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Basics of Coated Abrasives

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Coated abrasives consist of the following components:

Abrasive grain attached to some sort of backing material held on with some type of bonding agent.

We’ll start by taking each of these components; grain, backing & bonding - and breaking them down into their components. We’ll begin with the grains.

GRAIN

There are 4 common types of grains in regular coated abrasives. 

Aluminum Oxide – AO is formed by combining bauxite and other materials by firing them in an electric furnace.  The resultant mass is then crushed, and the pieces sifted through successively finer screens to assign a grit size (CAMI grading).  When crushed, the resulting pieces are naturally pyramidal in shape.  Due to the shape and the strength of the materials used in it’s making, AO is a very durable grain.  The grain is worn down during use, sanding finer the longer it’s used. So theoretically, you can start at 80 grit, and after you’ve worked the abrasive for a while, you’ll be sanding more like 100 or 120 grit.  Users take advantage of this characteristic by using the belts on more than one machine or application.  For instance if you have two wide belt machines doing intermediate and finish sanding respectively, you may use a 120 belt on the first machine to do your intermediate sanding, and when the cut for that application is no longer as sharp, the belt can be moved to the 150 grit machine and used there till the cut for that application is no longer sufficient.  This ensures you get the most production and the full value for the belt you purchased.  

AO is very tough and durable, as stated above, and is used on bare wood, most metals (especially the steels), leather and a wide variety of other materials.  It is a great general purposes grain and probably the most, common grain seen today.

Silicon Carbide – SC is made by combining pure white silica sand and coke, a by-product of coal production.  Again, these materials are combined by melting in an electric furnace, crushing the results and sifting the particles through screens to get grit grading.  SC grains are shaped like icicles with extremely sharp points and narrow grain bodies. It is second only to diamond in hardness; however it is very brittle due to the narrow grain body. Therefore when pressure is applied to the tip of this grain it fractures.  This characteristic is referred to as “friability”.  The benefit of friability is that a sharp edge is always against your workpiece providing extremely consistent finishing ability.  Because of the lack of grain body strength, which allows this grain to fracture with hand pressure only, the life as compared to AO or the other grains is less.  This is why when you see SC it’s normally in finishing type operations where the pressure required for working is lower than it would be in removal grits.  Examples of materials normally sanded or ground with SC would include glass, plastic, rubber, paint varnish, lacquer & sealers. It may occasionally be seen in sanding on extremely soft woods like Ponderosa Pine, but we would recommend open coat AO instead as the life would be much better. 

Alumina Zirconia – AZ is a grain that takes the best characteristics of AO & SC and combines them together to create one very durable yet friable grain.  Its main ingredient is bauxite, just like AO.  It is friable like SC but machine pressure is required to crack the AZ grain where the SC grain will fracture under simple hand pressure. Failure to achieve sufficient pressure to crack the AZ grain will result in glazing of the abrasive and reduced life. The best of both worlds, this grain provides you with a consistently sharp edge on the workpiece and extended life. It is commonly seen anywhere high removal rates and time are factors, including planing of wood, heavy grinding of metal and other dimensioning type applications.  It is normally mounted on heavier paper or cloth backings and the grain itself is 15% – 40% more expensive to produce, so using it in the removal grits, 24 – 60, is where you will achieve more bang for your buck. 

Ceramics – There are several different types of ceramic materials including ceramic AO, ceramic AZ and what is referred to full ratio ceramic.  Ceramic is manufactured by combining bauxite, just like regular AO, with other materials in a chemical bonding process. This chemical bonding results in raw grain that is very porous and coral-like in appearance. The full ratio would be the strongest, the ceramic AZ would be next in strength and the ceramic AO would fall last.  Of course, the higher the pure ceramic content the more expensive the material.  Ceramics were created for rough grinding on metal, but have crossed over into lots of other areas where sanding is done.  But keep in mind you’ll get the most value for your dollar using it in the grits where removal or dimensioning work is being performed and then using regular AO or SC for the more intermediate and finishing grits.

 

DID YOU KNOW?

Natural grain colors are:

AO – brown, pink, white

SC – black

AZ – blue, blue/gray

Ceramic - white

Abrasive manufacturers can make their product any color they chose to simply by adding dye to the size coat in the making process.  Therefore you should not rely on grain color to inform you of grain type. Also the addition of stearates and lubricants can make it hard to tell original grain color.  Always choose a grain based on the application and the machinery involved.

BACKINGS

The second component we’ll address is backings. There are 3 basic categories of backings.

