Miter saw Dust Collection HVAC part question

I have a 12" sliding miter saw on my bench at my new shop. My old shop, I had a 6 inch pipe from up above to just a square box around the saw.

In my new setup, I am running the 6 inch piping down low under the bench. I am going to cut a hole in the bench behind the saw to run the DC piping up to a vent.

I have a 12x6 Universal register box like picture 1 below sitting in my spare parts.

My question is, should I get a ceiling box like picture number 2 in a larger size like a 12x12 or 12x16 to where it actually goes partially underneath the back of the saw?

My original thoughts are the bigger the hole the more it'll catch falling. My secondary thought is, the bigger the box, the less suction...

Attachment 414987Attachment 414988

I forget what adapter I use, but my 6" duct joins a plenum under the whole back of the box, and there is a 3/4" wide slot all across the inside back of the box, so any way the saw throws dust, it goes towards somewhere pulling in air.

I thought that I had a great design, because I've used it since 2008 inside finished houses never getting even fine dust on anything, and then I had a friend "helping" me one day. He was throwing sawdust all in the air. I asked to watch him make a cut, and he pulled the saw all the way out, and pushed, also locking up the sawblade, which I had never done.

I grew up using a RAS, and still use a couple, so pushing never made sense to me. I have always lowered the saw, and pulled. The kerf directs the sawdust back into the box to be caught, and none goes in the air.

Assuming that you will lay the 12x6 facing up into some type of hood behind the saw, you will get better flow out of it. Using the square box makes you think you are getting better air flow, but in reality you will see little collection from the box itself. The restriction of flow in that square shallow box is huge. Most of the air flow is in a straight column into the round intake on it. With dust, unless you have the mother of all collectors behind it I do not think it worth while. any time you take a square or rectangle into a round the pressure drop in the corners is great. In a rectangle your effective draw ends up pretty much being an oval. I will be interested to hear how this all works out for you as I am thinking of building a chop saw station myself so hurry up and finish yours;) . Do not use back to back fittings. Give at least 3 pipe diameters between fittings for the air to stop rolling. Actually, I may be talking out of my hat there as I am using air as a reference. Dust will likely behave differently. Anyone?

Jack, basically the 12x6 would be facing up where the 6 inch port is straight down. So cutting a rectangle in the table top behind the saw and creating a box with angles shaped towards the middle if that makes sense...

Miter saw dust collection sucks...no pun intended. But by thought anything that falls should mostly fall into this port and anything else I can take a handheld brush and brush it into the port. I know it won't get the airborne stuff the greatest but I haven't found much that will either way. I cut PVC on it today and ended up with pieces on the back of my neck somehow... So I just want to take care of the majority of it.

It's just a 1.5 HP cyclone but it's only 8 feet away from the miter saw.