Two 4" dust ports or one 6"?

[Dan Case LR]

I'm creating a dust hood for my SCMS (a Dewalt DW708) that's mounted on a Rousseau 2875XL stand. I came close to popping for the Rousseau DownDrafter (they're available again) that was designed for this stand, but just couldn't bring myself to spend $149 for it. Instead, I'm making something loosely based on the DownDrafter but a little bigger. I never found dimensions of the DownDrafter, but I'd say my solution is a LOT bigger--the device I'm using as a dust-catcher is a 45-gallon storage container made by Sterilite that's almost three feet wide. The only time I'll need to adjust the position of the bucket is when moving from one extreme (45 left) to the other (45 right).

I'll be interfacing this with a 6" PVC trunk from my DC. If I put a single 6" port in the middle of this 36" box, I worry that it might not capture dust from the far ends. Would I be better off splitting it to a pair of 4" ports evenly-spaced? Should I come up with some sort of "trough" under the box that connects to the single 6"?

Any thoughts or ideas welcome.

D.

[Alan Schaffter]

I'm creating a dust hood for my SCMS (a Dewalt DW708) that's mounted on a Rousseau 2875XL stand. I came close to popping for the Rousseau DownDrafter (they're available again) that was designed for this stand, but just couldn't bring myself to spend $149 for it. Instead, I'm making something loosely based on the DownDrafter but a little bigger. I never found dimensions of the DownDrafter, but I'd say my solution is a LOT bigger--the device I'm using as a dust-catcher is a 45-gallon storage container made by Sterilite that's almost three feet wide. The only time I'll need to adjust the position of the bucket is when moving from one extreme (45 left) to the other (45 right).

I'll be interfacing this with a 6" PVC trunk from my DC. If I put a single 6" port in the middle of this 36" box, I worry that it might not capture dust from the far ends. Would I be better off splitting it to a pair of 4" ports evenly-spaced? Should I come up with some sort of "trough" under the box that connects to the single 6"?

Any thoughts or ideas welcome.

D.

That is a hard one. From a purely numbers standpoint one 6" is better that two 4"- the cross sectional area and CFM standpoint a single 6" pipe at 28.27 sq. in., will flow more air than two 4" at 25.13 sq. in (2 x 12.56 sq. in.) plus the friction is lower due to less pipe surface (circumference) in a 6" pipe at 18.84" vs two 4" pipes 25.13" (2 X 12.56").

That being said, it is always better to have the point of suction as close to the source of dust as possible to prevent it from escaping. If that means moving the 6" as you swing the mitersaw or having two 4" ports you'll need to experiment.

As I said in another post I think the best solution to the mitersaw is to (1) connect a shop vac to the blade port, then (2) have the DC connected to a hard enclosure with clear tenting in front so almost the entire saw is shrouded. The tenting contains any dust that wasn't picked up initially long enough for the DC to eventually suck it up. To see what I mean check out Tymes Dust Free Workshop video. (http://www.youtube.com/watch?v=pKSqjPuR1k4). The best shot of the mitersaw enclosure comes at the 7 min. mark.

[Chris Parks]

http://www.youtube.com/watch?v=W9Ll10MZMmg

[Alan Schaffter]

http://www.youtube.com/watch?v=W9Ll10MZMmg

I've seen that before and always wondered how big of a DC he has. Judging by the sound and dust being dropped out front it is really pulling some CFM!

In reality THE BEST rig to collect dust from a mitersaw would be to set it in front of a large window fan and send the dust outside. You get about the best CFM and capture the most dust that way.

[Dan Case LR]

I've seen that before and always wondered how big of a DC he has. Judging by the sound and dust being dropped out front it is really pulling some CFM!

In reality THE BEST rig to collect dust from a mitersaw would be to set it in front of a large window fan and send the dust outside. You get about the best CFM and capture the most dust that way.


According to his website (linked from the YouTube page) he has a 3HP Grizzly cyclone.

http://www.workshopaholic.net/workshop/dustCollection.html

D.

[ian maybury]

It looks like the sort of territory where the task is (1) to physically intercept the flying dust, to then (2) guide it to a collection point, and (3) suck it away from that point.

Sounds like it might be one of those scenarios where lot is down to specifics. Chris' video shows that some can get deflected into a location where the 'suck' can't pull it in. On the other hand if the hood is really good at (by gravitational and physical means) guiding the dust to a collection point then removal may not need a lot of airflow.

Only thinking aloud, and there may be problems - but some sort of hopper or downwards pointing pyramid shaped inlet under the saw forming the floor of the hood might maximise the use of gravity - resting the saw on a sheet table may increase the odds of chips getting hung up. Maybe the saw could even sit on cross members to allow air flow around it.

Against that there's also the issue of 'drifting'/airborne dust. As ever getting control of that probably means getting 50ft/min or better inwards air flow across the entire collection zone - this seems likely to rapidly head for the higher CFM numbers. Related to this is perhaps the question of how far forward the hood entry can be moved - the closer it can get to 'wrapping around' the blade (which isn't possible) the better it may do.

