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Thread: Plunge Line Question

  1. #1

    Plunge Line Question

    Gentlemen, I have a question.....In relation to the theoretically 90 degree intersection of the plunge line and the top edge of the bevel, what exactly causes this to become a curved intersection rather than a sharp 90 degree angle? I am still learning to grind properly, that being said, I have found that if I use more of the belt edge at the plunge line while slightly angling the blade tip towards my body, I can create a sharper more distinct angle. However, I have also found that if I am not careful and slow while doing this I can gouge the bevel surface fairly easily. To note, I am using a Norton blaze ceramic 36g to cut my preliminary bevels....would a J-Flex belt be more helpful in this situation?

    Also, I have got maybe 1/16" of belt or less hanging over the edge when doing this.

    Is this the correct technique? What could I be doing wrong? What am I missing?

    Thanks,
    Jeff
    Last edited by Jellis11; 04-14-2017 at 07:40 AM.

  2. #2
    Join Date
    Apr 2010
    Location
    Gladwin, MI
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    1,664
    It all depends on what you want it to look like. Generally speaking, having the plunges go all the way up to the spine and past, often called 'breaking the spine', is considered not good.

    You want to stop your plunge lines short of the reaching the spine. The resulting transition you're speaking of can take several forms. Generally speaking again, if you want a sharp transition, you would use a stiffer belt, with less belt hanging over your platen and more pressure. To get a sweeping, curving softer plunge you would use a more flexible belt, with more belt hanging over the edge and less pressure.

    Those would be the two extreme ends of the scale. You can adjust any or all of those variables and get results in between those two extremes.
    John Doyle
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  3. #3
    Quote Originally Posted by J. Doyle View Post
    It all depends on what you want it to look like. Generally speaking, having the plunges go all the way up to the spine and past, often called 'breaking the spine', is considered not good.
    I 100% agree with you, but perhaps I didn't explain my dilemma well enough....what I am referring to are the differences between this

    4384075853_7447fc280f_o.jpg

    And this

    18%20Harsey%20Mod%20II%20plunge%20line%20P1080376.jpg

    Not my work by the way, just images I pulled from the web.

  4. #4
    Join Date
    Jun 2014
    Location
    NW Arkansas
    Posts
    1,477
    What you are showing there is difficult, at least is is for me. I've tried to practice the rounded out plunges with varying success. For some reason, I've had better luck with hollow grinding rather than flat grinds with those round plunges. It comes down to the pressure you put on the blade as you start grinding. And, you need to have your belt overhanging about a 1/4" or so too. You kind of have to sweep into it, a little at time. You just have to roll into the plunge a little a time and keep that line tracking the way you want. Like everything in this hobby, it takes a lot of practice to perfect. My problem is that I'll usually nail it on one side, then can't ever get the other side to match.

  5. #5
    Quote Originally Posted by Self Made Knives View Post
    What you are showing there is difficult, at least is is for me. I've tried to practice the rounded out plunges with varying success. For some reason, I've had better luck with hollow grinding rather than flat grinds with those round plunges. It comes down to the pressure you put on the blade as you start grinding. And, you need to have your belt overhanging about a 1/4" or so too. You kind of have to sweep into it, a little at time. You just have to roll into the plunge a little a time and keep that line tracking the way you want. Like everything in this hobby, it takes a lot of practice to perfect. My problem is that I'll usually nail it on one side, then can't ever get the other side to match.
    Thanks for the reply Anthony! I WANT the nice 90 degree or so angle, but what I keep ending up with is more of the "sweeping angle". So, to try to get the 90 degree angle I am putting more pressure at the plunge area with the belt edge while pulling the blade end back towards my body at a very slight angle....So, not completely sure if this is the correct way of doing that, because one little slip and it is trashed.

  6. #6
    Join Date
    Nov 2011
    Location
    Santa Monica Ca.
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    6,433
    If you use a File Guide it will help insure a distinct 90 if you do your part, All so the image/knife at the top appears to have been ground with a wheel/Hollow while the bottom image/knife is a Flat grind
    Laurence

    www.rhinoknives.com

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    Maker and seller of Rhino Finger Skins Thermal protection for grinding harden steel.

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  7. #7
    Join Date
    Sep 2009
    Location
    Great Falls, Montana, USA
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    2,743
    I 100% agree with you, but perhaps I didn't explain my dilemma well enough....what I am referring to are the differences between this





    And this



    Not my work by the way, just images I pulled from the web.
    The major difference between those two is...... the top plunge was machined in (as in a milling machine)...... the bottom one was done with a belts/abrasives.

    In a grinding situation, the top image's plunge radius is generally accomplished on a "slack belt".

    When it comes to "cutting" the plunge lines on just about any blade, I "rough" them in pre-heat treat with a 50 grit, followed by a 120. The majority is done post heat treat with a 50 grit belt, and in the final stages of finish grinding, are cleaned up, with a slight radius on a 220 grit belt. Final clean up is accomplished with a 400 then 600 belt.

    Personally, I do not use 36 grit belts...... too aggressive for any type of fine control...especially when used on a machine where the belt is moving as fast as it does on a Square Wheel...... hence the reason my "heavy grit" belts are 50 grit.
    Last edited by EdCaffreyMS; 04-14-2017 at 10:00 AM.

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  8. #8
    Quote Originally Posted by rhinoknives View Post
    If you use a File Guide it will help insure a distinct 90 if you do your part, All so the image/knife at the top appears to have been ground with a wheel/Hollow while the bottom image/knife is a Flat grind
    Thanks Laurence, I could do that....I'm using a Wilton Square Wheel, so my left side is a bit wonky. Although I have made some "corrective" modifications, I'll have to make more to facilitate the use of a file guide.

  9. #9
    Ed, so I'll switch to a higher grit belt for roughing...easy to do, but why are all my plunges cutting the "sweep" instead of a semi-sharp 90 deg angle.

    What grinder function, action or otherwise determines and/or controls whether this angle will be a "sweep" or sharp 90 degree or anywhere in between?

  10. #10
    Join Date
    Jun 2014
    Location
    NW Arkansas
    Posts
    1,477
    Ha! I thought you wanted the rounded look, sorry. Laurence is right if you want the quick method. Use a file guide as a stop against the edge of the platen for your preheat treat grind. Then post heat treat, I use a J-flex weight belt overhanging platen about an 1/8' or so to smooth and soften that line a little.

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