Grit thoughts

[Ken S]

I find Tormek's grit designations puzzling. I have found that both 360 grit diamond wheels cut more aggressively than the 220 grit SuperGrind (SG) wheels. I suspect this is due to other factors than just grit size. Tormek recommends much lighter grinding pressure when using the diamond wheels. I think the diamond grains are sharper than the aluminum oxide of the SG.

Ionut, one of our most outstanding now inactive members, noted several years ago that the Tormek SG wheel has three grits (not two). In addition to stone grader coarse, supposedly 220, and stone grader fine, supposedly 1000, Ionut noted a third, coarser grit, the surface just after using the TT-50 truing tool.

Related to this, I have found that the stone grader is not limited to just two grits, full coarse and full fine. It has not entered into the handbook, however, there is a middle grit, often called "600". This name coincides with the 600 grit diamond wheel of the T2. With the SG, I find 600 grit an approximate number. I also find the two grit numbers assigned to the stone grader (220 and 1000) both approximate numbers. The TT-50 produces a coarser grit than the coarse side of the stone grader.

I did a simple test tonight. I have no way to accurately determine grit size. My not very scientific measuring system was to feel the surface of my SG wheels prepared in different ways. The freshly ground TT-50 surface was noticeably the most coarse. I divided my second SG in half with a black Sharpie. I used the coarse side of the stone grader on one half and a 325 grit diamond card file glued on a flat piece of steel on the second half. I found these two surfaces very close, with the diamond card file perhaps the tiniest bit more coarse. Even used very slowly for a smoother surface, the TT-50 was clearly the most coarse.

The file card may help keep the grinding wheel more true than the stone grader.

I place very little importance on exacting grit numbers. The SG wheel is quite versatile with the TT-50 and stone grader. I just think in terms of more coarse and less coarse.

Ken

[John_B]

I used the TT-50 grader the other day before sharpening a badly damaged knife. I enjoyed the added coarseness and it resulted in a quicker job.
I find for knives that I have sharpened previously I like to start out with the SG-250's grit somewhere in between coarse and fine extremes. I will then use the fine side of the grading stone for the final passes prior to honing. This process seems to work well when the knife does not need much metal removed.

[Ken S]

Very good post, John. The handbook is vague on which grits to use. I would second your combination use as being the most practical.

Ken

[Herman Trivilino]

In the last few days I've sharpened every one of the four pair of pruning shears we own. Every one was very dull. I first prepared the grindstone with the coarse side of the stone grader, and then used the fine side very briefly with light pressure. I needed something in between coarse and fine.

[Herman Trivilino]

I used the TT-50 grader the other day before sharpening a badly damaged knife.

The original truing tool produced a surface so rough you'd use it on a badly damaged chisel or axe.   

[Ken S]

Herman,
I almost always use the TT-50 slowly, producing a somewhat smoother finish. Occasionally, I use it quickly traversing the grinding wheel to produce a heavier cut. I like the versatility of being able to choose.

I think the original purpose of the stone grader was to make the SG aluminum oxide wheel cut like the original finer grit natural stone. "Coarse and fine" were essentially "SG and natural". I don't know if the Tormek engineers realized the potential for middle grades at the time. If they did, they chose not to include it in the handbook.

Although it is still not included in the handbook, a middle grade, called "600 grit" is now known to Tormek. It is produced by not fully grading the grinding wheel. It is useful for knife sharpening. (Incidentally, it is the same grit designation that Tormek uses for the DWF-200, the standard diamond wheel for the T2.)

Try it. I think you will like it. Please post your thoughts.

Ken

[Herman Trivilino]

Ken, I think I sort of stumbled onto the 600 grit thing. I do as I describe above, but I did't bother spending a lot of time or applying a lot of pressure grading with the coarse side, either.

Of course, it's occasionally necessary to apply the rough side with lots of pressure when the grindstone gets filled with swarf.

[RobinW]

With regard to Ken's original question, I understand that 'grit' is defined by the size of the particle on an abrasive sheet (which in turn affects the number of particles per square metre or square foot).

However, there are different abrasive materials - eg sandpaper; garnet paper; emery paper; aluminium oxide, silicon carbide etc.

Although two different types of abrasive sheet may have the same grit rating, they will perform differently on a given application.

