Thread: Ducting update.

Winning the battle against dust Bob, lots of great ideas for when I eventually get my own workshop.

What do you do with all the 140 - 150 mm diameter PVC discs that result from making a dozen 150 mm blast gates?

140mmdiscs.jpg

I needed to make some more 100 mm gate so this is what I can up with that uses 2 of the 150 mm discs.
This adapter/port has a couple of metres of 50 mm hose attached to it that I use on power tools.
Before I made this BG I was using one of those black plastic generic BG on it that leaked badly as it didn't really fit tightly enough.
I have several 100 mm ports around the shed I can move this around to and use the 50 mm hose.
10050mmBG.jpg
10050mmBG2.jpg

I also had a go at making a 100 mm Rocker Gate - not that I needed a rocker gate, I just wanted to see if the plans would work.
This uses some 6 mm white PVC that I rescued from a skip at work.
It uses a plain bearing/space at the rocking point that holds the front and back panels 0.25mm apart and this makes the movement very smooth and easy to switch.
The purpose of this one is to, when desired, cut the suck to the sander disc so a bit more suck is diverted to the belt.
I can really recommend this design
100mmrocker.jpg

I also made another 150 mm sliding gate for the thicknesser port.
The screw cap is somewhat redundant but it looks good.

Normally the thicknesser is only attached to this port when it is in use and so a blast gate is somewhat redundant but you know how it is.
if there is no blast gate I'll be in the middle of a thicknessing job and will need to use another machine on the same trunkline so will have to disconnect the thicknesser and close the post with the screw cap.
With a BG in place I can quickly open and close various BG so I can quickly use the other machine and the switch back again.
thickyBG.jpg

So now there are
- 2 x 150 mm sliding gates to manage each of the major trunk lines.
- 5 x 150 mm sliding gates to each of the; Lathe, BS, TS, TS guard, and Thicknesser
- 150 mm Rocker gate to the Belt/disc sander
- 100 mm sliding gates to the router and the power tool dust collection hose.
- 100 mm Rocker gate to the Disc Sander.
- there is also another 150 mm sliding gate under the such for the TS cabinet to the under router box that diverts more suck.
That's enough for a while - I think.

As part of my "Syney Arbor" bridge efforts on dust extraction in my shed I decided to have crack at updating my TS blade guard.

My TS guard up until now has been "cobbled" together from a commercially available "Dust Picker arm and hood" to which I had added a clear Polycarbonate surround and bristle guard edges and this is what it looks like


It works quite well but I thought I could improve it a little more.

A few months ago the skips at where I used to work came good with all manner of acrylic, SS rod, nuts and bolts etc.

So I started with this 20 mm thick piece of acrylic about 200 x 300 mm
It had some large holes already drilled in it so I had to work around these- you'll see what I did as this WIP developes
Acrylic.jpg

Which I cut with my negative raked tooth blade on my TS
And I also routed out a stepped hole for the dust port fortunate a few existing holes were lost in the process.
The smaller diameter step is that of the ID of a 100 mm DWV pipe (102 mm) and the large step (110mm) is the OD of the same pipe
It was an interesting exercise cutting the hole with without using a central hole in the acrylic - if anyone wants to know I can post a couple of photos.
Acryliccut.jpg

Then I rounded over the smaller hole with a 5/8 (~16 mm) round over bit to create a sort of a Bell mouth.
The ideal rounding over for a 100 mm diameter port is 25 mm but you can see I did not have enough room at the edges - it would still have been possible but it would have really weaker the acrylic at those points.
I could also have used a 19 mm bit but I didn't have one of those.
I then glued a piece of 100 mm duct on the other side of the port so the bearing on the round over bit would ride on the PVC making a fully smooth transition.
Then I sanded the acrylic edge cut by the round over bit with 400 grit W&D and glazed the cut edge with a hot air gun to make it even smoother.

roundover.jpg

Next I cut of the shroud from some 1mm thick Poly carbonate (PC) sheet.
PC is worth using because
- it can be cut with tin snips
- it has a much greater impact strength than acrylic
- it bends easily and can be "cold formed" - i.e. permanent bends can be generated at room temp.

