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Thread: Zero Clearance Insert, So, Zero Dust

  1. #1

    Zero Clearance Insert, So, Zero Dust

    Just kidding. But I would appreciate knowing how the percentage of dust created is apportioned differently top and bottom with a zero clearance insert installed in a table saw.

    As I am building this cabinet to contain an old contractor saw I anticipate that there will be less dust that gets into the bottom of the cabinet. So I am anticipating that the real action is in the dust collection at the blade. I picked up a used Biesemeyer T-Square Blade Guard and am contemplating how to increase the size of the dust port.

    But my real question is about the difference in need for collection below the table when using a zero clearance insert versus the big bags of sawdust below the saw cabinet I am used to with the factory insert. I imagine that there is the requirement for some dust control but am wondering if 6" of port diameter out the bottom connecting to the main run which is 6" is overkill at the same time that I am worrying that less than 4" (I forget the actual size of the Biesemeyer dust port at the blade guard) is sufficient.

    What difference does a zero clearance insert make below the table and what is the consensus on what it take to clear it to the central mains which are 6"?

  2. #2
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    IMO, the ZCI has zero effect on the path dust takes.

    Look at it this way.........if you are ripping a board [not just cleaning up an edge, but making 2 pieces of wood] the board has the insert completely covered, regardless of the clearance. The blade is cutting on the edge closest to you, in it's downward rotation, and the gullets theoretically will carry all of the dust down into the cabinet where you suck it away. But - as we all know, some of that dust remains in the gullets, or is caught up in the upward rotation whirlwind, and comes back up. I run my blades with the tips [the entire tip] just proud of the wood, and as much of the gullet still buried as possible.

    I've never put in overhead DC on my TS. It would be nice, and I might do it someday......but I would need to fabricate one of those versions that are supported from the ceiling, because I am not interested in the version that comes up and across the table itself - no likey obstructions at the corners of the table, because that 72" x 72" square is my primary finishing and assembly table. As is, it really does not create enough flying dust to bother me [and my air cleaner is right above the TS]...........unless I am just skimming the edge, in which case it is an Old-Testament-style nightmare - especially with the WO I most often use - brittle, sharp, very hard needles come flying back...gotta wear long sleeves for that one. I wear large-lens glasses, and if I didn't, then safety glasses would be a must. [I know, I know - my reading glasses are not impact resistant and are not the same as safety glasses.....don't start on me, OK? ].

    The ZCI, again IMO, has one purpose and one only: safety, so that thin strips do not get trapped between the blade and the edge of the slot. I'm not interested in being goalie for the archery team.
    Once more unto the breach, dear friends, once more;
    Or close the wall up with our English dead!

  3. #3
    I read this tip somewhere and followed it on a couple of my inserts.
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  4. #4
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    Glenn - did you notice a difference? I've thought about doing that, but haven't yet - just wasn't sure if it would help............
    Once more unto the breach, dear friends, once more;
    Or close the wall up with our English dead!

  5. #5
    I enclosed the rear, added magnet closures to control slots and installed a modified 0-clearance insert(like one in attachment above) on my contractor saw and actual dust coming at your face increased. There have been reports on some portables I've read about w/ two points of dust collection in which the 0-clearance insert works very well. I feel it's all a matter of air-flow and D/C pick-up points. My saw actually decreased it dust production from throat by installing wide open factory plate and running Thin Kerf 7-3/8". blade. Certainly a lot things you can change on a contractor saw. I feel a super slippery plastic large U-shaped, full length insert going from one side of saw to other inside blade cavity would result in much better funneling of dust to a 5-6" pick-up located as close as possible to blade assembly. A blade enclosure w/ separate dust pick-up would also IMO benefit you the most as far as dust control in general blade location.

    Mac
    Last edited by Mac McQuinn; 11-11-2011 at 4:15 PM.

  6. #6
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    My own observation tends to suggest that Kent's point about the dust getting carried down under the tables mostly in the gullets is true - which suggests that the task is to get it out of there before the teeth reappear above the table. The Robland I had when using their 1kW approx bag filter did a great job of spraying dust which had been carried around and back up over the table into your eyes if the top guard was removed.

    Get it all out of the gullets, and you don't need so much by way of a top guard, and vice versa.

    The other big factor is airflow. No airflow = no dust collection. Lots of airflow = large low pressure volume around the air intake = improved ability to gather in dust not intercepted by the hood + higher velocities which may do better in blowing dust out of the tooth gullets.

    I'm not familiar with enough saws beyond what I've had to know what the norm is, but there has to be enough open area (something of at least the cross sectional area of the duct) or the flow will be proportionally reduced. A zero clearance insert means that more or less all the air has to come from below the table.

    Combine this and the need to clear the gullets as well and it tends to suggest that the most effective lower hood will be one that (a) permits the free intake of air up to the capability of the duct, and (b) better still has it's air inlets sited and angled where they will be most effective at removing dust from the gullets.

