Help me wrap my brain around this

Started by HanS, November 02, 2013, 09:26:48 PM

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HanS

Hello everybody,

I'm a long time lurker first time poster. First, a big thank you for sharing all of this information. This is a prime example of what is great about the net!

Here's my situation. I am the proud new owner of a cnc router. A 48"x48" Cnc Mogul to be exact. The CFO of my house, the missus, agreed to purchase of the machine if I built her a craft room. The one remaining space to be developed is the back half of the basement. So the machine is going to have to live in the basement in her craft room. This creates a couple of issues to overcome. First is that the boss is asthmatic and allergic to everything from grass to dust. Second is noise, as a test I flicked on a router and shop vac in the intended room then went upstairs, her eyes were drilling holes through me as I reached the landing.......then the baby was woken up. Oh my!!!

My solution is to build an enclosure for the machine. This enclosure will house the dc system which will  be comprised of a down draft/vacuum table as well as a dust-shoe at the spindle, flowing into a separator with suction created by one (or two) ridgid wd4522 vacuums.

Which leads me to my first question. Understanding that laminar flow entering the separator improves efficiency how long of a straight run for a given cross section prior to the inlet is required to ensure laminar flow? In other words, is there an ideal placement for the collector (Y)?

I will generally want to use both streams together as the down draft will also serve as cooling air for the router and other electronics, how can I ensure good suction from both as friction losses will be unequal and flow will take the easiest path? Would I be better off having  separate systems each with their own vac source and separator?

A final question related to my enclosure design is about the container for collecting chips. I envision a vacuum tight cabinet with the separator hung from the ceiling, with a hatch for clean out. This is where my brain gives out. Is there any difference between having a top hat sealed to the top of the container or sealed inside of the container? I'm inclined to believe that the "delta P" above vs below the baffle would be the same in both cases....but then I second guess myself.

This makes me think of one more question: what exactly is the physical process causing the chips to drop out? Is purely the pressure differential or is it impingement caused by centrifugal forces or a combination of both?

If your still reading thanks for hanging in there.

HanS

galerdude

Probably none of this pertains to your situation but I'll share anyway  ;). I have a 4'x4' CNC in my insulated garage. I use a Thien Top Hat with a HF 2hp dust collector. The dust collector sits outside in an enclosed uninsulated attached shed. When in the garage I wear ear protection/headphones as the sound of the router is a little much. When I step outside the garage, the dust collector is a faint hum and the router (CNC) is a very faint whine, if that. I use my CNC all hours and have walked around at 2 in the morning and you don't hear squat 20 feet away. I live in a residential neighborhood on approximately a 50x100 foot lot. Never an issue.
Just some FYI.

revwarguy

#2
A little more food for thought:

From what you've described, the forces for holding material on a vacuum table while being milled are very different than for dust collection - I would start with 2 separate systems.  I am not sure about those Ridgid vacuums - are they meant for continuous duty?  Many of the smaller vacs are not, and you will probably want to do some 4-8 hour runs if you do any 2.5D (bas relief) type routing.

I have a basement CNC.  Here's my take - think hard about the difference in cost between an enclosure and a spindle.  Most of the noise of a router is its cooling fan.  I was shocked at how quiet the machine was the first time I saw a water-cooled spindle-based CNC table, both while jogging and cutting wood.  For the same reasons, those smaller shop vacs are almost as noisy as a router, but I don't know about that particular Ridgid model.

A well designed shoe should get at least 90% of the dust.  Do one of those first, and see what else you need to do from there, as it then really depends on what you are cutting.

I can sympathize about the noise, for sure.

Allen-in-Sheboygan

Regarding thy physical process of separation, I would like to know more myself as that knowledge should lead to design improvements.
My take so far is:
Step one: Particles are moved to the outer diameter of the separator by centrifugal action as the air swirls around in the separator.  This air flow should be smooth (laminar) as turbulence will tend to re-mix the particles into the air stream.
Step two:  Once the particles have reached the outer wall, friction slows them down and they fall toward the outlet at the bottom of the separator (i.e. through the slot in the baffle in our case).

If the above is correct, I would think that a smooth outer wall would improve the cyclonic separation until the particles reach the wall.  From then on, a rough wall (course sandpaper, perhaps) would enhance the slowing down process.

