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Thread: Grinder for CBN wheel for sharpening

  1. #1
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    Grinder for CBN wheel for sharpening

    I am planning to get a bench grinder to use with a CBN wheel for sharpening. After hollow grinding, I plan to use Spyderco stones to hone without a guide.

    Looking at past threads it appears the most desirable grinder is a half speed, 8" grinder. Although, looking at reviews for 8" grinders, the only one with consistently good reviews is the full speed Metabo. Of course, there is always Baldor, but 8" size is quite spendy.

    My question is, would a 6" grinder also work for hollow grinding plane blades and chisels or 8" is really what I should get? If 6" works, half speed Baldor grinder is a lot more affordable than the 8" Baldor.

    All of this is for hobby use.

  2. #2
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    Quote Originally Posted by Frank Martin View Post
    I am planning to get a bench grinder to use with a CBN wheel for sharpening. After hollow grinding, I plan to use Spyderco stones to hone without a guide.

    Looking at past threads it appears the most desirable grinder is a half speed, 8" grinder. Although, looking at reviews for 8" grinders, the only one with consistently good reviews is the full speed Metabo. Of course, there is always Baldor, but 8" size is quite spendy.

    My question is, would a 6" grinder also work for hollow grinding plane blades and chisels or 8" is really what I should get? If 6" works, half speed Baldor grinder is a lot more affordable than the 8" Baldor.

    All of this is for hobby use.
    CBN wheels are inherently well-balanced, so some aspects of grinder quality are less important than with standard wheels. For example mass/damping, and the degree to which the bushings support the wheel are no longer as critical.

    Among 8" grinders I've used and had good luck with CBN wheels on the 1 hp Rikon. There's also a 1/2 hp version that I've never tried. The 8" Delta variable grinder also works OK with CBN wheels, though I've relegated it to buffing/cleaning duties these days.

    Half- vs Full-speed isn't as important with CBN as it is with standard wheels, because CBN run quite a bit cooler. You can certainly burn tools on a full-speed grinder with CBN, but it's not *that* hard to avoid IMO.Just keep the pressure down and quench or rest the tool often.

  3. #3
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    Frank; from my limited experience of cbn wheels; they are only rated up to 3000 rpm; the Metabo DS 200 you mentioned has a higher rating of 3570 rpm. http://www.cpometabo.com/metabo-6192...bench-grinders

    The optional width with cbn wheels is another consideration; you have a choice of 1" (25mm) or 40mm; the wheel guarding on most 6 and 8 inch grinders will only suit a width up to 25mm. if you chose to fit the wider 40mm cbn wheels, you then have 3 options available; (a) remove the guard surrounds completely, (b) just remove the end caps, (c) have new guards fabricated to house the wider width. In my case, I chose to remove the guard surrounds completely. Personally, I am extremely happy with the decision to fit the wider 40mm cbn wheels to my 6 inch grinder.

    read the following attach: http://www.robohippy.net/featured-article/

    Important note; personal eye protection should always be worn when grinding.

    Prior to fitting the cbn wheels, I was using white Al- Oxide grinding wheels; at this stage I have not experienced any noticeable residual heat build up using the cbn wheels. That would validate what Patrick wrote on a previous post;I saw a paper a while back that claimed a >4X difference in heat dissipation per unit volume material removed compared to freshly dressed Al-Oxide wheels. Based on my own experience I think that's about right.

    Attached is a photo of the 6 inch - 2850 rpm bench grinder I have in my workshop; the cbn wheels are 6 inch x 40mm Vicmarc; 80 grit (L) 180 grit (R). http://www.cwsonline.com.au/shop/cat...er--cbn-wheels

    Stewie;

    Last edited by Stewie Simpson; 06-12-2016 at 5:13 AM.

  4. #4
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    I repeat : Half speed grinders are NOT necessary, unless you use CBN wheels that are only rated to 3000 RPM. They only make you grind twice as long. Most 8" grinders run half speed anyway, as the periphery of the larger wheels run faster to begin with. Simply grind for TWO SECONDS and quench when your bevel nears completion. LOOK AT THE WATER LEFT ON THE TOOL. When it SIZZLES,dip INSTANTLY. Keep a finger close to the cutting edge you are grinding to help feel when the tool is heating up.

