# Mitre saw dust hood which size hose



## Lazurus (10 Feb 2022)

Ok so i am going to build a dust collection hood for my mitre saw station, however before I start cutting holes foe the extraction what is best 32, 63 or 100mm I can use a van or chip extractor?


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## artie (10 Feb 2022)

I'd say the bigger the better, provided your extractor can shift enough air.


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## hlvd (11 Feb 2022)

I’d say the biggest diameter your extractors rated for.


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## robgul (11 Feb 2022)

I have a Y piece in the hose that has two flexible hoses (just 40mm) inside the "hood" - one is connected to the dust port on the saw and the other cable-tied to the back of the fence beside the blade - the second one near the blade improves things - but dust collection from a mitre saw isn't good at the best of times IME


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## sploo (11 Feb 2022)

Based on "hood" and "mitre saw station" I'd assume this would be a reasonable size. As such, you'd want something that could move a lot of air - so 100mm hose and the best/biggest HVLP extractor you have. A vacuum is highly unlikely to be able to move enough air to capture the dust.

If you undersize ducting/hoses for a low pressure (LP) extractor you kill the airflow. If you oversize the ducting/hoses you drop the air speed and dust will fall out of the air stream (e.g. trying to put a 100mm hose on a vacuum won't work well).

Note that most chip extractors won't capture fine dust in their bags/filters; so you risk making a dust pump (i.e. the extractor just pulls dust from the tool, then sprays the really fine - dangerous - stuff through the walls of the extractor bag).


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## Lazurus (11 Feb 2022)

I have the above in the workshop also a smaller vac with a 4" hose or a couple of Henry`s I think the above should be able to cope with the fine dust as well, so a 4" hose it will be. Many thanks all.


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## Inspector (11 Feb 2022)

You should follow hlvd's advise in the third post. A 4"/100mm duct can flow about 400CFM/680M3hr. A 5"/125mm roughly twice more and a 6"/150mm triple the 4"/100mm duct. So pull off the wye and put on a duct the size of the one on the Cyclone, keeping the length to the mitre saw a short as possible. The wye just in front of the cyclone introduces turbulence going into the cyclone and reduces the separation efficiency too.

Pete


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## Lazurus (11 Feb 2022)

Inspector said:


> You should follow hlvd's advise in the third post. A 4"/100mm duct can flow about 400CFM/680M3hr. A 5"/125mm roughly twice more and a 6"/150mm triple the 4"/100mm duct. So pull off the wye and put on a duct the size of the one on the Cyclone, keeping the length to the mitre saw a short as possible. The wye just in front of the cyclone introduces turbulence going into the cyclone and reduces the separation efficiency too.
> 
> Pete


Great advice Peter I will give it a try, I didn't realize the difference in going up just 25mm to 125. much appreciated


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## sploo (12 Feb 2022)

If you have the AW118CE you're in a great position. As advised by others; for maximum "flow"; remove the Y and get the right sized hose for the cyclone inlet straight into a hood and you'll be laughing. Directly hooked up you should get loads of air flow, and with the cyclone and filter the exhaust air should be pretty safe too.

If you can find some sort of interface/clip you could then quickly detach and hook the extractor up to other machines. Granted it's not as convenient as having ducting everywhere, but for a relatively small extractor like the AW118CE it's probably a good idea to avoid loses by using runs of ducting.

I don't have the CFM data to hand like Pete, but just the area difference between 125mm and 100mm ducting is 125^2 / 100^2 = 1.56 (i.e. 1.56x the area for air to flow). If it is twice as good in real terms I guess it might be down to boundary layer effects being worse with a smaller area. Regardless; even a small increase in diameter results in a good increase in area and therefore airflow.


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## Inspector (12 Feb 2022)

sploo said:


> I don't have the CFM data to hand like Pete, but just the area difference between 125mm and 100mm ducting is 125^2 / 100^2 = 1.56 (i.e. 1.56x the area for air to flow). If it is twice as good in real terms I guess it might be down to boundary layer effects being worse with a smaller area. Regardless; even a small increase in diameter results in a good increase in area and therefore airflow.



It is the boundary layer effect sploo.  

Lazurus try to keep the flex hose to a minimum as it has about three times the drag as smooth pipe. A 3m hose has the same losses as 9m of smooth duct. The corrugated walls of the hose make more turbulence. 

Pete


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