Bench grinder -- difference in 6" & 8" wheel heating object to be ground - Router Forums
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post #1 of 14 (permalink) Old 05-10-2012, 06:54 PM Thread Starter
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Default Bench grinder -- difference in 6" & 8" wheel heating object to be ground

6 wheel diameter (inches)
18.8495559 circumference (inches)
2,000 RPM revolutions per minute
37,699 inches per minute
3,142 feet per minute

8 wheel diameter (inches)
25.1327412 circumference (inches)
2,000 RPM revolutions per minute
50,265 inches per minute
4,189 feet per minute

25% 6" wheel less per min
33% 8" whee more per min


A 6" wheel at 2,000 rpm travels 3142 feet a minute
An 8" wheel at the same 2,000 rpm travels 4,189 feet a minute
If wheel material same for both wheels
The 6" wheel will run cooler having 25% less distance dragged across the piece being ground
The 8" wheel will run hotter having 33% more distance dragged across the piece being ground

Are my assumptions correct using a smaller wheel should give a cooler, slower gring?
I'm contemplating buying a bench grinder for light home use in my garage. I figure a low-speed grinder is better since I'm a precision guy and read the 3.450 rpm grinders go fast and heat up steel killing their hardness.

I was thinking of the Woodcraft 8in Slow Speed Grinder for $125. I don't want to spend a lot in that I can get a junky one for $50 from any big box but with super coarse wheels which I don't want.
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post #2 of 14 (permalink) Old 05-10-2012, 08:02 PM
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Quote:
Originally Posted by corbind View Post
6 wheel diameter (inches)
18.8495559 circumference (inches)
2,000 RPM revolutions per minute
37,699 inches per minute
3,142 feet per minute

8 wheel diameter (inches)
25.1327412 circumference (inches)
2,000 RPM revolutions per minute
50,265 inches per minute
4,189 feet per minute

25% 6" wheel less per min
33% 8" whee more per min


A 6" wheel at 2,000 rpm travels 3142 feet a minute
An 8" wheel at the same 2,000 rpm travels 4,189 feet a minute
If wheel material same for both wheels
The 6" wheel will run cooler having 25% less distance dragged across the piece being ground
The 8" wheel will run hotter having 33% more distance dragged across the piece being ground

Are my assumptions correct using a smaller wheel should give a cooler, slower gring?
I'm contemplating buying a bench grinder for light home use in my garage. I figure a low-speed grinder is better since I'm a precision guy and read the 3.450 rpm grinders go fast and heat up steel killing their hardness.

I was thinking of the Woodcraft 8in Slow Speed Grinder for $125. I don't want to spend a lot in that I can get a junky one for $50 from any big box but with super coarse wheels which I don't want.
Impressive array of numbers, I'm not about to dispute them, there just seems to be something intuitively wrong with the conclusion. Could be you are ignoring the cooling period between contact and the mass (heat sinking capability) of the wheels. Any given point on the six inch wheel would be in contact with the "Grindee(?)" for a substantially longer time during any given period... just thinkin... not always a good thing

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post #3 of 14 (permalink) Old 05-10-2012, 08:11 PM
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I have an 8" and I've had 6" grinders. I can't honestly say I've noticed a difference. The wheel itself is more important. You need the white ones with friable bond like Norton makes. The ones that come with most grinders are only good for lawnmower blades and splitting axes.
You should keep a quench tray close and use light pressure. I recommend a jig like Lee Valleys (05D13.02) to get really good results.

Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.
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post #4 of 14 (permalink) Old 05-11-2012, 06:03 AM
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The 8 inches will give a reduce curve in your sharpen chisel and if you really want to reduce heat, purchase a 1750 rpm, this is the best way to sharpen, I've posses both and there is a big difference between them.
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post #5 of 14 (permalink) Old 05-11-2012, 07:01 AM
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Quote:
Are my assumptions correct using a smaller wheel should give a cooler, slower gring?
YES - You are one of the few that comprehends this.

Quote:
I'm contemplating buying a bench grinder for light home use in my garage. I figure a low-speed grinder is better since I'm a precision guy and read the 3.450 rpm grinders go fast and heat up steel killing their hardness.
Most old-timers made their own grinders with changeable pulleys. I am a collector of 100-175 year old woodworking tools. Interestingly, pre-electricity wood workers sharpened tools mainly with files - TALK ABOUT LOW RPM! Some used stones - mainly mounted to a stick. You will find that a huge percentage of small motors run at 3,450, but machines (think about your drill press) with multiple speeds utilize multiple pulleys & an adjustable tensioner.

