Thread: 2hp dust extractor mod - build

Lots of progress - I have really broken the back of it now.

Jointer and bandsaw now fully connected, though I will add another 4" pipe to the bandsaw to collect the dust from above the table. See photos below

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I finished all the blast gates and I'm very happy with the result. Walls are made from 9mm mdf, mainly because that is what I hard to had. Gates are from 3mm ply, with the runners made from white fronted Masonite. This worked really well, as the Masonite is 3.2mm thick, and the ply runs nicely in the space provided.

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Lots of trial and error in making the gates. As I mentioned before, I sized some of pipes to fit inside standard 150mm pipe and others at the full size to fit into wyes and other junctions. Getting a strong bond between the mdf and PVC took a few goes. I can confirm that liquid nails doesn't work, the PVC just pulled right off.

I ended up using polyurethane glue, which worked an absolute treat. The bond is very strong, it fills gaps and is easy to apply. I'd never used polyurethane glue before, so I didn't realise it would expand quite so much, traps for new players!

Having made quite a few gates now (7, plus the 'practice' gates), I found the easiest process was to find the centre of the board you are using for the wall, mark the centre then use a compass to draw the outside circumference of the 150mm pipe. Then cut a smaller hole (I used a variable circle cutter in the drill pres, but you could use a drill and jigsaw) - it doesn't need to be neat or accurate. Then run a solid bead of glue around the line you drew and fit the pipe. Once it is dry, use a flush trim router bit to clean up inside the pipe.

For the inside-fit, I cut the pipe lengthways on the bandsaw then removed a small section to reduce the diameter of the pipe. Cut the mdf to fit this tightly, fit the pipe to the hole and use a solid bead of poly glue around the joint. Use paper or some thing over your benchtop. Don't ask how I know this. The poly glue is really good to fill in any gaps.

Last thing I worked on today was the table saw port. Inspired by Pete's work, I built a port for the back of my saw.
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I recognise that it is not ideal, nor as pretty as Pete's sheet metal work, but hopefully it should work ok. I fitted a new base to the dust cabinet of the saw, to block off access to the 4" port. I sloped the sides to help with dust collection, although the RHS could only slope slightly given the position of the saw carriage and the way it moves when the blade is tilted.

The hose will connect to the box hanging out the back, which will have the airflow smoothed by a bit of bendy ply or sheet metal to give a gentle curve into the cabinet. All the parts where the mdf meets the dust cabinet have been rounded over to smooth the airflow too.

I've not been able to try it, as I only glued up the joint between the hose and cabinet. Hopefully will get a chance to do this in the next few days. But I go back to work tomorrow, so time will be hard to come by.

As always, grateful for any comments or suggestions.

Trav

Square to round transition is what connects the pipe to the outlet (from the saw) , just google it and you will find lots of (how to's)
I would also put a curve (1/4 of a circle) inside the outlet (right hand side pic 9) best if it was the same width as the outlet and also a radius on the left hand side, this could be a piece of pipe (100mm dwv) or such, cut or formed so the circle was tangent with the inside edge of the outlet (the short piece)




Pete

Thanks Pete. The pic below was what I was planning. I'd a long curve into the cabinet, then a round over on the LHS of the entry. Is this what you meant ?

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Trav

Originally Posted by Trav

Thanks Pete. The pic below was what I was planning. I'd a long curve into the cabinet, then a round over on the LHS of the entry. Is this what you meant ?

Yep that looks good but what Pete means is then you will end up with a Rectangular exit port.
From there you will need a Rectangular to Round (ducting) transition to get a smooth flow of air - if not then you will get more turbulence and restricted flow.

I've been kicking this one around a bit. I've googled square to round transitions and there's no way I can make one from sheet metal as the plans I found suggest. I don't have the tools or skills. I could get a sheetie to make one up, but that would be expensive.