    Another factor regardless of whether or not the dust is dislodged from the blade that if the hood inlets are restrictive it may lead to transportation problems in the duct. Try to feed a 5in duct for example through a highly restrictive small opening and the air speed in the relatively large duct will be greatly reduced - well below the 4,000feet/min required for good transportation up verticals. Leakage in any of the duct joints (which unless the joints are taped can be very considerable) will further worsen the situation.

    My thinking anyway....

    ian
    Last edited by ian maybury; 11-11-2011 at 6:41 PM.

  7. #7
    Quote Originally Posted by Kent A Bathurst View Post
    Glenn - did you notice a difference?
    Sorry, I didn't mean to be vague on that one ;-) Seemed like there was a difference so I did it to another one that I use frequently. I didn't really do anything scientific so my perception is subjective on this one. I sealed certain openings into my cabinet and left others purposely open. Inadequate air in = impeded air out. Seal things up too much and your DC chokes. I agree that collecting the sawdust once the blade has channeled it under the table reduces that which can be carried back out.
    Last edited by glenn bradley; 11-11-2011 at 7:58 PM.
    “If liberty means anything at all, it means the right to tell people what they do not want to hear.” -- George Orwell


  8. #8

    clensing one's gullet

    First of all I have been educated, or schooled as my teen age kids would say. Imagine, the blade carries the wood it cuts away from a length of stock down into the cabinet regardless of the slop in the insert. Who would have thunk it? Not a cave man like I.

    Lots of folks have had great success using a brush at the juncture of a band saw blade as it returns up the lower wheels and makes some physical contact with the blade to dislodge saw dust. I don't know the relative speed of a band saw blade to a table saw blade but imagine it is significant. Table saws are beast.

    There are two issues for an enclosed contractor saw in my cave man cranium. One is the requirement of the electric motor to have air to cool itself and Two is the requirement to carry the saw dust away.

    I have not seen anyone separate these requirements. I remember a Fine Woodworking article by a retired airline pilot who talked about directing air flow. It included plans that I have stashed on some archive somewhere and used an enclosure that had intake slits behind the motor that met the cooling requirements of the motor as well as enough air to transport the dust down and out the bottom of the cabinet. But it now seems to me that one could get some pretty intense intake air flow directed at the blade that would blow the dust down and out the bottom.The most sophisticated dust system I have been privileged to view on the Creek is that of retired rear admiral Alan Schaffter. When he has shared the electric controlled gates and blade guard dust hood over his table saw, the one with the whiskers, I took notice. But clearly I had not understood the dynamics of the saw taking wood down and away from a single length of stock to create two lengths of stock. Plain as the nose on my face and did not consider it until now.

    I don't think it would be hard to direct vent the motor to the outside and let it fend for itself. It is powered and has its own dynamics to pull its own cfm of air. I am thinking that flexible ducts from the outside to each separate end of the motor would take care of its need. (Not like anyone needs invitation to disagree around here but I want to make it clear that I am inviting input.)

    This would leave lots of options to have quite a lot of cfm draw coming from a central dust collection system and very directed air ducts coming from the enlarged end section of a modified zero clearance insert as well as some smaller ducts coming from the outside and directed against the blade itself. Then again the use of a durable, light physical contact brush with the blade also seems reasonable. Its not like a space shuttle re-entering the earth's atmosphere or anything, but the issue of friction and heat is certainly something that would spank anyone being cavalier about the physics involved. So, what would a more directed dust removal duct look like below the table. I think it is worth thinking about, especially for some of the lower powered systems that would benefit by such an incremental efficiency of directed effort.
    Last edited by Bruce Seidner; 11-11-2011 at 9:05 PM.

  9. #9
    I slept on this and also read an email I got back from Forrest Blades saying that there is no change in dust below the table using one of their LeeCraft Zero Clearence Inserts. My understanding and experience of a zero clearance insert is to support the stock as much as possible around the blades cutting/pulling action to minimize tear out. It is also the case that this makes for a safer action of cutting because there is no room for unintended wood to lodge and play practical jokes with the dynamics of several horsepower throwing its weight around.

    What about making the insert the main inlet to match the CFM requirements of a 6" outlet?

    So if a typical insert is 14"x3.5" how many .25" holes would it take to prevent CFM constriction. This would be different for a given system driven by different motors, but if someone would be so kind as to provide a formula with some X's and Y's in place to particularize the application that would be enormously helpful and a real contribution to world peace. My guess is that there is not enough area on the insert to drill enough holes to meet the CFM requirements and to keep the insert intact enough to do its job. But that CFM could be made up with another shroud or inlet that directed flow across the blade.

    In the past I have had the typical plywood insert at the bottom of the contractor saw with an outlet to a dust collector or shop vac. This was like going from the stone age to the bronze age over a metal can or plastic bag sitting beneath the saw. But it seems that in the modern age of relatively powerful dust systems a pretty straight forward tweak is to be a bit more directed about making as much of the air flow past and over the bade as possible. I even had the thought that I could employ a small compressor that that I rarely use anymore to run a line into the bottom of the saw just to air blast the blade when it is running. This tiny compressor is not doing anything but taking up space at present and that would be like a dental pick blasting those gullets free of dust the way a dental pick gets those pesky particulates of food matter from between the teeth. Likely a bit over the top, but all done with things commonly found in a garage workshop.
    Last edited by Bruce Seidner; 11-12-2011 at 12:39 PM.

