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  1. #1
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    Default Improving machine cabinet dust ports

    I see some members are starting to open up their machine cabinet dust ports, which is good to see.

    The biggest restriction to air flow by DCs is the use of too narrow a ducting - remember a 100 mm duct can at most only carry about 40% of what has been deemed necessary to catch and transport fine sawdust.

    It's also not much use using 150 mm ducting but then use a collection port at the DC end restricted to 100 mm - the use of the bigger ducting has to be all the way to the filters/bags/cyclone.

    After this has been been attended to, the next major restrictions to flow are usually the dust ports on machines.

    Many so called 100 mm dust ports are only ~90 mm and some are even smaller with metal grids/grilles that further block the flow.
    If you intend to use the existing machine port, start by removing any grilles
    Use of multiple small ports can generate sufficient flow but remember that it takes THREE 100mm ports to equal the flow through a single 150 mm and more than 12, 50 mm ports to equal the flow of a single 150 mm port.

    Most dust collection ports on machines are poorly thought out both in structure and location.

    Step 1: Dust port location

    Step one would be to determine if the current location is appropriate, or has it been located for some other unrelated purpose i.e. ease of construction.
    It may even be better to close off the existing dust port and open up a completely new one.
    The dust port should be located as close as possible to the dust sourcee, and (as much as possible) opposite to any air intake(s) (e.g. a throat plate on a cabinet saw).
    Remember this is not "opposite the source" but " opposite the air intake to the cabinet"
    This may entail some sort of trade off between the inlet and outlet as they may not be aligned.

    Step 2: Air Inlets

    Next would be to look at where, and how much air, is able to enter cabinets to pick up and carry away the sawdust.
    If little or no air can get into a cabinet then no matter how hard the DC sucks it cannot remove the sawdust efficiently.
    Some example of this isusing a extra narrow gap throat plate, and the very restricted air intakes into lower cabinets of band saws.
    So be prepared to enlarge other intakes or generate additional air intakes.
    The sum of all air intakes should be at least twice the cross sectional area of the cabinet dust port.
    If any of the intakes are less than 10 mm in any dimension then only count half their area as contributing to the air intake area.
    Don't count any intakes less than about 3 mm in any dimension e.g. Small holes drilled in throat plates do bugger all for fine dust collection so don't count these.

    Step 3: dust port construction

    The most common form of dust port construction is design #3 - this sudden transition means it is restrictive to air flow and there are much better designs.

    If you have the length and are able to make tapered ports like #2 then this is more efficient than #3

    If you don't have the space then design #1, using a bell mouth exit involves some work but is as efficient as #2.
    These bell mouth ports ports can be easily turned up on a WW lathe out of MDF sandwiches or thick pieces of melamine

    Remember that in both cases 1 & 2 you will need to make the hole in the cabinet significantly larger than the ducting size.
    In the case of #1, if 150 mm ducting is used the hole will need to be at least 225 mm in diameter.
    In the case of the flared port like #2, 225 mm would be the smallest diameter to consider, 250 mm or bigger would be better.
    If the taper in 2 is long and wide then it can even be made rectangular in cross section

    Whatever you do, avoid using #4 as this generates the greatest turbulence and hence the greatest resistance to flow.



    Improving machine cabinet dust ports-cabinetport-jpg

    If anyone has any other port designs then I would be happy to comment on these.
    Attached Images Attached Images

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  3. #2
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    Sorry Bob, I know u've explained this, but I'm not understanding the pic

  4. #3
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    Fantastic info Bob. Thanks for sharing.

    If you were to do #1 or #2, is there a limit to the length of the transition?

    Cabinetport.jpg

  5. #4
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    Quote Originally Posted by johnredl View Post
    Sorry Bob, I know u've explained this, but I'm not understanding the pic
    Sorry I should have provided more detail
    The diagrams represent 4 different cross sections of the machine/cabinet ports.
    The blue lines represent the cross sections of the ducting walls.
    The reddish lines represents the cross sections of the walls of the cabinet
    The orange blobs represent the cross section of what is called a "Bell Mouth" transition.
    The labels "IN" and "Out" refer to inside and outside the machine cabinet.


    Quote Originally Posted by Afro Boy View Post
    If you were to do #1 or #2, is there a limit to the length of the transition?
    Well the longer the better but obviously you need to draw the line somewhere.

