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What is a "normal" surface grinder wheel speed?

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  • What is a "normal" surface grinder wheel speed?

    I was in KBC the other day and intended getting a few surface grinder wheels of various grits. I have enough already, but they're gradually getting smaller, and I wanted to keep a few relatively unused for when I need the larger diameter to get down into work to grind it.

    Fortunately I checked the max wheel speed when they bought them out and it was 3600 IIRC. That seemed quite low to me, and it let me to wonder what is the "typical" speed at which these grinders run. These were 6" wheels and my grinder spins at 4000 rpm. I have a VFD on that machine and it would be a 30 second process to simply adjust the speed down if I wanted to use those wheels, but instead I let them go.

    I was wondering at what RPM others' grinders are set at?

    Pete

  • #2
    3450 RPM should be about right. for a 7 inch wheel
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    • #3
      Ok, so my machine, specified as 4000 rpm is a "fast spinner" compared to others? That's quite a difference in speed!

      Pete

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      • #4
        "Normal" surface grinders usually run between 3000 & 4000 RPM using 6 inch wheels, as far as I know.

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        • #5
          Im considering building a 1800rpm grinder for 8" wheels. 3600rpms is.. just too fast for cool grinding of HSS. Vibrates too much too.
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          • #6
            Every surface grinder I've ever seen has been 3600 RPM (3400 RPM if you're in 50Hz land).

            Lane's 3450 is 3600 RPM minus the induction slip.
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            • #7
              Where did this 4000 rpm number come from? What is the nameplate info? ( all of it). 4000 rpm does not seem to correspond to any motor and frequency that I recall.
              As near as I can tell a 2 pole motor on 50 hz runs synchronous at 3000 rpm. Unless you are on 70z 4000 rpm would not be possible.

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              • #8
                It's probably belt driven, my Elliot 20 x 8 is and it has two speeds to allow better use of the ten inch wheel.
                Will

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                • #9
                  Originally posted by tdmidget
                  Where did this 4000 rpm number come from? What is the nameplate info? ( all of it). 4000 rpm does not seem to correspond to any motor and frequency that I recall.
                  As near as I can tell a 2 pole motor on 50 hz runs synchronous at 3000 rpm. Unless you are on 70z 4000 rpm would not be possible.
                  Well yes it is "possible" because it is 4000 rpm. The figure came from the manufacturer's "operating instruction book" which contains the specs. As noted by Will, the surface grinder doesn't just bolt the wheel on the end of the motor spindle, and it's driven by a toothed belt. They can have any speed they like by varying the pulley ratio, and in this case it's 4000 rpm

                  My wheels are rated at 4200 rpm IIRC and it was just as well I checked the KBC wheels as they were much lower.

                  Pete

                  Edit: Sorry you wanted the motor plate info. It's 2820 RPM 3 phase I rewired from Y to delta, being driven by a Sew-Eurodrive VFD. I don't use the variable speed facilities of the VFD however. I could, but figured I'd no doubt then start driving wheels in excess of their design speed and when the wheel exploded 2" from the end of my nose (I always seem to use that machine hunched over for some bizarre reason), that would lead to a foul smell in the workshop.
                  Last edited by PeteF; 03-16-2011, 12:26 AM.

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                  • #10
                    RPM isn't really "it".....

                    The real issue is surface speed. and somewhere from 4000 to maybe 6000 SFM is typical for "most" grinding with vitrified wheels, according to the wheel makers.

                    You can figure that out for any size wheel.

                    7" and 3450 RPM is about 6300 SFM.

                    For 8", it would be 7200 SFM, probably too fast.

                    The TPG I have uses a 3" wheel, turning around 7000 rpm, and that ends up as about 5500 SFM.

                    6" and 4000 rpm is 6280 sfm, at the high end, yes.
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                    • #11
                      Ahhhhhhh, you didn't tell us that. Most that I have experience with are direct drive, and it is very common , as the rated speeds on wheels suggest.
                      In any case, exceeding the rated speed is dangerous and to be avoided.
                      Since as you have seen, 3600 is pretty much the standard max speed, set it at that and pour a bunch of epoxy over the knob or you'll put your eye out. Just one if you are lucky. Yeah , I know , the peripheral speed decreases as the wheel diameter reduces, but the manufacturer rated the wheel in rpm, not sfpm. They know a lot about their product and give good advice in the area.

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                      • #12
                        I'll probably just buy more wheels locally of the same brand as they're all rated to greater than the grinder's RPM. It was just that KBC had wheels on special for 10 bucks each as I recall, and that's a bargain in any book.

                        As Jerry said, apart from the rating of the wheel in terms of max RPM, it's the surface speed that's more of a concern. I read somewhere that the faster you spin a wheel the more it behaves like a "harder" wheel. I'm getting absolutely beautiful finishes from this grinder, waaaay above any commercial grinding job I've seen anyway, they almost looked lapped and are like a mirror. So I don't want to screw around with running slower. The only thing I have noticed is that if the wheel needs dressing it will tend to burn the work on the leading edge. It gets cleaned up by the trailing part of the wheel and obviously the solution is to simply dress the wheel, as I do, but I wonder now if the faster speed doesn't make this machine more prone to that???

                        Pete

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                        • #13
                          Originally posted by lazlo
                          Every surface grinder I've ever seen has been 3600 RPM (3400 RPM if you're in 50Hz land).

                          Lane's 3450 is 3600 RPM minus the induction slip.
                          Funny, because every Jacobsen I've seen has "1700 RPM" sticker on the wheel cover. The wheel size is around 400 mm in diameter.

                          And yeah, those do run with huge surface speeds. Once calculated over 2000 m/min from a basic surface grinder
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                          • #14
                            It aint RPM. Its RPM that gives you the ccorrect peripheral speed.

                            Generally you want about 5500 FPM for vitreous bonded aluminum oxide grinding wheels. Work the math and you get the RPM for the wheel diameter your grinder is made for. As it happens a 7" grinding wheel is very happy at 3500 RPM. 5500 / dia x 4 = FPM thereabouts.

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                            • #15
                              What happens as the wheel wears

                              As mentioned, you can't simply dismiss wheel speed in RPM, since they're rated to a maximum RPM. In the case of KBC's that was 3600 RPM and too slow for my machine. I'm not interested in operating wheels beyond their rated speeds and if anybody else is, they can have it as far as I'm concerned. I have yet to have a wheel explode on any grinder, but I've heard from people who have and apparently it's rather a pants browning moment!

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