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Thread: CBN Wheels and Temper

  1. #1
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    CBN Wheels and Temper

    Hi Everyone,

    I very recently converted my bench grinder to CBN wheels. I bought a 180 grit and a 600 grit. I also have an 80 grit that I have not mounted yet. I have a couple of questions I was hoping some of you seasoned vets may be able to help with.

    1. I was under the impression that you could not burn the edge/take the temper on these CBN wheels. Anyone want to buy a new Lee Nielsen bevel edge chisel with a burnt edge? I guess that is my first question - is the burnt edge actually an indication of a loss of temper, or am I over-reacting on that? It happened almost immediately while changing the bevel on a 1/2" chisel. It was on the 600 wheel - I saw no issues at all with the 180 grit, but I burned the chisel on the 600 grit wheel pretty quickly. I am guessing that the higher grit = more friction = heat.

    2. My grinder is the ubiquitous Delta GR450 8" variable speed 1750 RPM grinder. I would like a heads up about the speed control, especially with these CBN wheels. Is there a better or recommended speed to use when grinding bench tools (chisels, plane blades, carving tools, etc)? I'd also appreciate anyone with a an idea or pics of a jig for using with chisels and plane blades to ensure a square bevel? I have the Wolverine rests on my stand.

    Any feedback will be much appreciated.

    Joe

  2. #2
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    Grinding with any kind of wheel, including CBN is going to generate heat. While CBN's design can help to channel heat away and reduce the risk, you absolutely "can" burn something with these wheels if you are not reasonably careful. My advise is to still use the same caution you might have used with previous grinding wheels with the CBN. Not only will it guard against the issue you've experienced, but being careful should also result in a better, more consistent edge on the tool.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
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    What Jim said. CBN wheels can burn an edge. I keep a finger close to the edge when grinding and when it gets hot, douse in water.

    I also have the Wolverine rests. I draw a line parallel with the side of the wheel on the rest and use that as a guide. I also draw a line on the chisel/plane blade square with the edge and use that as a second guide. I go slow and easy and check for square often.

  4. #4
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    As Jim notes, there is no free lunch. CBN wheels, however, get close. You are not invited to throw caution to the winds. You still required to have a light touch. After all, even though a table saw makes cutting wood easier, you do not take the cutting for granted .... there are always consequences.

    The higher the grit, the greater the heat generated. You will not use the 600 grit the same way as the 80 grit.

    Regards from Perth

    Derek

  5. #5
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    Quote Originally Posted by Joe Beaulieu View Post
    ...1. I was under the impression that you could not burn the edge/take the temper on these CBN wheels. Anyone want to buy a new Lee Nielsen bevel edge chisel with a burnt edge? I guess that is my first question - is the burnt edge actually an indication of a loss of temper, or am I over-reacting on that? It happened almost immediately while changing the bevel on a 1/2" chisel. It was on the 600 wheel - I saw no issues at all with the 180 grit, but I burned the chisel on the 600 grit wheel pretty quickly. I am guessing that the higher grit = more friction = heat.
    ...
    Are your chisels made from carbon steel? I use CBN wheels almost exclusively to sharpen lathe tools. I'm pretty seasoned in years but certainly no expert on grinding.

    CBN wheels are widely used now by woodturners but advice from that corner might be not be useful to a woodworking chisel guy. We woodturners, in general, don't even think about the heat because the tools are High Speed Steel instead of carbon tool steel. It is said that you can't destroy the hardness of the edge on HSS since the temperature needed to change the temper is higher than you can get with a grinder.

    Everything I've read says non-HSS hardened steels can easily be overheated. The general instruction is if the edge is blued it's too late. Without a hardness tester it would be hard to verify the exact effect of a given grinding. You might try the file test which I use to see if cheap lathe tools are properly hardened. (Some are not hardened at all, some are hardened only the first inch or so.) A small triangular file just skids and can't cut into hardened steel but can cut a notch into unhardened steel. I test on the side of the tool shaft but it should work as well with a burned edge on a carbon steel tool.

    I also cool the edge when grinding even HSS tools by dipping in water. Some claim this can cause micro-fractures in the edge but I haven't seen any evidence of a problem, either with the microscope or with use. When aggressively reshaping HSS tools if I don't cool the end it can get uncomfortable to hold. I always shape lathe tools with an 80 grit CBN wheel on a "half speed" bench grinder.

