GRINDING WHEEL and ABRASIVES BASICS

INDEX

General Abrasives - Construction and Identification

The specification

Contrary to what you may think, the long codes associated with grinding wheels actually can be interpreted and generally have a clear meaning.  Most all manufacturers will list the grit type, grit size, wheel hardness, structure and the bond in every grinding wheel produced.  It is important to note that there is NO STANDARD among grinding wheel manufacturers.  Each will use their own unique identifying method of marking; However, there are some common rules - at least here in the USA.  A typical grinding wheel specification might be 'A60-I10-VS'.  'A' is the grit type (in this case aluminum oxide), '60' is the grit size, 'I' is the relative hardness, '10' is the structure, and 'VS' is the type bond (VS for this company means vitrified bond.  There are variations too.  For example, one might see this '97C80 +F/-G -B11-DC'.  In this case, 97C is the grain/grit type (here 'C' probably is silicon carbide), 80 is the grit size, +F/-G indicates a zoned or graded wheel of an in-between hardness.  -B11 is the bond (B11 is probably a resin bond).  The DC is some process done to the wheel such as slots or grooves or holes or induced porosity.  Other suffixes are added by each individual manufacturer for special conditions.  Only by looking at the manufacturing record or process sheet will anyone know for certain what exactly made up that particular grinding wheel - The specification alone will not tell the whole story.

As noted above, generally one cannot take a grinding wheel from one manufacturer that is marked similarly and substitute for another manufacturer's grinding wheel of equal marking.  In the first place, it is highly likely they will not be marked similarly and secondly, variations in factory production methods typically make grinding wheels of different construction.  Unless one is not very picky about their grinding wheel or not doing exacting work and is willing to accept some grief, we would not recommend anyone try to make substitutions without some guidance from an engineer familiar with the process and the manufacturers.  This is where we come in: We work with the factories to get you the proper grinding wheel.  It is also important to note that the end user should expect some TRIAL & ERROR when converting from one brand of wheel to another.  Sometimes a manufacturer may need two or even three tries to finally hit on the exact hardness, grit, bond, etc..  Patience and time are key to any successful conversion and testing.

Grit type and color

Grit type is generally either aluminum oxide (white, pink, ruby red, brown, grey, etc.) silicon carbide (black or green), ceramic (blue and pink) or any combination of these.  Aluminum oxide is by far the most popular.  It is available in the following colors: White, pink, red, ruby red, brown, and grey.  Each color has it's own grinding characteristics.  Grey and brown grit are the workhorse grits used in bench grinding and production grinding.  Tough and inexpensive they are the most 'general purpose' grit found.  Can be used on low to high carbon steels.  The pink and white grits are typically used on your harder steels which need a cool, friable cutting action to avoid burns.  The ruby red grit is a special tough grit also used on tool steels.  These grits are a little bit more expensive than the grey/brown.  Ruby red is very expensive.   Silicon Carbide grits are commonly either black or green.  Black silicon carbide is used to grind non-ferrous metals such as aluminum and brass and also on plastics, rubber, and stone products such as marble and granite.  Black silicon carbide is a very sharp grit.  Green silicon carbide is an even sharper grit than black and is used primarily for carbides, titanium and plasma sprayed materials.  One interesting characteristic of silicon carbides is the effect they have on steels.  Due to the sharpness of these grits, one would think that they would be too aggressive and not provide a good finish.  In fact, on steels, silicon carbide is used as a sort of polishing/finishing grit.  It is used in tumbling processes as a surface finishing product.  Also, manufacturers will often blend a small percentage of silicon carbide in with aluminum oxide grit in grinding wheels and honing stones to achieve a better workpiece surface finish on steels.  The grit will actually dull and provide a rubbing action on steels which produces a better surface finish.

A newer grit that is available is ceramic (also referred to as SolGel® or SG®).  Ceramic grit has the characteristic of not dulling -- It will break down or fracture into sharp corners rather than dull and pull out of the bond.  This makes the wheel typically last longer and it will also provide excellent aggressive stock removal without heat build up.  This grit is only made by a couple of producers and is very expensive, typically two or three times as expensive as aluminum oxide.  You will normally not see a 100% ceramic grit wheel.  The grit is typically mixed with aluminum oxide in various percentages from 10% up to 50%.  Ceramic is used in tool steels and lower carbon steels equally well.  These grinding wheels typically require a good bit of custom engineering for your specific application and process to achieve profitable results.

Grit types are sometimes mixed in combination for achieving certain cutting characteristics.  Grits are also called friable (white) or semi-friable (pink, brown and grey, red, etc.).  Friable grit breaks down more easily and is useful for cutting harder materials. 
 

Grit size

Grit size typically runs from coarse (16 -24 grit), medium (36 - 60 grit) and fine (80-120 grit).  Superfine grits run from 150 and higher.  Grinding wheels usually will be between 24 and 100 grit.  Honing stones and jointing stones and other polishing abrasives will be 150 grit and higher.  Use a coarse grit for fast, aggressive stock removal and finer grits for less stock removal but better surface finish.

Grinding wheel hardness

Hardness is rated from A-Z with 'A' being the weakest bond and 'Z' being the strongest.  A weak bond is preferred for grinding harder materials while a stronger bond is desired for softer materials.  A typical weak bond for steel would be in the 'F, G or H' range.  A medium hardness would be in the 'I, J or K' range. And stronger bonds in the 'L, M, or O' range.  Hardness is dependant on the grit type, the material being ground, the amount of stock removed, and a number of other factors.

