I spent a goodly amount of time yesterday trying to put some reason behind various manufacturer’s differences in diamond paste grit size ratings when they have the same micron rating.
For those unfamiliar with abrasives (ie, sandpaper, stones, pastes, etc.) they’re typically spec’ed using a “grit” rating. Historically, grit size is equivalent to the “size” of the wire mesh screen used to sieve the abrasive particles, size being the number of wires in a linear inch of the screen. (E.g., 60 mesh has 60 wires each direction in a square inch and is the size commonly used in kitchen faucets, an 80 mesh has an opening which will barely pass a human hair.) All abrasives are made up of varying sized particles that fall within a size range… for example, a 100 grit stone might be made up of abrasive particles that pass through a size 100 mesh but do not pass through a 120 mesh (remember, the smaller the number the larger the hole in the mesh). For large grit sizes (e.g., for mesh numbers <240), the sieving process actually uses a wire mesh screen-like process. But for the grit sizes we’re interested in (>1000), the sieving process is much more exotic since we’re dealing with micron-sized particles.
We’ve already established that different parts of the world use different standards, but a micron is a micron, right? So why the conflict? Well, I found lots of information that pointed to nothing specific that we could hang our hat on. I found the usual references to JIS, CAMI (which is now apparently part of UAMA, so don’t be surprised if you start seeing references to that), etc.
On this document I found the statement “DIAMOND MESH EQUIVALENT - refers to micron size grading as performed by the National Institute of Standards and Technology (NIST)”, and a chart which included columns for both “mesh size” and “diamond mesh equivalent”. Here’s data for two entries in their table:
Code:
micron size mesh mesh equiv micron range
.5 25000 60000 0-1
5 2500 3000 4-8
I spent some time on the NIST website but gave up… their site is so large that you seemingly have to what/where you want to go to be successful. So I don’t know what context to apply to the NIST statement. I do know that NIST both 1)defines measurement/testing methodologies and 2)produces measurement standards which you can use to calibrate the measuring tools/devices.
For course stones it’s economical to sieve to reasonably close tolerances… for example:
100-grit = 149 - 125 microns
As the particle size gets smaller, it’s not only increasingly expensive, but becomes technically challenging to maintain such close tolerances… for example, here’s some data I found for a US made diamond abrasive:
Code:
mesh micron range average(*)
8000 4 – 2 2
15000 2 – 0 1
60000 1 - .25 .625
(*)calculated by me
Now, my hypothesis as to why manufacturers grit specs differ for the same micron-size…
According to
here US manufacturers (including Norton, although they are now owned by a European company) spec according to the
larger of the size-range of particles. Hence, using the table above as the example data, a 2 micron rated abrasive using the US method (micron range of 2 - 0) would equate to something around 15000-grit. The European convention is to spec to the
average particle size… thus a 2 micron rated product (4 – 2 micron range) would equate to approximately 8000-grit. So, the bottom line is that
the micron rating from some companies is the largest particle size in the abrasive and for others it's the average particle size.
Now, before we get too cocky about all this, most manufacturing processes create a non-linear distribution of particle sizes within the range… e.g., there might be more larger particles than smaller, so the average is not necessarily the midpoint of the range that I’ve used as examples above. Hence, it’s very difficult to compare product from different vendors even when you know the particle size range.