View Full Version : Another newbie tip

05-08-2006, 05:14 AM
I have an abrasive cut off saw as well as a nine inch angle grinder, when I use the cutoff saw I wear the discs down to nine inches or a bit smaller, The cutoff saw use one inch bore discs and the angle grinder uses 7/8 bore discs so I turned up a washer to fill the gap and now I can use the discs twice!

05-08-2006, 05:43 AM
Way to go Will, don't you just love recycling? You'll probably be warned about guards and all that, but you know that already if using a 9"er.

05-08-2006, 09:13 AM
I see guys do this all the time with used discs from 7" grinders. They are putting them on smaller grinders that operate at much higher rpm than the 7".
This is not a safe practice.
You should make sure the rpm will match the machine you are "swapping" to.

05-08-2006, 11:37 AM
While I understand Torkers concerns regarding RPM limits, it must be said that wheels are rated at a specific RPM for their largest diameter. As a wheel wears the diameter obviously shrinks and forces at the same RPM are less. Therefore, I believe that, within reason, that using a higher RPM on a worn wheel (smaller diameter) should present little trouble. In any case, use guards and eye protection and be careful.

05-08-2006, 11:46 AM
Firbikrhd1,Let me see if i get this correct,you say that the wheel wont turn the same speed when it is smaller?It will turn slower? there is not as much weight there ,why wouldnt it turn faster?

05-08-2006, 12:56 PM
IOWOLF, a larger diameter disc must travel more distance in one revolution than a smaller disc.
Just like an Airplane propeller, there is more pitch near the hub than at the tip to balance the thrust along the whole length of the prop.
In any case, one is taking liberties on the issue. Need to do a lab test to really know if it truly is safe.

Steve Stube
05-08-2006, 01:48 PM
Exercise CAUTION. There is a lot of difference between 3,600 rpm and 13,000 rpm (as already said) and another point is that the way the wheel is loaded in operation is very different too. Cutoff is primarily radial loading on the wheel, rightangle grinding is loading the wheel from (the side) a face. Many of the cutoff wheels won't tolerate much flex before they fail.

05-08-2006, 02:14 PM
:D An Rpm is an Rpm is an Rpm silly.

But I have noticed that wheels seem to deteriate faster twards the center like they are softer or something.
Perhaps Evan or his wife could enlighten us on that, or am I just imaganing it?;)

05-08-2006, 04:06 PM
One thing that does happen is that as a wheel gets smaller, its *tangential* velocity (the speed at the point that it is contacting the work...the only one that counts) is increasingly slower. While the number of rotations per minute stays the same, the number of surface feet per minute that contact the work goes way down. This is where the discussion of the forces applied being lower comes into play--There is much less centripetal force trying to make the wheel fly apart as it gets smaller.

Likewise, the number of surface feet that are doing work goes down....and the wheel wears quicker as you get smaller. Not to mention that cutting slows and the natural tendancy is to then bear down on things....as wrong as it may be:D


05-08-2006, 06:10 PM
Nice explanation PCarpenter - that was my initial thought as well. Same rpm but much smaller diameter means much less tangential velocity; the wheel should be fine. One little note though, centripedal acceleration is the acceleration towards the center of the circular path - that's what is responsible for maintaing a circular path since, technically, you continually change the instantaneous velocity. Centrifugal "force" is more what you are reffering to, although it is not a force at all but rather an intrinsic property of circular motion. The feeling of centrifugal force is cause by an objects inertia, or tendency to move in a straight line. A force must be acted on it to bend it to a circular path (this force keeping it in the circular path is of course the centripedal force) I'm sure you already knew this, but it didn't come across very clearly so just clarifying.

05-08-2006, 06:36 PM
Yup...good point. Centrifugal force....actually force due to centrifugal motion is what is actually acting on a grinding wheel making it want to become lots of little pieces :-) And....that is reduced with the reduced diameter. Now...if I could just remember the formula, we could figure out whether the reduction in force with the change in diameter makes it on par with the force of a smaller wheel at the higher rotational speed of the smaller grinders.:eek:

Still, the earlier posts about the different directions forces are applied to the various types of wheels is worth heeding. A cutting wheel should always be used as a cutting wheel as it may not be reinforced enough to stand any side load as in typical grinding.


05-08-2006, 09:04 PM
"Firbikrhd1,Let me see if i get this correct,you say that the wheel wont turn the same speed when it is smaller?It will turn slower? there is not as much weight there ,why wouldnt it turn faster?"