    If you are afraid to keep a finger tip close to the end of your tool, just keep a close eye on the water that is left on the tool after grinding. DON'T shake it off. That water is your friend. As a rule anyway, I use a white wheel,coarse as possible,and just grind for 2 seconds and quench instantly. Keep the water cup right in front of the grinder. Not on top of the grinder(Our old Delta had a large cast iron water cup on a swing arm ABOVE the grinder. You do not want to have to cover much distance reaching that quench. You do not want the water cup above the grinder: It is too easy to crash into it while raising the tool way up to dip it. I have noticed myself how easy it is to crash into a water cup that is suspended above the grinder. The most good an overhead cup does is to look cool!(No pun intended). My old flat face Craftsman has a rectangular water cup attached to its front via a slot it clips into. A good plan, if not as jazzy.

    About the finger near the tip of the tool: The worst accident I have had with a bench grinder was to have the fingerprints ground off perhaps down to where it will bleed. Mostly just annoying and of course painful,but not the worst pain by any means. You DO have to be INTELLIGENT ENOUGH to not get your finger jammed between the wheel and the end of the tool rest. You WILL lose that disagreement !!
    Last edited by george wilson; 06-12-2016 at 8:53 AM.

  5. #5
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    I use a Delta variable speed grinder 8". I have thought about buying a second grinder but the Delta is working fine for now. The largest challenge I had was getting my Batty tool rest to fit around the base.

    I have hollow ground: BU Veritas blades (A2 & PM-V11), Custom Veritas PM-V11, regular Stanley blades, Lee Valley PM-V11 blades made for Stanley planes and several different types of single iron blades for wood planes.

    My experience has been that unless you push hard against the grinding wheel the blades do not get more than warm. I keep a finger or two fairly close to the edge to monitor heat. I started out stopping every few seconds to check the heat level, recently I do 15 seconds or so of grinding between heat and bevel checks. I could go longer than 15 seconds but I like to check how even my bevel is developing at about 15 seconds. If the bevel gets uneven on one side, fixing it slows the process down worse than just checking it every 15 seconds or so. YMMV

    Pushing hard against the wheel always proves counter productive for me, in terms of getting a good even bevel anyway. I always have at least a little camber at the corners of the blades I hollow grind.

    I Believe David Weaver uses/used a Baldor grinder with 6" wheel and felt the smaller wheel was not an issue.

    I ran a similar thread when I bought my CBN wheels. I have seen several other threads asking what types of grinders people like. The Metabo has been mentioned a few times, Jet grinders are popular among turners, but it always seems like everyone has something different. The newer Rikon grinders seem to be building a following and the prices are reasonable. Baldor always seems to be the favorite higher priced grinder and the machines with 6" wheels are not as expensive.

  6. #6
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    grinder 005 (1024x768).jpgI'm the one with the full speed Metabo. If I kept anything on it, no matter how lightly, for 15 seconds, I would be WAY past what I was trying to do. The speed is no problem, but you do have to have a light touch. I bought several cheaper, slow speed ones, and sent them back. This is what I was looking for. It had been running for a few minutes with that stuff sitting on it while I went to the truck to get the camera. Now, something past two years later, it still runs like that.

  7. #7
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    Obviously how long you can keep grinding without quenching depends upon :1. How sharp your wheel is,and its composition. 2; How thick your bevel still is. 3; How light your touch is. 4; the RPM of the wheel. And various other less important factors. Your mileage may vary. I see Tom has the high priced spread!

  8. #8
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    An 8 inch grinder running at ~3500 RPM equates to an effective speed of 7000 SFPM (surface feet per minute). A 6 inch grinder at the same RPM runs at ~ 5500 SFPM. Half speed grinders deliver 1/2 the SFPM. Using the 8 inch grinder for 15 seconds exposes the chisel of plane blade to 1750 feet of abrasives, of course, pressure applied is highly important. My little 6 inch grinder slows noticeably with pressure applied. None the less, it doesn't take long to get the job done and there really isn't a big difference in 6 inch versus 8 inch practically speaking. I suppose you could say it takes 25% more time with a 6 inch wheel to do the same material removal but really just another second or two.

  9. #9
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    Quote Originally Posted by Pat Barry View Post
    An 8 inch grinder running at ~3500 RPM equates to an effective speed of 7000 SFPM (surface feet per minute). A 6 inch grinder at the same RPM runs at ~ 5500 SFPM. Half speed grinders deliver 1/2 the SFPM. Using the 8 inch grinder for 15 seconds exposes the chisel of plane blade to 1750 feet of abrasives, of course, pressure applied is highly important. My little 6 inch grinder slows noticeably with pressure applied. None the less, it doesn't take long to get the job done and there really isn't a big difference in 6 inch versus 8 inch practically speaking. I suppose you could say it takes 25% more time with a 6 inch wheel to do the same material removal but really just another second or two.
    I think that once you get above a certain level you're going to be thermally limited anyway, meaning that the things you do to avoid tool-burning (resting, quenching, whatever) take more of the time than the actual grinding. Once you get into that regime a 25% or even 50% difference in SFPM isn't going to make a huge productivity difference. I would argue that both full- and half-speed 8" grinders operate in that regime, though obviously opinions vary.