I decided to go for a wrap around on 3 sides shroud and getting the angles right was an interesting exercise which I trialled beforehand with a piece of paper.


PolyC.jpg


Then I had to make up a few bits and pieces
This is the T bracket that attaches by 4 CS SS screws to the acrylic base and the dust picker arm
I haven't done much TIG welding and this is one of the first SS weds I've done with my TIG and I was pretty pleased with the result - this was done without any filler rod.
Bracket.jpg

Then I thought a light of some kind would be useful

the base and cap are 50 mm DWV pipe fittings and the mesh is HD insect screen mesh.
The light is an 8W LED downlight that runs on 12V.
The Switch and the 12V supply jack are on top of the cap and the globe is held off the acrylic surface by a 12mm high Al ring (the globes get pretty hot) so I don't want it making a mess of the acrylic.
On top of the globe a 6 mm high Al ring holds the globe firmly in place.
The mesh is so the globe can vent.
Light.jpg

Here is a picture of the whole thing assembled.
An Al strip handle wraps around the acrylic acting as a handle and to clamp the PC shroud to the acrylic via 8 M6 SS screws.
The viewing is excellent and note how the back is completely open to allow enough air into the hood.
Wholething.jpg

And now with the light
whole2.jpg

Photos with lights often don't really show how much light is produced but it really is bright.
Wholething3.jpg

A just to prove I was visited by the FUF, the handle, which was also crooked on the old hood, and I kept reminding myself to get the new one right, still came out crooked.
The reason was, when clamping the PC, handle and acrylic up for marking out the holes, the "I want to be straight" piece of PC in place with the handle over the top was like holding down 3 cats with two arms. Only after I had drilled and threaded the holes did I realise OOPs.

handle.jpg

With the back fully open the PC is relatively floppy but that does not matter as all it needs to do is be strong enough to guide air flow.

Couple of more things.

The base of this design doesn't need to be made with acrylic - I could even be made of MDF.

The 1mm thick polycarbonate cost $90 for a 1.2 x 2.4 m sheet.
I'm going to make a Metal work lathe guard with it and I will also make a couple of guards for some Mens shed gear.
However, there will be some left over if anyone in Perth would like a piece at cost.

I have some small profile bristle guard that I might consider using - just got to figure a way of attaching it to the edge of the PC.

Hi Bob
Thanks for an interesting design. Adds to my ponderings regarding what I will build. So far I have not built one because I cant decide on a design that suits my work requirements.

Two Q if I may.
With your latest design how does the height of the guard adjust to suit different thickness wood. Doesnt seem as though the wood entering the saw could easily lift the hood. There is no apparent ramp at the front.

Also my fence is about 70mm high ( like yours) but I often have to cut thin strips off thin wood. Any suggestions how this would work. Still considering how I might allow the right side and front of the guard to rise automatically as the wood is pushed in.
Ron

PS your light addition is a marvelous idea.

Originally Posted by ronboult

Hi Bob
Thanks for an interesting design. Adds to my ponderings regarding what I will build. So far I have not built one because I cant decide on a design that suits my work requirements.

Two Q if I may.
With your latest design how does the height of the guard adjust to suit different thickness wood. Doesnt seem as though the wood entering the saw could easily lift the hood. There is no apparent ramp at the front.

Also my fence is about 70mm high ( like yours) but I often have to cut thin strips off thin wood. Any suggestions how this would work. Still considering how I might allow the right side and front of the guard to rise automatically as the wood is pushed in.
Ron

Hi Ron, just to give you more ideas, mine has independent side skirts that can move up about 40mm and have a front ramp as you describe that allows a piece of wood to feed in. The whole guard can be moved left to right on the rail to allow you to get the fence up close (within about 15-20mm).

