• January Turning Challenge: Thin-Stemmed Something! (click here for details)
  • Conversations are now Direct Messages (click here for details)
  • Congratulations to Scott Gordon for "Orb Ligneus" being selected as Turning of the Week for January 20, 2025 (click here for details)
  • Welcome new registering member. Your username must be your real First and Last name (for example: John Doe). "Screen names" and "handles" are not allowed and your registration will be deleted if you don't use your real name. Also, do not use all caps nor all lower case.

Where is RPM Measured?

Tom Albrecht

Tom Albrecht

Joined
Feb 8, 2014
Messages
1,206
Likes
648
Location
Evanston, IL USA
When I ask where it is measured I mean in reference to the motor listing. I am thinking specifically about grinders, but I suppose it applies to all motors. A slow speed grinder may have a motor label that states 1725 rpm. Is that the speed of the grinding wheel or the motor shaft? If it's the speed of the motor shaft, then a 10" grinder will have wheels spinning significantly faster than a 6" grinder, right? I guess I am wondering if grinder manufacturers take the wheel size into account when they manufacture specific sizes. That being said, I am wondering if can substitute wheel sizes and put larger CBN wheels that don't require wheel guards, on a smaller grinder and build up the base appropriately.
 
Tom I would hope that your grinding wheel is rotating at the same speed as the shaft of the motor 😉

Motor spec’s tell you the rotation speed of the motor, the size of the wheel has nothing to do with that.

The surface of a larger wheel will pass by a given point faster than a small wheel would do, FPM is used (feet per minute) to give the speed of something, like a bandsaws blade speed going through the wood.
 
RPM, revolutions per minute, is the speed of the motor shaft, surface speed of the wheel in relation to the RPM is dependent on the wheel diameter.

The larger the diameter (at a constant RPM) the faster the speed.

Surface speed = diameter times pi (3,14159...) < which is the surface length>times rpm which will be in inches per minute.

Yes you can put a larger dia wheel on but the speed at the tool tip will be faster and may cause problems.

Stu
 
The grinding wheel would be turning at the same RPM as the motor shaft unless the wheel is slipping, which isn't going to happen in the real world. When the motor shaft makes one complete turn, the grinding wheel also makes one complete turn. In one second the motor shaft makes about 29 complete turns and so does the grinding wheel. I think that you are confusing angular velocity with tangential velocity at the rim which is a linear velocity.

The tangential linear velocity of an 8" wheel is about 60 feet per second. If you had a 10" wheel, the tangential velocity would be about 75 feet per second. If your grinder motor has a 5/8" shaft, the tangential velocity is 4.7 feet per second. However, they all have the same angular velocity which is normally stated in revolutions per minute which in this case is 1725 revolutions per minute.
 
Whooo! "Tangential Linear Velocity" That's a mouthful, and a brainful for me.
Can I break this down Bill?
From what I understand, it is advised to run the CBN wheels on slow speed grinders. (And I've noticed that conventional wheels often have a "Max Speed" label on the side). Again, the various bench grinder manufacturers including Baldor, Jet, and Dayton to name a few, make slow speed(1725-1800 rpm) grinders in various sizes from 6" up to 14"-- same with the high speed 3450rpm sizes.
So what I am understanding here, and please excuse my ignorance/ density, is that a 10" wheel will be running at the same linear velocity on a 6" grinder with a 1725 rpm motor as it would be on a 10" grinder with a 1725 rpm motor. Right? And, that 10" linear velocity will be faster than the 8" linear velocity. ?
 
IMG_3756.jpg
Tom Albrecht said:
Whooo!
So what I am understanding here, and please excuse my ignorance/ density, is that a 10" wheel will be running at the same linear velocity on a 6" grinder with a 1725 rpm motor as it would be on a 10" grinder with a 1725 rpm motor. Right? And, that 10" linear velocity will be faster than the 8" linear velocity. ?
Click to expand...

Hi Tom,
All wheels on a 1750 rpm shaft will rotate 1750 times a minute
The edge of the whellmwill pass over your tool once ever RPM.
for a 6" wheel that is a foot and a half of grinding every revolution
For a 10 " wheel that is a bit over 2.5 feet of grinding every revolution.

Pulleys on our lathes increase or reduce rpm from the motor.