    I'm pretty conservative, and that may cause me to be more thermally limited than I should be. I quench when the tool gets uncomfortable to touch and keep quenching until it's lukewarm, but it takes about 450F to begin to change the temper of a typical edge tool.

  10. #10
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    I would have rather had a slow speed one, but none I tried were smooth. I really just ordered the Metabo to try it to see how smooth it was. It never went back. I do wish they sold a slow speed one. I get to deduct whatever I spend on tools, and that was close to the end of a year when I needed to buy some more tools. I have a Baldor on a pedestal in the mechanic/metal shop, but didn't want to remove it from where it is. The old Baldor belonged to my Dad, and runs pretty smoothly, but has friable wheels which require all sorts of maintenance. I don't want to shapen mower blades with a CBN wheel.

  11. #11
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    I must be using a great deal less pressure than others. I was just working a new blade. It is hard to time with an Apple watch or iPhone because they turn off before 15 seconds. I counted slow and looked at a watch... I can go 15 seconds and more easily and I never quench. I touch the blade to my bare arm to test the temp and my blade actually feels cool most of the time. The Delta grinder is a variable speed but I believe the slow speed is faster than a slow speed grinder. I run it slow to medium speed.

    I guess I am grinding very slow in order to make precise cambers, with the edges at steeper angles than the center of the blade. It does take me a while to do a new blade but I don't mind spending the extra time to get a relatively precise bevel without heating the blade up past warm.

  12. #12
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    Quote Originally Posted by Mike Holbrook View Post
    I must be using a great deal less pressure than others. I was just working a new blade. It is hard to time with an Apple watch or iPhone because they turn off before 15 seconds. I counted slow and looked at a watch... I can go 15 seconds and more easily and I never quench. I touch the blade to my bare arm to test the temp and my blade actually feels cool most of the time. The Delta grinder is a variable speed but I believe the slow speed is faster than a slow speed grinder. I run it slow to medium speed.
    The Delta's min speed is 2000 rpm, or about 10% faster than "half speed". You can adjust the off-time on your iDevices BTW.

  13. #13
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    Thanks for all the great responses. My worry is not speed at all, in other words, slower is just fine. In fact, if 6 inch wheel size is not an issue when grinding thick Veritas bevel up plane irons due to deeper hollow than one would get with a 8 inch wheel, I will likely just go with a 6 inch size. Other than being faster, is there any inherent advantage for 8 inch grinders? It seems they are a lot more common among turners, I am not and don't plan to get into turning any time soon.

  14. #14
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    There are tools, Japanese chisels for one, that many experts suggest not hollow grinding. Some knowledgable users still hollow grind some of these tools particularly with 8 & 10 inch wheels. The lesser bevel may be preferred in these situations.

    I thought about buying a grinder with a 6" wheel, but felt that the 8" would make better bevels on the drawknives, scorps, adzes and other green wood tools I use. I suspect someone will "do the math" and give you the actual numbers. Still, it just depends on the tools you use and what size bevel might benefit them. I was also thinking about getting a lathe for turning where the 8" grinder seems to be preferred.

  15. #15
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    Quote Originally Posted by Stewie Simpson View Post
    Frank; from my limited experience of cbn wheels; they are only rated up to 3000 rpm; the Metabo DS 200 you mentioned has a higher rating of 3570 rpm. http://www.cpometabo.com/metabo-6192...bench-grinders
    Please note that RPM for this type of motor depends on mains frequency - 50Hz or 60Hz. Stewie lives in Austrialia with 50Hz. Metabo DS 200 would run at 2980 RPM there. https://www.metabo.com/com/en/tools/...ml#description (See (220-240 V / 50 HZ) note there). Some other grinders can have a little less like 2800 or so, but always less than 3000. For half speed it is less than 1500 RPM. Search for AC motors or induction motor to learn more.

    I wonder now, why some or most CBN wheels have limit 3000 RPM - does it mean that any 50 Hz grinder is good? Should be, but then they often write recommended speed that is half of that.

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