  10. #10
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    A ZCI makes little difference with dust collection. The board you are cutting is making a kerf that is the perfect size. The kerf in the ZCI makes no difference. The primary reason for a ZCI is to improve the cut of the saw. The ZCI supports the edges of the kerf making for less "fuzz" on the cut edge.

    The dust that gets on the top of the table is primarily generated from the dust being carried around by the blade and coming off when the blade exits the insert. The best way to catch this is by using an over-the-table pickup system integrated with your blade guard.

    As far as internal collection, you must have at least the same number of square inches of intake for fresh air as you have for exhaust air. With a contractor saw, there is plenty of air intake space between the bottom of the table and the saw box. In addition, you have the open back that is a source of air intake.

    Finally, saw dust coming off the blade is traveling at over 100mph. No dust collector air speed (CFM) is going to be enough to change the direction of the dust ejection. Therefore, you need to install some sort of air/dust deflector to re-direct the air/dust into the exhaust port. More and more saws are being designed with dust chutes internally to do this.
    Howie.........

  11. Quote Originally Posted by Howard Acheson View Post
    Finally, saw dust coming off the blade is traveling at over 100mph. No dust collector air speed (CFM) is going to be enough to change the direction of the dust ejection. Therefore, you need to install some sort of air/dust deflector to re-direct the air/dust into the exhaust port. More and more saws are being designed with dust chutes internally to do this.
    Howie, can you name some examples of this and where you have seen them? I am curious about how they have accomplished this.

  12. #12
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    I'll give you "almost 90mph"...........

    10" blade diameter x pi x 3,000 rpm x 60 min per hour / 12 in per foot / 5280 feet per mile = 89.205 mph tip speed of the blade.

    Sorry........I was sitting here, read your post, and went for the calculator......involuntary, knee-jerk response.

    However, I have it on good advice that table saws at CERN in France do, in fact, eject dust particles at speeds faster than the speed of the blade.................
    Once more unto the breach, dear friends, once more;
    Or close the wall up with our English dead!

  13. I was proposing holes in the insert but I have holes in my thinking. Any such holes would be covered by the stock.

    I will go looking for some more ideas for the inside shroud for the bottom of the contractor saw. I recall someone using a length of 6" PVC and sliting the top the length of the base and leaving the rest intact for connecting to the rest of the system. The area around the slit is heated and opened up in a V shape forming the shroud beneath the blade. This reduces the volume of the contractor saw base and makes for a slippery no crannies duct for the dust out. On the front of my contractor saw base there is a great big smile of a slot for the angle adjustment of the blade. This front opening may or may not be sufficient to match the intake need of the system. I have a book on dust collection by Sandor Njowiefjyesejfe (phonetic spelling) and I remember a formula section I can dig out to get some general guidelines but if any other calculator wielding engineers can advise on the intake opening necessary at the front end of the base to vent through through the shroud inlet I would be grateful.
    Last edited by Bruce Seidner; 11-12-2011 at 9:54 PM.

  14. #14
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    As I've expounded in another thread, my personal experience gives a clear answer: Seal ALL extraneous holes in the cabinet - foam under the table top, foam around the edges of the cabinet floor, mag sheet over the big smile, tape over miscellaneous holes, foam and tape where the power cord enters the cabinet....anything and everything you can find. Seal it up as tight as you can.

    Then, add holes in judicious locations, designed to direct/manage the inflow. I have horizontal slots cut into the motor cover, to flow air over the motor [cooling and blowing dust off the motor and trunion gears] and holes low in the cabinet side opposite the DC connection [air flows across the floor of the cabinet toward the DC connection]. You can make too many holes, and close them off with mag sheets to play around with the effect, if that's something you want to do.

    Howie's explanation above is what I did - 5" dc connection means air out is pi x r squared = about 20 square inches, so I put in about 16-18 square inches of new holes [assuming that the Unisaw engineers were more clever at making extraneous holes than I was at finding and sealing them].
    Once more unto the breach, dear friends, once more;
    Or close the wall up with our English dead!

  15. #15
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    I'm beginning to think that the only thing besides an over head guard/pickup would be to pipe and air hose from a compressor into the cabinet with a nozzle pointed at the gullets at the point of them entering the cabinet, and try to blow the gullets clean, and in turn sending those particles into the cabinet to be carried away by the dust collection system of choice. It would have to be mounted so that it would move up/down/tilt with the trunnion, but that shouldn't be too hard to do.
    But honestly, my home build guard/pickup gets most everything on my saw. There are operations when I'm cutting something too thin that keeps the guard on top of the fence, and not on the wood being cut and misses more than I'd like, but when it's on the wood being cut it does a great job. dscn2619.jpg Jim.
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