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-   -   AIR in AIR out - analysing the air flow at the collection point (

Besser Mon 17 August 2009 20:56

AIR in AIR out - analysing the air flow at the collection point
Removing the dust from the cut and cut area seems to rely on the material laying in suspension in the air mass within the curtained foot print and that air mass being extracted. What I find "strange" is the airmass being removed needs to be replaced by sucking new air through the curtain. This airflow path isnt anywhere near optimal for:

A:getting cut material into suspension
B:removing the Air mass next to the actual bit

Have I missed something??

Just wondering if anyone is running a air injection system, directed at the cutting point?

Is this a worthy thread or is the current design satisfactory?

Doug_Ford Tue 18 August 2009 08:21

That's a good idea Besser. I've got a pretty powerful dust collector and 6" duct work but I still sometimes have some dust left in the narrow kerf. Maybe I ought to consider drilling a hole or two in the plastic shield I installed to divert the air from the router's fan.

Gerald D Tue 18 August 2009 09:05

I think a good curtain is just "breezy" enough to allow a good draft of air to convey the dust to the collection system. If one wants a sealed curtain/skirt, then it will be prudent to inject air at the side of the cutter furthest away from the hosed connection (so that the air flows across the dust source), but isn't it just as easy to have a hole in that section of the curtain/skirt?

lumberjack_jeff Tue 18 August 2009 17:43

My gut feel is that, given how much dust my routers fan kicks up, keeping the dust under the router into suspension isn't that difficult. Further, given that the dust collector is moving roughly 10x the air as the router...

I think that the key is keeping the vertical space between the dust collector and the workpiece minimized to keep the velocity up.

When I build a dust collector foot (very high on the list of priorities) it'll be height adjustable from the y-car and independent of the z slide.

servant74 Wed 19 August 2009 16:48

Being a dilettante in this area, I have wondered if it is reasonable to make an 'air sweep', blowing from one 'end' of a dust foot directed across the cutting area toward the vacuum end of the dust foot. If cutting very deep, a possible compressed air pointed toward the bit/cutting area but directed at an angle, still blowing the wood toward the vacuum end.

Just a thought.

Besser Wed 19 August 2009 19:04

I think Jeff has got it in terms of Air speed, the lower the cross section of foot clearance, the higher the air speed will be for the same vacuum pressure (remember the old PV=nRT). I also like the feed of compressed air idea and this is what the vacuum is creating on the exhaust side of its vane (almost like recirculating). Max air speed should be reached at the cutting tip so the foot needs to be as low as possible at that point. Jeff you could even raise and lower the curtain depending on the current draw (amps) of the blower.

Greg J Wed 19 August 2009 20:49

1 Attachment(s)
For the record.

My DC foot is a brush with a "door" at the front.

Attachment 5816

The DC system is a high volume cyclone using 6 inch ducting. It works great with zero fine particles and minimal chips (larger than particle size). I usually operate with the "doors" on the DC foot open. I just have to watch that bit going back and forth, up and down, back and forth, up and down .......

Do CNC operators lose their minds at a faster rate than the general public ?? ;)

Alan_c Thu 20 August 2009 16:15

I cant lose something I never had! :D

Besser Sun 06 September 2009 06:36

Hey Greg, like the pic (and flags) do you do deep plunges? How does the dust exit those deep pockets?

Seems to be a clear passage between the exit pipe and the "front door"

Greg J Mon 07 September 2009 10:42

Hi Besser,

If my memory serves me right, I believe the deepest plunge was about 1.00 inch (25.4 mm, [had to really think about that conversion]) The DC removed about 90% from the "slot". As the DC makes multiple passes, more debris is removed.

Originally Posted by Besser View Post
Seems to be a clear passage between the exit pipe and the "front door"
Not sure what you mean by "exit pipe".

Besser Tue 15 September 2009 17:39

Greg the "exit pipe" I was refering to is the ducting tube that comes off the foot..

I do like the minimal complexity of the setup and I suppose if it works don't change it.

inventall Tue 15 September 2009 21:39

If I follow the bit on my machine with a shop-vac holding it 1 inch to the bit there is almost no dust escaping. Even if I have the hose end in front of the cutter. And that is with a 2-1/2 inch hose and the thin tapered end on it. A router should only require a small hose, unlike a wide-belt sander lets say. The problem seems to be getting the suction close enough to the bit. Just food for thought.

sailfl Wed 16 September 2009 03:49


With my little experience in dust collection and what I have read, you need more than 2.5" hose to get the best collection. I have 4" and a dust foot that does not have a skirt and I am getting good collection. No system is going to get it all. I am very happy with my set up. Greg J likes his 6" set up. So there are a number of builds you can look at.

Besser Mon 14 December 2009 20:52

While a 4" extraction duct suits a balance between drag and buildup, the inlet works on the cubic volume rule. The further the stand off of the dust foot the greater the motor size to maintain the same air speed. A large stand off of the duct foot is counter intuitive as the cutting unit is normally very small for a router. High speed air is fundamental in picking up and catching cut material, reducing the square area of the inlet allows for higher air speed, then opening to a 4" duct suffices for transport.

When using brush curtain design, it slows down air flow due to the large foot print and offers an air path that totally misses the cut point. On the plus side the curtain makes any groove it passes over a high speed duct between each side of the curtain so can clean out these cut tracks.

High speed air at the cut ejection is a priority for efficiency.

servant74 Tue 15 December 2009 14:38

Cutter clearing air
I had been wondering if putting an air-compressor nozzle inside any 'dust boot' on the side of the cutter opposite the vacuum intake and 'blowing' air directly at the cutter would help chips clear better by getting the 'airborne' so they are more likely to be picked up by the vacuum. It would take shop air in addition to the normal vacuum.

After this discussion (and others) I have been thinking that providing air through the cutter (like some liquid coolant is done) wouldn't be best. But it would take modifying both the router/spindle and getting high $$ modified or specialty cutters.

Besser Tue 15 December 2009 19:49

Jack, this was on my mind as well. but then the curtain is less effective by the amount of shop air you are introducing.

I'm thinking of a new foot design to move the aperture opening closer to the cutter and smaller in diameter to increase velocity.

servant74 Tue 15 December 2009 20:08

Yes, a close fitting duct with a vacuum port near the cutter will help optimize the collection. Keeping up the total velocity of the air as it passes by the cutter to the vacuum port. The more area/volume that needs to keep up the airflow the more suction is needed, and a smaller diameter hose and/or opening will further restrict the airflow.

How much is enough is a real balancing job. I know starving the vacuum of air reduces its effectiveness also. ... [ As an aside: To relieve the vaccum starving for air, I have thought of a spring loaded gate (probably near the floor so as to help with picking up debris even when a floor port isn't needed otherwise) that would open if the vacuum pressure got to low indicating the vacuum fan is starving for air. How much vacuum is optimal is something I don't know currently. Might need to ask Bill Pentz or similar.]

MetalHead Wed 16 December 2009 06:44

2 Attachment(s)
You could take coolant hoses and an Air Regulator and setup a low pressure blast system that hooks to the dust foot.

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