Thread: Attacking my Bandsaw...

Here you go.






Ducts don't need to be round. A 160 mm duct could be squished into a flatter cross section with rounded ends. It would need to be opened up a bit more (e.g. 25% to retain air flow)

I don't know why BS come with a plastic or fibreglass annual tubular collar that fits around the underneath of the table with a duct for a 4" flexy. I guess that rules out table tilt although I have never used that feature on a TS.

Originally Posted by BobL

Here you go.

Thank you!
I know it is super basic stuff -- but how did you do it?

Originally Posted by BobL

Ducts don't need to be round. A 160 mm duct could be squished into a flatter cross section with rounded ends. It would need to be opened up a bit more (e.g. 25% to retain air flow)

I'm not quite sure how to do this. The trunnions in the picture have an internal diameter of 140mm across, and only 90mm vertically, between the concave internals of the trunnion and the underneath of the table. This appears to be a cross-sectional area of just over half of a 140mm flex, which is a far cry from a full 160mm diameter flex. What have I missed?

Originally Posted by BobL

I don't know why BS come with a plastic or fibreglass annual tubular collar that fits around the underneath of the table with a duct for a 4" flexy. I guess that rules out table tilt although I have never used that feature on a TS.

Again, not sure what you mean? What is an "annual tubular collar"?

Your reference to table tilt does give an entirely different option, which, with my limited metal working skills, I would find challenging. I have no interest in table tilt whatsoever, which means I could potentially ditch the trunnions altogether and come up with a "different" way of supporting the cast iron table which allowed for bigger ducting. Perhaps you meant something along these lines?

Details aside, is there consensus that below table extraction on a bandsaw ought to be a large duct as close as possible to the action, regardless of whether it comes in from the front, back, or side?

Originally Posted by LuckyDuck

Thank you!
I know it is super basic stuff -- but how did you do it?

Photo editing application. I use Photoshop but even the freebie versions will do it.

I'm not quite sure how to do this. The trunnions in the picture have an internal diameter of 140mm across, and only 90mm vertically, between the concave internals of the trunnion and the underneath of the table. This appears to be a cross-sectional area of just over half of a 140mm flex, which is a far cry from a full 160mm diameter flex. What have I missed?

If you can't fit a 160 mm diam tube underneath the table then fit whatever you can. See below.

Again, not sure what you mean? What is an "annual tubular collar"?

Think of a Mini Minor sized tyre tube. Now remove about 1/4 of the tube so you have a "C" shape. Then remove the inside 1/4 of the rest of the tube and then put a hole in the middle of the outside and attach a 4" flexy here.
Another shape that comes close is one of those travel pillows that wrap around the neck. Obviously the pillow can't be used but that's the shape that would collect the dust.
An eve closer shape is a modified bed pan. Duct gets attached to where purple lines are.

LD5.jpg

Details aside, is there consensus that below table extraction on a bandsaw ought to be a large duct as close as possible to the action, regardless of whether it comes in from the front, back, or side?

Unlike a sander, lathe TS or CMS where the spinning blade/drum/belt that rapidly changes direction and sprays dust away from itself a BS generates a cloud of fine dust under the table and some above the table and some sawdust gets dragged by the blade into the bottom half of the cabinet. Hence high speed air flows are not needed to capture the dust from this machine. 3 x 4" ducts strategically place should be sufficient.
On a small bandsaw Bill Pentz recommends a 5" inlet under the table and a 3" above the table - this adds up to about the flow that a 6" duct connected to a DC can generate.
On a drill press even less flow is needed - just one x 4" ducting is all that is needed.

Not sure about your saw, but both my band saws leak significant fine dust from under the table if I don't have a duct there. My testing showed the under table duct as necessary rather than optional. The trick seems to be to get enough velocity flowing over the blade to pull away as much dust as possible and to stop dust swirling around under the table. The old saw uses three 90 mm lines. The new saw uses three 100 mm lines. Ducting is 150 mm.

On my old saw I installed a plastic pipe to grab the under table dust, but that severely restricts table tilt, which I have used only once. It took all of a minute to remove the pipe so I could tilt the table. On the new saw I "clicked in" a 4" flexy under the table. The new saw has a port at the very top of the bottom cabinet, but when in use I still get dust swirling around under the table, so I blanked it off and clicked the flexy in between the cabinet and the table. Much better.

New saw ...
aaaFront (Finished).jpgaaaBack (Finished).jpg

Old saw ...
IMAG0036.jpg

Both saws are very clean when cutting thinish material. I do produce more dust on the table when cutting wide veneers with the new saw. However, I think the over table duct (which is attached near the upper guides) does a good job of keeping that dust away from my face (inlet for new saw has been improved since this pic was taken). The dust gets caught in the kerf and drops onto the table as the veneer bends away from the stock, so I doubt much of it is becoming airborne.