The particles for some abrasives break down when used, and the performance of the 'broken' particle is dependent on the type of abrasive, and this also affects how long it will last.

Another factor is whether the sheet uses closed (abrasive covering all of the sheet) or open (some empty space on the sheet). Closed is preferred for hand sanding, but can clog quicker than open; and open is preferred for power sanding.

Accordingly the various type of abrasive sheets on the market are designed for different applications (wood; metal; paint removal etc).

This would, in my opinion, explain the difference between the diamond and SG wheels. It comes down to the type of crystals/particles used and their inherent shapes. I would expect diamond particles to have different shapes to aluminium oxide particles.

[Ken S]

Robin,

I agree with you. Grit size is determined by the particle size. You have correctly noted that grit size is not the only determining factor in how an abrasive cuts. In my experiments with an SG-250, I found that the surface which was freshly trued with the TT-50 felt noticeably more coarse than either a surface freshly graded with the stone grader or my DMT 325 grit diamond file. Intuitively, I do not think that the trued surface should be more coarse than the stone's native coarseness. I believe that one of the SG-250 wheels I used in the test was previously unused. It did not feel as coarse as the
TT-50.

My testing is incomplete at this point. So far, I have not been overly impressed with the 325 grit diamond file or the coarse side of the stone grader, at least compared with the coarseness of the TT-50. I want to try a coarser diamond surface. I also want to work more with the middle grit (600) diamond file. It may be just right for knives. I will keep you all posted.

Ken

[johnmcg]

Hi guys, I'm new to the forum, not a beginner with the Tormek system, but not an expert by any means. I've been reading this topic with interest and have my own dilemma you may be able to help with.

I recently bought a 10,000 grit Japanese (ceramic - Sigma Power Select II) Waterstone, partly for that extra "zing" I sometimes need for gnarly woods and also to do final polishing on the backs of plane blades.

I understand that the Tormek Japanese Waterstone's 4,000 grit designation is based on the JIS (Japanese system) - am I right? However, I find that going from the Tormek JS to the 10,000 grit (JIS) stone produces a LESS polished face i.e. I seem to be going backwards. Am I missing something here? Anyone have any ideas why?

[RichColvin]

John,

When you grind on the Tormek with any stone, there is a slight concavity to the grind (very slight) as noted by others.  If you then hone that grind on a flat stone, there is a secondary, micro bevel at the top and bottom of bevel as noted in the exaggerated picture below.

Or possibly,

This is what is formed by your ceramic flat stone. 

If there is a dullness in the concave part of the grind, it is probably grit from the grinding, not a change in the surface. 

I could be wrong though, so I welcome other thoughts.

Kind regards,
Rich

[johnmcg]

Thanks for your reply Rich. The "dullness" is in the secondary bevels as shown in your diagram. The concave section polished on the Tormek SJ is much more polished than I can get the secondary bevels on the 10,000 grit stone (even after a fairly long attempt). At first I thought it might be a difference in grit systems (i.e. the 10,000 grit Waterstone may actually be a coarser grit than the 4,000 grit JS, if they were referenced to different grit systems), but my understanding now I that both grits are referenced to JIS (Japan system). I'm just not sure why a 10,000 grit Waterstone seems to be much less polished than the 4,000 grit JS - cudos to Tormek, I guess!

[Jan]

Not only the grit size and its hardness but also the properties of bonding agent determine the behaviour of a sharpening stone. As far as I know the Sigma Select II sharpening stone does not contain bonding material. The stone has amorphous structure similar to glass.

E.g. Naniwa Professional Stones are bonded with magnesium.

Jan

[johnmcg]

Thanks Jan - so I shouldn't expect to be able to get a polished finish with the Sigma?

[Jan]

John, I do not have datasheets for the stones under discussion, so my opinion is only speculative. 

In my understanding the carrier of the abrasive particles is for Tormek SJ wheel softer than the one of the Sigma stone. For this reason the exposure of the abrasive grains may be for the SJ wheel smaller than the exposure of the grains of in the Sigma stone. This may result in finer surface you got using the SJ wheel, despite the fact that its grit size is larger.

There exist also an alternative explanation. The finish from the Tormek SJ stone may be better because the blade was guided and so the edge surface is geometrically more consistent than the surface from free hand sharpening on the water stone.

Jan