20161010_070510.jpg

Hi Dom
thanks for your reply. I had seen your design before and it comes closest to meeting my requirements.
When I look at where most of the table dust comes from it seems to be thrown forward presumably carried round by the rotating blade.
Does your design with what appears to be some form of brush/ fingers at the front stop this dust escaping?

Also I had hoped that my guard would allow the right side to lift high enough that the fence could slide underneath. This doesnt solve the problem of what happens at the front when the fence is half way across the guard close to the blade when making thin cuts.
Still contemplating my design
Ron

With your latest design how does the height of the guard adjust to suit different thickness wood. Doesnt seem as though the wood entering the saw could easily lift the hood. There is no apparent ramp at the front.

Thats right there's no ramp because I just grab the front handle to raise or lower the hood to suit the wood but I don't lower the hood so that it seals up against the wood as this is not necessary and is in fact counter productive to air flow (see below). The hood is attached to an elbowed arm from the roof of my shed and is balanced by a lead counter weight over by a nearby wall. The counter weight and hood are connected by a SS wire thru a couple of pulleys so the the hood just stays where I put it. It also moves from side to side but that requires loosening and tightening a threaded knob.

This approach suits setups which have a fixed TS location. However this hood is also suitable for attachment to an articulated arm attached to the saw.

The main reason I don't like using a ramp is because it impedes air flow by forming a turbulent zone inside the hood. Folks do get obsessed about having zero gap around and over the wood but what they need to appreciate is that if you have a decent size port and duct to begin with (i.e. at least a 100mm) then gaps will aid air flow. Yes some chips will escape but I don't care about these as they both worth worrying about .
A solution to chip escape is to used bristle guard edging that the wood can push thru. This the means that the hood height does not need accurate good positioning, but it does reduce air flow.

Also my fence is about 70mm high ( like yours) but I often have to cut thin strips off thin wood. Any suggestions how this would work.

This is indeed a problem. My hood does move laterally +/- 50 mm so I can position it so the side of the hood is closer to the blade and up against the fence but I am only game for this to be no less than about 25 mm. For strips less than this the hood has to be raised above the fence.

Still considering how I might allow the right side and front of the guard to rise automatically as the wood is pushed in.

Originally Posted by ronboult

Hi Dom
thanks for your reply. I had seen your design before and it comes closest to meeting my requirements.
When I look at where most of the table dust comes from it seems to be thrown forward presumably carried round by the rotating blade.
Does your design with what appears to be some form of brush/ fingers at the front stop this dust escaping?

Also I had hoped that my guard would allow the right side to lift high enough that the fence could slide underneath. This doesnt solve the problem of what happens at the front when the fence is half way across the guard close to the blade when making thin cuts.
Still contemplating my design
Ron

Like Bob says it's probably mainly chips that will have enough energy to be thrown forward but i don't like the mess and i'm sure those chips carry some fine dust with them.

I have a reinforced rubber sliced into thin strips that holds it's shape against the vaccum but allows wood to be pushed through. It does a really good job. Normal cutting of timber less than 40mm leaves no mess on the table in front of the blade (or anywhere else). Even edge cuts or dado cuts aren't a problem.

20160825_204523.jpg

Originally Posted by DomAU

Like Bob says it's probably mainly chips that will have enough energy to be thrown forward but i don't like the mess and i'm sure those chips carry some fine dust with them.

The ability of an air stream to separate fine and coarse chips in an interesting one .

My most detailed testing of this has been while using my lathe.
With 6" bell mouth hood (BMH) dust extraction running behind the work a large number of large chips are still squirted all over the place (many towards me) but a particle counter at operator head height does not detect any fine dust (<5 microns) above that of the variation of outside air dust particle concentration. This is because the 1000 CFM is able to suck all that fine dust away.

