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Tool Grinding and CBN wheels? OK or Not?

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by BFHammer, Jun 21, 2017.

  1. BFHammer

    BFHammer United States H-M Supporter - Premium Content H-M Supporter-Premium

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    For my wood lathe HSS tools I use 8" CBN (Cubic Boron Nitride) wheels mounted on a slow speed bench grinder at 1750 RPM. These work great for my wood lathe tools, 80 grit for roughing in and 120 grit for maintaining a great edge.

    Being new to metal lathe tools - is there a reason or reasons that this would be a bad idea for grinding HSS lathe bits? It makes sense to me on the surface but I'm wondering if speed is an issue or are there other problems. In the metal lathe world of the inter-webs I never see this method mentioned.

    Thanks,
    Mark
     
  2. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    They are used all the time in the production environment for sharpening HSS endmills, drill bits, etc. Just not worth a flip for grinding carbide. In fact, they are not made for grinding carbide, just HSS.
     
  3. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    For a sharpening touch up of a tool bit, the slow speed grinder will work well. For roughing out a bit from a blank, it will be S-L-O-W... HSS is not particularly prone to losing its hardness by grinding to higher temperatures, even heating it to dull red only softens it slightly. Do not worry at all about blue color developing on the bit when roughing it out.
     
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  4. 4ssss

    4ssss United States Active Member Active Member

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    I've used them in OD/ID grinders to grind hardened steel. They work very well.
     
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  5. mikey

    mikey Active User H-M Supporter-Premium

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    I sent a query to Ken Rizza of Woodturners Wonders about this:

    Me:
    I am a hobby machinist who prefers using HSS tool bits on my lathe. Are your CBN wheels an appropriate tool for shaping metal lathe tool bits from square HSS tool blanks or will it be too a slow process? If you think it is a good choice, which grits would be appropriate for rough shaping and finishing?

    Ken:

    CBN is fine for that- 60 or 80 grit for rough shaping and 350 for finish work.

    Given that a CBN wheel is intended for use with HSS, I would give it a go and then report back on your experience if you would. (That means show us the tool, hint, hint!) There are some distinct advantages to using a wheel to grind lathe tools; a non-explosive CBN wheel that needs no dressing is very attractive.
     
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  6. BFHammer

    BFHammer United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Thanks for the replies guys and Mikey thanks for reaching out on that.

    I do like the CBN wheels for the reasons you described, solid (no exploding wheels) and they stay pretty clean. Hopefully I will get time this weekend to give it a try. I will report back - you guys can critique what will be my very first attempt at grinding a HSS bit. I have read and read and watched and watched - nothing left now but to just do it!
    :chemist:
     
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  7. mikey

    mikey Active User H-M Supporter-Premium

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    Hope your grinder has a wrap around rest to provide support on the right side of the bit. This helps to grind the all-important rake angles. If you have another grinder with aluminum oxide wheels, practice with mild steel keystock before trying HSS - it helps.

    Good luck - we'll be waiting to see what you come up with. No pressure, of course!
     
  8. benmychree

    benmychree United States John York H-M Supporter-Premium

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    I have always heard that in grinding HSS tools, that repeated heating and quenching is not good practice, as it tends to cause fine cracking at the cutting edge and subsequent breakdown of the cutting edge, although I think this much more important in grinding mill cutters than lathe tools, where I have never seen any evidence of it causing a problem; I agree that the blue color is not a problem, but holding onto the tool bit gets to be a problem as heat builds up and travels back to where the fingers are, although grinding the lathe tools in their holders does make it a non problem.
     
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  9. Cavediver

    Cavediver United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Care to elaborate on that statement? If that's better left for a different thread I'd be happy to start one.
    (Written by a guy who grinds with wheels and is looking to switch to a belt due to YOUR previous posts :D)
     
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  10. mikey

    mikey Active User H-M Supporter-Premium

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    Heh, heh, not enough advantages to make me even think about going back but the key ones to me are that 1) the wheel doesn't snap like an old belt will, leaving you stranded without a back up and 2) the hollow grind can make honing easier and more accurate because you have two edges orienting the tool on the stone. It is very quick to touch up a hollow ground tool between uses because you're only hitting the edges.

    On the other hand, a belt sander is far faster when shaping a tool, is easier to avoid facets with due to the flat platen and grinds very cool compared to a solid wheel. I won't go back to a wheel myself but I want to encourage others to grind tools, regardless of the machine they use.
     
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  11. RHayes

    RHayes United States Active Member Active Member

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    Mark, I'm sure you have found out by now that your CBN wheels are the ultimate for sharpening HSS bits. I have a pair in 180 grit and the list of benefits over traditional grinding wheels is very long. My only regret is they are located in the wood shop area of my split shop and I have to walk around to touch up a tool (bypassing two perfectly good grinders with traditional friable stones) 180 grit is a little fine for shaping, and a 60 grit would be better. That being said, I use them anyway to shape 3/8, and 1/4" HSS bits from scratch.
     
  12. mikey

    mikey Active User H-M Supporter-Premium

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    Sure wish they made a seamless CBN-coated 60 grit sanding belt. :cry:
     
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  13. Tozguy

    Tozguy Active User H-M Supporter-Premium

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  14. mikey

    mikey Active User H-M Supporter-Premium

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    Yeah, those ceramic belts are on my wish list. When my current set of AO belts start snapping, those ceramic belts are coming to my house. Thanks.
     
