# Bench grinder -- difference in 6" & 8" wheel heating object to be ground



## corbind (Apr 17, 2012)

*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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## jschaben (Jun 21, 2009)

corbind said:


> 6	wheel diameter (inches)
> 18.8495559	circumference (inches)
> 2,000 RPM revolutions per minute
> 37,699 inches per minute
> ...


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:wacko:


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## Cherryville Chuck (Sep 28, 2010)

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.


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## thrinfo (Jan 21, 2012)

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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## OPG3 (Jan 9, 2011)

> Are my assumptions correct using a smaller wheel should give a cooler, slower gring?


YES - You are one of the few that comprehends this.



> 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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## wbh1963 (Oct 11, 2011)

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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## CharleyL (Feb 28, 2009)

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.

Charley


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## kp91 (Sep 10, 2004)

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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## wbh1963 (Oct 11, 2011)

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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## OPG3 (Jan 9, 2011)

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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## CharleyL (Feb 28, 2009)

What bicycle? These old grinder wheels had a shaft with a crank shaped bend on one end. A vertical board with a hole in the top end connected to this crank. The bottom end attached to a horizontal board that the user would place their foot on and use it much like the old treadle type sewing machines to keep the grinding stone turning while they were using it. The wood frame that the stone was attached to was long enough for the user to sit on while facing the stone and using it. I can't believe that you've never seen one. Fourty years ago they seemed to be everywhere. If you watch Roy Underhill's show on PBS you will sometimes see him use one of these old foot powered grind stones. You can also frequently find them in antique/junk stores and deserted barns.

Charley


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## wbh1963 (Oct 11, 2011)

OPG3 said:


> 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.


I did recall that a repetitive cooling and heating process was part of the 'magic' that tempers steel. The use of beeswax or motor oil in concoctions that might enhance the process are new ideas that I have not heard of before, and must consider.

In my case I want to get good at transforming existing surplus plane irons into ones with custom skews & bevels for their current 'duty assignment'. Logic suggests that in the easier cases the iron won't be overheated during the grinding. The same logic also suggests that some of the re-grinds won't go that easy.

Thanks for responding!


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## OPG3 (Jan 9, 2011)

You're very welcome Bill and other readers!
I'm in the process of cleaning out my dad's basement. He died in January. Daddy had a lot of old tools - many of which came from his grandfather. There's just me and a brother to divide the tools among. We both have grown and married kids. I am the only one even slightly interested in the *old* tools. Some of these date back as far as 1839, which was before electricity, hex nuts or philips head screws were used. Some of the old tools have not been used since 1940, yet they are in amazingly good condition. There are two old crank-style grinders and a lot of files. I'm thinking that my (full-time carpenter) maternal great-grandfather spent a *good part of his time sharpening plane irons, chisels and sawblades.* He built the house my dad grew-up in with these very tools. Back then, he mixed the cement to make concrete for the corner and pier footings, layed the brick foundation, cut and hauled (with mules) the trees to the sawmill to make the lumber, stickered and dried the lumber, framed the house, built the stairs, attached the wooden lath (for the plaster to adhere), BUILT THE WINDOWS, MADE THE TRIM WORK, plastered the interior walls and ceilings, trimmed the house and brick veneered and painted the house and brick white throughout. It was a surprisingly nice house. During the years to follow, plumbing was added, electrical was added, and; eventually central HVAC. He added a fish pond, an enclosed porch and a concrete (two-strip) driveway (grass grew between the strips). Initially, he had dug the well and built the wellhouse. I got most of the tools the old man used to build houses - one at a time without the help of electricity. I am finding these old tools very interesting - many of which I never knew were in my Dad's severely over-cluttered basement until the past couple of months. The ability of the tools to "hold-an-edge" was nothing like the steels of today - so *SHARPENING SKILLS* had to be super-keen! Beyond grinders (very slow speed) stones were used. I have also accumulated two very old "razor strops" that were used to put super-sharp edges on some of the knives and chisels. Those "razor strops" were some of the old things that were used on me regularly - so they came as no surprise. They were regularly used to adjust my and my brother's attitudes. I am very glad I never had a sister - she would have been one tough girl!


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## denniswoody (Dec 11, 2011)

OPG3 said:


> 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.


Have look at this video and the use of beeswax. The Birth Of A Tool. Part 2. Chisel Making (by John Neeman) - YouTube


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