Bell End Hoods to improve air flow efficiency

R

Rushton

New member
Has anyone experimented with building bell shaped hoods for the end of collection pipes or the connection inside cabinets?

Bill Pentz refers to using these in the section of his website on Tool Ducting under Dust hoods. At the Australian Woodwork Forum, there is a very active section on dust collection and one member, Bob Loss (BobL), posts extensively about improving dust ports and the connections to our machines. He is a big advocate of using bell shaped hoods to improve air flow for fine dust collection by as much as 20%. His principle thread on the topic where he discusses the four typical connections into a machine cabinet and then shows how to create bell shaped hoods in both 4" and 6" PVC pipe using a mold is here:
Improving machine cabinet dust ports

And the post within that thread where he shows his molding process is here:
Creating Bell Shaped Ports with a Mould

Two examples of the dust flow created with a bell mouth hood are illustrated by the sawdust flowing off of the wood being sanding in these two videos:

Dust Capture.JPG
https://www.youtube.com/watch?v=xMlX1wavcWs

Dust Capture2.JPG
https://www.youtube.com/watch?v=xjtHCmR-N3M


Some illustrations BobL provides in the FAQ - Dust Extraction (Practical Aspects) - well worth reading the entire FAQ!


Normal BMH.JPG
BMH compared to Naked Duct.JPG
 
Interesting post as I ponder the best way to make a miter station dust collection hood for my saw. This is one messy saw to use but extremely useful. It allowed me to get rid of my old radial arm saw and I'm just finishing up my 10'+ saw station with 5 base cabinets and 20 drawers. I'm now trying to figure the best way to collect the saw dust it produces. I've come to the conclusion the hood will be 1 to 1-1/2" taller than the sliding arm and wide enough to cover the extreme left/right swings for angled cuts plus 2-3". Then I'll taper the sides with 1/4" plywood baffles to direct the sawdust to the DC pickups. I am going to use the same blast gate I used on the router table to split the one 6" to two 4" hoses and have those about 8" apart. My thinking is this will be better than a single 6". I'll close off the front as close as I can so it doesn't interfere with the swing of the saw. Hopefully after all the work it will perform well. I don't expect 100% pickup but a reasonable amount hopefully.
 
I'm planning to make a similar dust hood for my RAS. From what I've read about bell mouth hoods, using a radius wherever one's duct terminates through the wall of the dust hood should improve the efficiency of the airflow into the duct and extend the reach of the suction. I plan to mount the 6" duct through a 2" MDF panel that will give me enough thickness to create the recommended radius in the MDF and shown in the diagram above. With the bell mouth hood, I'm thinking a single 6" will be better than two 4" for my application.
 
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Unfortunately, no. I've gotten diverted entirely to other family matters. Hope to get back to this later this fall.
 
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