Grit confusion

[nissnn]

Hello everyone,

I have been looking into the grits of the Tormek diamond wheels and am left with some questions.

What I know is that the grit values given on the product pages are FEPA values, but I could not find clear information on whether they are given as FEPA P or FEPA F values.

However, some information can be found on particle size, both directly from Tormek and from third‑party sources. For example, 600 grit diamond wheels have a particle size of around 25 µm.
FEPA P600 corresponds to a median particle size of 24.8 ± 1 µm, while FEPA F600 corresponds to a median particle size of 9.3 ± 1 µm. Therefore, in this case the grit value given by Tormek must be a FEPA P value.

This makes sense, because FEPA P values describe coated abrasives, while FEPA F values describe bonded abrasives, and the diamond wheels are of course diamond‑coated steel. However this can be somewhat confusing, because FEPA P is commonly described as "for sandpaper" while FEPA F is described as "for sharpening stones", but I guess traditionally stones are usually bonded abrasives and sandpaper is a typical coated abrasive.

So I assume that Tormek diamond wheels all use FEPA P values for their grit values, but I am not sure about the SG wheel. A finely graded SG wheel is supposed to have a 1000 grit, but I am not sure how I should compare this value to the diamond wheels. On one hand, it would make sense for Tormek to use comparable grit ratings for their wheels. On the other hand, the SG wheel is definitely not a coated abrasive.

So if the SG wheel grit description were based on FEPA P values, it would be somewhat rougher than the extra fine diamond wheel. If it were based on FEPA F values, it would be around halfway between the extra fine diamond wheel and the Japanese waterstone (JIS 4000 ≅ FEPA F1200 ≅ FEPA P5000).

I guess my question can be boiled down to:
How does a finely graded SG wheel compare to the extra fine diamond wheel, both in surface finish and in material removal rate? Is it a bit rougher, or is it significantly smoother?

If anyone can share their experience on this, I would be very grateful! Also, if anyone has some theoretical knowledge and/or corrections to my assumptions, please share them as well!



Additionally, for future reference, but not directly relevant to my main question:
Both FEPA P and FEPA F are not supposed to be used for diamond abrasives. For superabrasives:

The grit designation is prefixed with a "D" to denote diamond and a "B" to denote cBN.

- from https://fepa-abrasives.org/abrasives/standards/

According to this standard, the actual FEPA grit size of the 600 grit diamond wheel would be FEPA D20 (particles between 15 and 25 µm). This seems to be confirmed by Tormek here. But it of course makes sense to use a more familiar grit rating for customers, it would just be helpful to actually know which one is being used.

Best regards,
nissnn