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Need Ideas For Bench Grinder Table Vacuum System

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  • #16
    Losing the suction through small diameters is certainly a factor that needs to be closely examined. I've got a couple of shop vacs which came with the small 1 7/8 hose but with an inlet fitting that would take the 2 1/2" hose too. When fitted with the larger hose the difference in both volume and velocity was like night and day. The increase in air loaded the motor down but it moved so much air and would suck things in from a lot further from the end of the hose. Just what we'd want for a vacuum system on the grinders like this.

    Now if the vacuum had a lower power motor going to large on the hose would not maintain the FPM in the hose. And I saw this in writeups on dust collection systems when I was trying to pick the size of ducting for my wood shop collection system. Basically these instructions all said the same thing. Keep the duct or hose as large as you can while still allowing for a fast enough FPM in the duct/hose to stop the particles from settling. And it actually warned against using reduced size diameters at the collection funnels as this would act as a restriction and reduce the FPM in the duct/hose. But at the same time it also stressed the need for smooth inner surfaces.

    Bottom line is that for a good shop vac I'm thinking that between 2.5 and 3" is likely the optimum size to get the most volume and still have a good flow rate in the hose. And if using the commonly available stuff I'd certainly go with the 2.5" hose and ensure it stays at 2.5 through the whole collection system from connector funnels to the vac.
    Last edited by BCRider; 05-01-2019, 12:46 PM.
    Chilliwack BC, Canada

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    • #17
      Originally posted by BCRider View Post
      Losing the suction through small diameters is certainly a factor that needs to be closely examined. I've got a couple of shop vacs which came with the small 1 7/8 hose but with an inlet fitting that would take the 2 1/2" hose too. When fitted with the larger hose the difference in both volume and velocity was like night and day. The increase in air loaded the motor down but it moved so much air and would suck things in from a lot further from the end of the hose. Just what we'd want for a vacuum system on the grinders like this.

      Now if the vacuum had a lower power motor going to large on the hose would not maintain the FPM in the hose. And I saw this in writeups on dust collection systems when I was trying to pick the size of ducting for my wood shop collection system. Basically these instructions all said the same thing. Keep the duct or hose as large as you can while still allowing for a fast enough FPM in the duct/hose to stop the particles from settling. And it actually warned against using reduced size diameters at the collection funnels as this would act as a restriction and reduce the FPM in the duct/hose. But at the same time it also stressed the need for smooth inner surfaces.
      I'm not the engineer who sizes up this stuff, but I can tell you it doesn't work out by accident. The CFM needs to be adequate, then you workout the static pressure necessary for the ducting run which has to do with length, curves, dia etc. You often see them get larger as they move away from the pick up, but that's so that many pickups can work at the same time. If you're only using one (with no others or others shut), you don't increase the diameter. Often when the diameter is increasing (to accommodate several pick ups), you need to have a way to introduce air so that the velocity doesn't fall too much going from small to large
      in Toronto Ontario - where are you?

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      • #18
        ... you need to have a way to introduce air so that the velocity doesn't fall too much going from small to large
        In my reading that was a big point that was made too. For the air to move we have to let it get in is so it can move.

        In the case of my table saw I stuffed the openings between the top and the sheet metal sides and also made up overlapping panels to close off the big opening in the back and still allow the motor to swing for angled cuts. And on the bottom it has a sloping side box to catch the sawdust with the 6" duct outlet on one side. But at the same time I included a 4" gate on the opposite side of the "V" to let in air to make up for the small openings. This also created a strong cross wind that keeps the dust from piling up in the sloping sided box.

        There's online calculators for figuring this out that I found at the time. I took the CFM and static pressure ratings of the dust collector I bought and the calculator suggested 6 point something to 7" ID's for the short runs I was able to use. So I opted for 6" since there's more stuff available for that size. Also it was the size of the inlet at the blower fan housing.

        Oddly enough I use it between my table saw, the sliding compound saw sitting in a big shroud to focus the dust collecting and also on the jointer in the same way that you want to use with your grinders. The machines are all grouped right around the dust collector and I've got an 8' piece of 6" flex hose that I shift from duct to duct to pick up the chips from each machine as needed. Not as convenient as a multi path system but it sure provides excellent dust grabbing. And since the wood working shop is in the basement and I don't want the dust flying around the house I find that this works well.
        Chilliwack BC, Canada

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        • #19
          I did not put any actual pipe diameters on my sketch above. This was deliberate. I did mention starting with a 3" diameter in my text, but that is not an absolute. You could easily go larger.

          You should start with the diameter (internal) of the vacuum that you are going to use and work from there. But work BACKWARDS from the intake at the grinder wheel. So if you have a 2" vacuum intake, go with a 2" intake at the grinder wheel end. And work up from there as you proceed towards that vacuum itself. At no point should the opening be smaller than that two inches or whatever your vacuum has.

          With PVC you can use thin wall pipe to allow the telescoping without getting ridiculous with the final diameter. Or even alternate thin and thick wall pipe so the thumb screws will rest on thick wall while the sections that they fit into are thinner. A few minutes at the local hardware or home supply store would be a good place to pick the pipe diameters. Some of the stores in my area sell short lengths of PVC that make testing easier.

          This may not produce the "ideal" system in terms of suction, but it will probably be as good as it can be, given the restraints of the grinder work station.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #20
            To McGyver's point, you really DO need to "design" it to do the job.

            You can make it easier by enclosing the wheel as much as possible, and not letting the sparks/dust get outside the enclosure. If they did, you would need enough air velocity to capture them, which is difficult.

            A funnel behind the wheel is not doing the job, even though you may think it is. You just do not have "wind tunnel" type volume and velocity, some of the dust, perhaps most of it, is escaping. You DO have some advantage because much of the dust is directed at the funnel by how it comes off the wheel. But not all.

            With limited volume and velocity, enclose, then exhaust. That way, the limited volume of air is all passing the dust generator (wheel) and picking up dust. The enclosure is responsible for capturing the dust into the airflow.
            Last edited by J Tiers; 05-01-2019, 07:47 PM.
            CNC machines only go through the motions.

            Ideas expressed may be mine, or from anyone else in the universe.
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