Dust Collector versus shop vac

[alexy]

I'm a relative newbie with my BT3100. Great machine for the money!

I've hooked mine into my DC system, with a 2-1/2 stepdown at the machine. But reading Bill Pentz's excellent site on dust collection, I wonder if a shop vac (with good filters so it is not just a dust blower!) would give me better dust collection results. Here's what Pentz says in response to a question about the need for a shop vac when you have a good DC system (and I have a DC system, but not one that Pentz would call good<g&gt: http://billpentz.com/woodworking/cyclone/Equipment.cfm

quote:Most of us continue to need a good shop vacuum even after getting a big dust collector because a vacuum is the only way we can get enough pressure to pull the air we need for good fine dust collection through our smaller machine ports and restrictive internal machine ducting. Air at the low pressures generated by our dust collectors is virtually incompressible, so just like a water valve, any small opening acts like a partially closed valve and kills the airflow we need for good fine dust collection. This leaves our dust collectors pretty much worthless with any port connection smaller than about 3” because we get no air movement. To get the needed air movement for good chip collection at our smaller tool ports, we need to use a vacuum.

This makes intuitive sense--the shop vac with its much greater suction can get more airflow out of a small port even though it can't begin to match a DC's airflow from larger ports. Anyone who has tried both be willing to share their experiences? Anyone tried modifying the outlet from the BT to open it up to 4" (not just step it up after the restricted outlet)?

[L. D. Jeffries]

Can't say about others but I have a PennState 1hp DC, wall mounted with elbow thru wall and an 8' straight run to dropdown flex hose. Before my RIDGID 6amp vac hooked to BT3 saw would throw dust/shavings all over floor under saw. Hooked DC 4" hose to adapter to saw port and now only get a very LITTLE dust when in use. Built a hood for chopsaw that worked fairly well, planner works great, router table as well, am going to build collector for DP next and then a sanding fixture from peg-board.

[Warren]

Used a shop vac for a year or so on the BT and was satisfied. Put a 1hp Delta unit in the shop for the sander and DP. I added a first stage collector for the table saw and planer and was impressed with how much cleaner it is under and around the BT. I've never considered enlarging the attachment point on the bT until you rought it up. Couldn't hurt. I wonder if I have the necessary and skills and whether I should buy a new tool to accoplish it. Decisions. Decisions.

[jbalders]

I have the HF 2HP DC, 4" hose up to the adapter connected to my BT3K (4" to 2-1/4" Reducer Fitting #144534 at Woodcraft), and another 2-1/2" line that runs up and over to connect to my Shark Guard. The only time anything escapes my DC is when I'm trimming the edge of a board with my ZCTP on. Chips sometime get thrown out the side in this case.

My understanding is that a very short restriction will cause the air to accelerate past it (think of the venturi a carbuerator) and reduce the CFM only a little. Longer restrictions, like reducing it down to 10' long run of 2.5" hose at the low pressures generated by a DC will cause a severe reduction in CFM like Bill Pentz mentions.

I could be wrong though. Maybe someone who's more familiar with this branch of physics could chime in.

Jeff

[just4funsies]

Your physics lesson is right-on. It's just like a momemtary restriction in a water pipe. water speeds up through the restriction, and then slows down on the other side. If it's momentary (very short run), it barely reads into the calcs. If it's a longer run, then the equation adopts the physics of the smaller sized pipe, and that's where you will get reduced performance.

[jbalders]

What do you know? I guess I was paying attention in high school physics class after all!

Jeff