Thread: Dust collection article in Australian Woodsmith issue 96

Hi Doug
some comments on

Originally Posted by doug3030

Lets use the example of my belt and disk sander which is a work in progress in my shed as we speak. I have attached a photo of the machine. I have a 2hp dusty with a nominal flow of 1200cfm

My first step was to work out where the dust needed to be collected from. Obviously from the 2" dust port on the end of the machine, and also from the cavity beneath the machine because there are vent holes into there from the bottom of the sanding belt and also to create a flow into the machine to try to encourage as much invisible dust as possible into the collection system.

Second step is to design the collection system/ducting to achieve the best collection. SO I look for readily available plumbing or dust collection fittings and come up with a way of assembling them to maximise collection. I also look at custom making fittings if it appears to be worth the extra effort and cost.

Third step is to cut the pipes and assemble and test the system i have designed based on the predictions I have made. The attached picture shows what I came up with as the first design to try, a four inch pipe from under the cavity of the machine with a 2" branch pipe to the manufacturer's collection port. I was hoping that the air flow restrictions of the vents and other openings in the machine cavity would provide enough choke on the 4" pipe that the 2" pipe would provide enough suction to work on the manufacturer's collection port.

To test the machine, first of all I went around the machine holding strips of tissue paper to see how much suction was created through any of the inlets. I use the paper strips because the angle they are deflected gives a better indication of the strength of the flow rather than just putting your hand near the opening to feel the draft. It may not be very accurate or give a reading you cluld plot on a graph, but I got a lot of change out of $10,000 when I bought it. There is a pic of my HIgh-Tech testing equipment below too.

The other test is to obviously use the machine and just see how effective it is.

In this case, I was not getting sufficient extraction through the 2" branch to be effective, so I started blocking of vents in the cavity to see if that would help. It did not work with all the machine cavity vents and openings effectively blocked off (yes its the high pressure/low volume - low pressure/high volume thing).

The next stage of course is to redesign to correct the design faults and rebuild. In this case I am going to remove the 2" branch completely and retain the 4" main line from the machine cavity, so now instead of wanting to restrict the flow into the body cavity I will be looking at opening it up to collect more air and therefore more invisible dust.

The 2" branch pipe on the manufacturer's dust port will be replaced by a connection to the shop-vac, which was what it was made for in the first place. I was trying to avoid using it because it does not (yet) vent outside of the shed.

Once I have made the modifications I will repeat the testing stage and modify again and retest again and so on until I am satisfied with the result.

I welcome comments from anyone on the procedures I outlined above. Hopefully it will help someone and If someone can show me a better way I would be delighted to learn.

1st, your connection to the 2" port is way inefficient with those 90° bends. The major point of collection is where the belt disappears at the motor end of the machine

2nd, on the assumption that you mostly use the sander with the belt horizontal, I'd be looking at how I could disrupt the laminar air flow / increase the suction where the belt is out of sight.

Originally Posted by ian

Hi Doug
some comments on1st, your connection to the 2" port is way inefficient with those 90° bends. The major point of collection is where the belt disappears at the motor end of the machine

2nd, on the assumption that you mostly use the sander with the belt horizontal, I'd be looking at how I could disrupt the laminar air flow / increase the suction where the belt is out of sight.

You are quite correct, Ian. those bends are inefficient but there was no other real practical way to do it from the one dust collection unit and as I said above I was going to replace the 2" pipe with the shopvac, which should have solved that problem, but read on to find out why it didnt. That port was supposed to be clearing from the end of the belt as you said would be the best place to collect from but once again read on...

So I fitted the shopvac and it was a dismal failure too! Checking further into it, the 2" dust port on that machine is nothing more than a cruel practical joke perpetrated on the unsuspecting public to make us believe you can extract dust from the sander with a shopvac. Its a 2" port, mounted on a hollow plastic cowling which encloses the rear of the machine up to the back end of the sanding belt. On entering the machine the port divides in half horizontally. The bottom half is connected to a rectangular duct leading into the back of the disk sander, and the top half is supposed to provide enough suction to clear the dust from the back end of the belt down a very narrow duct within the cowling and out through a 90 degree bend. I am surprised that I had not noticed before how totally inadequate this is.

So now its back to the design phase. I think I can safely chuck the cowling in the wheely-bin and design a shroud to cover the end of the belt, being careful not to disrupt any of the functions of the machine including the ability to tilt the bed, which is necessary to change belts. then I will need to work out a way to collect from the disk sander, which will probably have to be from within the cavity of the machine itself as the small rectangular duct I mentioned earlier is too small but that is all that will fit through the hole in the cast-metal base of the unit.