Is there any reason I couldn't smooth the transition with builders bog or putty? A 150mm pipe fits neatly into the end of the box. If I tapered up to the lip of the pipe, over a length of 100mm or so, would that provide enough of a smooth transition?

As a last resort, how much turbulence would it create to have an abrupt change?

Trav

Originally Posted by Trav

I've been kicking this one around a bit. I've googled square to round transitions and there's no way I can make one from sheet metal as the plans I found suggest. I don't have the tools or skills. I could get a sheetie to make one up, but that would be expensive.

It's actually relatively easy to make.
The way I do it is make a mock up using cardboard - then use that as a template.
It will be a lot easier if you use thin sheet metal you - if you can use Al that is even better as you can bend that by hand
The tools I use are set of tin snips, a pair of clamps, a pop rivetter and some sealant.
I make the bends from each corner of the the square by clamping the sheet metal along the lines between 2 pieces of 25 x 25 angle iron and using a wooden mallet to initiate a bend or a crease.
Then once all the creases are started it should bend more while you are bringing it all together.
A couple of pop rivets at the join and a bit of sealant and belt it around into better shape a bit with the wooden mallet.
A bit of sealer to bog up the holes and its done.

Is there any reason I couldn't smooth the transition with builders bog or putty? A 150mm pipe fits neatly into the end of the box. If I tapered up to the lip of the pipe, over a length of 100mm or so, would that provide enough of a smooth transition?

That would work but you will need a lot of bog.

As a last resort, how much turbulence would it create to have an abrupt change?

Could be as much as 10% of the air flow.

Originally Posted by Trav

Thanks Pete. The pic below was what I was planning. I'd a long curve into the cabinet, then a round over on the LHS of the entry. Is this what you meant ?

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Trav

The long curve is good, however, the thing to try and avoid is reducing the area of the outlet, as you have it drawn there the long curve is encroaching on this by a small amount so I'd finish the curve on the long side about what looks like half way along the long side, for symmetry whatever the width of the outlet is make the curve finish along the long side this same distance.

The short side (LHS) is still too much of a sharp edge, the air that is coming around that edge crashes into the air that is coming from the long side and causes turbulence and this then is reduced flow, Bob talks about bell mouth entry and this is what you should try to do on this short side edge, the solution will be a little different but the same, cut a piece of 100 pipe the height of the outlet and just sit it on the floor of the outlet so the outside of the pipe is inline with the short side outlet, hopefully things become clearer at this point and you will see how now you have a much longer radius for the air to flow around, I'd draw it in there but...... there's still a bit more to do to fully resolve this corner......

The square to round transition can look a bit daunting and there's a few things that have to be right such as the true length lines if you draw it as a plan, alternatively you could set up a dummy square outlet and pipe and physically measure each line, cardboard and scissors will also work.
As Bob says thin sheet can be bent by hand or clamped between some angle iron or just held over the edge of the bench and use a piece of wood (saves your hand) to force a crease into the metal as it only is a few degrees bend each time, depends on how many divisions.




Pete

Thanks Bob and Pete.

For the outlet itself, I think I understand what you mean Pete. I've done another dodgy sketch - let me know if you think this is better.

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I've shortened the curve on the outside to end where the intake ends. I've also drawn in (not necessarily to scale) what I think you mean about the short side of the intake. Is this what you mean? I am conscious not to penetrate into the cavity of the cabinet, which could lead to the airflow problems I marked in green. I presume that rather than the full red piece I marked, I'd be better off cutting at the yellow line?

In terms of the transition, I'll think more about whether I could make something up. I hadn't thought about making it from aluminium rather than sheet steel, so that would certainly be more possible. But I can't help but thinking that bog would be easier. A few off cuts to pack out the shape, smoothed over with bog. If I am going to lose 10% of flow, I will need to do something.

Thanks

Trav

I've shortened the curve on the outside to end where the intake ends. I've also drawn in (not necessarily to scale) what I think you mean about the short side of the intake. Is this what you mean? I am conscious not to penetrate into the cavity of the cabinet, which could lead to the airflow problems I marked in green. I presume that rather than the full red piece I marked, I'd be better off cutting at the yellow line?

