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Sharpening Pressure

Discussion in 'Maintenance, Tinkering & Embellishment' started by MtnHawk1, Jun 29, 2019.

  1. MtnHawk1

    MtnHawk1 Gold Member Gold Member

    May 22, 2019
    I've always heard that diamond sharpeners cut the fastest, which makes sense since diamonds are the hardest material, but the instructions on the ones I've seen say to use only light pressure. When I use heavier pressure on my silicon carbide stone (120 grit: about the same as the diamond stone, to keep the comparison as equal as possible) I remove metal faster and raise a burr quicker than on the diamond plate. I've used both types of stones at even lower grits and found the same to be true.

    It is usually taken as common knowledge and common sense that the harder a knife edge is pressed into a stone (up to a certain point, of course), the faster it is to relief grind and sharpen. Even though diamonds sharpen faster, it seems this advantage is diminished or negated by the light pressure that must be used.

    I don't have a lot of knives or sharpen a lot, which is too small a sample to draw any definitive conclusions comparing diamonds with silicon carbide, aluminum oxide, or other coarse-grit stones that are used to remove metal quickly. Given that, I'd like to know if anyone else has had the same results I have.

    Thank you….
    Last edited: Jun 29, 2019
    bucketstove likes this.
  2. kreisler


    May 11, 2012
    same experience here me too.
    when i use less pressure on Spyderco UF stone, less metal gets removed.
    when i use much pressure (e. g. on 302UF) , much metal gets removed.

    i dunno why that is.
    doesn't necessarily make sense to me.
  3. sickpuppy1

    sickpuppy1 Basic Member Basic Member

    Sep 27, 2018
    Some of it may depend on the mfg of the diamond stone your using too. Many of the diamond stones are dust that is glued or epoxied to the surface of a material. Those want to you to use lighter pressure as to not clog and to not pull the diamond from its base. And doing it that way, they will last a long time. Other mfg use a epoxy resin with the diamond material impregnated throughout the resin and will keep working till the "stone" is gone, in which case you only have to worry about clogging. the epoxy stones for me so far just seem to work better at lower pressure. I think one other thing too is heat. If the stone is dust glued to a base, then the friction at contact generates heat which could loosen the dust from its "glue" and cause premature wear.
    I've been using a set of Diamond Matrix stones which happen to be mfg by one of our members here. It is the later type of stone where the diamond dust is in a epoxy slurry evenly throughout the entire "stone". It just seems to work faster at lower pressures which is fine with me as I have more control over material removed. An he may chime in here and be able to shed more light on as the research that's been done.
    MtnHawk1, Edgeoflife and The Zieg like this.
  4. Gritomatic

    Gritomatic Dealer / Materials Provider Dealer / Materials Provider

    Jan 4, 2016
    Low pressure is dictated by engineering principles. Low pressure hardens metal surface while high pressure weakens it. It directly affects the edge retention. For coarse grinding, the maximum pressure is approximately 2 kg/sq.cm. For superfinishing, the maximum pressure is approximately 0.2 kg/sq.cm. For freehand sharpening, it's much less than the weight of the knife. For guided sharpeners, it's less than the weight of the sharpening stone.
  5. ToddS


    Jan 15, 2015
    I think the fact that pressure depends on contact area is often lost/forgotten in these discussions... Pressure increases with decreasing contact area. As well, there is a minimum threshold pressure required for grinding and it's easy to not achieve threshold this on coarse diamond plates.

    eKretz, dantzk8, kreisler and 4 others like this.
  6. The Zieg

    The Zieg

    Jan 31, 2002
    And from a practical point of view, my best results are when I imagine trying to shave off the surface of my sharpener/stone as though I were using a cheese plane. Just like the cheese plane, the harder I press, the more difficult it is to pull through.*


    *I hereby absolve readers from needing to make "cheese cutting" remarks. ;)
  7. MtnHawk1

    MtnHawk1 Gold Member Gold Member

    May 22, 2019
    kreisler, unless I'm missing something, wouldn't that be what you'd expect: less sharpening pressure = less metal removed, much sharpening pressure = much metal removed.

    sickpuppy1, thanks for the education. Much of what you wrote about the manufacturing processes I did not know, and it's helpful to better understand how diamond stones sharpen.

    Would you mind telling me who manufactures the Diamond Matrix epoxy stones you mention? I don't think I've used that kind before. Maybe they would work better for me. Since the diamond material is imbedded throughout the stone I am guessing they would last longer with more pressure than stones with diamonds just on the surface.

    I don't like to spend any more time reprofiling and sharpening than I need to. It would be nice to sharpen with the hardest stones so that I know I am removing metal freehand as fast as possible, but using light pressure is counterproductive to rapid metal removal, at least in my experience.

    I have read some posts where people sharpen with more pressure than the manufacturers recommend. I am hesitant to do this because diamond stones are expensive, but my curiosity will probably get the best of me one of these days.