Not sure how I am going to work all this out yet but it might involve an evening sitting in the shed in front of the machine consuming sufficient quantities of that bubbly amber thinking enhancement fluid until the solution comes to me. It might be time to experiment with bending some acrylic more than just a gentle curve.

Doug

I've got to thinking, which is a dangerous thing. It is kind of to do with the idea of a jet of air used to disrupt the path of dust within a machine that is wont to kick it out into the workshop, it's a little bit of the push/pull we talked about in another thread and your 2" port got me to thinking, though it's not really for your situation I don't think cos of the problems you mentioned with that port. Geeze will you get on and just say it... We used to have an old electrolux vacuum that was capable of blowing quite hard as well as sucking. I'm wondering about the feasibility of making a closed system either by using one of those suck/blow vacuums or an enclosed and vented vacuum where the vent goes back into the machine. I could imagine with that electolux set up with a blowing port right across from a sucking port and dust generation in between that it might be quite effective. It would be impossible to seal every opening so you run the risk of blowing some dust out into the workshop, but if the unit was also rigged with DE on a 4" or 6" port it might reduce the cabinet pressure enough to restrict the dust to the DE and vacuum. More food for thought?

Gee I thought the idea might have been good enough at least for someone to tell me why it wouldn't work! :

Originally Posted by mic-d

Gee I thought the idea might have been good enough at least for someone to tell me why it wouldn't work! :

Hi Mic-d, yesterday while I was at work I started writing a reply several times but kept getting distracted by work so I never got very far.

At the end of the day I had to rush off to catch up with my son and my 3D printer kit birthday present which finally arrived and we spent last night working on.

I think your idea has some potential but rather than use a high volume low pressure inside a cabinet, LVHP bursts of air might be better. The reason for this is that a LVHP would impart gas flow direction without adding much volume and potentially pushing dust out of the cabinet. I am keen to test this put some time.

The other place I see LVHP being useful is to break up the air flow and dust spray resulting from a spinning circular blade - something else to try

I gather that a vacuum cleaner is still in the HVLP category? I've thought more about it and I reckon it might deserve a closer look, it is a gut feeling . The supply and extraction of air by the vacuum cleaner component should provide a net zero change in cabinet pressure because the one pump is doing both jobs. If the extraction port could be designed to collect the entire air input or nearly so, then the DE component would mop up the vagrant dust. I'm thinking if you were fitting to a table saw then the push/pull ports would be perhaps little further apart than the width of the saw blade (with ports designed for efficient jetting and collection of air) and the gullet of the teeth buried in the air stream and no more. It's a matter of whether the velocity of the air would be sufficient to at least dislodge the blade-entrained dust away from the slot so it's not ejected out into the workshop, if not cleanly collected by the pull port. Perhaps that is going to require a greater velocity...

Originally Posted by mic-d

I gather that a vacuum cleaner is still in the HVLP category?

As a DC, a vacuum cleaner is considered a LVHP, i.e. 100 cfm, 3.5psi even though they a significantly different from conventional DCs which are 1000 cfm at < 0.5 psi
As a push mechanism or flow disruptor a vacuum cleaner is still in the HVLP category as push mechanism from a compressor would be something like 8 cfm at say 90 psi.

I've thought more about it and I reckon it might deserve a closer look, it is a gut feeling . The supply and extraction of air by the vacuum cleaner component should provide a net zero change in cabinet pressure because the one pump is doing both jobs. If the extraction port could be designed to collect the entire air input or nearly so, then the DE component would mop up the vagrant dust. I'm thinking if you were fitting to a table saw then the push/pull ports would be perhaps little further apart than the width of the saw blade (with ports designed for efficient jetting and collection of air) and the gullet of the teeth buried in the air stream and no more. It's a matter of whether the velocity of the air would be sufficient to at least dislodge the blade-entrained dust away from the slot so it's not ejected out into the workshop, if not cleanly collected by the pull port. Perhaps that is going to require a greater velocity...

The more I think about it, I wonder how much dust (especially invisible) is blade entrained.
This is how I envisage the dust spinning off the blade.
TSpred.jpg
The visible dust should have enough momentum to be ejected tangentially which will create some tangential air currents and carry some invisibles with it.
The rest will be carried by the air currents generated by the spinning blade but at the blade exit from the cabinet/throat plate some air should be sheared off.
Then passing through wood should shear off even more.
I really need to do some measurements on this but I won't be doing these I my own, maybe next time Interwood comes around we'll give this a crack.

I have my 6" DC inlet at A and as the bottom of the cabinet is hopper shaped there is very little dust in the cabinet corners. When I had the inlet at B so much sawdust built up inside the cabinet that sawdust leaked out of the gaps in the cabinet and it also bunged up the blade lifting mechism.

