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Thread: Hollow grind question

  1. #16
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    Quote Originally Posted by Phil Mueller View Post
    I​t just occurred to me that I’ve spent another $200 on....
    When does this slippery slope end!
    My best projection, based on long observation of the woodturning, other wood endeavors, metalworking, pottery, motorcycles, boating, and a variety of passion-inspiring pursuits - the slope ends when you finally slide off the Big Slope of Life.

    JKJ

  2. #17
    I like to follow a bench grinder with a few strokes on a similar grit stone, then move on up the grits.

  3. #18
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    Quote Originally Posted by bridger berdel View Post
    I like to follow a bench grinder with a few strokes on a similar grit stone, then move on up the grits.
    Hi Bridger

    This is an interesting issue.

    If one is using a vitreous silicon carbide grinding wheel - the recommended white wheels are typically 46 grit - then the resulting bevel is quite coarse and serrated. Your suggestion to follow with a coarse stone is one I would follow when I used such wheels. In fact, it was recommended that one did not grind to the edge of the blade ... not just because of the resulting heat, but to preserve as much of the old smooth bevel as possible. Unfortunately, leaving this edge comes complete with a rounded wear bevel. It needs to be removed.

    The advantage of a CBN wheel is not just they are great at keeping their shape and grinding quite cool; the advantage is also that one can use a reasonably smooth grit, such as 180, and that this produces a smoother surface at the end of the hollow. Indeed, one can grind safely to the edge (enough to raise a fine wire, and no more than than).

    I have recommended a step beyond this, and that is to use the Tormek BGM-100, since this will enable the grind to be dead straight. It is not just that the resulting hollow grind is super clean and straight, but when ground to the edge of the blade, it is possible to move directly to honing with higher grits. I start with 6000 grit. This requires about 3 strokes to raise a smooth and straight shine with a wire to the back of the blade. A couple of strokes on a 13000, and you are done. The hollow will remain this friendly for quite a long time. Grinding like this means less grinding.

    As before ...



    Regards from Perth

    Derek

  4. #19
    If you are grinding with a fairly fine grit on a jigged grinder you can likely forego the first stone being of similar grit. I grind and hone freehand, and have multiple grinder options. The first stone being near the grit of the grinder needs only a few swipes before moving on up the grits- in my hands this is quicker than jigging the grinder. However, i don't have a tormek, so i'm not in a position to address the amount of time spent fiddling with those particular sets of gizmos. What i do works for me, is quick and efficient in my shop. YMMV.

  5. #20
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    None of this matters, much, since you rarely grind tools all the way to the edge. If you are under the impression that you do, you have a fundamental misunderstanding of the process. There's little need to spend a lot of money on grinding wheels.
    Last edited by Charles Guest; 08-21-2018 at 7:15 PM.

  6. Grinding to or past the edge mostly happens when repairing a damaged edge.

  7. #22
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    Quote Originally Posted by bridger berdel View Post
    Grinding to or past the edge mostly happens when repairing a damaged edge.
    imo; its not sound advise to grind away the damaged edge from the bevel side of the tool.






    Last edited by Stewie Simpson; 08-21-2018 at 10:15 PM.

  8. #23
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    Quote Originally Posted by Charles Guest View Post
    None of this matters, much, since you rarely grind tools all the way to the edge. If you are under the impression that you do, you have a fundamental misunderstanding of the process. There's little need to spend a lot of money on grinding wheels.
    Charles, can you expand on this a bit more please for those of us who don't understand the process. Thanks.
    Chris

    Everything I like is either illegal, immoral or fattening

  9. #24
    Quote Originally Posted by Chris Parks View Post
    Charles, can you expand on this a bit more please for those of us who don't understand the process. Thanks.
    Stewie just posted a few pictures how you repair a nicked edge. Present the chisel perpendicular to the grinding stone to grind out the damage. Only then remove metal from the bevel at the usual angle, only doing this until just a little remains of the obtuse grind done previously. That little bit is quickly removed on a coarse flat bench stone, if you dare to go close enough.

    It does take some decent skills to do that evenly and accurately on the grinder.

  10. #25
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    Quote Originally Posted by Kees Heiden View Post
    Stewie just posted a few pictures how you repair a nicked edge. Present the chisel perpendicular to the grinding stone to grind out the damage. Only then remove metal from the bevel at the usual angle, only doing this until just a little remains of the obtuse grind done previously. That little bit is quickly removed on a coarse flat bench stone, if you dare to go close enough.

    It does take some decent skills to do that evenly and accurately on the grinder.
    I have never found the need to do that with a CBN wheel as it cuts so quickly and the wheel absorbs much of the heat produced is absorbed by the mass of the wheel and the edge never gets hot enough to suffer any damage at all. I had a chisel handed to me as a challenge where the owner had deliberately driven the edge into a nail several times and I took those deep nicks out by grinding it to the edge and the chisel blade was still cool enough to touch right at the bevel. CBN wheel sharpening has been a game changer in this respect for anyone who uses one.
    Chris

    Everything I like is either illegal, immoral or fattening

  11. #26
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    I've been grinding to the edge of blades for over a decade, and without any damage to the steel. For several years this was with a Tormek, which is a wet grinder. For the past 3 years I have done so with a CBN wheel on a half-speed bench grinder. So far I have not damaged a blade on this either.

    The edge is not weakened by this method. I have used 25 degree hollow grinds (to the edge) for shooting end grain, and never chipped nor rolled an edge. I have hollow ground to 30 degrees bench chisels, and chopped with them. Also no chipping or rolling of the edge. In short, in my experience, the method is perfectly safe when the grinding is cool.

    The trick is to keep the edge cool. This is not for the heavy-handed or impatient. Take it slow, light, and with a cool-running wheel, and she'll be right.

    Regards from Perth

    Derek
    Last edited by Derek Cohen; 08-22-2018 at 4:55 AM.

  12. #27
    When my current grinding wheel wears down too much I'll certainly think about the CBN option. But that'll take a few more years, and in the mean time I use the method posted above to grind a nicked edge without overheating on a conventional coarse wheel.

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