Another possibility is if you have access to a lathe your could turn a 100 mm wooden cylinder and add that to the outside edge like this.

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Bob's on the money, Any reason why you don't want to encroach into the cabinet? Is the concern possible turbulence as you have drawn with the LHS green arrow, if you make it as Bob has drawn it I would think turbulence would be minimal, a possible improvement here would be to add a piece of sheeting from the cabinet side of the 100 and go back into the cabinet, the aim being to help flow into the outlet box as much as possible.

I was thinking of dam spillways today as a good visual way to explain air flow, I just googled dam spillway and found a few interesting pics....
Blowering Dam spilling.jpgimages.jpeDam & Spillway Lake Berryessa, Glory hole, Napa California.jpgmonticello_dam_glory_hole4.jpg
A top view of the first one would have been better but the curve on the far side is the same thing we are trying to recreate on the short side on your cabinet, as is the second one, this is a section thru the spillway wall but still shows the curves you want, the next two are bell mouth entry (for water in this case) but same thing for air




Pete

Good examples Pete.

Originally Posted by pjt

Bob's on the money, Any reason why you don't want to encroach into the cabinet? Is the concern possible turbulence as you have drawn with the LHS green arrow, if you make it as Bob has drawn it I would think turbulence would be minimal, a possible improvement here would be to add a piece of sheeting from the cabinet side of the 100 and go back into the cabinet, the aim being to help flow into the outlet box as much as possible.

A plain piece of sheeting back into the cabinet with a sharp end may still induce some turbulence at the edge.
See B where the red cross is.

If you do that then a rolled edge to that sheet as per D or with a wooden cylinder like B would be better.
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The rolled is sometimes seen on the edge of rectangular ducting. The squirrel cage fans in my shed both have them.

Thanks again for your advice Bob and Pete. Hopefully we will get there soon.

I like the dam photos Pete. Good example of a bell mouth in action!

I am only concerned about encroaching into the cabinet to avoid turbulence. I think I could add in a 100mm PVC "tower" as shown in the image A that bob produced, and then a new "wall" to smooth the air as it comes into that area. I think this is what Pete was suggesting.

I've made an alteration based on Bob's diagrams. Would this work well?

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An alternative might be to cut 1/4 of the column out, and reverse it, as in the next diagram. This is probably easiest as I will need to cut out a section of the column anyway to attach it to the cabinet.

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Anyway, thanks again for help to date.

Trav

Turbulence will only be significant where there is significant flow. The amount of flow in the areas indicated by your green lines in the diagrams above will be relatively low (i.e. 10% of 10%) so you can probably ignore those.

Originally Posted by Trav

Thanks again for your advice Bob and Pete. Hopefully we will get there soon.

I like the dam photos Pete. Good example of a bell mouth in action!

I am only concerned about encroaching into the cabinet to avoid turbulence. I think I could add in a 100mm PVC "tower" as shown in the image A that bob produced, and then a new "wall" to smooth the air as it comes into that area. I think this is what Pete was suggesting.

I've made an alteration based on Bob's diagrams. Would this work well?

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An alternative might be to cut 1/4 of the column out, and reverse it, as in the next diagram. This is probably easiest as I will need to cut out a section of the column anyway to attach it to the cabinet.

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Anyway, thanks again for help to date.

Trav

Yep the top A is what I was thinking or something like it, when not looking directly at the machine it is a bit harder to be more definitive but the minimum I would do is A in Bob's set of sketches, I also agree that any additional sheeting (the green lines) is small cheese with regards to improving flow/reducing turb. However, looking a A I think benefit could be had by taking a green line to the back of the cabinet (basically rotate the green line around 50 ish degrees) and up to the underside of the table effectively isolating the portion of cabinet to the left of the round, this reduces a big dead flow area.



Pete