Plunge Line Question

Discussion in 'New to Knifemaking' started by Jellis11, Apr 14, 2017.

  1. Jellis11

    Jellis11 Well-Known Member

    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?

    Last edited: Apr 14, 2017
  2. J. Doyle

    J. Doyle Dealer - Purveyor

    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.
  3. Jellis11

    Jellis11 Well-Known Member

    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.
  4. Self Made Knives

    Self Made Knives Well-Known Member

    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. Jellis11

    Jellis11 Well-Known Member

    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. rhinoknives

    rhinoknives Well-Known Member

    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
  7. EdCaffreyMS

    EdCaffreyMS Forum Owner - Moderator

    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: Apr 14, 2017
  8. Jellis11

    Jellis11 Well-Known Member

    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. Jellis11

    Jellis11 Well-Known Member

    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. Self Made Knives

    Self Made Knives Well-Known Member

    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.
  11. Boss70

    Boss70 Member

    I had a similar problem a while back but only in one side. I found the cause was that I had a gap of about 1/16" between the belt and my platten on one side. Might check and make sure that your platten is adjusted correctly.

    Sent from my SM-G900V using Tapatalk
  12. Jellis11

    Jellis11 Well-Known Member

    Oh Wow! So, what was out of alignment, the belt or the platen? How in the world would you adjust that on a horizontal axis? Washers?
  13. Jellis11

    Jellis11 Well-Known Member

    No worries at all Anthony! I appreciate your taking the time to answer my questions!
  14. Boss70

    Boss70 Member

    I cut some shims out of a Coke can and put them between the platten bracket and the main body of the pattern. Worked perfectly for me.

    Sent from my SM-G900V using Tapatalk
  15. Kev

    Kev Well-Known Member

    It's funny, how all this stuff works together, and small changes make big differences. Usually I attempt to get a more rounded line, just because it's more visually appealing to me. And it's nice for me at least because I can change it to a more square line if I need to, to "fix" things if I'm too far out to match. It's just a trick I've saved a blade or two with. My next tool purchase is going to be a file guide. My experience is "training wheels" help teach me the muscle memory I need once I choose to take them off. But in my research I've found that a good file guide can be much more than a traning tool.
    Post a couple pictures of your results, I'd love to see what your getting, and what the symptoms are causing.
  16. Jellis11

    Jellis11 Well-Known Member

    I'll do that this evening Kev...thanks!
  17. EdCaffreyMS

    EdCaffreyMS Forum Owner - Moderator

    I'm finishing up a basic class with a gentleman who has never ground blades before..... he was having the same issue, and for him it was simply that he wasn't paying attention to the horizontal angle he was holding the blade at when grinding...... basically was holding the tang end higher or lower on one side then he was the other. Think of the edge of a grinding belt as an immovable "straight edge".... in order to achieve/maintain a given angle, you have to insure that you hold the blade horizontally to achieve the plunge cut angle you desire...... THEN, the trick is to match/maintain that angle on the opposite side of the blade. Along with that, body motion could also be the you move to the side, you MIGHT be dropping that side hand/arm.

    There can of course be other explanations, but often time the simplest, tends to be the correct one. Check into that, and if it's not a solution, let us know and we can go from there. :)
  18. Jellis11

    Jellis11 Well-Known Member

    Thanks Ed....I've spent at least an hour each weeknight and nearly the entire weekends in front of that thing for the last several weeks....learned a lot, worked a lot of things out, but still have a few struggles.....I'm getting there though!

    I was able to finish one so far..I struggled with the straight bevel and finally had to take it to the spine to make it work. But I think it turned out okay.

    Last edited: Apr 14, 2017
  19. EdCaffreyMS

    EdCaffreyMS Forum Owner - Moderator

    Hey! That's really nice! Keep going!!
  20. Kev

    Kev Well-Known Member

    I can certainly appreciate the KISS philosophy. I am a learn by doing guy and find my groove by trial and error most of the time. That said it's great to have a place like this where experts chime in. Most times I learn to watch for things, I didn't even know I was missing.

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