Thread: Life Ain't meant to be Easy

I think that most of us run (or would like to) 150mm as the main trunk with 100mm branches/risers from the machines. I can't say that I know of anybody running 5" (125mm) ducts.

Most machines have both a minimum CFM air-flow and a minimum air-speed they need for scavenging and for most of these a 4" port is all that's needed with a std. DC... so that's all the mfr's provide.


FWIW, I think there may be a table for general CFM values on Bill Pentz' website, but I'm not sure on the air-speed. I'm pretty sure it's around 60-70 kmh from the average machine and for vertical risers - thicknessers, etc. need more - and about 45 kmh for the main horizontal trunk.

Good luck with the sheet metal work.

Originally Posted by Skew ChiDAMN!!

I think that most of us run (or would like to) 150mm as the main trunk with 100mm branches/risers from the machines. I can't say that I know of anybody running 5" (125mm) ducts.

Skew, yes and you would have to ask why CT would import a machine that has a 5" opening. Sell it and who gives a damn is most probably the answer to that one.

BP makes a big point of using 150mm right to the machine and I think he is right. As soon as a 100mm duct is introduced that is what the duct operates at and the 150mm becomes a decoration. Air speed is god when it comes to dust extraction as the slower the air moves the less dust can be suspended in that air. Also, if the machine cannot supply the air by replacing it through vents or cavities/openings in the machine body the air will slow down also.

I just looked at my Jet bandsaw and it is going to be an interesting exercise getting a 150mm duct into that, though of all the machines that may not be necessary. The tablesaw is easy as are the rest.

I too have looked at a few of the dust collecting systems along with the ports on machines and felt frustrated.

I think the critical thing to remember is that you need to maintain both air speed and air volume.

As far as I can work out, on any system, the minimum pipe diameter needs to be 150mm in order to maintain both sufficient air speed and volume to remove fine dust.

If a machine only has a port less than 150mm then you need to either:

a) introduce a shallow angle "Y" piece to split the airflow to your machine to maintain the same overall volume as your minimum trunk diameter.

b) introduce a shallow angle "Y" piece to split the airflow with one line to your machine port and one to create a "bleed" so the the total air volume and speed is maintained.


c) enlarge the port size and any subsequent pipework on the machine so that it is 150mm

My reading leads me to the understanding that any constriction (or sharp bend) of a pipe no matter how short will result in a permanent reduction in airspeed/volume that will severely compromise the efficacy of the overall system.

You also have to take into account the fact that two smaller pipes will not flow the same amount of air at the same speed as one larger pipe even if their radii sum to that of the larger pipe. This is caused by simple mathematical factors as well as the additional reistance that air flowing through two pipes experiences when compared with the same volume of air through one single pipe. For example: if your main line is 200mm then two 100mm lines would more than halve your airflow!

Originally Posted by Mini

Also, if the machine cannot supply the air by replacing it through vents or cavities/openings in the machine body the air will slow down also.

It always amazes me when I see people using zero clearance plates on router tables and table saws and sealing the cabinets, etc., without giving any regard as to just how the air is supposed to get into the machine to be extracted in the first place.

You may find that you can get a new plate with a 6" inlet just as easily as you can get a 6">5" reducer. I seem to remember several dust suckers that came with interchangeable plates, 2x4", 1x5" & 1x6", so if the hole in the impeller casing underneath the adapter plate is 6" or greater, you just have to remove the screws (or pop rivets?) and get the local sheet metal place to make a matching plate if you don't feel like making your own.

The only problem will be if the inlet pipe comes in from the side at 90 degrees to the impeller shaft rather than parallel to it - not common but I've seen them, usually in DIY setups - it really belongs in a cyclone separator but some people get confused & meld the two designs with unhappy results.

Also remember this. There is nothing a roll of dust tape can't do.