have you taken in account the the Venturi effect???

.

 

I don't see how a the sawdust can be removed without incoming air. My home built router table has a plexiglass front door that has 5 - 1" holes just for incoming air and that keeps the compartment almost completely dust free. Close those holes and nothing gets removed except a very little from what small amount of air infiltrates. If possible I'd add adjustable air vents. Maybe I'm missing something here.

 

In my case, the air is coming into the dust collection box from above the router table. This is why it is important for the throat plate to have as much open area as possible as in the case of the Incra system that I have. The system works quite well.

 

I am familiar with the "Venturi effect" in Fluid Mechanics but do not understand what you are referring to for a router dust collection system. Please amplify.

 

Ben I'll point out a few important facts about RT dust collection.
First is that not a single speck of dust is created below the table. 100% is created at the top of the table and up from there.
Second, most of us use insert rings in our table plates to keep the gap as narrow as possible. This doesn't allow much room for either the dust or the air needed to transport the dust to get through. The Clean Sweep rings help by allowing more air but the combined surface area of the openings is maybe 2 square inches of cross sectional area on average where as a 4" dust port is about 12.5 sq. in.
Third, the router is trying to blow air upwards through the router windings to cool itself. You are pulling air in the other direction. These two air flows tend to cancel each other out. You are also pulling air out at the intake end of the router.
Fourth is that cooling happens by transferring heat from the windings to the passing air molecules in the air stream. In a vacuum there are fewer molecules so cooling is reduced.

The only particles that would normally make it past the bit and go under the table are due to the randomness of the collisions between particles being ejected from the bit. Most will stay above the table if you don't try to move them somewhere else. So the logic of trying to suck all that dust down through that small hole by putting your router in a box that will probably compromise it's cooling while simultaneously making the router hard to get to for locking height, changing bits, and changing speed absolutely escapes me.

I'm considering building an extra pickup that will sit under the table at collet height that can capture the tiny fraction of dust that my fence mounted pickup misses but that also won't interfere with me getting at the router and that design will actually improve cooling. I'll probably mount it with magnets as It has to be able to attach on either side because I work from both sides of my table. I just haven't figured out what I'll make it out of and how yet.

 

Chuck you are stating the obvious about where the dust is created. However, that does not mean that there will be no dust below the table. My first router table had only dust collection through the fence on top of the table and I got a lot of dust below the table. There was no enclosure around the router.

In my experience, the Incra system with the clean sweep rings works quite well. I am really happy with it.

My experiment suggests to me that the air flow induced by the dust collector completely overwhelms the air flow induced by the built in fan on the router.

The reduction in air density due to the vacuum is hardly noticeable. Sea level atmospheric pressure of 14.7 psi is equivalent to 407 in of water. The maximum suction (no air flow) on my dust collector is about 10 in of water. A more typical operating suction pressure would be closer to 2 in water. If you pulled a hard vacuum, then what you said about reduced cooling effectiveness would be correct; you would also likely have collapsed duct work.

The only reason I ever have to open the access door on the Incra clean sweep is to change router speed, which I do infrequently. Bit changes, height adjustment and locking the router all take place from above the router table.

 

I think Ben (bfblack) has made an effort to add a real scientific evaluation here and should be commended for his sharing it with us. It shows us the importance of air flow and the huge difference there actually is when more air movement/exchanged to dissipate heat. There is no presumption here, just facts.

I also agree, that a snorkel from the end of the router to the outside of the box may help as long as you also use your DC at the same time. I presume a snorkel would cut airflow without the added DC.

 

I think Ben (bfblack) has made an effort to add a real scientific evaluation here and should be commended for his sharing it with us.

I agree but I still have to stand by my comments. As a power engineer with a background in physics I didn't grasp what I said out of thin air.

I presume a snorkel would cut airflow without the added DC.

Only very marginally. The longer it is the more impact it would have as there is some friction between the air molecules and the side of the tube but it would be over a longer distance than anyone would need to go with the snorkel. Since the snorkel would probably be much larger than the router's intake and the distance air would have to travel to get to the intake would be very short I suspect you would need some pretty good instrumentation to see the difference.

 

I would like to THANK Ben for sharing that information with us!

Otis Guillebeau from Auburn, Georgia

 

Ben,

Thank you for sharing that info. I have a question about the procedure; why did you not include a result for DC ON door open ???
This would be necessary to compare all facets equally ! and would show similar to what we are saying about air flow holes on the door or elsewhere. I would like to compare and see why so many people and designs use this technique.

Dan

 

I did not consider running a test with DC On, Door Open because that was not a mode of operation that I normally considered. It would take 30 min of my time and would be straight forward to run the test. My engineering judgement says that the dust collection efficiency would be reduced with the door (partially ?) open. The air flow induced by the DC system wants to take the path of least resistance. Having the door open will likely reduce the air flow coming from above the router table, and hence the dust collection efficiency. I do not have a data logger so I can't record temperatures while the router is being used for its intended function.