  15. Kernbigo

    Kernbigo United States Active User Active Member

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    i used a cbn wheel for thread grinding gages years ago worked great for grinding, but the angle form was not precise enough for gages
     
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  16. BFHammer

    BFHammer United States H-M Supporter - Premium Content H-M Supporter-Premium

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    This past weekend I did finally get around to giving the CBN wheels a whirl and attempting my first hand ground HSS lathe bit.

    All in all the process went well. On the slow speed grinder with and 80 grit CBN wheel the progress is not lightening fast, but neither is it painfully slow. It took me about 15 minutes to completely grind including time for snapping pictures.

    I had my water can at the ready but the tool never really got hot enough to worry about. Many thanks to Mikey and his tutorial as I studied it at great length and even referred back to it during the process. As promised here are some pics.


    I used my robo rest which is a regular part of my set up for wood lathe tools. I adjusted to 15 degree angle to the wheel and added a couple of 15 degree reference lines as well.
    IMG_3697.JPG


    IMG_3698.JPG

    It's difficult to see in the picture but I marked the line on my blank at 15 degrees (note to self - get a fine line sharpie!)
    IMG_3699.JPG


    The block was a great idea and made the grinding very easy.
    IMG_3701.JPG


    As I mentioned it wasn't lightening fast - I would guess it took about 5 minutes to get the initial side ground. You can see I cheated a little bit as my 15 degree angle doesn't extend the entire ground surface, but I don't think it will effect performance.
    IMG_3705.JPG


    Measured with my protractor which is more accurate than my sharpie. Marked off my second angle to give me an 85 degree included angle.
    IMG_3707.JPG



    Almost there....
    IMG_3709.JPG

    That does it for the face.
    IMG_3710.JPG

    Same thing but with a little better picture.
    IMG_3711.JPG

    And with the back rake completed.
    IMG_3713.JPG

    IMG_3714.JPG

    IMG_3715.JPG


    Of course I had to immediately try it out. It certainly faces and cuts, I need some more experimentation to get the best profile, feed and speed. Here is the initial result on a piece of mystery steel. The left 2/3 was cut with my newly finished bit. The final third with my trusty carbide insert - so I have something to shoot for.
    The chips I got were of the razor wire variety, have to study up on chip breaking. I did take a quick video with my phone, but I'll have to figure out how to upload that.
    IMG_3720.JPG

    Thanks for all the comments and checking out the results! Any advice is always welcome!

    Thanks,
    Mark
     

    Attached Files:

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  17. mikey

    mikey Active User H-M Supporter-Premium

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    For a first tool bit, that is an awesome job! Congrats, BFHammer!

    I have a few suggestions:
    • You have side rake but almost no back rake. The tool will cut but it does so with higher cutting forces and all the cutting is at the side cutting edge. Adding back rake will focus forces near the tip of the tool and will improve chip ejection and finish.
    • Your tool looks like it has no nose radius, or very little. I would suggest at least a 1/64" nose radius to reduce the chance of breakage and improve the finish.
    • You are using the tool with a negative lead angle, meaning the side cutting edge is perpendicular to the work piece. This is fine when cutting to a shoulder but for general turning, try using the tool with the shank of the tool perpendicular to the work. This will improve your finish.
    • I suspect the stringers can be changed to chips by using a faster feed.
    Great job on this tool!
     
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  18. BFHammer

    BFHammer United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Mikey,

    Thanks for the tips. I will have to revisit the tutorial and study the back rake.

    I did actually put a nose radius on the tool, I did that with a quick pass on my 180 grit wheel and with a diamond stone. In my haste to try it out I neglected to take anymore pictures at that point. Now that I'm over the hurdle of grinding that first tool, I'll go back and experiment with more and less nose radius.

    As to the angle of the tool, yes you are right. I was playing around with different angles - also different speeds and feeds - unfortunately my approach would fail scientific method. I'll be a little more measured in my testing going forward.
     
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  19. BFHammer

    BFHammer United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Figured out the mysteries of posting a video -

     
  20. mikey

    mikey Active User H-M Supporter-Premium

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    I want to encourage you to work at this grinding thing because once you figure it out, the tool will work well for you. You're doing great and things will only get better as you gain experience.

    The tool angle thing is called changing the Lead Angle. The tip of the tool has three cutting surfaces - the side cutting edge, the nose radius and the end cutting edge. As you change the angle of the tool, you bring one or two of these edges into prominence. When the tool is perpendicular to the work, the nose radius does most of the cutting if you have enough back rake. When you angle the tip of the tool back toward the tailstock, you bring more of the side cutting edge into contact with the work and finishes get better but it can chatter on thin work pieces or deep cuts. When you angle the tip toward the chuck, less of the side cutting edge contacts the work and you can take deeper cuts with less potential for chatter.

    So, when you're roughing, try positioning the tool so the shank of the tool is perpendicular to the work. When finishing, try angling the tip slightly more toward the tailstock and you will notice the finish improve. When turning into a shoulder or when facing, angle the tool so the side cutting edge just clears the face of the shoulder. You can actually see the width of the chip widen and thin out as you turn the tool toward the tailstock - try it in aluminum so you can see this easier. You will also see the finish improve significantly. Fun to play with but useful, too.

    By the way, back rake does several important things. It shifts the cutting forces to the tip of the tool and this can greatly improve finishes. It also helps the tool to penetrate better so cutting forces are reduced. It also helps to improve chip flow and together with side rake, this greatly reduces cutting temperatures.

    So yeah, try grinding some back rake and see what happens. Let me know if any of this is confusing or if you have questions.

    Mike
     
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