond. Resinoid(B) bond is tougher and less brittle than vitrified bond making it ideally suited to heavy duty grinding and cutting on portable machines at high operating speeds.
  3. Electroplated bond is used widely with superabrasive CBN and diamond. A single layer of abrasive grain is electroplated onto a metal hub. Highly complex forms can be manufactured and higher surface speeds can be attained. More on electroplated diamond and CBN wheels……

 

Grades. The grade indicates the relative holding power of the bond, which holds the abrasive grain together. The amount of bond will determine the wheel’s hardness (grade).

Use soft grade wheels: 

  1. For hard materials such as hard tool steels and carbides.
  2. For large areas of contact.
  3. For rapid stock removal

Use hard grade wheels:

  1. For soft materials.
  2. For small or narrow areas of contact.
  3. For longer wheel life.
  4. On high horsepower machines.

Balance.

Often grinding wheels will have to be balanced. This is where weights are aligned around a retaining flange to counteract the greater mass of one part of a wheel. A wheel that is out-of-balance will exhibit excessive vibration and will cause chatter marks on the work surface.

  
     

 
 


 


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