Thread: Sanding hood

Seems like a good idea with a sanding hood.

Originally Posted by robo hippy

By 'dusty' I am guessing that you mean the dust collector. Nice thing about the newer ones with the pleated paper filters, they filter out down to 1 to 0.5 microns, so not a problem as long as all joints are sealed. My old cloth bag on my little 1 hp dust collector would send off a small mushroom shaped plume every time I would fire it up. It may not get 100%, but I would bet that it gets 99.99%.

I would be careful about betting, most standard pleated filters for dust collectors are only about 90% efficient at 1 micron and 70-80% efficient at 0.5 microns. There are special filters for cyclones and dust collectors that will go to 99.5% but 99.99% is another league altogether. It may sound like there is very little difference between 99.99% and 99.5% but in terms of dust filtration it's a factor of 2 times more effective at cleaning up dust.

Another test I have heard of, and don't know how accurate it is, is that if you can smell the wood, you are breathing in particles. I can not smell the wood. .

This is not a valid test. In most cases a human nose quickly becomes desensitized to smell. I completely lost my sense of smell for 6 weeks working with MDF outside. It ruined my sense of taste - beer tasted like soap.

Figure less than 4 sq. ft of open area/funnel area. 55 cubic feet per minute flowing through that 4 sq. ft. That is just over 6 cubic feet per second moving around you and into the hood. That is a lot of flow in a small space.

55 cubic feet per minute = 55/60 = ~1 cubic feet per second.
55 cubic feet per minute is actually a very very slow volume transfer rate. Bill Pentz recommends 1000 cubic ft per min with a linear air speed of 4000 ft per minute as a minimum to remove fine dust at source.

Oops, I should proof read better. The smaller 1 hp dust collectors move around 500 cfm, not 55. The size of the funnel opening is probably as important as the air flow.

robo hippy

Originally Posted by robo hippy

Oops, I should proof read better. The smaller 1 hp dust collectors move around 500 cfm, not 55. The size of the funnel opening is probably as important as the air flow.

robo hippy

No, air flow as Bob points out is the premium part of the equation. A duct at of a given size and at a given rate of flow has what I call a "catchment bubble" at the entry to the duct. It is the size of this bubble that is critical and it is not as big as we might initially think

From BP's site and with thanks for his efforts.......


At first these larger air volumes do not make sense because we know it takes 50 FPM to move the fine dust and 4000 FPM to move the heavier sawdust and chips. We already know from experience why it takes more air. When we use our shop vacuums they only pickup up right next to the end of the hose. The reason is unlike blown air that holds together for quite a distance, air being pulled or sucked by a vacuum comes from all directions at once. This means that the area being pulled from is roughly a sphere. It also means that our airspeed will fall off at roughly the same rate as that sphere area grows. Airspeed for sucked air falls off at roughly the same rate as the area of a sphere expands given by the formula Area=4*Pi*r^2. Most air engineers target for a duct speed of roughly 4000 FPM because this is what we need to pickup most woodworking dust and keep our vertical ducts from plugging. If we use that 4000 FPM in our air formula where FPM=CFM/Area we can compute how much air is moved in different sized pipes. We can then divide those airflows by 50 FPM to see how big of an area each will cover, translate into square inches then convert to the surface of a sphere. Although the math is fun, the bottom line is airspeed drops below the 50 FPM we need for good fine dust collection very quickly. Our 2” duct at 4000 FPM only supports 87 CFM and that turns into less than 50 FPM at only 4.47 inches from the center of our duct. Our 2.5” standard vacuum hose only supports 136 CFM at 4000 FPM which turns into only 50 FPM at 5.59” from the center of the hose. This explains why we see almost no pickup just 2” inches from the end of our vacuum hoses. A 4” duct at 4000 FPM airspeed only supports 196 CFM which turns into less than 50 FPM at about 6.71” from the center of our hose. A 4” duct that only supports 349 CFM at 4000 FPM only gets 50 FPM about 8.94” from the center of our duct. Many round this to 9” and use this as the standard for 4” duct. Our 5” duct that supports 545 CFM at 4000 FPM only supports 50 FPM out to about 11.18 inches. Our 6” that supports 785 CFM at 4000 FPM only supports 50 FPM out to about 13.42”. And, our 7” duct which supports 1069 CFM at 4000 FPM only gives our needed 50 FPM out to about 15.65”. In short we need to move a lot of air to ensure capturing the fine dust.

End of quote

The above is a bit of a hard read at first or even a bit confusing for some so read, think about it and then re-read as needed. A 6"/150mm duct only has a collection area of a bit more than 300mm at a the premium air speed we require for good collection. No matter how good the hood is it can't trap all the particles long enough for the duct to pull them in if you have a small air flow and four inch duct is generally regarded as too small.

If anyone thinks they are getting good DE from 4" I will lend them my Dylos dust meter when I get it and they can publish the results. The OP has made a very good and well though out dust hood that other could do worse than copy and good on him for showing us, it just needs more air flow. If you need to wear supplementary breathing protection I guess that says it all.

We need to build on efforts like Robo has shown us and improve them as we go. I would run two six inch ducts into that hood spaced around it and I bet you still would not capture all the dust as it is plainly impossible unless the work piece was fully enclosed. For the sanding operation why couldn't it be more enclosed with a drop down section from the top? Dust would be still adhering to the workpiece as it turns and getting flung out the top and front IMHO.

I like the term 'catchment bubble'. This is the reason for a hood/funnel. The catchment bubble is inside the hood, which defines it to a certain area. Sure, if I have a 5 inch hose with 1000 cfm. air flow, if I drop a spoon full of dust 12 inches away from it, most will fall to the floor, and some of the air born stuff will find its way into the hose. Same with the hood, but with the hood, what falls to the floor falls inside the hood, and everything else goes into the hose. There is some throw out from the power drill, and from the spinning bowl, but for me, I am a fan of slow speed sanding, both the drill (600 or less rpn), and the bowl (15 rpm). This keeps anything from going outside the hood area.

robo hippy

Thread from the dead time. Robo, the video has disappeared from the link in the first post, is there any chance of us seeing it again as I recall it was a very good answer to the problem. Thanks.

Hi Chris

Check it out on You Tube.

Cheers,
John

Chris, is this [ame="http://www.youtube.com/watch?v=AZsVc7qVx7A"]it[/ame]?

Thanks Pat, that's it.

Originally Posted by Mini

Thread from the dead time. Robo, the video has disappeared from the link in the first post, is there any chance of us seeing it again as I recall it was a very good answer to the problem. Thanks.

Fixed now.

Thanks .

Thanks for fixing that up. If you do lose it again, you can type in robo hippy on You Tube and it will come up.

I did see a guy on Wood Central Message Boards, who took a funnel (bowl with no bottom, and put his dust hose on the back of it, and put that on an articulated arm on his wall so he could move it in to use, or away as needed. Much classier than the plastic big gulp hoods.

robo hippy

Robo, do you have vision of the hood in operation?

Originally Posted by robo hippy

Thanks for fixing that up. If you do lose it again, you can type in robo hippy on You Tube and it will come up.

I did see a guy on Wood Central Message Boards, who took a funnel (bowl with no bottom, and put his dust hose on the back of it, and put that on an articulated arm on his wall so he could move it in to use, or away as needed. Much classier than the plastic big gulp hoods.

robo hippy

I reckon that two 150mm ducts off the back of that hood would be pretty damned good.