    In the case of 1, the minimum "radius of curvature" (r) of the Bell Mouth Hood (the orange blob in the diagram) shoulder should be 1/2 the radius of the ducting (lets call the radius of the ducting RD).

    So, if 150 mm diameter ducting is used, then RD = 75 mm , and "r" = 37.5 mm
    This means the radius of the opening in the cabinet (let's call that RC) should be 37.5 mm more than RD i.e 75 +37.5 = 112.5 mm, or a diameter of 225 mm.

    Of course you don't need to be mm accurate - as long as "r" is more than 37.5 mm and RC is more than 225 mm.

    If 100 mm diameter ducting is used, RD = 50, r = 25 and RC = 150mm.

    It turns out that there is little gain by going for larger r or RC, but if you have the space then it may be worth doing.

    For the sloped transition the KPI is 5 duct diameters or in the the case of 150 mm that is 750 mm but that can get awkward to make and place.

    In the case of #2 if even a slightly rounded duct connection to the cabinet is used (e.g. r = 10 mm for a 150 mm duct), the slope can be much less (e.g. 300 mm) but unless there were extenuating circumstance I probably wouldn't bother making one any short than this.

    #1 has a big advantage it can be made much shorter than #2.

    BTW, where possible ALL ducting should be arranged so that any junction or transition is 5 duct diameters away from any other junction or transition. In small DYI setups this is virtually impossible to achieve but if you have a choice about where to located a junction or transition then try to apply this consideration.

  6. #5
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    Thanks bob. I'd missed this thread before. Great thread, especially as it relates to both cabinet ports and dust extractor inlet ports.

    I'd posed the question in my thread, but it is probably more relevant here. Apart from making a bell mouth inlet, has anyone found a suitable product that can be simply modified to form a bell mouth? I don't have a lathe but would be willing to hack into a pot plant or similar if that would work. Otherwise it's option 2 for me.

    A question for BobL - would a 'bell mouth' with a large diameter round over bit work better than a taper?

    Trav
    Some days we are the flies; some days we are the windscreen

  7. #6
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    Quote Originally Posted by Trav View Post
    A question for BobL - would a 'bell mouth' with a large diameter round over bit work better than a taper?
    Trav
    I've answered this at least 3 times before but for completeness I will answer it here as well.


    For the same size opening a bell mouth shape will collect slightly more air than a taped end but the bell mouths has the major advantage of collecting more air from in front whereas the taper still collects some air from the back and the sides of the opening. This is a significant advantage because it targets specific dust sources and grabs more dust before it has a chance to escape into the shed.

  8. #7
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    This is great information Bob. Thanks
    Regards
    Al .

    You don't know, what you don't know, until you know it.

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    This is indeed great information, no doubting that.

    However, I have just looked at how I could apply this to dustports on my machines. Looking at example A above.

    I started out checking the exact size of the "150mm pipe, It actually has an ID of 153mm and an OD of close to 160mm.

    So, to find the diameter of the dust port we need it is the the inside diameter plus the transitional curve, which is half the radius but you need two of them. You also need a lip around the outside of that to fasten it to the machine. Bearing in mind that this will be on a working machine in a workshop environment it will need to be somewhat robust so I think 25mm would be a good figure.

    That adds up like this: 153 + 76.5 +50 = 289.5. Lets say 290mm.

    I drew a circle 290mm diameter on a piece of cardboard and cut it out. I also drew on circles for the ID and OD of the pipe and the outer edge of the transitional curve, to make it easier to visualize where everything would fit on the real article.

    Now off to the shed to see where I could locate this monster dustport on my machines ... (it takes only a minute or two to make this cardboard 2D mockup. I suggest everyone do this. It is an intereting exercise.

    Well I could put a port in the door of my 21 inch bandsaw, but that would not be very practical. I could make a bigger shroud for my 15" thicknesser which would accept a port this size. In both these cases it would be difficult if not impossible to open up the cabinets to allow the required air intake.

    With a small amount of adjusting I could fit it to my 8" jointer and allow enough air into the cabinet.

    So the jointer is looking good. The rest of the machines may benefit from multiple 4" ports built to the same principles, but even they may be too big for many of the machines. I guess its back to the old compromise of ports of type 3 above for their smaller size and sacrificing some efficiency.

    Now I stress that I am not knocking the value or accuracy of BobL's information, just pointing out that before you go and cut all your 25mm MDF into 290mm circles, have a think about where you can use them.