 

heres my logic;
If you run the router in the table without a cutting bit in it for a short time, and place your hand above it, you can feel the air being pushed up through the motor by the rear fan.
If you seal the router into a box and then suck the air out, then the router fan will be starved and the router will overheat.
If you seal the router in a box and do NOT remove air and dust, but allow some air in to mix then the router fan will blow dust into the router, windings, bearings, everywhere.

Therefore, if you seal the router into a box, run dust collection from it but install a tight fitting tube over the router fan entry and run that tube through the box to outside free air, you will remove the dust AND keep the router cool AND free of clogging.

Thats what I've done, and my large makita router (NOT adjustable speed) has never shown signs of overheating.

 

I have two DC ports on my router table. One for the Incra LS fence and another for the box. Over the years I've noticed that when the DC is attached to the fence, my Milwaukee will warm up considerably during extended use. When the DC is attached to the box, no problem with the router heating up even under hard, extended use. Suction if provided by a 2 1/2" PVC run of about 25' into a dust deputy followed by a 6hp craftsman shop vac. Dust collection is quite acceptable with this setup, however, knowing what I know now, I'd probably go with a 4" line into my regular DC'r just for air flow. But after 10 years of relatively regular use, can't say I've experienced any problems relating to overheating.

 

Is the router still inside a box but with no air movement in the box? If that's the case I would expect it to get hotter than normal. My first experience with a router mounted in a cabinet was at the mantle factory I worked at for a short while. It was a big Makita and the cabinet was at least 30 inches in all 3 directions, maybe more. They had basically just boxed in the stand it was mounted in. After running the machine for 15 minutes or so I could feel the heat coming through the sides of the box. That was in about 03 or 04 so I can't remember now if there was any DC going or even hooked up to the box at the time but I was surprised at just how hot that router got and I knew after that that I never wanted to mount one in a box after that.

As I said earlier I'm looking at making a half moon shaped pickup to mount under the table and that should catch the maybe 5% that gets thrown downward without creating issues of getting to the routers controls or built up heat. The pickup design is easy enough but I still have to figure out what to use for nipples to attach the hoses to and how to Y it all together with the main pickup in my fence.

 

Stupid question: do all routers take in air from the housing-end and blow out towards the collet? Sounds like the logical flow, but I seem to remember at least one that blew the other way - perhaps the internal fan had been exchanged incorrectly.
Might account for the different observations made by members here.

Bill, I would be particularly interested to know which way your Milwaukee blows, as theoretically the top suction should increase airflow, if anything?

 

It would be counter intuitive to make a machine that blows (or sucks) dust into its own motor.
Also, when the router is used by hand, the dust would blow straight up into your face
If one does exist, please let me know so I can make sure never to buy one.

 

@Biagio
Thank you for doing this test, However I think that this is not what concerns most of us in that you will need to do further testing with a longer duration under actual load conditions over an extended length of time. Also compare actual air flows to the Dust collector under actual full loads and with the door open and then closed. Then after a goodly length of time running continuously under load, remove the motor and dismantle to check the amount of chips and dust inside. This is one of the causes of motor failure is the chips and sawdust abraiding the field winding's insulation and shorting out the motor, from what I have read.

Herb

 

Herb, I take your Fifth - I only asked a stupid question.

 

I think what you did was good, I just think that your test should be expanded more to duplicate actual working conditions.

Most people don't use their routers to that extent anyway.

Herb

 

Folks, as a(n apparently mutual) lover of physics, I would gently note a few things:

- The venturi effect occurs during constriction of airflow.
- Most router tables are going to provide for airflow not just through the tiny aperture around the bit/fence, but through a secondary source. Mine is the shop-built Abrams table and it provides additional airflow via holes in the 'window' on the face of the table.
- Pressure is only part of the equation; volume is critical - ergo, providing an additional intake source for air is crucial to dust collection

Empirically, I used to have the table connected to a Fein - it did okay. It's now connected to an Oneida, and you would need a microscope to find any dust anywhere inside the table - router housing included.

The 7518 would disappear if it wasn't firmly mounted.

 

Folks, as a(n apparently mutual) lover of physics, I would gently note a few things:

- The venturi effect occurs during constriction of airflow.

You are correct. It was the Bernouli Principle I was thinking of which says that when you have a stream of air going through a constriction the pressure will drp as the velocity increases, as when passing through a boxes ports. https://en.wikipedia.org/wiki/Bernoulli's_principle

- Most router tables are going to provide for airflow not just through the tiny aperture around the bit/fence, but through a secondary source. Mine is the shop-built Abrams table and it provides additional airflow via holes in the 'window' on the face of the table.

They had certainly better have extra porting or all of the things previously pointed out will apply such as heat and decreased vac efficiency. Where the porting is can be very critical. Having flow across the router casing and lots of it should help but it doesn't solve all the problems associated with a box.

- Pressure is only part of the equation; volume is critical - ergo, providing an additional intake source for air is crucial to dust collection

Empirically, I used to have the table connected to a Fein - it did okay. It's now connected to an Oneida, and you would need a microscope to find any dust anywhere inside the table - router housing included.

The 7518 would disappear if it wasn't firmly mounted.