    I don't often sharpen chisels and plane irons but when I do it's usually on a 1200 grit CBN wheel on a Tormek. The wheel turns so slowly that it's difficult to overheat the edge.

    I have also reshaped carbon steel tools on my 80 grit CBN wheel and sharpened them on my 600 grit CBN wheel on a half-speed bench grinder. I used a light touch and cooled often and didn't see any bluing on the edge.

    The heat should be less with the 80 grit, especially with a light touch. If I had a chisel with a burned cutting edge I'd probably grind through to the unburned steel with the 80 grit wheel.

    I also wonder if it shaping a carbon steel tool would be cooler on a coarse belt sander. I grind cabinet scrapers into curved hand scrapers for smoothing woodturnings. If I shape the thin scrapers on a CBN grinding wheel the edge is easily overheated. If I shape on a 1" belt sander with 60 grit the edge is much cooler. I still dip in water every few seconds. Perhaps that would work with a chisel.

    Something else to keep in mind about CBN - when new the wheel will probably not leave as smooth a surface as it will after being used a while. Apparently the process of bonding the grit to the wheel surface can leave random particles sticking up higher than others. These are eventually broken off with use.

    JKJ

  6. #6
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    Hey Guys,

    I very much appreciate all the answers.

    I think I may have been a little misleading with the way I phrased my OP question. I was very surprised at how fast the CBN wheel actually burned the edge. It was two passes - once back and forth with what I thought was a very light touch. I am very careful with this stuff - its dangerous and I know you can ruin a good tool. My conclusion is that the 600 grit is creating a ton of friction heat. I have to be much slower and lighter on the 600 grit that I do on the 180 grit. It is even more so that what I had to do with my old ceramic stones. The chisels are A2 steel - I guess that means medium carbon content. My original point was just that I was surprised that the CBN wheel, advertised as a much better heat dissipative material, would burn faster than the old stone based wheels. They obviously have their uses - no truing the wheels, no worries about wheel shatters, etc. I am not complaining... :-)

    I dont think anyone actually answered my second question. What is the speed control on the grinder used for? When do you slow it down vs speed it up, and what speed setting do I use with the CBN wheels?

    Thanks again for all your support.

    Joe

  7. #7
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    I can't answer your question, but just want to mention that your LN chisel is not ruined. Heck you probably already know that.... but just in case.... you can simply grind, (coarse grit stone), off the section that lost temper and then grind a new bevel.

    good luck,
    JeffD

  8. #8
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    My LN chisels have never seen a grinding wheel.... 15 years now.

  9. #9
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    I was very surprised at how fast the CBN wheel actually burned the edge. It was two passes - once back and forth with what I thought was a very light touch. I am very careful with this stuff - its dangerous and I know you can ruin a good tool. My conclusion is that the 600 grit is creating a ton of friction heat. I have to be much slower and lighter on the 600 grit that I do on the 180 grit.
    Joe ... 600 grit ... seriously?

    They would generate a large amount of heat. When we say that CBN grinds cool, it is understood that this a relative statement. The standard was grinding by a 38 grit white wheel on an 8" grinder. The result was quite coarse, and even then the heat generated was high enough to heat steel to blue. Imagine what an 80 grit white wheel could do!

    For O1, A2 and PM-V11 plane and chisel blades I use a 180 grit CBN wheel, and the heat generated is far, far less than the 38 grit white wheel. There is heat, and if I were cavalier with the blade, it could be blued. However I have not done so in 5 years of using this set up.

    600 grit is for turners who use M2 and M4 steel. These are designed to withstand heat. You can grind then red and they will not be affected.

    Keep the 600 for lathe chisels, and use the 80 for plane and chisel blades. Use waterstones, etc to hone to a sharp edge.

    CBN wheels have been around a long, long time - John has been using them for decades. I only discovered them 5 years ago, and was so amazed that flat woodworkers did not know of them. As a result, I wrote this article: http://www.inthewoodshop.com/Woodwor...ningSetUp.html CBN seemed to take off after this. Rob Lee (of Lee Valley/Veritas) visited my shop and I demonstrated them to him. He had not seen them before. Now Lee Valley stock them. These wheels are a real game changer. How did they remain a "secret" for so long?

    Regards from Perth

    Derek

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