    I think they would make excellent free standing "gulp" collectors for lathes or for scrubbing the air around sanders etc but their application to the scale of machinery found in most small workshops/garages you may be surprised at how few places you could use one in practice. this is both from the point of view of the sixe of the port and being able to ventilate the cabinets to the required extent without compromising the structure, safety or function of the machine.

    Cheers

    Doug
    I am too poor to buy cheap tools.

  10. #9
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    Quote Originally Posted by doug3030 View Post
    I think they would make excellent free standing "gulp" collectors for lathes or for scrubbing the air around sanders etc but their application to the scale of machinery found in most small workshops/garages you may be surprised at how few places you could use one in practice. this is both from the point of view of the sixe of the port and being able to ventilate the cabinets to the required extent without compromising the structure, safety or function of the machine.
    Yep - agree mostly.
    The problem is further exacerbated because there needs to similarly sized large openings made to allow the air into cabinets.

    Just remember that the 1.5X Bell mouth hood (BMH)recommendation is just that - it's the optimum (or near optimum, and there is no benefit from going to larger factors) BUT any sort of round over is better than none.

    A 1.3X BMH opening with round over would be almost as good as a 1.5 and even a 1.1X would be worth doing.

    Even if you were to use a small rounder router bit on the PVC itself it would be better.

    Oh yeah one more thing, a half a BMH is better than no BMH. If for example your can only fit your BMH in by having the port along one edge of a cabinet then this reduced the space required from the 1.5X to 1.25X.

  11. #10
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    This info is pretty good. It's rather amazing it isn't pushed elsewhere or even made into common woodworking knowledge.

    Why don't "they" sell BMH's with their hose and fittings? Seems like an obvious thing....

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    Quote Originally Posted by Evanism View Post
    Why don't "they" sell BMH's with their hose and fittings? Seems like an obvious thing....
    I think the reason is that it is easier to customise the fitting for each individual and unique application than it would be to modify a prefabricated "standard" fitting. Of course the "ideal" solution would be to build the proper ports into the machines at the design phase.

    Evanism, I suggest you cut out the 290mm circle as I suggested above and take a walk around your shed and think about the radical design changes incorporating one of these babies would have on every machine you own.

    Doing that this morning was a real eye-opener for me as to how difficult it would be. Give it a try.

    Cheers

    Doug
    I am too poor to buy cheap tools.

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    Quote Originally Posted by doug3030 View Post
    I think the reason is that it is easier to customise the fitting for each individual and unique application than it would be to modify a prefabricated "standard" fitting. Of course the "ideal" solution would be to build the proper ports into the machines at the design phase.

    Evanism, I suggest you cut out the 290mm circle as I suggested above and take a walk around your shed and think about the radical design changes incorporating one of these babies would have on every machine you own.

    Doing that this morning was a real eye-opener for me as to how difficult it would be. Give it a try.
    Im here right now scratching my noggin doing a job for Trav on exactly this. Even just imagining a 30cm plate on the surfaces is big. 150+75+fat to secure it adds up fast.

    The BS, TS and lathe are a yes, the jointer will have to be murdered at the back.... poor old CJ196 is in for The Big Chop. Nice jointer that. Not fancy, but decent. Be better with a helical though...

    I think I'll absolutely get the biggest benefit from the lathe. I saw BobLs setup on the little woodfast and that exactly what I do, but with an occy-strap .... the BMH will be a huge improvement while I sand.

    On another matter, it doesn't take very much research to see this is obviously used EVERYWHERE.... turbos, carbies, jet engines, air inlets on big air conditioners for buildings, even water treatment plants use them. I personally thank BobL for flogging this horse. Dust awareness and management is an A1 critical for everyone dealing with wood dust - hobbyist or pro or shop.

  14. #13
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    Quote Originally Posted by Evanism View Post
    I personally thank BobL for flogging this horse. Dust awareness and management is an A1 critical for everyone dealing with wood dust - hobbyist or pro or shop.

    I second the motion.

    BobL has provided a wealth of knowledge to help us help ourselves, but there is no "one size fits all" option.

    Look at the examples of my machinery and Evanism's. I can fit a optimum 150mm port on my jointer, he cannot. He can fit one on his bandsaw but I cannot.

    Its a matter of taking the knowledge and using it intelligently to do the best you can in your own shed.