    Thanks again for the good info! :thumbsup:

    I probably should have mentioned in my OP that I am not interested in smooth, polished edges. I like coarse, toothy, utility edges. I also appreciate the more technical posts and information, and all the work that it takes to get there, but physics and engineering are not my strong points so my understanding is limited.
    Good points! It's often easy to overlook the simple and obvious, which are also often the most important.
    Would you please explain this further?
    Last edited: Jun 29, 2019
    kreisler likes this.
  8. Wowbagger

    Wowbagger Basic Member Basic Member

    Sep 20, 2015
    . . . and Brondo has what plants crave.

    Lets back up a little from the end of the post about damaging metal from high pressure. That's all about the edge and final edge refinement.

    . . . some other abrasive materials WILL HAVE SHARPER POINTS and can cut faster in some cases (most . . . all cases) than the diamond stones.
    From MUCH experience using extremely high pressure on large DMT diamond plates cutting A2 woodworking blades I have found even super high pressure (relative to what you and I can generate at home while sharpening without powered machines) some times is not enough to remove material in a timely manner. Think : stone on the floor, plane blade double back tapped to a large block of wood, me bent over with as much of my weight on the blade as I can . . . I assume well above 100lb. To be clear I was flattening a large area of the blade on the back / flat surface of the single bevel blade. I was trying to remove a minute thickness but over an area of a couple of square inches at the least.

    Now :
    • This in no way damaged the Extra Coarse (220) stone. I still have that stone and use it to reprofile S110V blades and it cuts like all get out and fast too.
    • The comment about surface area and "the fact that pressure depends on contact area is often lost/forgotten" is exactly right. My 220 grit was still too fine and the surface area too great for what I was working.
    • I then went to 36grit alumina zirconia belts glued to a flat plate. Even with much less pressure applied to the block the blade was attached to I was able to get the job done rather than just wish about it.
    • The grit of the Alumina Zirconia is much, much, much more sharp than the diamond and was able to either be hard enough to over come the A2 and or fracture and refresh to be effectively self sharpening which the diamond will not do to any significant degree.
    • Cliff Stamp explains very well pressure required for abrading various hardness blade materials correlated with various stone materials and I will post a link.
    • Finally and I think I will shout this for all the world to hear : THE DIAMOND PLATE IS ONLY DAMAGED FROM HIGH PRESSURE WHEN ONE GOES EDGE FIRST (edge leading) and with a small amount of blade area on the stone. If one gets back off the edge and uses high pressure to repfrofile with the emphasis on the edge trailing stroke very high pressures can be used on the diamond plates.
    Mostly Cliff says the higher hardness of the blade (for material removal not edge refinement) the higher the pressure required per git. For diamond or other abrasive; doesn't matter.
    Last edited: Jun 29, 2019
    MtnHawk1 likes this.
  9. Wowbagger

    Wowbagger Basic Member Basic Member

    Sep 20, 2015
    The results and application of my failed high pressure sharpening can be seen at this link. See the third photo down of the backs of the stack of wide blades. And there are many more than that stack.
    I say failed with 220 diamond. I succeeded by going to the 36 grit and then working through progressively finer grits from 120 through many stones to 8,000.
    LINK > > > TO PHOTOS
  10. jux t

    jux t Gold Member Gold Member

    Jan 10, 2018
    I find that it depends on the blade that’s being sharpened.
    willc likes this.
  11. sickpuppy1

    sickpuppy1 Basic Member Basic Member

    Sep 27, 2018
    Mtnhawk, I didn't fully realize you were talking about freehand stones when I wrote that, although the same principle applies. The diamond matrix stones I was referring to are made by Diemaker and sold through EdgePro , for the edgepro line of guided sharpeners. So while it could be used freehand, using a 1"x6" stone that way would be a pain in the butt,lol
    I use them on a Hapstone M2 sharpener, and its been very enlightening. I still have and use the SIC stones I bought from Gritomatic, but the stone "feedback" and cutting between the two are very different.
    Last edited: Jun 30, 2019
  12. David Martin

    David Martin Gold Member Gold Member

    Apr 7, 2008
    That's a problem when using diamond plates. Use light pressure so you don't fracture the attached diamonds off your stone. Thus,
    causing more wear and blade steel is also removed slower. Thus, the sharpening process takes longer. Whereas when using conventional SiC stones (using the coarse) one can use more pressure and grinding the edge bevel goes faster. DM
    willc, jpm2 and MtnHawk1 like this.
  13. eKretz


    Aug 30, 2009
    No, you don't need to use only light pressure universally on diamond plates. That's the point Todd was trying to make. If you have a narrow bevel, use light pressure. If you are sharpening only a tiny portion of the blade - for instance a curved part of the blade - use light pressure.

    If you are sharpening a straight section of blade with a wide bevel making good contact, bear down and apply some pressure. I promise you will not hurt the plate, nor strip off diamonds. If you are trying to sharpen something like a plane blade's back - as Todd noted - you very likely won't be able to apply enough pressure.

    Pressure is force divided by area. So the force that would actually damage a diamond plate is greater for every additional diamond your item being sharpened is in contact with. The more diamonds in contact at one time, the more pressure that can safely be applied. I have a DMT C that I have borne down upon with all the force that I can muster (at 6'3" and 220 pounds, considerable). It still cuts very well. After a cleaning with BKF it doesn't look much different than a brand new plate. It is approaching 10 years old. The key is not to apply too much force over too small an area.
    Eli Chaps and MtnHawk1 like this.
  14. DeadboxHero


    Mar 22, 2014
    You can add as much pressure as you can control when setting the shoulder and making the burr. Move back and forth to speed it up and lock your wrist.

