Thread: To branch or not to branch extractor hose to table saw

Originally Posted by mordrax

Disclaimer: Apologies if this has been asked before.

I have a single 4" connection coming out of my impeller that runs for ~4m via steel pipe across the ceiling of my shop and I want to connect it to my table saw on the other end of the shop (for space/sound reasons, this was the closest i could put the two).

My options are:
1. Connect the 4" directly to the hood
2. Connect the 4" directly to the cabinate (which i'm going to have to build)
3. Have a 4" to 2x4" Y connector and connect to both the above options.

I'm leaning towards 3, but my concern is, because the inlet is only 4", is there any point to have a branch? Would it reduce the cfm that each sucks by half? And if it does do that, then is just having one connection more effective?

I've read elsewhere (from BobL methinks) that a 4" connection only sucks 400 cfm max, logically then, if the main 4" pipe can only suck 4", then the branches should be doing 200cfm right?

The primary limit to your systems sucking capability is indeed set by the 4" duct coming out of the impeller at ~400 cfm.

The next limit will be the restrictions imposed by the TS cabinet. These are usually so restrictive that unless you provide additional openings in the cabinet (usually opposite the location of the dc connection) you will be lucky to get 200 cfm into the ducting. The good side of this means if you put a 4" connection to the hood above the saw you could get nearly 200 cfm at the hood which is better than nothing but still will let a lot of fine escape into the shed.

Another problem is 200 cfm through a 4" pipe equates to a linear air speed of 2300 fpm.

This has two problems,
1) It will not pick up fast flying chips (although this does not really matter health wise) and
2) if the air flow stalls the sawdust may not be resuspended on resumption of airspeed and it will clog the ducting

To be sure about grabbing you need is 1200+ cfm split 800 cfm to the cabinet (6" duct needed) and 400 to the hood).

Going back to your original question I would go for option 3 as the best of those three options.

Thank you for that in depth reply.

Regarding your comment on cabinate restriction:

The next limit will be the restrictions imposed by the TS cabinet. These are usually so restrictive that unless you provide additional openings in the cabinet (usually opposite the location of the dc connection) you will be lucky to get 200 cfm into the ducting.

My initial thoughts were that cabinates should be more or less enclosed to prevent dust getting out especially if the dust extractor is restricted.
I have a makita 2708
MAKITA-2708.jpg
and I was planning to cover up all the holes that goes right around under the table top, some grills on the sides and the two depth/angle holes at the front (pictured).

But if I'm understanding you correctly, I need to leave open the equivalent of what I'm sucking in to keep the airflow going? So practically, that would mean if I put the DC at the bottom of the cabinate, then I can leave the gap that goes around under the table untouched so the air would be sucked downwards from under the table of the saw.

Originally Posted by mordrax

. . . . . But if I'm understanding you correctly, I need to leave open the equivalent of what I'm sucking in to keep the airflow going? So practically, that would mean if I put the DC at the bottom of the cabinate, then I can leave the gap that goes around under the table untouched so the air would be sucked downwards from under the table of the saw.

Correct, on no account should you plug the holes - if anything cut a few more. I made a shallow inverted pyramidal hopper out of sheet ally for the underneath of my contractor saw and then cut a hole in the tip of the pyramid to match the thread size of a threaded PVC coupler and just screwed the coupler into the hole. IT does not matter if the coupler thread leaks a bit as the pressure will be inwards.

I have that same saw and what Bob has described was pretty much how I was planning to set it up. 6"underneath and a 4 inch line to the blade guard. Glad to see I'm not too far off in my planning.