    Oh and I just made up a cardboard mockup of the optimized 100mm port. I measured the pipe for confirmation and the ID is actually 104mm and the OD is actually 111mm. That makes a 206mm circle, allowing for 25mm all round for securing. Even that is harder to find suitable spots for than you would think too.

    Cheers

    Doug
    I am too poor to buy cheap tools.

  15. #14
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    Cheers Guys,

    Here is a diagran illustrating my previous post about location of ports along the edge of a cabinet.

    This means that 2x is not essential in two dimensions to fit a BMH like this.
    Fixing using the 5 black points shown is all that is needed.
    There's no need to seal the flange, it doesn't matter if it leaks a little - after all the more air it can collect the better.
    You can see that this port has about 3/4 of the port still acting as a BMH - definitely worth doing.

    Improving machine cabinet dust ports-bellmouth6-jpg
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    Quote Originally Posted by BobL View Post
    You can see that this port has about 3/4 of the port still acting as a BMH - definitely worth doing.
    Yes Bob definitely worth doing. Unfortunately on my machines, it does not make that much difference.

    I folded my cardboard mockup n line with the circle of the ducting and walked around my machines again and it is still a big ask to find somewhere to put a port that size too.

    I still find the information useful. It is good to know what "optimum" is so that you know how far from it you are.

    Right now I am thinking about the other factor you mentioned in the first post of this thread: opening the cabinet enough to let the air in so it can collect the dust and transport it away.

    Here's what I am thinking. Comments most welcome.

    Example 1: my 8" jointer.

    I can put the fully optimised 150mm port on this machine by just cutting out a bigger hole than what is there. In front of the port is a steel ramp that directs the chips to the dust port. Great, but we are not really worried about the chips from a health point of view but the more we get the less sweeping up we need to do. Plenty of air can get into the cabinet. there are vents pressed onto the sheet metal sides of the cabinet but at less than 10 mm only count as half their size.

    Plenty of air can get in under the cabinet sides and will be sucked up into the port. I see that as a good thing as it will minimise dust (visible and invisible) escaping from the bottom of the cabinet. If you can get 1000 cfm through it you could possibly push it around the workshop instead of sweeping the floor. I think it is well worth any effort to increase airflow through the DC system for this machine as any increase in CFM would translate into better air quality in the shed, with an added bonus of lots of chips collected in the process.

    Note: I realise that the above only discusses collection within the cabinet, and a lot of invisible dust would be coming out the top, despite the air being drawn in around the blades. Other collection methods are needed to deal with this.

    Example 2: my 15" thicknesser.

    Ok this is a very different scenario. The main collection point is above the cutterhead through a sheetmetal cowl. To even get an optimised 4" port on the cowl I would need to do a lot of sheetmetal work to replace the cowl. SO while I am at it why not make a cowl big enough for an optimised 150mm port?

    Well for one thing, I am not sure I could open up enough area for enough air to get in to make it worthwhile. All the air needs to get in around the cutterhead and the infeed and outfeed rollers. There is a fairly heavy sheetmetal cover over the top of the cutterhead and rollers that may possibly be modified by cutting away an area along the length of the infeed side, but there are safety issues.

    It would require fabricating some sort of additional guard to prevent anything entering the newly opened space above the infeed roller. Without this it would be a definite no-go on safety grounds alone. In any case there is really only room to open up a vent maybe 330mm * 30mm maximum (9900 sq mm, so with the other narrow airways between ctterheads and rollers I think saying 11000 square mm of openings to get air in is being kind. Now Bob says that for a given size port you need twice as much inlet. 11000 divided by 2 is 5500 sq mm. So what is the diameter of the ideal duct to deal with 5500 square mm of inlets?

    So, 5500 sq mm divided by pi = 1750mm. Square root of 1750 mm is 42 mm. Thats the radius so we only need an 84 mm (2 x 42) port to get all we can out of the thicknesser because of the limitations of the inlets we can create

    It already has a sub-optimal 100mm port and has not had the modification that would provide any meaningful airflow at all. Why would I go to the trouble of trying to improve on the existing port? That's why I always use my thicknesser outside of the shed in the yard and let the breeze work for me.

    Yes, it may be possible to increase the flow through the thicknesser but safety could become an issue if not careful. It would be a lot of work and there is no perceivable benefit I can see of trying to improve on the existing port.

    Example 3: Never mind, lets talk about examples 1 and 2 for now, its getting late

    Cheers

    Doug
    I am too poor to buy cheap tools.

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