Understanding Abrasives archive

Tuesday 24 March 2009

Steven Russell sorts out the best smoothers and shapers

Gallery

Furniture makers divide into three groups: those who love everything about the finishing process; those who regard it as a necessary evil; and those who absolutely loathe it.

Whichever group you fall into, the fact remains that the job has to be done, and it is a task that can be rendered more satisfactory if the most suitable abrasive is used. Here we look at popular abrasive types and list the pros and cons of their different grains, backings and coatings.

We will look at gradings and talk about the 11 most useful types of abrasive.

Abrasive grading comparisons

Two primary standards exist for classifying the grit size of abrasives. In the US the CAMI (Coated Abrasives Manufacturing Institute) standard is used. Europe uses the FEPA (Federation of European Producers of Abrasives) standard. Japan uses the JIS (Japanese Industrial Standard), which is equivalent to the FEPA standard. FEPA-graded abrasives can be easily distinguished by the letter P in front of the grit number, such as, for instance, P240. There are sizing differences between the CAMI and FEPA standards, so the two are not universal. See the PDF download at the end of this article for more information.

Abrasive Types

1 Powdered glass

~5 Mohs scale

Although still available, this is rarely used today except for hand finishing. Glass papers are usually pale yellow. This type of abrasive has been superceded by those which can be used dry or wet and which cut quicker and leech colour. It is available in sheets.

Pros

- Inexpensive

- Still popular with French polishers for cutting back cured finish surfaces

- Can be used for abrading leather, plaster of Paris, polyester and epoxy fills and glazed ceramics

Cons

- Very slow cutting

- Wears and loads easily

- Useful for hand sanding only

2 Emery

~7.5-8.5 Mohs scale

A dark grey, blocky-shaped impure form of corundum, a natural aluminium oxide. The use of emery abrasives has been largely replaced with newer high-performance synthetics. Available in sheets, strips and rolls.

Pros

- Useful for some metal finishing and glass grinding operations

- Available in sheets, strips and rolls

Cons

- Slow cutting

- The shape of the grains can create deep scratches in wood and its grains can cause tannin-rich woods to discolour

3 Garnet

~7.5-8.5 Mohs scale

A naturally occurring, very sharp dark reddish-brown abrasive grain made from semi-precious garnets. Of the seven species, pyrope - a magnesium aluminium silicate - and almadine - an iron aluminium silicate - dominate. Available in and for sheets, belts and discs.

Pros

- Inexpensive

- When used by hand, garnet abrasives can leave a smoother finish, grit for grit, than aluminium oxide-based abrasives

- Works well on softer woods

Cons

- Abrasive grains wear out quickly when used in power-sanding operations

- Limited availability

4 Aluminium oxide

~9.0 Mohs scale

One of the most popular man-made abrasives featuring very tough wedge-shaped grains that lack a clearly defined crystal structure. Aluminium oxide is manufactured using a fusing process from bauxite in a Higgins furnace at very high temperatures. Aluminium oxide abrasives include those made from brown - the most common type - white and pink aluminium oxide grains. Available in and for sheets, rolls, belts, H&L discs, flap wheels, fibre discs and others.

Pros

- Very tough and durable

- Recommended for use on hard woods and for grinding some types of metal including carbon steel, alloy steels and bronze. Heat-treated products offer extended life

Cons

- Lack of a clearly defined crystal structure in lower grades causes the grains to round over when worn instead of fracturing

- Higher grades feature friable grains and have a significantly longer life

5 Stearated coatings

Some abrasives are available with stearated coatings which significantly reduce loading of sawdust and resins on the surface of the abrasive during sanding.

6 Silicon carbide

~9.25 Mohs scale

A black synthetic material with a hexagonal crystal structure and blocky, sharp-edged grains, manufactured by the reaction of silica sand and coke in an Acheson furnace at very high temperatures. It has very good friable grain structure that breaks along crystal cleavages to expose new sharp-cutting surfaces. Available in and for sheets, H&L discs, strips and belts.

Pros

- Can be used wet or dry

- Grains are among the hardest synthetic abrasives, being surpassed only by diamonds, cubic boron nitride and boron carbide

- Excellent for cutting back hard lacquer, epoxy and urethane finishes

- Can be used on some metals and plastics

Cons

- Dark, brittle grains can release during sanding and discolour light-coloured timbers

7 Alumina zirconia

~9.2-9.5 Mohs scale

A synthetic abrasive with self-sharpening crystalline structure with blocky grains, manufactured from aluminium oxide and zirconium oxide and quenched when molten, producing a crystalline structure that fractures well in use, revealing new cutting edges. Available in and for sheet, disc, belts, fibre discs and flap wheels.

