1750 RPM Grinder & Chisel Sharpening

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I need to get the initial edge on new chisels (the "hollow grind"). Everything I've read says to use a slow speed grinder with aluminum oxide wheels to prevent overheating of the steel (which ruins the chisel).
I've been told that the grinder should not exceed about 1200 RPM. One local woodworker actually keeps a cup of water next to his slow speed super expensive Baldor grinder. He grinds for about 5 seconds, dips the chisel in the water to cool it, and repeats the process.
I currently have an $70 Delta 1750 RPM grinder. I can get aluminum oxide wheels for it, too. How risky is it to use it?
I'll have the cup of water to prevent overheating.....
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Never Enough Money wrote:

(with some practice) you wont even need the water. Remember a light touch.
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Never Enough Money wrote:

<snip>
That's why I use one of those 1"x40" sanding belts.
Does a great job for this application.
Lew
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I thought a belt made a flat surface. The diameter of a wheel gives the "hollow grind" which makes subsequent sharpening much faster.
I also have a 1" x40" belt but didn't think it would be better than a grinder. Am I wrong?
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Never Enough Money wrote:

Never gave it a thought.
I use it to quickly sharpen things like scissors, scrapers, construction chisels, etc.
HTH
Lew
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I use a 1750 grinder with all my lathe tools. As long as you can keep the bevel right it will work great. Using water is helpful but the whole process shouldn't take but a few seconds.
Tim

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I have been doing it for years. Never used a slow speed job. Pay attention and there is no problem.

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I was just reading in Leonard Lee's book _The Complete Guide to Sharpening_ about this. His power sharpening tool of choice for the primary bevel is a vertical belt sander because it is cooler and creates a flat bevel. Then he puts a microbevel that is one degree greater than the primary bevel on using either the Eclipse or Veritas honing guide and waterstones(800x followed by 4000x to 8000x).
Lee does not like hollow grinding at all.
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I agree, it removes metal from behind the cutting edge. the OP could look at this: http://www.leevalley.com/wood/page.aspx?c=2&pD884&cat=1,43072 and add the motor.
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Alex - newbie_neander in woodworking
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If you are comparing to a single bevel then that is true. But if you are comparing to a primary/secondary bevel method then it depends how you do it. Hollow grinding followed by flat honing like Krenov recommends seems like better geometry in my investigation. Link below.
-------------------------------------------------------------------------------------------------------- What I've learned about sharpening plane blades http://members.shaw.ca/petermichaux/workshop/BevelDownSharpening.html
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On 1 May 2005 14:55:09 -0700, "Never Enough Money"

You certainly don't need a hollow grind on anything except razors, and they're different again.
That said, I'd be lost without a powered wheel for initial shaping on boot-sale pre-war chisels.

Aluminium oxide (white) and a "half speed" grinder are a good start.
The woodturners are probably the experts though, as they do a lot of sharpening with this type of equipment (rather than finishing with a bench stone). They're now using pink stones rather than white, as they run cooler.
It's also worth finding a grinder (or at least spindle) that can take a 40mm wide stone, not the 20mm or 25mm that are more common at the cheap end. You can always upgrade the stone later, so long as the spindle is long enough.

Nice idea, but impractical. Induction motors don't run at that speed, so you'd need reduction belt drives. A 4 pole induction motor will run at 1750 rpm (1425 in the UK) and that's close enough for a cheap direct drive grinder. 2 pole motors run at around 3000 rpm and so the really cheap metalworking grinders are too fast for this.

Not the best idea. You're better cooling with airflow (which the stone provides) or cooling much more frequently than this. 5 seconds is just enough to set up hot/cold cycling that's going to avoid drawing the temper, but may also give rise to microcracking in a hard steel.

Sounds fine.
Read Leonard Lee's book too.
--
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Andy, thanks for the thorough answer. One minor update to your RPM part of the answer: "Nice idea, but impractical. Induction motors don't run at that speed, so you'd need reduction belt drives. A 4 pole induction motor will run at 1750 rpm (1425 in the UK) and that's close enough for a cheap direct drive grinder. 2 pole motors run at around 3000 rpm and so the really cheap metalworking grinders are too fast for this. "
Garrett Wade is selling one that runs at 1120 PRM. How do they do that? http://www.garrettwade.com/jump.jsp?lGen tail&itemID1879&itemType=PRODUCT&iMainCat=0&iSubCat=0&iProductID1879
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On 2 May 2005 05:46:43 -0700, "Never Enough Money"

It doesn't appear to have any reduction mechanism, so I guess it's a 6 pole induction motor (which fits with the claimed speed). That's simple enough to design, the only problem would be finding enough orders to make it worthwhile for the motor maker to tool up for this "special".
It's the frst one I've seen, but I doubt it will be the last. Once one does it, others tend to follow.
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Andy, I'm trying to understand how 60 hertz and the number of poles maps to RPM. I'm not too bright when it comes to motors so please indulge me an ignorant question.
I'm visualizing an amature and 4 windings in the stator. Now 60 hz going in seems to mean that the strongest e-field (or B-filed) woudl change at 4 tiems that rate. This would give 240 RMP. So I must be missing something...what? Please avoid the tempatation to say "A brain."
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Revolutions Per MINUTE. So you are only off by a factor of 60 for openers. But that only applies to synchronous motors.
del cecchi
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It applies to induction motors too. There's a slip frequency, but the seconds/minutes mixup is still valid.
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1) you multiplied where you should have divided
2) you forgot to account for the number of seconds in a minute
60 hz == 3600 cycles/minute
4 pole motor == 3600/4 == 900 rpm.
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Thanks for the good answer, Robert.
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If you have an hour or two, this has to be one of the fun places to spend it.
http://www.sea.siemens.com/step/default.html
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Useful site. Thanks for the reference.
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