PAPER – Paper backings are available in different weight or thicknesses using the designations A (the lightest wt.) thru F (the heaviest weight).  Light weight papers A-C are commonly used for power sanding with sheets or discs or for hand sanding.  The heavier weights (D, E & F) may be seen in sheets or discs for power sanding or in the case of the E & F weights for belts. Only the E & F weight products are considered heavy enough to be belt materials.  Some parameters that need to be met before considering a paper belt are:

  1. Work must be dry. The waterproof papers we have are all A/C weights and therefore unsuitable for belts.

  2. Work must be flat.  Paper will not follow a contour easily and is more likely to crease, kink or tear on contoured work than cloth.

  3. Work should be limited to grits 80 and finer.  If you ran coarse grit belts (24-60) in paper, the backing would wear out before the grain was fully utilized.  Paper is extremely well-suited for belts in grits 150 finer where it will result in a finer finish per grit than cloth and will cost less.

  4. Belts should be at least 6-8” in width and over 150” long. Short belts like portable belts, belts for edge sanders, backstands or dynafile machines are usually fairly short and narrow and therefore have less cool down time in operation.  Heat is the enemy of any abrasive. These shorter belts do not provide time for the heat built up during sanding to dissipate in use and while they may at first glance appear to be less expensive, they are in fact more costly in number of belts per piece worked and downtime for belt change.

Paper can be chemically treated to be waterproof and should be marked as waterproof or with the abbreviation W/D or WP.

CLOTH – There are 3 basic weights of cloth backings: J, X and Y.  J and X are cotton materials. The J has a slight degree of conformability to it and should be used where a small degree of conformability to the workpiece is needed in belts or sheets.  The X is heavier weight cotton and is an all purpose/general purpose backing used in a wide variety of belting applications including portable belts, intermediate belts and wide belts.  Y backings are 100% polyester (or in some cases a 60/40 poly/cotton split) and are used for flat, heavy duty dimensioning or other high heat or rough applications.  The J and X weight backings have flexed versions available that are referred to as J-Flex (JF) and X-Flex (XF).  These would be suitable for applications where the X or J is not quite flexible enough.  The most common uses for JF backings are as pump sleeves and for mold sanding belts.  The XF backings are used mainly for slack of belt operations.  The Y backings are not flexed but come in a lightweight poly (YX) and a heavy weight poly (Y or YY).  Normally 24-50 or 60 grits will be the heavier poly backing and the 80-120 will be the lighter poly backing. 

No cotton backings are naturally waterproof.  They may be chemically treated to be waterproof however.  If this has occurred, the backing should say waterproof or be marked as W/D or WP.  If you do not see such a marking on the back of a cotton belt, do not run with water or the backing will stretch.  You may use 100% oil as a lube if that is not detrimental to your application.  Polyester, on the other hand, is inherently waterproof.  As it is not chemically treated to attain this resistance, it is not normally marked as W/P on the backing itself, but may be noted in a product description by the manufacturer.  Just remember that any 100% polyester back is waterproof.  Polyester is in fact naturally waterproof, shockproof and tear resistant, which is why it is often seen in heavy duty, rough applications and wet grinding applications. 

FIBER – Fiber backings are basically very heavy paper backings.  Several sheets of paper stock are combined together using chemicals, heat and pressure in a process called “vulcanizing”.  The results are very strong, heavy, paper backings that are used for fiber discs or “vulcanized discs”.  Fiber backings are not waterproof but may be used in conjunction with grease sticks or 100% oils as lubricants.  Many companies also offer lubricated fiber discs negating the use of grease sticks or oils as lubricants.

FILM/LATEX

You may also encounter film or latex backings. Film/latex backings are almost like a plastic paper.  They provide the finishing characteristics of paper with the strength of a lightweight cloth.  We see a lot of film or latex products enabling sanders to be used incorrectly for specific applications.  For instance, we commonly see people using orbital sanders to sand corners.  As round objects are not meant to fit into square areas, normal paper backed products will tear when orbitals are used in this fashion.  Not only is this inappropriate use for the abrasive disc, but the pads on the sanders themselves, also being round in shape, are not meant to do this type of sanding.  The film and latex backed discs are normally quite a bit more expensive than regular paper backed products and considering you’ll have to replace back up pads for the sanders more often, and at some point , the sander itself will need repairs involving bearing replacement, you’ll eventually  realize you have paid for a rather expensive convenience.  Jitterbug and other sheet sanders are reasonably priced and designed to perform corner sanding while using normal paper backings. They will provide as good a finish in the same amount or less time with less up front and hidden costs overall. Remember the old adage “Use the right tools for the job”?  I don’t remember ever once hearing an adage saying “Use the wrong tool for the job with a higher priced product that will allow you to do so, and end up paying extra replacement / repair charges on top of that.”  Just doesn’t have quite the same ring does it?