I have noticed that the old saw does a better job of keeping the lower cabinet clean. I think this is due to three things:

1. The new saw uses my only 15 Amp circuit, so when it is in use the cyclone is connected to a 10 Amp circuit and is only running on 50 Hz. Once I get another 15 Amp circuit installed, I'll get 20% odd more air moving.
2. The port on the cabinet of the new saw is further away from the blade and the air intake into the cabinet. Don't plan to do anything about this until I am running the cyclone at 60 Hz.
3. The new saw is used exclusively for re-sawing and cutting veneers. It produces a lot more dust that the old saw ever could, but I still think the major issue is insufficient velocity/volume of air (running at 50 Hz).

Originally Posted by BobL

Unlike a sander, lathe TS or CMS where the spinning blade/drum/belt that rapidly changes direction and sprays dust away from itself a BS generates a cloud of fine dust under the table and some above the table and some sawdust gets dragged by the blade into the bottom half of the cabinet. Hence high speed air flows are not needed to capture the dust from this machine. 3 x 4" ducts strategically place should be sufficient.
On a small bandsaw Bill Pentz recommends a 5" inlet under the table and a 3" above the table - this adds up to about the flow that a 6" duct connected to a DC can generate.
On a drill press even less flow is needed - just one x 4" ducting is all that is needed.

Thank you for your input Bob. There is some new information here for me -- in that the difficulty of capturing the fine dust is not that it is somehow inherently difficult to capture, but that its behaviour in the presence of fast moving machine parts makes it difficult to contain.

But I'm not sure I've got this right? Sure, a bandsaw blade has far less mass than a tablesaw blade. I have no idea if it runs at the same speed or not, but I would have thought the two bandsaw wheels would generate significant "wind currents" which would need to be overcome by the required 1000CFM recommended by Pentz. Hence, I'm scratching my head about your "high speed air flows are not needed to capture the dust from this machine"? What have I missed?

Thanks John for your input. Much appreciated.

Originally Posted by John Samuel

Not sure about your saw, but both my band saws leak significant fine dust from under the table if I don't have a duct there. My testing showed the under table duct as necessary rather than optional. The trick seems to be to get enough velocity flowing over the blade to pull away as much dust as possible and to stop dust swirling around under the table. The old saw uses three 90 mm lines. The new saw uses three 100 mm lines. Ducting is 150 mm.

Yes, my saw has woefully inadequate collection/extraction at the moment. The single 120mm port down in the bottom corner of the lower cabinet does virtually nothing -- and this on an Italian-made industrial machine!

Originally Posted by John Samuel

On my old saw I installed a plastic pipe to grab the under table dust, but that severely restricts table tilt, which I have used only once. It took all of a minute to remove the pipe so I could tilt the table. On the new saw I "clicked in" a 4" flexy under the table. The new saw has a port at the very top of the bottom cabinet, but when in use I still get dust swirling around under the table, so I blanked it off and clicked the flexy in between the cabinet and the table. Much better.

I have seen your three port solution which looks very good. I was thinking that since larger diameter pipes get bigger airflow, I would just split my 200mm into two ducts, with the bigger 160mm duct below table. But my problem is that these machines were not made for 160mm diameter ducts. Is that the reason why you opted for two below table ducts of 100mm rather than going with a single, larger diameter duct?

Assuming that I can satisfactorily fit (= bodgy) a 160mm duct close to the blade, under the table, do you think that would be the best solution? I don't mind effectively eliminating the option of tilting the table. I've had the machine for 8 years and have never tilted it. Perhaps your partial duct, with a slot, or something similar to the "bed-pan" idea of Bob's, will help me get the suction closer to the blade? I will likely experiment once I have the ducting partially in place.

Do you happen to have a shot of your ducting as it comes right up to the blade underneath the table?

Originally Posted by John Samuel

I have noticed that the old saw does a better job of keeping the lower cabinet clean. I think this is due to three things:

1. The new saw uses my only 15 Amp circuit, so when it is in use the cyclone is connected to a 10 Amp circuit and is only running on 50 Hz. Once I get another 15 Amp circuit installed, I'll get 20% odd more air moving.
2. The port on the cabinet of the new saw is further away from the blade and the air intake into the cabinet. Don't plan to do anything about this until I am running the cyclone at 60 Hz.
3. The new saw is used exclusively for re-sawing and cutting veneers. It produces a lot more dust that the old saw ever could, but I still think the major issue is insufficient velocity/volume of air (running at 50 Hz).

I think I have missed something here between your post and Bob's? Bob is saying that we don't need a huge amount of air to capture the fine dust made by a bandsaw, but you seem to think it is desirable to eek out the best performance possible from your cyclone (i.e. by running it at 60hz). Like you, I do a fair bit of resawing, so I'm keen to get the best dust extraction solution I can for my machine.