Depending on the design of the TS guard, the guard has a max of 1/3rd the flow of a 6" BMH so it has less air to fight the dust cloud with but the guard shroud should constrain the dust much more than a naked hood.
Remember the fine dust will move where ever the air is moving. If the rubber fingers on the end of the guard are pointing even slightly into the hood that's where the fine dust will go. OTOH the visible dust has more momentum and so can more easily go against the air stream. This is shown by the spray of visible chips from the lathe being up to 3m, and fine dust spreading all over the shed with no DC running.
With the DC running there is no fine dust left behind in the shed and the spray of visible dust is reduced to about 1.5m from the lathe.

DomAU's nice setup has a Clearvue at the end providing the suck so little of the following applies, but folks who don't have a ClearVue and need to wring every bit of performance out of their DC might want to consider this.

The greatest air speed of the dust being generate is that being flung away from the saw blade at A.
The highest air speed into the flex will be at at B.
The air thus has to make somewhat of a right angle turn to get from A to B
To collect dust at source, ideally A and B should be as close together as possible, so moving B so that it above A should improve collection and also improve visibility of what is going on at the edge of the blade.
Additionally a BMH dust port will nominally provide the same air speed at double the distance from the port as a non BMH.
So whatever the air speed is for a non BMH is at C , a BMH will produce at D.

All these should add up to make a useful difference.





Sawguard.jpg

The design of an effective tablesaw guard/ dusthood is an interesting problem. On one hand we want it to collect ALL the fine dust but many of us would LIKE it to also contain the larger sawdust as well.

The problem is that these two particle sizes have vastly different properties. Fine dust has virtually no inertia and once it leaves the saw blade it goes where the airflow goes. The larger particles have much more inertia and can travel greater distances before air friction reduces their velocity so they settle out. Collecting both creates problems for an effective design.

Firstly the tablesaw hood is a partly enclosed system that almost certainly has significant turbulence inside. As the tablesaw hood is a partly enclosed system I am not sure that comparing it to an open bellmouth behind a woodlathe has much relevance.
Secondly the top of the saw blade where the dust is released is above the wood being sawn and normally above the bottom of the hood.

Consider the following scenarios.
A rectangular hood sealed flat all round on the table saw table would have no airflow except through the throat slot, but would leak no dust or sawdust. As the hood is raised off the table air would begin to flow under all the edges. All the airflow should be inward and no fine dust would escape. Remember fine dust goes where the airflow goes. If the airflow is fast enough saw dust sized particles would also not escape. This assumes that the dust extractor has sufficient suction to produce sufficient negative pressure inside the hood so that all airflow is into the hood. As the hood is raised higher the air velocity under each edge will decrease and while all the dust should still be caught, more and more of the larger particles will escape against the lower air velocity. Removing the back of the hood would exacerbate this problem.

The problem with this analysis is that it does not take into account turbulence in the hood. Turbulence may allow fine dust to escape if any air leaves the hood other than via the ducting.

If my analysis has any validity I am beginning to think that a design that produces only small gaps around the base will be the most effective for both fine dust and sawdust. Doms design with auto lifting sides may be the go.

The problem for me is that I dont have a particle counter so I can test my designs. It easy to see if sawdust leaves the hood but impossible for me to detect escape of submicron dust.
Whatever design I construct it MUST allow me to use the TS unobstructed or it will not get used. I suppose any hood will be better than what I have now which is no hood.!!!
Ron

Originally Posted by DomAU

Hi Ron, just to give you more ideas, mine has independent side skirts that can move up about 40mm and have a front ramp as you describe that allows a piece of wood to feed in. The whole guard can be moved left to right on the rail to allow you to get the fence up close (within about 15-20mm).

20161010_070510.jpg

Dom, if the main body that the skirts and extraction pipe attach to were to be higher than the fence the skirt nearest the fence could be lifted to sit on top of the fence and the fence can then be moved as close to the blade as you wanted. This puts the fence inside the perimeter of the hood and a flexible front skirt should provide enough seal to prevent a huge leakage of chips. My design was similar in principle but the side nearest the fence was sliding on brackets and lifted to allow the fence to get closer to the blade. I also had short brushes at the base of the hood as well that sealed against the saw table.