Q&A with Stan Watson – Technical Director, DMT

I was recently contacted by Stan in response to some articles here that have been referring to DMT Diamond Whetstones, and he kindly offered to field any questions I might have.  Unfortunately I certainly did have some that I was very pleased to be able to put to a real expert in the field, and Stan has agreed to allow me to republish them here. (I have taken the liberty of adjusting the questions and responses a tiny amount (like using the Ex Ex Fine stone!) to fit this format, and open forum).  I found the answers quite fascinating, and they will take a bit to really assimilate them properly into my expanding understanding of sharpening processes.

So to the Q&A with Stan A Watson, Technical Director, Diamond Machining Technology. www.dmtsharp.com

Stu’s Shed:

Sharpening is obviously one of those topics that deserves a decent amount of coverage, which is how diamond stone sharpening became a topic.

In the first instance, I was wondering if I was indeed on the right track when comparing different sharpening media by converting all the different grading systems used to that of the actual abrasive particle size in microns, and if my hypothesis is then correct that you can step from one form to another and back again, so long as you are progressively moving from a larger abrasive to a smaller one?  In saying that, I understand that some abrasives work by breaking down and continuing to abrade as they get smaller so they cover a range of sizes, but the majority are treated as consistent in size from new to exhausted.

Stan A. Watson:

Abrasive particle sizing is at best a huge mix of systems. The single best way of comparing abrasive to abrasive is the micron system which is an actual measure of grain size in absolute terms. The other systems in use today; CAMI / UAMA (Coated Abrasives Manufacturers Institute / Unified Abrasives Manufacturers Institute) FEPA (Federation of European Producers of Abrasives) USS (United States Standard Sieve) JIS (Japanese International Standards) and “grit size” are all arbitrary size designations with no relevance to any true physical dimensional scale. As far as abrasives breaking down during use, you have touched on the vast difference between the action of loose and bonded abrasives and that abrasive which remains solid or which tends to be more friable during use. A loose rolling abrasive sharpening system (lapping, SiC paper and waterstones) tend to produce much different results that a true fixed bonded system such as DMT Diamond Whetstones.

Stu’s Shed:

With that in mind, does the measure of particle size of diamond abrasives fixed to a medium (where only a portion of the particle is actually exposed above the surface) compare with those where the particles are free to move (such as in diamond paste)?  I note from your website that only 1/3 of the diamond is exposed, so is the particle size the site provides the size of the actual diamond, or the size of the exposed portion of the diamond?  (In other words, when I have stated the Ex Fine DMT stone particle size is 9 microns, does it actually perform as one with a particle size of 3 microns)?

Stan A. Watson:

We designate the actual size of the diamond itself so for instance when we state that the Fine / 600 mesh / red / 25 micron grade of diamond, we are actually using a 25 micron size diamond where only about 8 to 9 microns is exposed above the metal bond. That being said, let me state that there is a size range about the mean particle size or a Gaussian distribution of particle sizes which would be from about 23 to 28 micron. But, rest assured that DMT takes advantage of a highly sophisticated system of fluidized bed micronized particle separation to ensure there are no oversized / undersized rogue diamond crystals in each diamond grade.

(Editor’s note: This feature cannot necessarily be said about all diamond stone manufacturers products, this technique ensures a dependable crystal size for DMT stones, but your mileage may vary with other manufacturers, and this can be assessed in part by observing the performance and surface finish obtained when using other manufacturer’s diamond stone products. The problem with a rogue stone in the matrix is it will cause gouge marks in the material being ground that will cause significant amount of additional work with the next stone grade to remove, if that is even possible.)

Stu’s Shed:

By looking at the particle sizes of the 4 sides of the DiaSharp stones I have, there seems to be a large step from the fine to the extra fine stone, and if I was dealing with silicon carbide sandpaper for example, I would use a few more grades in between before jumping to the extra fine particle size.  Is this actually valid, or have I headed off on a bit of a tangent there?  I would expect that DMT wouldn’t put out ‘fine’ and ‘extra fine’ grades without intermediate stones if you couldn’t go from one to the other, but it does seem quite a step.

Stan A. Watson:

The actual step from “Fine” to “Extra Fine” is from a 25 micron to a 9 micron (8-9 micron  to 3 micron exposure) and progresses nicely in the range of micron sizes we offer. You can control the surface finish quite nicely by controlling the applied force during sharpening so as to finish up each step in the progression of sharpening with lighter and lighter strokes. The comparison with diamond to SiC paper is not really an apples to apples comparison as the SiC paper while being a bonded abrasive is friable, looses the particle bond quite easily and could be of either “open coat” or “closed coat” type.

(Editor’s Note: There are two common patterns used when bonding the abrasive to the backing material, called “Open Coat” or “Closed Coat”.

An Open Coat pattern is when there is a lower density of abrasive particles, so the entire particle can dig deeply into the work, facilitating faster material removal, and less likelihood of waste particles clogging the abrasive material.  The particle size in this case – the working size of the particle as it were, is closer to the actual measured size of the particle.

A Closed Coat pattern is when the particles are tightly packed together on the backing material – a denser pattern. With the gap between particles being very small, the abrasive cannot bite in as deeply, resulting in a finer finish (or being realistic about what we are talking about here – a finer scratch pattern), and the effective size of the abrasive particles is much smaller than the actual particles themselves.  These materials tend to need some form of lubrication because of the increased heat buildup.)

Stu’s Shed:

In the article on the Alisam sled, one assumption I have made is the consistency of thickness of the DiaSharp stone (and in particular that the two sides are parallel).  Do you have a listed tolerance for the thickness, or should the stones be mounted in an adjustable holder to ensure the top surface is parallel with the base?  The Alisam sled obviously assumes the sharpening material is parallel with the surface their sled runs on.  I was reluctant to use the diamond stone on my precision granite block, because being double sided, I would expect the diamonds on the underside to cause some damage to the comparatively softer block.

Stan A. Watson:

The DiaSharp product is produced to a parallel,  thickness and flatness tolerance and you should be confident in your ability to step from stone to stone with out having to adjust the iron in the Alisam jig.  Yes the diamond on the bottom of the double sided stones would adversely affect your granite surface plate.

Stu’s Shed:

I am planning on doing a video feature on diamond sharpening (as I have with slow speed grinders, and will for Japanese waterstones and “Scary Sharp”). Do you have any particular advice about the use of these stones that are at the extreme ends of the diamond whetstone scale – the XX Fine and XX Coarse DMT Stones?

Stan A. Watson:

Remember that especially with the performance of the XX Fine there is a break in period and the stone will produce better and better results the more it is used. Also if you are using the XX Coarse to flatten waterstones, do so under running water to flush out the abrasive slurry as quickly as possible.

Stu’s Shed:

Sorry about the long list of questions – writing these articles often raises as many questions as they answer!

Stan A. Watson:

No problem, I am more that happy to correspond with you about anything that I may be able to contribute to.
Best Regard! Stan.

So a big thank you to Stan Watson for not only obliging me with very comprehensive answers to my questions, but also for allowing those responses to be published here.  There will be a video in the near future demonstrating the use of the DMT Diamond Whetstones in action, so keep an eye out for that in Stu’s Shed TV / iTunes.

2 Responses

  1. […] have amended this list in light of recent discussions with the Technical Director for DMT, and the micron size now listed is based on the size of the […]

  2. […] Q&A with Stan Watson – Technical Director, DMT (stusshed.com) […]

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