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

[Rod Sheridan]

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, the EU machines such as the types that Ian and I own must meet EU dust emmision standards. I don't think the manufacturers of those machines are guessing, I think they have a pretty clear idea of how to meet the standards, and all the designs I've seen include a lower blade shroud and an above blade shroud.

If a manufacturer could achieve satisfactory resuslts without the top collection, I assume that they would as it's a cost savings during operation of the machine.

Regards, Rod.

[ian maybury]

Agree on EU dust collection requirements Rod, although they don't seem (unless there are less widely published regs covering machine design) to be highly specific.

In general there tends to be a pretty specific requirement in terms of shop air quality (the typical 1mg/cu m TLV), but more generally stated requirements on dust hooding arrangements. They don't for example get much beyond saying that all saws should have 'adequate' top and bottom dust collection hooding.

Here's the page published by the HSE in the UK on the topic - their stuff normally reflects EU requirements. It pretty much reflects current practice in Europe on the various types of machines referred to:

http://www.hse.gov.uk/pubns/wis24.pdf

I've a feeling as before that the likelihood that a machine may be used with a wide variety of types of dust collector forces quite a lot of compromise in the design of stock dust hoods - they can't just design to suit only high CFM dust systems. (e.g. the stock Hammer top guard while pretty good is fitted with a small bore collection hose stub)

Hammer and Felder machines have pretty good collection, but there are examples of European machines that fall far short of them.

I guess in the end I'm probably trying to do better than Hammer or the EU regs may require - a blue printed system you might say. In that for me even small amounts of dust can kick off a sore throat and eyes, and a runny nose. A short release in the context of a specific and difficult to collect from cut might not have much influence on average air quality (the 1mg/m3 number), but if I get spritzed with dust from that single cut then away I go....

ian

[Chris Parks]

To clarify my previous comment, I think we as a group of interested people are guessing because we do not know what happens below the table. The behaviour of the dust stream below the table has to be known if we are to take measures to improve things on all saws. I also think a lot of us over think the issues involved and have said that before, no one agrees with me but that's life.

[David Kumm]

I think the problem for Ian and me is the sheet metal shrouds amount to not much more than a channel down into a hose. Most of the dust stays in the machine but it isn't easy to modify the design vs the full cast iron shrouds that are integral to arbor and motor mounts. My easy way out solution was a 4" sharkguard and a high pressure blower to move 8000 fpm through both ports. Dave

[Chris Parks]

David, you have just simplified two pages of discussion. I wonder why we have solid tops on TS's, a perforated top through which air goes through like a down draught table sounds good to me. It would be nearly impossible to entirely block the air but then I am talking of conventional fixed tables, not sliding versions.

[ian maybury]

It sure helps, but there's places where even 8,000cfm is probably going to struggle to capture dust without the help of some sort of effective shrouding David - for example high speed stuff coming off the top of the blade. Or when the top guard is positioned 6in above the cut - the inlet area becomes so large that the inwards flow slows right down.

Nothing (much) for example would end up in the top guard if the bottom chute actually worked.

Shroud/guard layout, and the creation of higher air speed choke points seems always to matter.

The (fond) hope in starting the thread was that some knight(s) in shining armour would step forward to say that they have spent 20 years investigating dust shrouds, and that the key points in any layout are xyz. Or that we might get some sort of pooling of experience going that would point to what the likely critical success factors are.

Here's a shot at one. The picture starting to emerge suggests that within reason no matter how much air you put through it the bottom chute doesn't properly clear the dust from the tooth gullets.

This if true suggests the need for some new method to be incorporated in dust chutes that can achieve this. We mentioned cross wise air movement as on option before, but it's by no means certain that the low pressure available from a dust system can generate an energetic enough flow to really work. Chances are that putting a 45deg angle/chamfer/single sided knife edge on the blade gullets might help too....

ian

PS the possible issue with table perforation Chris tends to be that it'll probably stick sheet material to the table. It probably won't clear the teeth either...

[David Kumm]

Ian, given the difficulties with improving the shroud, I'm wondering if it would be more effective to plug up all the openings between the top and the base, bottom if you don't have one- etc. other than what you need for make up air and take that from the front of the saw so in effect the only dust escaping will be from the blade rotation which is picked up with the overhead. Dust that stays in the cabinet and can't get out won't hurt you. Dave

[ian maybury]

That's definitely a promising option I think David. The K3 doesn't seem to leak much dust inside, but against that there quite a large (couple of inches?) gap between the top of the chute, and the underside of the table/slider.

I know the guys on FOG advised concentrating on the top guard, and it's clear that clearing most of the dust out of the blade gullets in the bottom chute is a bit of a tall order - but it'd be a far more elegant solution if it could be pulled off. It'd make the consequences of having to remove the top guard for certain cuts pretty minimal too.

The question is what to do with the chute on the K3 - it really only covers about the first 1/4 of the blade circle, and relies on a tight slit through which the blade exits it to create a fast inwards flow and hence a seal. The second quarter is fully exposed, albeit inside the cabinet. i.e. to fully box in the lower blade like the cast dust chutes on the bigger Felder machines may pretty much require removing it and starting from scratch.

It's time to take a close look at it. There's no rush as it's not doing a bad job at present..

ian

[Ted Friesen]

I have a Grizzly 715P Hybrid table saw. With a 6 inch hose connected to the cabinet port, I have only a small amount of dust collecting in the corners, but the floor of the cabinet is completely clear. I also fabricated an articulated overhead dust extractor using a PSI blade guard connected to a 2½ inch hose. Both the 6 inch and 2½ inch hoses are attached via one blast gate to a Grizzly G0440 Dust collector that is rated for 1,354 CFM. There is lots of air flow! But, I'm still not happy with the amount of saw dust above the table especially when using a cross cut sled, which means I can't use the over arm dust collection, which means that the below table post is only able to extract the dust that the blade takes below and not the dust that the blade throws at me from above. I've come to accept that the pockets of my shop apron will always have some saw dust in them.

[Jim Dwight]

I use a Ryobi BT3100 which has a pretty good duct to channel most of the dust outside the saw enclosure. It came with a bag that worked reasonably well but my shop vac hooked to the 2.5 inch dust port works better. I use refrigerator magnets to close off other openings in the base to direct airflow around the blade. I also use a zero clearance plate that is tight to the blade on the front but deliberately opened up to 3/8 or even 1/2 inch on the back half. I did some testing years ago of holes in the plate and wider slots and found that a wider slot resulted in less dust on top of the saw. But you don't want stuff falling in, so there is a limit.

A lot of dust comes off when the blade is not buried in the wood. If you are taking a little off the edge. I don't think anything over than overhead collection will solve that. Track saws have the same problem.