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  • Another newbie tip

    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!
    Will

  • #2
    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.

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    • #3
      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.
      Russ
      I have tools I don't even know I own...

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      • #4
        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.

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        • #5
          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?

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          • #6
            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.

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            • #7
              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.

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              • #8
                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?

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                • #9
                  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

                  Paul
                  Paul Carpenter
                  Mapleton, IL

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                  • #10
                    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.

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                    • #11
                      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.

                      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.

                      thanks
                      Paul
                      Paul Carpenter
                      Mapleton, IL

                      Comment


                      • #12
                        "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?"


                        Iwolf,
                        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.

                        Comment


                        • #13
                          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...
                          Last edited by Fasttrack; 05-08-2006, 09:49 PM.

                          Comment


                          • #14
                            Another newbie tip

                            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)
                            Jim (KB4IVH)

                            Only fools abuse their tools.

                            Comment


                            • #15
                              Guys...you all should read this...
                              http://www.mmsonline.com/articles/0303rt3.html
                              Here's a death from using the wrong wheels
                              http://72.14.207.104/search?q=cache:...&ct=clnk&cd=20
                              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.
                              Russ
                              I have tools I don't even know I own...

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