Perplexing dust collection enhancement fail

[Joe Jensen]

I have a Felder KF700SP saw/shaper combo machine and the dust collection with the stock blade guard was insufficient. The stock guard has a very small port, maybe 1.5” I.D. I have been running a modified Felder guard where I heat formed a 3” plastic coupler to the Felder guard and I enlarged the opening of the guard itself. Collection was pretty good but I centered the plastic coupler so it ran straight vertically and the air clamps I use would sometimes hit the duct. Also the airflow was still less than ideal. The I.D. of the 3” coupler is 2.7” for a cross sectional area of 4.24”. But, the enlarged opening of the Felder guard is only .9” by 3.3” for a cross sectional area of 2.97” which is about the area of 2” diameter port.
Here are pics of the original modified guard.




With the new modified guard I had two objectives; 1) increase airflow and 2) Improve clearance to the air clamps. I decided to use a 3” round to 7” by 1” (part #51026) nozzle that I purchased from Air Handling Systems. I sawed each corner about ¾” so I could bend tabs on the ends. I also trimmed them the match the profile of the top of the Felder guard and I drilled small holes for some self-tapping mounting screws.





Next I started cutting the top of the Felder guard out. First I cut the original 2” end off flush with the rest of the guard with a band saw. Then I used and air die grinder with a carbide burr to cut the rectangular slot. I ended up with a slot .9” by 6.5” long. That gives me a cross sectional area for the slot of 5.85”. The I.D. of the 3” sheet metal adaptor is approx. 2.9” which is a cross sectional area of 6.6”. So I didn’t lose much in the transition from round to flat. Here are some pics of the guard after trimming.




I decided to attach the sheet metal transition to the Felder guard with small self-tapping screws and silicone adhesive. The screws really just held it in place while the adhesive set. Here are pics after mounting the transition.





The transition is mounted at a slight angle to keep the duct away from the air clamps on the slider. You can see that it tilts in one direction which is away from the slider. The transition is tall enough to clear the fence so it doesn’t change how narrow I can rip. Here are pics of the guard mounted on the saw.




So now, how well did I achieve my two objectives? First the good news, I did improve the clearance for the air clamps. But the airflow measured with an anemometer in line with the 3” flex duct did not change. It measured approx. 6970 ft per min with both blade guard assemblies. I do use an Oneida Smart collector that speeds as CFM decreases up to utilize full HP of the system to generate the max possible pressure and therefore maximizing the CFM. Maybe the Smart collector is compensating? I can't see why the CFM did not change even though I almost doubled the cross sectional area of the port on the guard. Thoughts?...joe

[Scott DelPorte]

Just kinda guessing here because I dont know your set up very well. Is it possible that the loss caused from transition from inside the shroud to the 3 inch duct is small compared to some other loss that is in series with it in some part of the air flow circuit? Its hard for me to tell, but for instance if the blockage caused by the blade and table insert causes a greater loss then the transition, then reducing the loss of the transition wouldn't change the overall CFM a lot. You could test that by comparing CFM with your two set ups mounted above the table surface so there is no effect of blockage at the inlet to the blade shroud. Interesting problem.

[Joe Jensen]

Scott, to clarify, I measured the blade guards with them off the saw, I did this for simplicity sake.

[Scott DelPorte]

I am not familiar with the Smart Collector system, so not sure how that affects it. Perhaps your system adjusts the impeller RPM to compensate somehow.

But using a circuit analogy; if you change one "resistor" in a series, and it doesn't change the "current" very much, it means that it's resistance is small compared to something else in the circuit. If you had a very long and duct run for instance, that could be the limiting factor on CFM compared to the area that you increased. Or the blade blockage, which you ruled out already.

Aside from all this speculation on my part, thats a pretty cool transition duct you fabricated.