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post #6 of 14 (permalink) Old 05-11-2012, 08:09 AM
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Yet another totally awesome thread!

Thanks to Corbin for bringing the topic up and starting it out with lots of math on tap.

Chuck's common sense reminder that the wheels included with my yet unused Orange Box boat anchor of a grinder are pretty nasty stuff is right on time. Thanks man!

Otis, ya just got me again for the second time in an hour...ripping the wheels off the grinder and putting a pulley on one and a buffing wheel on the other is a very do-able beginning. Of course the next step would be to actually source and use a multi-pulley axle/arbor/shaft & the rest of the stuff needed to create a flexible multispeed grinding powerhouse. This has me wanting to surf arround in search of a shop-smith type widget with issues bad enough its almost free, but not so bad it couldn't be morphed into something useful.. Thanks again Otis...

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post #7 of 14 (permalink) Old 05-11-2012, 09:09 AM
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Slower with finer grits is best for getting cutting edges really sharp. For example, there are several very slow speed (40-60 rpm) wet stone sharpening systems on the market that do a great job of sharpening if you want razor quality sharp. Torit and Jet are 2 of the brand names. Do you remember the old 24" diameter foot pedal operated stones of the old days with a drip can above the stone to keep it wet? Farmers and woodworkers of 100 years ago always had one of these to keep their cutting tools sharp. These had very fine grit stones and pedal power kept them turning slow enough that the speed and wetness of the stone kept the blades being sharpened from ever getting too hot. The water also kept the surface of the stone clean. For the final hone my grandfather always had a strip of harness leather hanging nearby so he could stroke the blade on it for that final super sharp finish. His knives and chisel blades were always as sharp as razor blades and I was always in awe of what he could accomplish with such ancient sharpening methods. Sadly, I wasn't the one who inherited my grandfather's sharpening tools.

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post #8 of 14 (permalink) Old 05-11-2012, 09:12 AM
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It's definitely an all of the above kind of question.

If you have a large wheel or high speed grinder, keep the time the tool is in contact with the wheel shorter.

Always try to use only as much pressure as is needed to achieve the result you need.

Use wheels appropriate to the job at hand. At work we (try to) use different grinders for general work and for sharpening, and dress them regularly to make sure that they haven't been loaded with debris.

I've seen large diameter drill bits spinning less than 300 rpm lose their hardness because of overfeeding and insufficient cooling. But I can also sharpen these bits fairly well on a 12 bench grinder... but it requires a lot of quick, light passes.

In my opinion, if you've got room for multiple grinders, get a slow speed one to make your life easier. If you've only got room for one, get a 8 inch grinder that can serve a lot of purposes, and get yourself a couple of nice wheels for sharpening.

Of course, there are a bunch of different versions of the scary sharp sandpaper method that are powered by drill presses, scavenged motors, and elbow grease out there as well.

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post #9 of 14 (permalink) Old 05-11-2012, 09:33 AM
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I'm still trying to envision a bicycle with a grinding wheel instead of a tire! Thanks Charley..

Doug's comments are right on the spot to! Excess pressure doesn't 'really' get the job done any faster

Any of you all have a link to someplace with good instructions on how to put the temper back into something that has lost it? I vaguely recall that part of my metal shop days some 30 years back but not in nearly enough detail I would actually try it without some refresher reading.

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post #10 of 14 (permalink) Old 05-11-2012, 11:06 AM
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Bill, I've not personally needed to do this, but my recently deceased dad and father-in-law used to make and sharpen cutting tools. The hobby / occupation of knife-making incorporates tempering techniques that I've been told are as much about quenching as they are about the heat. Joy's dad always quenched cutting edges with beeswax - he swore by it. My dad, however; used some concoction of motor oil and something unknown to me. Water is not the best of choices and the beeswax (non-flammable ?) seems the safer alternative as I'm typing this. There are numerous ways to heat metal for this purpose, I'm thinking Google and YouTube may have some good answers. I, very simply just never make or overheat chisels, bits, etc. so never needed to learn this skill.

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