    If using coated diamond plates don't add lots pressure.Those tear out. That's why Vitrified diamond/CBN is faster, one can add more pressure.

    When removing the burr use alternating passes with the lightest tough possible.
    willc, Eli Chaps and MtnHawk1 like this.
  15. Twindog

    Twindog Gold Member Gold Member

    Apr 6, 2004
    That's an interesting concept. I Googled around, but didn't find anything other than case hardening, where just the surface is hardened over a softer interior steel. But that seems to be an entirely different process for low-carbon steels.

    By intuition alone, I can't see how this works. Do you have any links that would explain it in more detail?
    David Martin likes this.
  16. kwselke


    Jan 5, 2018
    I've found I get better results with light pressure on all my sharpeners; KME, Sharpmaker, DMT bench stones, strops, and hones. If I wish to remove material faster I utilize a coarser grit instead of adding additional pressure. I do not know if there is a scientific reason for my experience or if I am just better at maintaining my angles with light pressure. All I know is that once I learned to use the right tool for the job and to let the tools do the work I started getting much better edges.
    The Zieg and sickpuppy1 like this.
  17. eKretz


    Aug 30, 2009
    That is all fine and dandy if you are not working on a wide bevel or similar high surface area item. If you are, light pressure won't cut it. See what I did there? :confused: :D

    It takes a minimum pressure to cut also, especially after the diamonds dull a little - Todd alluded to this as well...

    Regarding the "hardening" effect, this could theoretically be possible if the steel involved work hardened because the abrasive merely rubbed and didn't cut the steel.
  18. kwselke


    Jan 5, 2018
    Please explain "wide bevel or similar high surface area item"? I thought the question was about simple sharpening, including re profiling. Dull diamonds? OK
  19. MtnHawk1

    MtnHawk1 Gold Member Gold Member

    May 22, 2019
    jux t, I agree, but in my case I don't have the time, money, interest, or storage space to do the research and purchase the best sharpening stone for every type of knife steel I own. Plus, I use knives in the wilderness and keep sharpening tools to a minimum when I have to carry them. Even at home I like to stay minimalist. I prefer having stones that will do everything, even if they may not do everything as well as possible.

    sickpuppy1, thanks for your follow-up post and info. Some of my knives are big wood choppers that I use in the wilderness, and I prefer using 8x3 benchstones for them.

    I appreciate you mentioning diamond matrix stones. From the little I know about diamond sharpening stones, diamonds imbedded in a matrix make more sense than ones on the surface. Now that I know about them I’ll see if I can find them in a larger size.

    I'm checking out some low-grit (80 and below) non-diamond stones that look like they will do well in hogging off metal at a fast rate, without having to be concerned about using too much pressure.

    David Martin, this is exactly what I mean! Thank you for saying it so well and succinctly! :thumbsup: I wanted to find out if forum members who are more knowledgeable and experienced than I am had experiences similar to mine. I didn't want to come to any conclusions about diamond stones based on mistakes I may be making that I was unaware of.
    jux t likes this.
  20. HeavyHanded


    Jun 4, 2010

    My take - I love to use heavy pressure for some operations - flattening plane irons, final work on a regrind sometimes.
    But...the harder you press, the further into the steel you are disturbing the lattice, potentially weakening the steel below the surface. I try to stop short of the apex.

    You are also increasing the possibility of a pressure burr/wire edge.

    Diamonds tend to cut faster once they begin cutting to their depth, similar to how a file will not get good purchase on a piece of steel until it has cut to its depth. I noted this in a recent review of a diamond grit axe puck - once the coarse side cut for a minute, the stock began to come of with increasing rapidity even with light pressure.

    I agree you can use more pressure on diamonds if the surface area is larger, but I have also ruined a DMT XXC doing this, and TBH I still don't think the amount of pressure used or work hours was anywhere near what should have been required to kill it. I play it safe now, and use light pressure on all my diamonds. When leaning on my vitreous stones I accept I'm also glazing the surface and will like as not need to recondition them more often.

    Once the initial profile is good, they don't need a lot of force even with large surface areas. This being true of waterstones as well. Once the surface is established, a run through a progression goes pretty quickly even on broad bladed chisels, and leaning on them only loads up the stone surface and requires more downtime with a nagura.

    Edit to add:
    It is an interesting conversation re unit pressure and stock removal by grit rating. It is important to remember that with really coarse abrasives not only is the relief are (space where removed metal can go) much deeper, but the abrasive themselves determine shoulder to shoulder spacing. Unit pressure increases very rapidly as the spacing increases, even as applied pressure stays the same. This is the inverse of applying more force on a larger surface area.

    In many cases applying more pressure fails to improve grind rate much due to limited abrasive depth and limited space for the swarf to go, irrespective of how sharp the particles are.
    Last edited: Jul 1, 2019
    MtnHawk1 likes this.

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