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Thread: Just how effectively can below table collection capture dust on a table saw?

  1. #1
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    Just how effectively can below table collection capture dust on a table saw?

    I'd appreciate hearing your experience on this.

    We've been discussing the vagaries of saw top guards on the other thread, and the Ryan made the point that (presuming correct blade alignment so that it's not cutting on the back stroke) much of the dust the top guard has to deal with may be the result of the below table dust collection arrangements failing to 'empty' the blade. i.e. dust created on the downward cutting stroke being carried around and up the back in the tooth gullets and thrown into the air.

    What's your experience? Are there lower dust chute arrangements fitted to some models of saw that do a good enough job that for example it's possible to work when required with the top guard removed without finding the air filled with dust - or worst still having dust thrown forward into you face?

    Dust system performance is surely a factor in this, in that decent air flow should make the job easier. What's the experience in this regard?

    Have you formed any views as to what the critical success factors are?

    Thanks

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    Clean blades shed spoil more readily than a grungy one. Once shed, adequate airspeed will capture the spoil and carry most of it off. In my experience a bit of dust gets carried back to the top of the material being cut via the gullets that don't completely shed the spoil. Tooth count doesn't seem to affect the amount much but material type really does. I find an overarm more useful for rips than for other cuts. Just my experience.
    "A hen is only an egg's way of making another egg".


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  3. #3
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    I agree with Glenn. My saw has a pretty good below blade dust shroud and it gets the larger percentage but a small amount gets carried to the top. It is small enough that I don't worry about it.
    Ken

    So much to learn, so little time.....

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    Thanks guys. I'm not familiar with US pattern table saws, but there's a photo first down below the header on this page of the dust chute on the Hammer K3. http://www.ukhammer.co.uk/products_d...6&region=gb-en

    The rectangular bottom of the chute is open, and the dust hose fits over it. You can see the way the blade is only partially covered (for about 1/4 of its perimeter), meaning that it relies on negative pressure inside the chute and the inward air flow through the slot to not leak dust. It's also pretty much wide open at the top - there's nothing like a seal or a tight gap to the underneath of the table.

    Air intake into the cabinet is fairly non specific too, there's a gap all round between the table and the top of the cabinet, plus various slots for the height and tilt adjusters. Sealing off part of the slot could if needed significantly increase the rate the air is drawn in through the remaining openings, although it actually seems to work fairly well as is. (in that it doesn't deposit much dust inside the cabinet)

    What it may (or may not - it's way ahead of the Robland combo i had before) do such a good job on is the removal of the dust from the blade gullets before it clears the chute/shroud.

    It sounds like you guys do pretty well on this. i.e. relying primarily on the bottom chute/shroud. Do you happen to know off hand what the layout of the chutes on your saws might be? Is it partial like the above, or does it completely enclose the blade? Is there anything fancy/special about it's layout? Does it for example have narrow gaps or anything else special to e.g. generate high speed air flow across the blade? Does it run a large hose. e.g. 5in?

    We've previously discussed the possibility that siting an air inlet to create a flow across the tooth gullets might be useful, but nobody has tried it yet....

    ian
    Last edited by ian maybury; 07-17-2012 at 7:49 PM.

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    dad's sheet.jpg

    Here is a pic of the cast iron shroud on the SCMI. It does a pretty good job. As I was about to take a picture of the Knapp I realized the saw was full of dust - a testament to how lousy it works- so I didn't bother. It is sheet metal much like the Hammer only worse and modification doesn't look easy. I'm running a 7.5 hp high pressure impeller so velocity into the shroud is over 6000 fpm and still dust fills the base. Dave

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    Quote Originally Posted by ian maybury View Post

    We've previously discussed the possibility that siting an air inlet to create a flow across the tooth gullets might be useful, but nobody has tried it yet....

    ian
    I was kind of wondering the same thing. I get a fair amount over the blade. I think Im gonna try it when I get home Sat

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    Thanks David, that looks quite similar to the cast chute/housing on the 700 and 900 Felders too. The open side is presumably pretty well closed off when the sheet metal sliding cover is closed. There doesn't seem to be any great magic to it or the Felder version - just a reasonably close fitting blade housing with air pulled through it.

    No fancy tweaks to clear the tooth gullets etc.

    There seems to be a high degree of similarity between the SCMI and Felder sliders in terms of overall layout anyway - looking at a review of one in a UK mag recently even the slider extrusion looked quite similar...

    I guess it'd be nice if it was to turn up that there's an XYZ saw that gives absolutely superior under table collection - with design features that could be copied.

    The Robland combo I had before the Hammer sounds a bit like the Knapp - on a 1 1/2 HP bag filter it'd block unless the filter was clean and the filter flowing freely. The Hammer is in comparison very good indeed...

    You'd imagine though that with the sort of air flow and level of suction you have that the Knapp would work at least moderately well...

    ian

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    There are very few weaknesses to the Knapp but DC is one of them. Felder does it as well as anyone I've seen but have not directly compared the MM. My T130 2000 vintage isn't as good as the newer Felder shapers but I haven't looked at the new SCMI models. It would seem a good sheet metal guy or someone with welding experience could make a whole new unit. The integral cast iron shrouds seem to be the best. Dave

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    I have owned at least a dozen tablesaws, from an old 8" Atlas, to a Felder. Believe it or don't, the best dust collection was on my Shopsmith. It had a shroud enclosing the bottom half of the blade, a lot like Davids pic, with a direct 2 1/2" port, and this was back in 1983 or so. The Felder is a fairly close second.

