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Thread: Hollow grind with cbn wheel on low speed grinder- how far do you go?

  1. #1
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    Hollow grind with cbn wheel on low speed grinder- how far do you go?

    Hi all, I'm getting a slow speed grinder with a cbn wheel. I've never done hollow grinds myself. Would appreciate it if any experienced woodworker could advise how much to grind on the blade before you risk edge temper. Pics would be welcome. Thanks!

  2. #2
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    Don't wait for it to get hot. Take a couple of passes on the grinder followed with a swish in water. Practice on something you don't care about and just make sure the tip never turns blue.

  3. #3
    Quote Originally Posted by Assaf Oppenheimer View Post
    Hi all, I'm getting a slow speed grinder with a cbn wheel. I've never done hollow grinds myself. Would appreciate it if any experienced woodworker could advise how much to grind on the blade before you risk edge temper. Pics would be welcome. Thanks!
    Unfortunately there is a pretty good correlation between people who are sure they can safely grind their tools on a high speed grinder and those who think that tools made of plain carbon steel will not hold a good edge. They tend to favor high chrome steels.

    Some even avoid putting their Japanese chisels to the grinder (as if they know better), but their western chisels they grind.

  4. #4
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    Hi Assaf
    I have a 8" CBN wheel on my Rikon slow-speed grinder to put a hollow grind on old plane blades. My experience is that the CBN wheel is much less prone to overheat the edge, but it can happen, especially at the corner of the bevel edge. I use a Veritas guide which helps to create a consistent straight bevel edge - it allows you to move the iron left/right in a straight path across the surface of the wheel. With that I've learned to move the iron at a constant speed across the wheel while grinding to avoid overheating one spot, and especially don't stop the movement when the corner of the blade is on the grinding surface.
    As you probably know, only use it on HHS, and yes, practice on something first.

  5. #5
    I use 180 and 600 grit cbn wheels on a slow speed (1040 rpm) 6" grinder for all my edge tools. I bought them initially for hss turning tools but have not seen a problem with carbon tool steels for flat work. As James says, keep them moving and keep a water bath at hand. If you keep a thumb near the edge you can feel before the tool gets hot enough to burn. I used white al-ox wheels for years but cbn is far superior in several ways including running cooler. I can remove material almost as fast with my 180 cbn as with an 80# white wheel with far less risk of overheating. I would recommend 180 as a good all-around grit to start with.

  6. #6
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    The only grinding I do is to get a damaged edge back to the point that it can be honed correctly, all the way across. None of my plane irons have seen a grinder for over a decade. The chisels that see rough work visit the grinder when one loses a chip.

    To answer the question, only far enough to get to the point the edge can be sharpened correctly on stones.

    Some sharpening methods require grinding often. I avoid those.

  7. #7
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    I'm with Tom on this. I would rather spend the time on a 400 grit stone unless I managed to chip or ding a blade. I haven't so I never use my grinder again once my tools are sharp. I don't usually go below 1000 grit to re-sharpen. I know it's faster to hand sharpen with a concave grind, easier to find the bevel on the stone and all that. I just don't care about that stuff or how long it takes. When I'm sharpening, I'm chillin. It's another part of woodworking that I enjoy.

  8. #8
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    Quote Originally Posted by Assaf Oppenheimer View Post
    Hi all, I'm getting a slow speed grinder with a cbn wheel. I've never done hollow grinds myself. Would appreciate it if any experienced woodworker could advise how much to grind on the blade before you risk edge temper. Pics would be welcome. Thanks!
    You need to understand that you don't grind all the way to the edge regardless of the kind of wheel you have unless you have a really bad gash on the edge. Most gashes can be taken out on a medium oilstone by tipping the blade upright and rubbing across until the edge is blunted to below the gash. This completely eliminates the danger of overheating. Then you hollow grind back in to just behind the edge, and take out the remaining bluntness and bring to sharp ON YOUR HONING STONES, not at the grinder.

    A hollow grind does not shorten the blade, or produce a burr. That work is done at one's honing stones.

    You don't need a CBN wheel to hollow grind. A $15 coarse wheel does the job just fine, if you understand what hollow grinding means in the first place.

  9. #9
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    Quote Originally Posted by Assaf Oppenheimer View Post
    Hi all, I'm getting a slow speed grinder with a cbn wheel. I've never done hollow grinds myself. Would appreciate it if any experienced woodworker could advise how much to grind on the blade before you risk edge temper. Pics would be welcome. Thanks!
    Assaf, have you read my article on my website: http://www.inthewoodshop.com/Woodwor...ningSetUp.html

    My main CBN wheel is an 8" 180 grit. I also have a 80 grit. The ultimate grinding set up, in my opinion, uses the Tormek BGM-100 blade holder. The half-speed is a bonus, not a necessity, but I would recommend this. It runs a little cooler.

