A frequently asked question is: Why is there no international micron diamond powder standard, like there is a standard for almost any other product, including diamond mesh sizes. The answer is quite simple: No dimensional standard can exist without a common, reliable and repeatable measurement method.

Here is a history of some attempts to design a micron diamond powder standard:

In 1976, FEPA had organised comparative microscopic measurement tests between 6 different laboratories on a single, coarse micron size. For a 40µ median size, the results varied between 36.54 and 41.11µ, i.e. 11.3%. On a 90% size span basis, a distribution width including 90% of all measured particles, the maximum variance was 25.0%.

In 1979, FEPA nevertheless designed a micron diamond powder standard that was in fact rarely used, mainly because of the lack of a common measurement method, but also because the standardised micron sizes did not match the industry’s ever increasing demand for accuracy and diversity in micron diamond powders. It had the virtue, however, to define some optical measurement procedures and the 'circumscribed circle' measurement method. Since that time, most manufacturers abandoned microscopic powder measurement anyway, to concentrate on easier and less time consuming measurement procedures.

In 1998, in an attempt to revise the FEPA micron diamond powder standard, another test was organised between 6 major companies, on 5 samples of approx. 2, 8, 20, 35 and 80µ median sizes. By then, due to the many different instrument types used (optical, electrozone, laser diffraction), the variances had grown considerably, to 23.5% on Median Sizes and 82.4% on the d95-d10 span. The correlation had not improved, it had become significantly worse. It was then decided to repeat the exercise on all samples except 2µ, in considerably more detail and using optical instruments only. This choice was made in the assumption that a product standard should be capable of being used not only by the manufacturers, but also by the major users, with a reasonable investment.

In 1999, the variances between measurements were found to be 21.5% on Median Sizes and 124% on the d95-d10 span! The study included various other analyses, such as operator variances within a single company, or variances between different slides of the same product, etc. The standardisation project was there­fore abandoned, on the following grounds:

• No dimensional standard can exist without a reliable and repeatable measurement method. The current variances cover approximately 2 micron sizes, which is enormous compared to the grading consistency expected by the users.
• The skills of optical measurement are no longer available in most of the companies concerned and no other instrument can at this stage be expected to produce matching results in different locations, by lack of common measurement methods and calibration.
• Most users now require grading accuracies and batch-to-batch con­sis­tency far greater than could be granted by any such standard. Many of them showed little interest in a new standard anyway. In addition to the basic measurement problem, no standard could probably go into the accuracy and detail that is now required by the industry.

In 2002 a project for a DIN/ISO standard was abandoned for the same reasons. Simultaneously, a Japanese project was also aborted.

ANSI B74.20-1997
This US standard covers 'Specifications for Grading of Diamond Powder in Sub-Sieve Sizes', using the Coulter Multisizer (electrozone type) in the coarse range and the Horiba LA910 (laser diffraction) below 3µ. It cannot really be regarded as a product standard, but rather as a recommended inspection method. On the Coulter Multisizer, spherical calibration is prescribed, which causes the measured values not to match the product denominations. Example: On a nominal grade 6.00µ, with a 4-8µ size range, the prescribed d50 (median size) tolerance is set at 4.20µ±8%. The underlying 1.43 mean multiplying factor between the two figures reflects the difference between spherical and length measurement.

Micron powders are produced by sedimentation methods, in which the size distribution is guided by particle mass. Mesh sizes are sieved and the sieve apertures determine the maximum particle dimension allowed to pass. Their distribution is therefore roughly based on the smallest particle dimension. Mesh size quality inspection was ruled by the FEPA Standard, first published in 1972 and revised 1997 and now by the internationally accepted ISO 6106 Standard, first published in 1979 and revised in 2006, which are both based on a sieving method.

There is an overlap between micron sizes 45-70 to 70-120 and mesh sizes D46 (325/400) to D76 (200/230). This subject is further discussed under Technical Information/Micron-Mesh Overlap.

It is worth mentioning a number of other standards, which relate to the superabrasives micron and mesh sizes:

ISO 565 and ISO 3310-3
These standards define the test sieves, metal wire sieves, perforated and electroformed sieves that are used in sieving and testing sieve sizes.

ISO 9284
This defines the sieving machines.

FEPA, in 1994, published some further standards relating to:
-Measuring the relative strengths of saw diamond grits.
-Measuring the metal coating weight on coated diamond and CBN grit.

ISO 13322-1:2004)
This standard describes the static image analysis theory in great detail.

ISO 13320-1 and 13319
ISO 13320 describes the principles of laser diffraction methods for particle size analysis. It is worth reading, as it adequately describes the many problems encountered with laser diffraction instruments. ISO 13319 describes the electrozone measurement methods.

ISO 4287
This describes the Geometrical Product Specifications, i.e. surface texture or surface roughness, a subject frequently referred to in diamond lapping and polishing applications.