1750 RPM Grinder & Chisel Sharpening

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.....

Reply to
Never Enough Money
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1750 RPM is fine. Just take it easy, if you take a light enough cut (with some practice) you wont even need the water. Remember a light touch.
Reply to
Vijay Kumar

That's why I use one of those 1"x40" sanding belts.

Does a great job for this application.

Lew

Reply to
Lew Hodgett

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

Reply to
TDUP

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?

Reply to
Never Enough Money

Never gave it a thought.

I use it to quickly sharpen things like scissors, scrapers, construction chisels, etc.

HTH

Lew

Reply to
Lew Hodgett

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

Reply to
CW

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.

Reply to
petermichaux

I agree, it removes metal from behind the cutting edge. the OP could look at this:

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and add the motor.

Reply to
AAvK

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.

Reply to
Andy Dingley

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?

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Reply to
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.

Reply to
Andy Dingley

Use the 1750 machine. As others have wisely said a light touch with short contact period will work fine. If you haven't already, you will soon learn with things are getting warm. Most tool grinding operations happen with very brief contact anyway.

Sharpening technology is like a lot of over shop techniques - there are thousands of ways to spend or save your money.

Reply to
RonB

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

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Reply to
petermichaux

Just one guy's opinion, but I sharpen my bench and lathe chisels on a regular old Delta bench grinder with the original wheels, and after a little practice, it has worked well for me for some time. I *do* dunk the chisels between grinding passes, and do *not* grind for as long as

5 seconds. With a slower speed, I'm sure you can keep the tool on the wheel a little longer, but with a standard grinder, one quick pass is all you get before quenching it, or you will burn the blade. A light touch and a bit of patience rules the day here, but it's certainly possible. After gettting the hollow grind in, I just touch it up quick on my whetstones, and they're plenty sharp for me.

Aut inveniam viam aut faciam

Reply to
Prometheus

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."

Reply to
Never Enough Money

for openers you missed the difference between HERTZ or cycles per SECOND and Revolutions Per MINUTE. So you are only off by a factor of 60 for openers. But that only applies to synchronous motors.

del cecchi

Reply to
Del Cecchi

It applies to induction motors too. There's a slip frequency, but the seconds/minutes mixup is still valid.

Reply to
Roy Smith

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.

Reply to
Robert Bonomi

If you have an hour or two, this has to be one of the fun places to spend it.

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Reply to
George

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