I thnk what I was trying to say is pretty well covered in Fastrack and PCarpenter's posts. In any case, the RPM of the (electric) grinder in question is not determined by the size of the wheel, rather by design. It may run slower under a load but not significantly faster than it's rated RPM. (Air Grinders may react differently given different air pressures and volumes) A smaller diameter wheel will be subject to less centrifugal force at a given RPM than a larger wheel at the same RPM, therefore, it must be run at a higher RPM to achieve the same centrifugal force. That said, I have no idea how much faster a particular wheel could be safely spun to acheive it's design forces. That would probably depend on several factors such as wheel diameter in question, thickness and other design parameters that affect the weight of the wheel and forces involved.

05-08-2006, 09:47 PM
I think i can help with the equations; the equation for centripedal acceleration is a = (V^2) / r
Read : Centripedal acceleration equalls rotational velocity squared divided by radius, and the centripedal force, via F=ma, is just m(V^2) / r or mass times centripedal acceleration.

The rotational velocity which i believe is actually the average of the vectors of the instantaneos velocities, is really just your rpm times your circumfrence. Make sure the units all come out right though - multiplying by rpm should give you inches per minute as your velocity provided you measuer the circumfrence in inches.

The centripedal force is the amount of force required to move the particles of the wheel in a circle, if the centripedal force neccessary to induce such a path is too large, the wheel will no longer continue in a circle; the pieces will fly off in a straight line. So we should be able to figure out the centripedal force of the wheel at full size and then the force at say 7inches and compare them. As long as the force is the same or the 7inch is less then everything is happy-go-lucky except for the very important point already emphasized about how the wheel is loaded. Finding the mass of the wheels at different times is going to be difficult though...

some days i really wish i could spell... :)

05-08-2006, 10:21 PM
At the risk of beating this thing to death, we are talking about surface inches per revolution (speed and force at the outer edge of the wheel), I think. At the smaller diameter you won't have the same centrifical force (trying to pull the wheel apart) as you do at the larger diameter.

I know folks that do this with larger wheels, use them on smaller grinders once they have worn down to easily fit within the guard(yes, keep the guard on it) of the smaller grinder, and have never had a problem. The advice about not applying side load to a cutoff wheel is absolute truth. Most of the guys have a small grinder with a cutoff wheel on it to slice through bolts, bar stock, and the like. The main thing with any grinding operation is eye protection. Safety glasses are good, full face shields are better. Been there, seen that done!

Jim (KB4IVH)

05-09-2006, 01:28 AM
Guys...you all should read this...
Here's a death from using the wrong wheels
I had to sit through an accident investigation at a pulpmill a few years ago.
A guy was using a wheel meant for steel on aluminum and it blew up in his face, smashed his face shield and blackened his eye from the force of his glass frame hitting him so hard.
They brought an expert in to give us a couple of hours worth of speel on the dangers of grinding wheels/discs.
Using the bigger wheels on smaller grinders was a definate no-no according to him.
The safety committee but an abrupt halt to the practice of using lower rpm wheels on the smaller grinders.
The above link covers a lot of what he said.
Figure this...
I have a 5" grinder that operates at near 15,700 sfm.
One 7" grinder that operates at 12,838 sfm.
Another 7" @ 9170 sfm.
I just looked at some of my grinding discs.
Some of the 5" ones are rated at 13,500 rpm.
None of the 7" dics are rated above 8600 rpm.
I've had maybe a dozen discs blow up on me over the years and it ain't fun.
FWIW, I've used worn down discs from my chop saw on my 5000 rpm 7" grinder for cutting grooves and for cutting deck screens. I don't after the talk from the Wurth Abrasives rep.
I'm not trying to rain on anyones parade...just sharing something I was taught.

05-09-2006, 04:30 AM
Formulas, eh? Well I got you some. As mentioned before, a=v^2/r, which gives you the force "pulling" the outer edge into a strait line. However, this can be changed a little, since v, or sfm, =rw, where w is angular velocity, or rpm. Substituting a little, we get a=rw^2. Which is a little different from the above, but basically saying the same thing. Then again we could get into talks about if the same torque is applied to each wheel, what would be the velocity since each wheel would have a different I value (cause of the larger radius) so hence different max w values, and I could talk a little more about that but don't really want to right now.