I have also thought that slots placed in the lower outer wall would help remove the outer few mm of the air stream that contains the particles.  Obviously, the slotted portion of the separator would have to protrude into the collecting bucket.  This would not require the particles to "fall" out of the air stream which to me would seem to improve small particle separation - think of how dust "hangs" in the air and takes minutes to settle out compared to fractions of a second available to "fall" out in the separator.

I hope the above speculation is of interest and provokes some responses to correct and/or improve on the explanation and the improvement ideas.

jdon

Okay, I'll give it a shot, but I must emphasize that I am not in any way an expert- don't say I didn't warn you!  :)

As far as having a rough (sandpaper,etc.) wall, instead of smooth at the bottom of the cylinder, some small particles might be caught there, but I think that after a short time that area would be caked over with dust. Since the dust isn't physically attached (stuck) to the wall, it would tend to be pulled off the wall by the airstream. My guess is that small irregularities (sandpaper particles) would also cause microscopic turbulence and disruption of laminar air flow, which could actually be detrimental, by "kicking up the dust" that's on the wall- I picture miniature dust devils being created.

As I see it, the whole idea is to get dust out to the wall of the separator, where it can eventually fall by gravity through the slot, while the circulating air eventually goes to the center of the separation chamber, and exits. The baffle separates the rapidly moving air above from a relatively stagnant (less air flow) area below. If I understand correctly, your idea is of having side ports in the lower part of the wall, so dust particles can escape out the side, instead of dropping out the bottom. The problem I see is that air would go through those slots as well, and end up causing a lot of turbulence in the waste area below the baffle. This would stir up dust already collected, and since the air has to go someplace, it would go up through the baffle and exit, carrying dust with it. So, I don't think it would work.

Of course, there's a pretty good chance that I'm completely wrong! But this is what I like about this forum- lots of interesting ideas!

Allen-in-Sheboygan

I was thinking more of short vertical slots a couple of inches tall by maybe 1/4 inch wide especially with a lip kind of like a cheese grater to "scrape off" the outer layer of air containing the majority of the particles.  I don't see why this would cause more swirl in the bucket than the long slot in the bottom except that it will add a little more air flowing into the bucket.

I've thought it would be interesting to make a double bucket where the inner bucket only received debris from the bottom slot and the outer bucket from the side slots.  One could quickly see the effectiveness of the side slots.

As to the sandpaper idea, I see your concerns.  However, from what I've been reading, getting the particles and the air containing them slowed down is part of the key to separation.  Compared to a cyclone, the Thien design has little flow length to slow the air stream.  So is it more important to slow the air stream or to maintain laminar flow (I wish someone knowledgeable would give us an idea if there is ANY laminar flow in the separator at all. My impression is that laminar flow is VERY difficult to get except with very low flow rates.)

retired2

Quote from: Allen-in-Sheboygan on January 14, 2014, 03:06:28 PM
I was thinking more of short vertical slots a couple of inches tall by maybe 1/4 inch wide especially with a lip kind of like a cheese grater to "scrape off" the outer layer of air containing the majority of the particles.  I don't see why this would cause more swirl in the bucket than the long slot in the bottom except that it will add a little more air flowing into the bucket.

I've thought it would be interesting to make a double bucket where the inner bucket only received debris from the bottom slot and the outer bucket from the side slots.  One could quickly see the effectiveness of the side slots.

As to the sandpaper idea, I see your concerns.  However, from what I've been reading, getting the particles and the air containing them slowed down is part of the key to separation.  Compared to a cyclone, the Thien design has little flow length to slow the air stream.  So is it more important to slow the air stream or to maintain laminar flow (I wish someone knowledgeable would give us an idea if there is ANY laminar flow in the separator at all. My impression is that laminar flow is VERY difficult to get except with very low flow rates.)

The baffle is the cheese grater and it scrapes off the lower layer of air entrained with dust.

Allen-in-Sheboygan



The baffle is the cheese grater and it scrapes off the lower layer of air entrained with dust.
[/quote]

Wouldn't the air stream along the side contain more dust due to centrifugal force than the air along the bottom of the air stream?

retired2

#8
Quote from: Allen-in-Sheboygan on January 15, 2014, 03:53:47 PM

Wouldn't the air stream along the side contain more dust due to centrifugal force than the air along the bottom of the air stream?

Watch some of the videos of the separator in action and you'll see there is a sloping air stream that almost pushes the waste into the drop slot.  I think it is an over-simplification to think the spinning air slows down and the waste drops out due to gravity.