Pros

- Extremely sharp and durable self-sharpening grains with excellent friability

- Excellent for use in heavy-duty stock removal or production sanding operations

Cons

- Expensive

- Extreme hardness of grains makes it difficult to crush into fine grit sizes, generally limiting availability to 150 grit or larger

8 Ceramic aluminium oxide

~9.5 Mohs scale

An exceptionally hard and sharp synthetic abrasive with a very uniform crystalline structure that is formed when alumina gel is dried and crushed. The name is derived from a manufacturing process that is similar to manufacturing industrial ceramics. Available in and for sheet, PSA discs, flap wheels and belts.

Pros

- Exceptionally hard-wearing. Lasts 5-7 times as long as standard abrasives on wood

- Grains fracture on a sub-micron level

- Overall, the best abrasive for heavy shaping and the production sanding of wood

Cons

- Expensive

- Grit sizes are limited to 80-220

9 Diamond

~10 Mohs scale

The hardest-known substance, diamond abrasives are made from carbon and are manufactured using natural diamonds and man-made diamonds. Industrial diamonds occur as three main types: bort (single crystal non-gem-quality fragments), ballas (spherical masses of crystals) and carbonado (impure diamonds). Available in and for H&L discs, flexible foam hand pads, PSA discs.

Pros

- Extremely durable abrasives offering exceptionally long service life when used on a variety of hard and tough materials

- Excellent grading and uniformity of particle sizes ensures a superior finish

- Best abrasive for sanding stone and glass inlays

Cons

- Expensive

10 Open coat

Open coat abrasives have grains coving approximately 50-60% of the backing surface. This system offers a faster cut and increased flexibility when compared to closed-coat abrasive systems, see below. In addition, the open coating offers more resistance to loading on the surface on the abrasive.

11 Closed coat

Closed coat abrasives have abrasive grains covering 100% of the backing surface. The increased density of abrasives on the surface makes these abrasives less flexible than open-coated abrasives. Closed coat abrasives offer increased stock removal rates when compared with open-coat abrasives.

12 Non-woven systems

Non-woven-based abrasives offer significant advantages over traditional products like steel and bronze which tend to load easily and wear out quickly. Non-woven abrasive pads are constructed from flexible non-metallic materials and do not compromise the surface with metallic fragments that may eventually rust or discolour pale timbers.

The primary abrasives used in non-woven abrasive pads, from most aggressive to least abrasive, are: aluminium oxide (brown, tan, maroon and blue pads), alumino-silicate (green pads) and talc (white pads).

These are available in and for sheets, H&L discs and belts.

Non-woven surface-conditioning pads offer consistent performance with an easily controlled cutting action that resists loading.

Non-woven abrasives are also available as polishing papers in roll, sheet and disc form, in sizes ranging from 1-30 microns.


David Preece

Tagged In:

steven russell , Abrasives

Glossary Rollover a term to view its definition

Finishing

"Closed coat abrasives have abrasive grains covering 100% of the backing surface"


Abrasives Glossary

Mohs Mineral: hardness scale introduced by Friedrick Mohs in the early 1800s, going from talc with a value of one to diamond with a
value of 10
Friability: The ability of an abrasive grain to cleave easily along weak crystallographic planes, producing new sharp fracture facets
Micron: One millionth of a metre, or one twenty-fifth of a thousandth of an inch. This term is used to describe the average abrasive particle size of very fine microgrits and abrasive powders
Micron-graded abrasives: A more precise grading system than the traditional grit grading process that utilises stringent controls for producing abrasives with a more consistent and superior scratch pattern. Typically used with diamond and micro-finishing abrasives
Zinc stearate: A dry white-grey non-abrasive lubricant added to some coated abrasives that reduces loading on the surface of the abrasive
Abrasive grade: Specifies the grit size of an abrasive product. There is an inverse relationship to the size of the abrasive grains and the grade number, thus the higher the grade number, the smaller the grain size
Loading: Term used to describe the accumulation of resins, sawdust or finish on the surface of an abrasive