[Jim Becker]

The guard, itself, is your restriction...only so much air can traverse it to get to the hood and beyond. The OEM port was likely sized with that in mind. While I rarely use the guard on my MM slider, it's natively sized for a 2.5-3" hose. (actually the metric equivalent)

Just as an observation, your hood seems to be inclined to the right in such a way that it's going to interfere with very narrow rips with the fence in the "tall" configuration. I tend to run my fence in "short" configuration most of the time and it would seem to be a way to get up close with your setup, too.

[mike mcilroy]

Probably a dumb question but, are all the seams and joints on the new metal fixture sealed?

[Joe Jensen]

The guard, itself, is your restriction...only so much air can traverse it to get to the hood and beyond. The OEM port was likely sized with that in mind. While I rarely use the guard on my MM slider, it's natively sized for a 2.5-3" hose. (actually the metric equivalent)

Just as an observation, your hood seems to be inclined to the right in such a way that it's going to interfere with very narrow rips with the fence in the "tall" configuration. I tend to run my fence in "short" configuration most of the time and it would seem to be a way to get up close with your setup, too.

Hi Jim, the way the guard leans to the right it clears the fence with the minimum rip possible with the guard in place. Any narrower and the guard itself is in the way. The base of the guard has a lip on the sides where the blade enters to stiffen it I assume. When I measured the new guard it's 5/8" wide and on the old guard the lip is mostly gone from the blade hitting it when it's banged while the saw is running. So the old guard has an outlet at the base that's about 9" by .9" for a cross sectional area of 8.1". The new guard has a 5/8" by 9" for a cross sectional area of 5.6" so maybe that's the difference. Also that lip would cause turbulance.

[Joe Jensen]

Probably a dumb question but, are all the seams and joints on the new metal fixture sealed?

The seams are sealed.

[Joe Jensen]

I am not familiar with the Smart Collector system, so not sure how that affects it. Perhaps your system adjusts the impeller RPM to compensate somehow.

But using a circuit analogy; if you change one "resistor" in a series, and it doesn't change the "current" very much, it means that it's resistance is small compared to something else in the circuit. If you had a very long and duct run for instance, that could be the limiting factor on CFM compared to the area that you increased. Or the blade blockage, which you ruled out already.

Aside from all this speculation on my part, thats a pretty cool transition duct you fabricated.

The Smart system increases the impeller speed to increase suction when the CFM is low (motor is underloaded). It increases to keep the motor near FLA all the time.

[Wade Lippman]

Do you have anything else open besides the overhead guard?

[Joe Jensen]

There is a 6" main branch and the overhead guard taps off that. Further down there is a 5" drop to the lower part of the saw. Inside the saw it drops down to a 4" flex hose which connects to the cast iron blade shroud which is less than the area of a 4" duct. Both the upper and lower ducts were running but nothing else.

[Don Kondra]

No matter how "smart" your system is, it can only draw so much from a 3" line.

Move up to a 4" or 5" and you will see some improvement...

For example, I did your standard SDD mod on a 2 HP dust collector. At the end of a 5" line I get ~ 500 CFM, if I use a 4" line I get ~ 300 CFM.

Cheers, Don

[Kevin Jenness]

Does it pick up the dust adequately?

[Jim Becker]

No matter how "smart" your system is, it can only draw so much from a 3" line.

Generally true, but in this case it's the hood that's the limiting factor

Move up to a 4" or 5" and you will see some improvement...

Again, the hood/guard itself is the limiting factor. As he's noted, the branch to the machine is a 6" branch with the smaller hose to the guard and a 5" (120mm likely) port on the slider's body. And these Euro sliders don't pull from the cabinet like typical North American cabinet saws...they have a shroud around the blade so dust/chip collection is concentrated on the blade area. I have essentially the same setup for my MiniMax slider.

[David Kumm]

Joe, are you measuring just the guard cfm or is the bottom saw port open too? Your mod would show more improvement with a traditional collector than a smart providing the smart had speeded up to reach its designed airflow with the small port. Dave