Hardness grades are typically linear: If you increase the hardness by one letter grade (An H to and I for instance) it could give you double the wheel life.  Many people mistakenly believe that such a move (from an H to an I) would only be marginal -- Don't be misled here: A move of just one or two hardness grades could have a dramatic effect on your process!

It is important to note that it is almost impossible to match one grinding wheel manufacturer's wheel hardness to another manufacturer:  Differences in factory kilns, measuring instruments and the lack of a standardized hardness system do not allow for direct cross-overs.  One company's 'G' hardness would be a 'F' with another and even a 'H' with another.  We get calls all the time on this:  We simply cannot guarantee one wheel to be the same as another.  Even when our manufacturers switch their production to a different factory in another state or country we will see some variance.  This can sometimes be considerable.

Structure or grain spacing

Structure is basically the spacing between abrasive grains.  An open structure would be 12 or higher while a closer structure would be 6 or so.  Here again, the structure depends on a variety of factors not the least of which is how difficult the material is to grind.  One would think that a closer spacing would make a tougher wheel but this is only true to a point: With less bond holding the individual abrasive grains, the softer the wheel would be.  Also, the same holds true for a very open structure: If the grains are wide spaced you have fewer grains to grind with but a greater amount of bond holding each grain -- This could make the wheel tougher.  Grinding wheel engineers will typically adjust the BOND STRENGTH depending on the application.

Bond type

There are various bond types but the most common are vitrified and resin.  Vitrified is basically a vitreous glass much like pottery or glassware fired in a kiln.  Resin wheels are plastic resins mixed and cured at lower temperatures.  Vitrified wheels are commonly used for bench, surface and tool room applications such as surface grinding while resin wheels are commonly seen in cutoff wheels, centerless wheels and superabrasive wheels (diamond & CBN).  Newer bonds are Plastic bonded wheels based on high technology from companies such as RESEARCH ABRASIVE.

Diamond and CBN basics

Diamond and CBN wheels come in several bond types: Resin (most common), vitrified, metal and electro-plated.  Resin is used in most tool room and production applications.  Vitrified and metal bonds are newer bond types with specific applications (We won't go into a lot of detail with these as they are somewhat rare and more expensive and almost always are custom made special order items).  Electro-plated wheels are very common and are typically found in cutoff wheels and low demanding abrasive grinding such as for plastics. 

Resin wheels are made much like a traditional grinding wheel with a thick bond/grit layer usually between 1/16" and 1/4".  Electroplated wheels are a much thinner thickness.  In both cases, the bond layer is applied to a hub which is either aluminum or steel made to the specific profile required.

Like traditional grinding wheels, Diamond and CBN wheels are used in a variety of processes and with a variety of materials.  Typically, diamond wheels are used strictly on carbides and CBN is used on steels.  Some manufacturers produce a 'hybrid' wheel which is a special grit that will grind both steels and carbides (Typically used on parts that require grinding of carbide and steel at the same time).  Plated diamond wheels are used on non-ferrous materials such as plastics, rubbers, nylons, fiberglass, etc.

Identifying your diamond or CBN wheel

Diamond and CBN wheels are classified by their shape, grit size, concentration and the bond.  A typical diamond wheel specification might be D1A1-150R100-B4 where D1A1 is the wheel shape, 150 is the grit size, 100 is the concentration and B4 is this particular manufacturer's bond (B4 is most likely a resin bond). 

Grit sizes of 120 to 180 are typical for tool room applications.  Finer grits of 220 or above are generally special order and for extremely fine finish work. 

The concentration is, in layman's terms, simply the amount of grit in the mix.  Concentrations of 75 or higher are preferred but it also depends on the specific application.  Some jobs may do better with less concentration.  Generally, the higher the concentration the longer the wheel will last and the more expensive it will cost up front. 

Bonds are weak or strong depending on the application but usually there is one main bond for 90% of the wheels made.  Exotic bonds like copper and polyamide are very expensive and are utilized in demanding, precise operations where close attention is paid to both wheel life and wheel cost.  Typically, a manufacturer will need to know if a diamond wheel will be run in coolant or dry -- This determines the bond.  CBN wheels should always be run in coolant. 

You will also see in the specification a callout for the bond layer thickness, for example: "X=1/8".  This is very important in the life of the wheel and it's initial cost.  A bond layer of 1/8" thick will be half the thickness of 1/4" thick and thus half the life.  Some manufacturers will supply wheels with as little as 1/16" or as much as 1/2".  The most common are 1/8" and 1/4".  The price can vary dramatically and this is an important factor to consider when comparing wheels from one company to another.

Diamond wheel on steels and CBN on carbides?

We get asked quite frequently if a diamond wheel can be used on steels and CBN on carbides.  We do not recommend this as the wheel life will be greatly reduced and in some cases, the wheels may not even cut at all.   A diamond wheel is specifically used for carbides, plastics and other synthetic materials.  It will not grind steel well at all.  CBN wheels should only be used on steels.  There is a hybrid grit available that will grind both; However, it is a compromise in wheel life and grind-ability.  But in cases where you must grind both materials at the same time, it can be a real time saver.
 

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