Sorry not to be more specific Dan, but the 1 or 2 inlet question probably depends on how good a job the hood does, and how it's laid out. Maybe it'll collect from a single point (and in doing so probably create a more regular inwards airflow), or maybe it'd pay to have a second intake to deal with 'dead' areas or the like.

Maybe some trials with a cardboard mock up and some flexible ducting (while trying the differing cuts you anticipate, and watching carefully how the dust behaves) would be an idea?

ian

[David Kumm]

My SCMS is in a box with a hole behind the saw. Attached is a 10x12 boot with a 6" flex hose. The overhead port just has a 2" vac hose long enough to stay down into the boot. Picks up most dust but the floor of the box still needs cleaning. At least most of the airborne stuff is directed into the box and seems to stay out of the air- at least according to the Dylos. I am pulling about 6500 fpm through the 6" hole though with a big system so that helps. Dave

[Chris Parks]

The approach used in that video is about the best practical approach that I have come across, I use it myself driven by a Clearvue 1800 and it works as good as any method. You can get all technical about this stuff but in essence common sense usually gets it done. I have a customer who is installing a commercial sanding room as we speak and he is degree qualified in this field having engineered and sold fume and dust extraction systems. He asked me how I would design it and after discussion he agreed that the method I suggested would be as good as any. What I am trying to say is that six inch pipe, a 15" impeller/ 3-5hp gets the job done for all practical purposes and this is exactly what BP says. To background the above a bit more when we were discussing the design I had no idea of his qualifications as that did not come out until later.

Back to the Miter saw hood, don't expect good results unless you are using a good CFM capable extractor. Also another thing to be kept in mind seeing as this is an international forum is that American DE equipment is operating at 600 RPM faster that any other due to the 60hz factor of the electricity supply, a fact that precipitated the design of the CV Max. The hood shown works though better on some saws that others due to saw design. Here is the page link from where the link was taken..http://www.youtube.com/results?search_query=miter+saw+dust+collection&oq=miter+saw+dust+&aq=0&aqi=g2g-m1&aql=&gs_sm=e&gs_upl=34507l40420l0l44098l15l14l0l1l1l0l1290l2669 l2-3.2.7-1l6l0

You are never going to collect it all no matter what you do, so being blunt about it, get over that fact and move on. Design the best you can, steal good ideas, weld it all together and see what happens. For instance the overhead collection idea from Allan S which I commonly link to is the best I have seen, but never having used a Shark Guard for instance I can't compare the two but to me it appears to be a good approach and one I am half way through building. Is there a solution that traps more than his? maybe but I bet for all practical purposes it works as well as any other idea. I unashamably pinch ideas unless it is clearly patented but will always acknowledge the original source if possible as Alan knows.

Sorry for the ramble but I don't think this stuff is rocket science for the average hobbyist if the basic equipment parameters are followed, what tends to bring it undone is trying to use short cuts in equipment capability. It bugs me that Fred Bloggs will spend thousands on things to cut and shape wood but won't look after himself health wise by using cheap inferior DE equipment, that is exactly backwards from what should happen. Mind you there are those who think it is all a bit of hot air but as I pointed out recently it is their perrogative and I hope they enjoy the health issues that their decision brings. Has this advanced the discussion? most probably not but I feel better.:)

[ian maybury]

You can safely count me in to the higher CFM lobby Chris....

ian

[Dan Case LR]

And speaking of higher CFM, methinks that's what I need.

I tried a single 6" port on top of the box. With the saw centered (90 degrees), Cutting a 2x4, I'd say it caught maybe 40% of what the saw spewed forth, with the remainder on the floor of the box and around the saw. With a 1 1/2" ID hose on the saw's dust port with the other end taped inside the 6" port, capture improved to maybe 50-60%. I tried the same with the box rotated (the port on the bottom), with less effective results. I believe the combination of the bend in the 1 1/2" hose and the too-sharp 180 degree bend in the 6" hose connecting the box was responsible for the loss in effectiveness.

I believe that an arrangement like the guy in the video has--a transition to a slot at the bottom of the box with the port on top, plus the hose from the dust port--would be the most effective option. I'll try experimenting with a cardboard version, but based on what I've seen thus far I think I'm swimming up a waterfall without more suckage from the DC. The 1 1/2HP DC ain't gonna cut it.

Bottom line is that I'll have to make the most of what I have for now and try to budget for a better DC down the road.

D.

[ian maybury]

Hi Dan. I hadn't picked up you were on low CFM. Getting collection on low CFM (if it's possible) probably as above hinges on coming up with a layout of the saw and the hood such that you can guide the dust and chip by bouncing it off the hood/gravity into the inlet of a relatively small duct. Even then it's not going to move enough air to pull off the 'inwards air movement across the face of the hood to capture floating dust' deal on any normally sized hood either.

