Effects of enlarging the dust port
Size does matter
Last update: 2/16/10
Many woodworking tools have dust collection ports installed. However, while many larger collectors have 6" or even 8" inlets, most tools have ports that are considerably smaller, usually 4". So the question often comes up: Do you reduce the pipe down to the size of the port or install a larger port?
You'll often see people planning to run a 6" main line, with 4" drops to each machine. Why? Usually because they've seen industrial shops designed that way. The problem is that those shops are meant for the dust collection system to service multiple machines at once, with all the lines open all the time. Home shops aren't used that way. We almost always only run one machine at a time, so it makes sense to run a single pipe size all the way around.
When you step down the size of the line, you're choking your dust collection system for no good reason. You paid a good bit of money for that dust collector, you should be trying to get the maximum airflow you can. Generally speaking, 4" lines induce too much drag to do a good job of dust collection. 6" lines are adequate for all but the largest installations. 8" will do quite nicely, but the cost of ducting over 6" really skyrockets with little extra benefit for most shops. So 6" seems to be the sweet spot in price/performance.
So, if we're going to commit to 6" lines, it's silly install all that capacity and then choke it down at the machine. Remember that a 4" line isn't 2/3 of a 6" line, it's only 45% (28.25 sq in vs 12.5 sq in). So a 6" line being choked down to 4" will move less than half as much as it normally could.
Also, you have a problem with dropout. Since the 6" line has more than twice as much square area as the 4", when the air hits the 6" section it slows down by more than half. You have to maintain a certain airspeed for the sawdust to remain airborne, so when you force it to slow down that much you can have sawdust settling out and blocking your ducts. Generally speaking, you need to maintain 4000 feet per minute to keep the sawdust moving. To keep up that speed in the 6" section, the air has to start off moving in the 4" section at 9000 feet per minute or over 100 mph! It's pretty unlikely your dust collector can achieve that much pull in a 4" port, so you're in trouble from the get-go.
So, the real answer is that you have to enlarge the ports. Since I was about to upgrade the port on my jointer, I took the opportunity to document the practical effects of the larger port. In the images to the right, you can see the standard 4" dust port that comes with the the jointer. The dust collector was hooked to it with a "sewer and drain" (S&D, ASTM D2729 or D3034) reducer, which actually works quite nicely. The internal flange is perfectly sized to fit tightly over the port, the screw keeps the adapter in place.
I made a few passes through the jointer with a board picked at random. As you can see, quite a number of chips escaped from the jointer. I also saw quite a lot of chips sitting in the flex hose.
Making a 6" port on the jointer was fairly simple. Underneath the 4" port is a fairly sizable chute, about 6" tall. I simple took a piece of birch plywood and, using the original port as a template, drilled four holes and counterbored them on the back. The counterbores allowed the plywood to sit around the threaded inserts for the bolts. Also notice the slot to allow room for the lip of the chute, which was easier than grinding off the lip.
The connector is a standard 6" HVAC connector from Lowes, with an adhesive seal. I cut a 6" opening and routed a roundover, then attached the connector with sheet metal screws. I've found that these connectors work perfectly with the 6" flex hose I purchased from McMaster-Carr. The hose fits tightly over these connectors and tightly inside the S&D fittings.
The results were quite dramatic. Just putting my hand on the jointer head opening (not running, of course) I could feel a tremendous increase in the airflow. I ran the same board for the same number of passes with the new port. Looking at the photos, you can see the amount of chips escaping the jointer has been almost completely eliminated. And there were no chips at all in the flex hose.
Since the materials to make the new port cost about the same as the reducer you no longer need to buy, all this will cost you is about 15 minutes of your time.
These tests were conducted using a Grizzly G0490 jointer modified with a Byrd shelix cutterhead. The dust collector was a Grizzly 3HP cyclone connected with approximately 20' of 6" PVC and several feet of 6" flex hose.