(sorry guys just finished Dynamics course, kinda happy to actually contribute here- one more lurker bites the dust! :p)

05-09-2006, 08:04 AM
You know all those formulae are in your "Bible", but it all comes down to common sense. Just remember that no machine tool has a conscience; they just don't care if you lose digits or have your face torn off. A guy about 20' away from me had a 2" by 12" inch wheel explode in his face. He was setup to grind 2 small blocks on a surface grinder,everything was looking good until the wheel touched 1 of the blocks..Bang! Now this guy had over 20 yrs. experience, surface grinders were his specialty. Guess what he forgot to do? By the way, not one chunk hit him directly although he was shaking for at least 30 minutes afterward. We razzed him for that for at least a year.

05-09-2006, 08:44 AM
Millman....You do make a point. One of the first things the rep asked us was "What is the most dangerous machine in the shop?"
Answer...A bench grinder! He had pics of some pretty nasty looking wheel explosions.

05-09-2006, 06:45 PM
Scatterplot - that equation only works when your "rpm" is actually in radians per second. Otherwise your rpm has to be converter into a circumfrence and then it again gives you rotational velocity or as Scishopguy put it, surface inches per minute. As you said, still the same thing.

Secondly - i don't care whether anyone uses different wheels or not, i probably wouldn't just to be on the safe side, but there should not be any difference. I repeat: there should not be any difference - provided it has a reduced radius, a reduced mass and the same width. In fact i will crunch some numbers based on a general purpose 14inch wheel later tonight and will post again. Torker - i don't mean to come off confrontational or anything; i always respect your imput - especially all the help you've given me with my truck! That said, those articles are assuming that you are using too large a wheel on a faster grinder, not a small wheel on a fast grinder. Also as a note to everyone else - there is no such thing as centrifugal force!! Seriously. Thats a pet peeve of mine. That "force" is not a force at all. What we feel when spinning something in a circle is an intrinsic characteristic caused by inertia of the spinning object. Thus the greater the velocity or the greater the mass, the more inertial resistance it will have to being moved into a curved path instead of a straight path.

Still though - i agree that caution should be excercised with grinding wheels in general. They are used so often with out problems that i think sometimes people forget just how dangerous they can be.

05-09-2006, 06:53 PM
Fastrack, I think you may have something there."!! Seriously. Thats a pet peeve of mine. That "force" is not a force at all. What we feel when spinning something in a circle is an intrinsic characteristic caused by inertia of the spinning object. Thus the greater the velocity or the greater the mass, the more inertial resistance it will have to being moved into a curved path instead of a straight path". Sounds like a credible explanation of that "mysterious force" that we all grew up with. It's actually SFPM that counts in grinding and cutting.

05-09-2006, 07:19 PM
I know I may sound like I am on both sides of this argument, but there is one potential difference between a cutoff wheel for a chop saw worn down to 9" and a 9" cutoff wheel for an angle grinder and that is reinforcement.

The former is held in on an arbor that swings in such a fashion that it should never be exposed to any side load at all.

Freehand cutting with an angle grinder is a wholely different issue and as I recall, the cutoff wheels I have for use in my 4" angle grinder seem to have some fiber reinforcing that you can see in the back side. They are thin and would quickly explode the first time they bound just a bit in the cut if they didn't have something to hold them together....which always happens to some degree when freehand cutting.

Edit: This part of the discussion is a wholely different topic than the forces applied by rotational speed...but still may be important in avoiding injury.

Just another thought.

05-09-2006, 07:32 PM
Absolutely! I was assuming the same composistion. The thought occurred to me earlier that they might be different but then i saw they were all advertised as the same abrasive material but that, of course, is not really what is important here! Thanks!

05-09-2006, 09:27 PM
Good call about converting to radians. That has screwed me on much homework :)

05-09-2006, 09:57 PM
hahaha - i've been there before!

05-09-2006, 11:55 PM
Some numbers on edge speed. If a 14" chop saw wheel runs at 4000 rpm, the cutting edge is moving at pi x diameter x rpm so that's 14660.8 SFPM on a full sized disc. Assuming the same saw, the driven hub speed is constant as is "pi" so that same disc worn down to 5" diameter has an edge speed of 5236 SFPM. It's easy to see why the blade cuts so much slower.

Running a 5" used disc on a 10,000 rpm right angle grinder will yield an edge speed of 13090 SFPM so that is within the range of the original rating. However, using the disc worn only to 6" puts the edge speed up to 15,708.

The manufacturers should be rating their wheels based on top speed with a full size wheel (plus a safety margin - we hope...). Other factors such as side loading may cause a catastrophic failure.

I do not recycle chop saw blades only because I am concerned by the stress put on them by previous use so my disclaimer is - USE CAUTION IN ALL CASES AND I AM IN NO WAY RECOMMENDING THE PRACTICE - but your mileage may vary...