It's good that you get those results. But some of the variables can be critical. One of the issues if you don't get results like yours is that the corners and possibly the joint between sides and bottom depending on port locations are dead air spaces. There is no air movement there and so debris can build up. This is likely what happened in the earlier photo of the DW router. I had the same issue with my fence DC. When I built the fence I notched the base piece for the bit but the square edge caused turbulence and I got a lot of debris buildup against it that would eventually cause problems at the bit. I had to slope that part so that the turbulence decreased enough to carry the debris up and over the base of the fence and into the vac pickup. So if you don't get the box design and porting right you risk the chance that you can overheat and/or get debris buildup and cause some to go through the router. Those are pretty serious consequences.The solution to that risk is simple. Just don't box the router in and if you want under table DC then just add a pickup at collet level. Me adding that to mine would be easy if it wasn't for the fact I want to be able to work from both sides so mine has to be switchable from side to side.

 

"Ben, what were your findings for the temp readings while utilizing installed independent systems and w/ the router under extended load and not under load???...
did you use a free standing router as a baseline??..."

Since I don't have a temperature data logger, I have no way to simultaneously load the router and record the temperature. I am contemplating buying an inexpensive data logger.

 

A lot of knowledge here. That´s why I love this forum.
I do not use DC system but I did some temperature measurements on one of my trimmers and found that the temperature raised until 70 Celcius deegres making it impossible to touch the trimmer armature with the bare hands. I still use the trimmer as a milling machine but keep my hands away from it. See my post "Too hot"

 

Seems to me that in addition to a "snorkle" to the router's bottom, that the 4 inch DC connector should be ABOVE the table, behind the fence, and the 2.5 to the box beneath the router. Of course, this probably means redoing your table to allow for the snorkle, and modifying the fence to allow for the 4 inch DC port. To me, the logic favors that port arrangement.

The snorkle doesn't really need to drop straight down, it could be a 90 degree connector that goes off to one side to reduce its size. It might also benefit from a filter so you're not directing airborne sawdust into the motor. Because I have put shelves and sides in my steel Rockler stand, I see a wall cabinet going up above the router table to store bits, at least in my shop.

 

From your document... "The “theory” behind providing room air directly to the bottom of the router is that the dust collector causes a downward air flow while the internal router fan causes an air flow in the opposite direction. In my 50 plus years’ experience as a mechanical engineer with a specialty of heat transfer, this argument was counter intuitive. Consequently I have investigated this problem experimentally to confirm or deny this router folk lore."

Ben...I'm not sure the folk lore is related to opposing air flow...it seems you have, as stated, limited your testing to just that. The "folk lore" is broader than that and so your tests might be somewhat skewed.

There are many components to the discussion. Note that I did not call it an argument or debate. Most woodworkers have developed dust collection and air supply techniques after years of experimentation on their particular configuration. This would include router manufacturer, air flow requirements of the router, amount of dust produced that goes into the box (bit dependent) and extent of use.

Different routers provide cooling differently...for example, the Bosch 1617's internal fan blows air directly upwards (whens mounted in the table) versus the Triton router which diverts air sideways as it exits the router. The Triton also diverts air away from the insert and it's dust collection plasticware under the bit. So while the "folk lore" might contain some of your assumptions, it does not include all.

Equally important is dust not entering any of the components in the router as a result of it's internal cooling system. This includes switches, slides, armature, etc... This can only be determined over time for the different varieties of routers and their cooling configurations.

There are two separate systems going on. First the air flow requirements of the router need to be satisfied. Secondly, the dust collection system needs to do it's job. I can state categorically that any compromise created as a result of mixing the two is just that...a compromise.

That dust enters the box is a dust collection issue. Edge profiles versus bottom profiles (grooves) create different dust problems. If one can produce enough suction above the table, dust will not enter the box. For those routers that push their air directly our of the insert towards the bit would only aid in that circumstance. Desert Rat Tom explained his reasons for putting the greater dust collection capability on top.

All this assumes that one has worked diligently to separate the two systems. The Snorkel approach does exactly that. It keeps router cooling separate from dust collection..When using the snorkel, the positive pressure created by the router's internal cooling needs to be equalized in the box...that is the reason for allowing some suction from the dust collection system to evacuate the box's otherwise positive pressure.

My conclusion is that you need to be a bit more specific in defining what "theory" you are working with and what part of the "folk lore" it contributes to. As I read your document you are specifically dealing with dust collection below the table, no snorkel, using a router that blows air directly up through the insert. I think your assumption is also that there is no dust collection at the fence...did I understand your environment correctly...? And your tests deal directly with the heat generated at the router as a result of overcoming/not the opposing air flows...?

If you separate the two systems, there will be no opposing air flows...except for evacuating the positive air created by the independently supplied airflow of the router's cooling system.

Good of you to have spent the time to perform some tests and publishing them here...many will benefit from your tests and the subsequent discussion of the many views. Since you took the time to test and publish I thought it only fair that I read your document and the responses you got carefully before commenting.

I'm not sure you want to confirm or deny the entire "folk lore" but maybe you can state the specific part of it that you will be resolving...and more specifically, maybe that part of it that does not require years to prove/disprove. I'm sure you noted some responses indicated different experiences.