    There are reasons to get a CBN wheel. These include longevity of the grinding media, retention of the circumference size (which means no alteration to settings), and the wheel runs cool, almost as cool as a Tormek if you use a light hand. Yes, it is possible to burn steel, but I have not experienced burning an edge in some 30 years with a white wheel, and these are more "dangerous" than a CBN.

    In short, I hollow grind all non-laminated blades to the edge of the blade. This includes chisels and plane blades. The hollow acts as a jig for repeatable freehanding angles. All Japanese laminated blades are freehand honed on a full, flat bevel.

    Hollow grinding "to the max" means that there is less steel to hone, and this translates in faster sharpening, and the sharpening medium becomes less important. The hollow also lasts a long time. Ironically, the better you get at hollow grinding, the less you need to grind.

    Hollow grinding is not just efficient in terms of time, but it is not wasteful of steel, as some imagine. Refreshing a hollow means that you remove steel from the hollow - one does not shorten the blade this way. I stop at the edge when I can feel the slightest wire. This is no different from grinding on a stone and achieving a wire edge that way.

    Regards from Perth

    Derek

  10. #10
    I start by adjusting the stand so the wheel is centered on the bevel, and I grind until there's about 1/16" to the edge.

    That's my "secondary bevel".

    If you grind to the edge with a stone you risk heat damage. I wouldn't do that.

  11. #11
    I rarely use my CBN wheel on a chisel or plane blade - I have a WorkSharp and use that most of the time to re-establish a primary bevel or to correct a damaged edge. But when I do use the CBN, I do like Charles G. recommended. Don't take it all the way to the edge. Use your stones to finish the sharpening and to establish a (sharp) secondary bevel.

    And just for specifics, I have an 8 inch CBN wheel on a 1725 RPM grinder. I forget the grit but it's not too aggressive.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  12. #12
    Mike. Btw, they make cbn discs for the worksharp. Even better than the diamond lap discs .

  13. #13
    You can take the temper out of carbon steel with any type of grinder if you are not paying attention. CBN makes it more difficult. I think CBN wheels are a good investment if you use a grinder much if only for economy - the white stones I have worn out over the years would have paid for CBNs. For turners they are definitely the way to go.

  14. #14
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    Quote Originally Posted by Derek Cohen View Post
    Assaf, have you read my article on my website: http://www.inthewoodshop.com/Woodwor...ningSetUp.html

    My main CBN wheel is an 8" 180 grit. I also have a 80 grit. The ultimate grinding set up, in my opinion, uses the Tormek BGM-100 blade holder. The half-speed is a bonus, not a necessity, but I would recommend this. It runs a little cooler.

    There are reasons to get a CBN wheel. These include longevity of the grinding media, retention of the circumference size (which means no alteration to settings), and the wheel runs cool, almost as cool as a Tormek if you use a light hand. Yes, it is possible to burn steel, but I have not experienced burning an edge in some 30 years with a white wheel, and these are more "dangerous" than a CBN.

    In short, I hollow grind all non-laminated blades to the edge of the blade. This includes chisels and plane blades. The hollow acts as a jig for repeatable freehanding angles. All Japanese laminated blades are freehand honed on a full, flat bevel.

    Hollow grinding "to the max" means that there is less steel to hone, and this translates in faster sharpening, and the sharpening medium becomes less important. The hollow also lasts a long time. Ironically, the better you get at hollow grinding, the less you need to grind.

    Hollow grinding is not just efficient in terms of time, but it is not wasteful of steel, as some imagine. Refreshing a hollow means that you remove steel from the hollow - one does not shorten the blade this way. I stop at the edge when I can feel the slightest wire. This is no different from grinding on a stone and achieving a wire edge that way.

    Regards from Perth

    Derek
    I have a 6" 80 grit CBN wheel mounted on a 3450 RPM grinder. I was reluctant to hollow grind fearing disaster, but I remember Derek telling me how happy I'd be with the results. Though I have a right-hand tremor from Parkinson's Disease, I find that the quick metal removal helps me. I use both hands of course to move the chisel or the iron from right to left and back but spending just seconds rather than interrupted minutes is easier. I've not burned an edge nor even heated a blade to discernible warmth. I freehand sharpen and usually finish with David Weaver's Unicorn Method. I get great edges. You can use me as the poster boy, Assaf.

  15. #15
    Quote Originally Posted by Prashun Patel View Post
    Mike. Btw, they make cbn discs for the worksharp. Even better than the diamond lap discs .
    I didn't know that. Thanks!

    I'll go look for one.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

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