Where the true advantage of latex or film backed discs lies is in their ability to produce finer finishes in the very fine grits and their ability to be used wet or dry.  If you have fine or very fine finishing applications or if you have wet finishing applications the extra money paid for these backings may well be justified.

BONDING

Every coated abrasive receives two layers of adhesive bonding in the making process.  The first layer, referred to as the maker coat, is what actually adheres the grain to the backing.  The second layer, referred to as the size coat, is what ties the individual grains together (so that they act as a unit as opposed to acting as individual grains) and provides protection against heat.

 In the early days, glue bonds were the only bonding agents available.  These glue bonds were animal based products that were not phenolic or thermo-setting in nature.  In other words, when they got hot during use, they re-softened.  The advantage of this re-softening is that the bonding then acts like a cushion for the grain, which results in softer finishing characteristics.  The downside is you loose aggressiveness of cut and life, as the protection for the grain from heat offered by phenolic resins is absent in glue bonds. 

With the advent of the synthetic resins, the productivity and life of abrasives were greatly enhanced.  Resin bonds are phenolic and therefore, offer the grain excellent protection from heat which in turn extends life.  Most of the abrasives in use today are resin bonded.  When you do encounter glue bonded items, they usually fall into the “finishing” category of products as that’s where the workload is the lightest and the softer scratch most important. 

Summation – For every sanding/grinding job you have to do, you should be able to look at these components and their components and come up with the qualities an abrasive will need to possess to help you accomplish the job.  From the grain to the backing to the type of bonding, there are reasons, or should be, for each choice you make.

 The goal is always to determine what is required and then get it done in the most cost effective manner possible.  You’ll find there are no real short cuts in sanding.  Sure you can skip grits, but you will eventually sand the same amount.  If you jump from 80 to 220, you will only buy two grits, but you will use double the 220 you would use AND have the time for the extra sanding required because 220 is not designed to remove an 80 grit scratch.  In general, as long as you skip no more than one grit in a sanding sequence, you should not be overworking any of the grits involved. Be aware that just because things may SEEM to save you money on the front side, you will be paying in time or aggravation or some other commodity likely just as valuable as the money itself, on the backside.

 

Abrasive Grains

Aluminum Oxide - A blocky, hard grain best suited for sanding and grinding of ferrous and non-ferrous metals, wood and solid surface materials.

Silicon Carbide - A sharp, very hard and brittle grain best suited for sanding of glass, plastics, rubber, ceramics, solid surface materials and some non-ferrous metals.

Alumina Zirconia - A very hard and sharp grain that works well for grinding of stainless steel, spring steel, titanium and other hard steels and for dimensioning of wood.

Crocus -A natural abrasive of iron oxide particles. Used mostly for cleaning and polishing soft metals.

Emery - An abrasive that is a natural composite of Corundum and Iron Oxide. The grains are blocky, cut slowly, and tend to polish the material being abraded.

Garnet- A very sharp grain that cuts very quickly when new. Fractures quickly, keeping it sharp. Perfect for sanding wood end grains or for final-finish sanding of wood. Very economical.

Stearate - An additive that deters loading when sanding soft resinous woods, after sealer coats and when working with soft ferrous or non-ferrous metals. Not an abrasive grain.

 

Cloth

 J - A light weight, flexible Egyptian cotton cloth
JF - A light weight, very flexible Egyptian cotton cloth
X - A heavy, stiff Egyptian cotton cloth
XF - A heavy, yet flexible Egyptian cotton cloth
YX - A lighter weight polyester backing
YY - A very heavy, stiff polyester backing


Fiber - A very hard, strong, coated abrasive backing material consisting of multiple plies of chemically-impregnated paper. Used primarily for disc products.


Paper

A - A very light weight paper - typically for sheet use only or light PSA or hook & loop disc usage
B - A light weight paper - typically for sheet use only or light PSA or hook & loop disc usage
C - A medium weight paper - typically for sheet use only or light PSA or hook & loop disc usage
D - A medium to heavy weight paper - typically for sheet use only or PSA or hook & loop disc usage 
E - A heavy weight paper - typically for stroke or wide belt sanding
F - A very heavy weight paper - typically for stroke or wide belt sanding


 

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