  10. #10
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    That cast saw enclosure on the higher end Felder and SCM machines is very nice.

    I've been thinking a little about it, and (now) remember reading a piece in FWW which reckoned that the dust collection on traditional table saws can be greatly improved by sealing the dust chute or cabinet top to the underside of the table, and if needed (when there is no dust chute per se) forming a ply hopper in the cabinet leading to the dust hose. Slots for hand wheels etc in the cabinet need sealing up too (magnetic sheet was mentioned), especially if the dust system delivers a fairly low flow.

    Which fits with all of your inputs - it would have the effect of ensuring that a much stronger air flow is drawn down through the throat plate past the blade. Which should do a much better job of clearing dust from the blade teeth. It should also deliver proper emptying of dust from the cabinet.

    One major advantage Felder and similar cast enclosures may have is that they seem to mimic this situation - without filling the cabinet with dust.

    If the cast dust chute more or less encloses the blade and largely seals under the table (allows in only the make up air required for transport) then most of the intake air can be drawn down through the blade slot past the up coming teeth at the back of the blade.

    The slot could probably be widened a bit in that area to improve this flow too - not too much or work might catch, and it might cause suck down problems when feeding sheet material.

    In practice it'd probably need more intake area than just the slit in the throat plate (to ensure there's enough air flowing to transport chips back from the chute and up the duct) - especially with a high flow dust system and 5in duct. A strategically placed hole or two in the side of the chute/blade enclosure as originally discussed above should fill this need and might add some more to clearing the teeth.

    The relatively large gap between the top of the chute on the K3 and the underside of the table (several inches) seems likely to be poor in this regard - and may explain why it seems to chuck a lot of dust back up over the table. In that (since the top opens into the saw cabinet which takes most of its air through the gap under the table) most of the heavy duty suction available will likely be dissipated by drawing air from there.

    Sounds like it's time to take some time to see if DIY shrouding might be not an option to improve the situation - by bringing it closer to that which obtains with Felder's cast enclosure....

    ian
    Last edited by ian maybury; 07-18-2012 at 8:07 AM.

  11. #11
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    Not on my Hammer B3, if I don't use the above table dust collection a lot of dust escapes.

    It's just the same as all the other saws I owned, they needed above blade dust collection as well, of course the Hammer actually manages to capture most of the below table dust, as opposed to the cabinet saw that developed an equilibrium when a certain amount of the cabinet was filled with dust.

    Regards, Rod.

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    Hi Rod, that's basically what I was trying to say in a long winded way too - that while the K3 does a decent enough job under the table that it still very much needs the top guard.

    It's maybe appropriate to inject a note of caution on the topic of improving bottom chutes. Since the above several of the guys on FOG with experience of both the cast full enclosure chute and the Hammer type chute seem to be suggesting something similar. That the difference in performance between the two types is not all that marked, and that the top guard (a) remains important, and (b) is often the easier source of improvements.

    The logic is clear - if it was possible to very effectively clear dust out of the teeth below the table then top guards wouldn't be such an issue. Trouble is that the blade is moving very fast, so it's probably tough to get it working so well with the sort of air speeds we use that the top guard becomes unimportant.

    That doesn't necessarily mean that there are not improvements to be had with tuning below the table, but it sounds like there may be limits to what is feasible. It's also fairly clear that what works well on e.g. a high air flow/CFM dust system may hardly work at all if less air flow is available - and vice versa in that a more restrictive chute set up may not flow a lot more air with a larger dust system.


    ian

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    Everyone is guessing at what is needed, that much is plain. I have been meaning to put a camera inside a saw for a while now to see what actually does happen.
    Chris

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    It's as they say Chris at this stage a thought experiment, and with thought it's obviously not a simple topic.

    The point of this and the other thread is to offer an opportunity to pool some of the perceptions/bits of experience we all have to see if there's any conclusions or pointers for further work that can be extracted from it.

    For example there are places where clearly it's important to generate pretty high airspeed (e.g. getting the top guard down close to the work/getting fairly high speed inwards flow plus physical interception of flying dust is one way of gaining good collection through it), and others where it's necessary to open stuff up to generate high CFM.

    It's as before quite linked to dust system capability too. A larger impeller can generate enough CFM and suction to work in situations where a smaller item doesn't move enough air. Against that it seems likely that carefully tuned hooding can often do a decent enough collection job (less so on air cleaning) - but it's going to struggle on a set up optimised for higher CFM.

    ian
    Last edited by ian maybury; 07-19-2012 at 5:47 AM.

  15. #15
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    Ian, as you know the EU equipment is tested for dust emmisions and must meet the EU standards for such. That drives the design of the machine with respect to dust collection, so I would expect that there has been some optimisation of it.

    It would be a marketing disadvantage to produce an inferior design that required much more airflow to capture the dust, as air volume is a consumable that must be paid for continuously during use.

    Regards, Rod.

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