Thanks for your help!

Originally Posted by LuckyDuck

I have seen your three port solution which looks very good. I was thinking that since larger diameter pipes get bigger airflow, I would just split my 200mm into two ducts, with the bigger 160mm duct below table. But my problem is that these machines were not made for 160mm diameter ducts. Is that the reason why you opted for two below table ducts of 100mm rather than going with a single, larger diameter duct?

I am not familiar with your dust collector, but if you have a 200 mm duct then you are going to have bags of air for a band saw.

BobL is right. The blade speed of a band saw is slower than a table saw or drop saw, therefore so too is the speed of the dust. The blade guides can make it swirl around, but whilst that can complicate matters a little, the guides are likely to slow down some of the dust. The slower the dust speed, and the less momentum the particles have, the easier it is for the air flow to grab it.

As you might guess, I like the three duct solution. I did fairly extensive tests on my old saw to convince myself it was the right solution. It was. I definitely needed all three intakes to go a really good job of grabbing the dust. On my set up most of the dust is collected by the duct between the lower cabinet and the table. The next biggest dust collection point is the lower cabinet, and the least dust is collected by the over-table duct.

On your machine I would be inclined to put a 4" line over the table, another 4" line under the table, and whatever remains (perhaps a five or six inch duct) to the lower cabinet (providing your dust collector can keep the velocities up ... about 4,000 FPS). If you can fit a bigger duct under the table, go for it. However, I reckon a 4" line close to the blade will do the job. For the under the table inlet it was important on my machine to keep the inlet close to the blade so my air velocity stayed high.

When thinking about the Hz my cyclone runs at it is important to recall that my new machine has both lower velocity and lower volume because it is currently running at 50 Hz. Also, it has 100 mm ducts, the old machine has 90 mm ducts. The duct being bigger alone reduces my velocity a bunch, even if the volume was the same. Then because I am only at 50 Hz, I lose volume and even more velocity. I think Bob's comment relates to blade speed, as discussed above. We still need to do a first class job on ports/shrouds to get good dust collection. I will do no more fine tuning on the new saw until I am running the cyclone at 60 Hz. However, I expect that when sawing wide veneers and the like I am probably always going to get more dust on the table than when sawing thinner pieces.

I get enough air through a 150 mm main duct to do a pretty good job. With a 200 mm duct you should have no problems, but unless your saw is very different to mine, I reckon you will need three ports, not two.

Here is a shot for my new machine. Note how the flexy is at most 35 mm from the blade. It pulls air from both over the guide (which you can see) and from under that guide,(which does not show in this shot).
IMAG0558[1].jpg

This is my old machine. The fitting at the top of the pipe is a 45 degree bend cut away so I could get the airflow as close as possible to the blade.
IMAG0559[1].jpg

Finally, look at the two blades in the pics. They are a world apart. The re-saw blade has big teeth with huge gullets compared to the small blade on the old saw. The new saw produces MUCH more dust, especially when cutting boards 250 - 350 mm wide.

Originally Posted by John Samuel

I am not familiar with your dust collector, but if you have a 200 mm duct then you are going to have bags of air for a band saw.

I hope so! The FELDER RL250 is a mighty beast of a machine, at 7.5HP or 5.5kW S1 duty rated, and 840kg. While Pentz "grudgingly" admits the RL units are "good enough" (with reference to the RL160 and RL200), he is not impressed with the cost of the units which is very high indeed. However, apart from the fact that I do not want to vent externally (my house is double brick and I'm really keen not to punch holes in the walls or roof), I very highly value the fact that I have 500L of chip collection capacity in the form of two large bins lined with heavy plastic bags. Over the course of several hours of machining, I can fill ten such bags, pile them high on my trailer, and get rid of them at the tip and then start over again (the bags are reusable). I don't have the luxury of emptying a smaller bin outside, as most cyclone owners appear to do. I have too much volume going through my shop.

The RL250 has a 50m2 filter system with integrated pulsed air cleaning which, when put in automatic mode, means that the filters will (touch wood!) not clog up. The RL250 is "rated" for multiple gates open, to service multiple machines at once. But we both know that that "rating" is not good enough. My plan is to use one machine at a time, with just the right amount of gates open to approximate the cross-section of the 250mm mainline; I should be able to optimise the number of gates open to obtain the best extraction possible for the unit. I plan to use 120mm, 140mm and 160mm ports on 200mm verticals. I know I still have a lot to learn; but I'm a willing learner! None of my ducting is up yet; still a lot of work to do.