I don't want to draw controversy, but realistically a saw hood of this sort is a classic example of a situation requiring quite high CFM to do a decent collection job. The one to be careful of is that given that most hoods of this sort have a fairly large open/entry area it's likely that even with high CFM a properly laid out hood will be required even in this case....

ian

[David Kumm]

Yes Dan, the system you described which is like mine needs lots of CFM. I'm pulling 1200+ and still have some dust in the bottom of the box. At least it stays inside the box and the volume of DC air keeps the airborne stuff pulling towards the box and away from my lungs. Dave

[Chris Parks]

Hi Dan. I hadn't picked up you were on low CFM.

And that unfortunately is a real problem when trying to advise someone on a forum such as this, we don't know what capability the DE has. Someone posts here and asks for advice because his TS has bad dust collection but we don't know what is attached to the other end of the duct or even if the duct is installed properly. Another respondent says I can't understand your problem as my exact same saw has good dust extraction. It is an a point that should be borne in mind in these discussions. The other issue is someone says they have a 3hp extractor that flows 5,000 CFM :eek: not understanding that the quoted figure was done with a blueprinted manometer and anemometer and a lot of marketing thrown in as well. The whole DE market is a minefield of mis-information, hyperbole and downright lies which we have no way to disprove until the whole thing is installed and it does not work. As I have said before most DE just recycles the dust it picks up, it does not pick up all it should in the first place and just spews the fine dust back to the workshop through bags that only hold the big stuff. Damn, I have to get a life and go fishing or something. :(

[Jim Andrew]

I'm wondering if it would help to bring your pipe to the bottom of your dust catcher. Wish all this discussion would have come about before I bought and set up my 2 hp cyclone. Works pretty well, but 3 hp would be better. There was a post here a few years ago where a guy said his 5 hp cyclone would suck the chrome off a bumper hitch.

[Chris Parks]

Have a look at the videos I linked to and you will see what he meant by that statement.

[Dan Case LR]

Update: I've played around a bit with a cardboard baffle to bring the 6" port down to a 1" slot across the bottom of the box. Even with the 1 1/2HP DC, the results are a substantial improvement. The biggest problem is that the degree of suction present makes it difficult to keep the cardboard from collapsing. I may try a more permanent version of this at some point, but for the moment I have way too much else to get off my bench first.

However, I've learned another important point about SCMS dust collection: The blade makes a HUGE difference. Admittedly, the stock blade on my SCMS has taken a beating. Now that I'm done with cutting PVC pipe with it (at least for a while), I decided a new blade was overdue. I put a shiny new 80T Forrest Chopmaster on it last night, and the improvement in dust collection is substantial. The Chopmaster is throwing most of the dust in the same direction--right into the saw's pitiful excuse for a dust chute.

And I'm making fast cuts that are glass-smooth and need little or no sanding. A genuine win-win!

D.

[Chris Parks]

Update: I've played around a bit with a cardboard baffle to bring the 6" port down to a 1" slot across the bottom of the box. Even with the 1 1/2HP DC, the results are a substantial improvement. The biggest problem is that the degree of suction present makes it difficult to keep the cardboard from collapsing. I may try a more permanent version of this at some point, but for the moment I have way too much else to get off my bench first.

The area of the slot must equal the area of the 6" duct or you will have a choke in the system.

[Terry Hatfield]

Another idea for you to consider....

Here's what I did for my Hitachi. I used 6" PVC to a chip "corral" I built on the back of the bench. The 6" PVC is attached to an HVAC offset register boot and the wings of the corral are attached to the boot. I then ran a seperate 4" line to the port on the saw. The Hitachi has a bigger port that most but not 4" for sure but running the 4" there gives a good duct speed which really helps pickup the maximum amoutnof chips at the saw port. I'd say that this system captrues 85% of the chips but as has already been noted, any setup that is this wide open needs maximum cfm. My cyclone produces a little over 2100cfm and I could still stand some more for the SCMS. I feel like any good solution will have to address the port on the saw as well as some kind of hood or surround for sure.

223217

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Hope that helps and doesn't just muddy the water further.

Terry

[Michael W. Clark]

Hi Dan,
The more CFM, the better. I have a 1.5 HP DC that, as Chris mentioned is rated to 1100 CFM, but probably pulls about half that or less once connected to hoses, ducts, and hooding. I have a 10" CMS and built an MDF hood behind the saw. The saw sits on a bench against the wall. The hood is rectangular with sides and a top. The saw is recessed into it as much as possible without interferences. I put the dust port in a trough at the bottom of the hood and it is recessed below the base of the saw. The dust port on the saw has a deflector that throws the dust downward toward the dust port. It is no where near high efficiency, but is better that connecting to the saw port only with a shop vac and it does get most of the fine dust. Some of the large dust has too much velocity and does not get captured and lays in the trough.

Maybe something here you can use, but I know yours is different with having the sliding function.

The blue tape in the picture is over a 220V outlet. My floor plan changed after I buit the shop. Ooops:)223233223234