I wondering if a compressor blast at C would redirect the invisibles back into the cabinet. I bet it would make a fair old whistling racket though.

I only clicked on this thread as a whim......now you have me thinking.

I think you would still want some negative pressure inside a saw cabinet. This way any "loose" dust cannot fall out of any gaps, it will be sucked inwards.

My felder has a simple shoot which works very well. It has a 5" port that leads to an adapter that changes it to say 6" x 1 1/2". Now this is a big reduction in cross section, but it accelerates the air flow where it is needed.....right under the cut. The square tube sits right under the table, (it has neat cutouts for scriber but sits forward of the main spindle) The blade is dead in the middle of the tube and most importantly it has an adjustable ally back that can be moved right up to the arc of the blade. It works very well and the only stuff that ends up in the base of the machine is either thin offcuts that fall through on the far side of the blade or big chips. I have never seen fine dust in the cabinet. At times if i have a bind an get some smoke, often the first thing i know about it is the smoke coming from the dusty......

The big flaw with all circular saws though is one type of cut. The one were the blade is up to 99% in the cut. Shavings will fly off everywhere and there is nothing that can be done, unless you deploy movable magnetic hose etc. If you take a 1mm offcut off though, the dust goes downwards. When cutting whiteboard and MDF in the trade we always tried to have trimmings, just to keep the horrid dust down.

Blunt and incorrect blades also add to the fine dust particles, so keep you blades sharp!

Originally Posted by Ueee

I only clicked on this thread as a whim......now you have me thinking.
I think you would still want some negative pressure inside a saw cabinet. This way any "loose" dust cannot fall out of any gaps, it will be sucked inwards.

That was a given.

My felder has a simple shoot which works very well. It has a 5" port that leads to an adapter that changes it to say 6" x 1 1/2". Now this is a big reduction in cross section, but it accelerates the air flow where it is needed.....right under the cut. The square tube sits right under the table, (it has neat cutouts for scriber but sits forward of the main spindle) The blade is dead in the middle of the tube and most importantly it has an adjustable ally back that can be moved right up to the arc of the blade. It works very well and the only stuff that ends up in the base of the machine is either thin offcuts that fall through on the far side of the blade or big chips. I have never seen fine dust in the cabinet.

The dust we are talking about is sub 10 micron, it cannot be seen.

At times if i have a bind an get some smoke, often the first thing i know about it is the smoke coming from the dusty......

That's a good sign in terms of invisible dust collection but a bad sign in that sounds like your DC is inside your shed. If so that is an excellent demo that invisibles are passing straight through your DC and back into your shed.

The big flaw with all circular saws though is one type of cut. The one were the blade is up to 99% in the cut. Shavings will fly off everywhere and there is nothing that can be done, unless you deploy movable magnetic hose etc. If you take a 1mm offcut off though, the dust goes downwards. When cutting whiteboard and MDF in the trade we always tried to have trimmings, just to keep the horrid dust down.

I agree with this observation and thanks for that useful tip.

Blunt and incorrect blades also add to the fine dust particles, so keep you blades sharp!

Yep Agree 100%.

One method of stopping/reducing dust coming off conveyer systems especially at head/tail ends and loading stations is to use an air curtain, basically this is an airstream directed and positioned such that the rouge dust is blasted back into the main dust extraction, they usually cover the width of the dust hood where the belt exits, a similliar thing is the air curtain at the main doors of shopping centers, I think they are supposed to stop critters flying in when the doors open

Just thinking about this tho.....the dust exiting a conveyer hood would have much less velocity so this dust is easier to stop.....but it's kinda what we're talking about here, I googled air curtain dust control here's one...Powered Aire: Air Curtains and Air Doors

Food for thought....



Pete

Originally Posted by pjt

One method of stopping/reducing dust coming off conveyer systems especially at head/tail ends and loading stations is to use an air curtain, basically this is an airstream directed and positioned such that the rouge dust is blasted back into the main dust extraction, they usually cover the width of the dust hood where the belt exits, a similliar thing is the air curtain at the main doors of shopping centers, I think they are supposed to stop critters flying in when the doors open

They are designed to reduce the loss of airconditioned air to the outside. Some have an intake vent in the floor across the door. We have one at work at the entrance from a busy corridor into a semi clean facility. As well as minimising air exchange and hence dust that gets into the semi clean facility it also acts as a bit of an air shower and blow some dust off people as they walk through the curtain.

If you search for "air knives" and variations of, you might get some good ideas. Exair is a big supplier of them in the US. They do use a lot of air for the industrial ones. A home made unit for a saw blade might work with less air. An air knife across a belt or disc sander might work well at stripping off the dust on those chokers too.

Pete