HARDNESS TESTING OF METAL:
is an indefinable property which finds wide practical application when it is correlated with some service experience, other property or characteristics of a metal. Most hardness tests for metals yield a numerical value and are based on their resistance to permanent indentation under the conditions imposed by the particular test. Resistance to scratching is another basis for hardness testing. There are many different ways of measuring this property called "hardness" but no one way of defining it. Hardness tests are made under arbitrary conditions and there are no basic correlations for converting numbers from one scale to another. The best that can be done is to calibrate one scale in terms of another, but the hardness conversion relation for one type of metal does not usually apply to another type of metal.
In the Brinnell hardness test, a known load is applied, for a given length of time, to the surface of a specimen through a hardened steel ball of known diameter.
The scleroscope hardness test is based on the height of rebound of a diamond tipped hammer falling from a fixed height. The hammer is approximately 1/4" in diameter and 3/4" long. The diamond striking tip is rounded to 0.01 inch radius and is mounted in the hammer's normally rounded end.
The File hardness test is more of an art than a science. It does serve as a very quick and useful "go or no- go" hardness test when applied by a person skilled in the art. In this test a file is hand held with the index finger of the operator extended along the surface opposite that which is used for cutting. The material surface to be tested is slowly and firmly rubbed until it can be determined whether the file will bite into the metal.
In lapping, the application of the file test is similar to the method used in determining whether a particular abrasive will abrade metal in the desired length of time and whether, in doing so, will break down to a fine particle size thereby leaving the desired microinch surface.
The MOHS scale of hardness, devised by Friedrich Mohs, a German mineralogist in 1826, was originally based on the susceptibility of a material to be scratched. When adopted many ears ago, only ten known materials were listed. Talc, the softest, was numbered No. 1 and Diamond, the hardest, was numbered No. 10 with other materials falling in between. Since that time, calcined and hydrate alumina have been developed and their place on the MOHS hardness scale has been approximated.
The Knoop hardness test consists of applying a known load for a specified time to the surface of a metal through a diamond having unequal longitudinal and transverse included angles. The Knoop hardness number is the applied load divided by the unrecovered projected area.
MOHS SCALE OF HARDNESS
HARDNESS RULES OF THUMB:
ABRASIVE GRIT - SIZES 8 TO 240:
These are called "screened" sizes. The U.S. Department of Commerce has specifications for each screen number.
ABRASIVE GRIT - SIZES 280 AND FINER:
There are no standardizations for the "subsieve" or finer grit numbers from 280 and finer. Considerable variation exists in both nomenclature and sizing practice between producers. Grit sizes differ from one producer to another though they try to remain competitive. Personal testing is the only sure way to determine if a particular grit size meets your needs.
AVERAGE PARTICLE SIZE OF ABRASIVE GRAIN