Al
 
The maximum speed is always stated as angular velocity so you don't have to do any calculations to determine if it will work with the motor you have. I have Vicmarc 485 mm bowl jaws and they are marked 600 RPM MAXIMUM. My Talon, Stronghold, and Vicmarc chucks also have a maximum RPM rating. The Norton matrix grinding wheels on my grinder also have a maximum RPM rating ... I think it is 4000. You could calculate the tangential velocity at the outer edge of your grinding wheel, but that's just mostly useless information from the perspective of the user who just wants to sharpen his tools.
 
However, the RPM all other factors being equal, a larger wheel will sharpen faster and heat up faster than a small wheel.
 
Dennis said it so my Math challenged brain can understand. I did do the math one time trying to understand if a 6" wheel at 3450 rpm was much different than an 8" wheel at 1750. I'll let you do the math again but what it really amounts to is simply, are you comfortable with the speed your sharpening.
To answer Tom's question, yes you could put a larger CBN wheel on but they weight more and can your grinder handle that.
 
Al, If I were to guess, I would guess that you are a mathematician. 😀

The difference between an engineer and a mathematician:
To an engineer anything beyond about two decimal places on the circumference isn't meaningful because of manufacturing tolerances of the grindstone. The engineer would say that the RPM of your motor is no better than 1750 +/-10 RPM under nominal load conditions so the surface velocity could be rounded off to the nearest whole number or even nearest decade.

To a mathematician all of this rounding off and approximation would be intolerable. 😀
 
A craftsman takes the tools he has to work with and uses his skills and finesse to get the job done,
while the engineer and the mathematician are arguing over proper speed versus diameter of the wheel.
You can always adjust the amount of pressure you exert on the tool while sharpening so it does not heat up.
And a little cold water goes a long way in keeping your tool cool while sharpening. 🙂
 
Bill Boehme said:
Al, If I were to guess, I would guess that you are a mathematician. 😀

The difference between an engineer and a mathematician:
To an engineer anything beyond about two decimal places on the circumference isn't meaningful because of manufacturing tolerances of the grindstone. The engineer would say that the RPM of your motor is no better than 1750 +/-10 RPM under nominal load conditions so the surface velocity could be rounded off to the nearest whole number or even nearest decade.

To a mathematician all of this rounding off and approximation would be intolerable. 😀
Click to expand...

I would disagree. The mathematician is satisfied that a solution exists, and moves on to another problem. Figuring out the actual speed is just an application, and unimportant.

The physicist wonders if the tool actually makes contact with the wheel, because of the vast empty space between the atoms, the electrons, etc.

The woodturner obsesses about a 40 degree vs 41 degree grind 😛
 
I think that you are right. The mathematician has no interest in the answer ... only that it exists and can be found ... by somebody else. Then maybe that means Al must be a bean counter. 😀

The physicists are wondering if there is anything left to do now that they have verified the existence of the Higgs boson. 😛
 
Bill Boehme said:
I think that you are right. The mathematician has no interest in the answer ... only that it exists and can be found ... by somebody else. Then maybe that means Al must be a bean counter. 😀
. 😛
Click to expand...

I used to have to predict where the bean fields were and how many beans would be harvested.
🙂. Mostly I was an applied mathematician - my job was getting the wrong answers fast. 🙂

I made up answers on lots of problem for which we could prove no solution existed.....
 
This has a lot of information here. I just bought the Rikon slow speed grinder that was on sale at Woodcraft. Can't pass up a bargain. Thought of giving it to my wife for her birthday later this month. Changed my mind, though.
As my friend said, I learned more behind the schoolhouse than inside. 😀
 
John Torchick said:
This has a lot of information here. I just bought the Rikon slow speed grinder that was on sale at Woodcraft. Can't pass up a bargain. Thought of giving it to my wife for her birthday later this month. Changed my mind, though.
As my friend said, I learned more behind the schoolhouse than inside. 😀
Click to expand...
Feel free to give your wife the Rikon slow speed grinder; I'm sure she will greatly appreciate it!

Do expect to receive for your birthday a diamond bracelet, though 😛
 
While we're off subject, I'd like to second giving your wife practical presents.

I gave my wife a Stihl 360 Pro about 15 years ago. While she's never run it, she did pose once while pregnant, and the pictures were well received by her family. The saw has kept our house warm during those years. It cut the ground through the forest so we have electricity now, and her saw has cut most of the wood for her favorite wood bowls.

Probably the best present I ever gave her! 😛
 
You guys are really short hitters, Do you only drink half a beer at a time?

I gave Nancy a fully loaded Robust American Beauty for Christmas 2 years ago. She loves it because I'm in the shop for a lot more of the day now.
 
You must log in or register to reply here.
Back
Top