Originally Posted by John Samuel

BobL is right. The blade speed of a band saw is slower than a table saw or drop saw, therefore so too is the speed of the dust. The blade guides can make it swirl around, but whilst that can complicate matters a little, the guides are likely to slow down some of the dust. The slower the dust speed, and the less momentum the particles have, the easier it is for the air flow to grab it.

This is very useful to know. I assume that the TS, SCMS, Thicknesser/Planer and Lathe are different propositions, and we need as much airflow and volume as possible for them?

Originally Posted by John Samuel

As you might guess, I like the three duct solution. I did fairly extensive tests on my old saw to convince myself it was the right solution. It was. I definitely needed all three intakes to go a really good job of grabbing the dust. On my set up most of the dust is collected by the duct between the lower cabinet and the table. The next biggest dust collection point is the lower cabinet, and the least dust is collected by the over-table duct.

This is a revelation to me, and I appreciate you taking the time to explain your position. From all I've read -- and I've read extensively on this forum, and the Bill Pentz site, etc. -- the message has come home to me that larger diameter ducting produces better dust extraction, as long as the air volume (e.g. 1000CFM) is maintained. From what you're saying, however, it is actually better to split the ducting into smaller (e.g. 4") sizes. The reason why I am surprised -- not doubting you -- but surprised, is that so many posts "poo-poo" the multiple 4" ducts and strongly recommend combining the ducting into one bigger duct of 6". Is it just that the BS is a different machine with different extraction requirements, or what have I missed?

Originally Posted by John Samuel

On your machine I would be inclined to put a 4" line over the table, another 4" line under the table, and whatever remains (perhaps a five or six inch duct) to the lower cabinet (providing your dust collector can keep the velocities up ... about 4,000 FPS). If you can fit a bigger duct under the table, go for it. However, I reckon a 4" line close to the blade will do the job. For the under the table inlet it was important on my machine to keep the inlet close to the blade so my air velocity stayed high.

Would you say that your 3-line solution is better at chip collection, in that it is the most efficient at getting the visible chips that collect on top of the timber being cut, and collect in the bottom of the cabinet? May I clarify that you believe your solution is also better for capturing the invisible dust too? The reason why I ask, is that just about every other post from the gurus on this forum advocates a 6" port (with bell mouth) as the most efficient way of capturing that finest dust. Please understand that I am not doubting you, just asking for clarification! Thank you.

Originally Posted by John Samuel

I expect that when sawing wide veneers and the like I am probably always going to get more dust on the table than when sawing thinner pieces.

This has definitely been my experience, even with my "woefully inadequate" dust extraction (ahem: chip collection) at the moment. There is a lot more visible sawdust up on the table when resawing. My biggest blade is 1" although I quite like the 1/2" size as a general all-rounder and rarely change it up or down.

Originally Posted by John Samuel

I get enough air through a 150 mm main duct to do a pretty good job. With a 200 mm duct you should have no problems, but unless your saw is very different to mine, I reckon you will need three ports, not two.

All bandsaws appear to be, more or less, the same construction. Mine seems a bit tighter up under the table than some, as the cabinet is only just as wide as it needs to be. It would be impossible to introduce a decent sized flex side-on as you do, as the trunions and associated steel supporting the trunnions are too bulky and in the way. I like Bob's idea of the "bed-pan" design, coming around either side of the trunions. I will have to "play"!

[QUOTE=John Samuel;1754406] Here is a shot for my new machine. Note how the flexy is at most 35 mm from the blade. It pulls air from both over the guide (which you can see) and from under that guide,(which does not show in this shot).
IMAG0558[1].jpg

This is my old machine. The fitting at the top of the pipe is a 45 degree bend cut away so I could get the airflow as close as possible to the blade.
IMAG0559[1].jpg

Originally Posted by John Samuel

Finally, look at the two blades in the pics. They are a world apart. The re-saw blade has big teeth with huge gullets compared to the small blade on the old saw. The new saw produces MUCH more dust, especially when cutting boards 250 - 350 mm wide.

Thank you for the pictures. I can see that you must get excellent extraction by having your flex so close to the blade and the blade supports. Of course, I'll let you know how it goes for me! Cheers.

Originally Posted by LuckyDuck

Would you say that your 3-line solution is better at chip collection, in that it is the most efficient at getting the visible chips that collect on top of the timber being cut, and collect in the bottom of the cabinet? May I clarify that you believe your solution is also better for capturing the invisible dust too? The reason why I ask, is that just about every other post from the gurus on this forum advocates a 6" port (with bell mouth) as the most efficient way of capturing that finest dust. Please understand that I am not doubting you, just asking for clarification! Thank you.

One (impractical) solution for collecting fine dust from a Band saw (or an wood working machine) is to put the machine in a sealed box and then suck real hard on the top of the box with a big duct connected to a big dusty.
The problem with this is how do you get the wood in and out of the box.
In the case of a SCMS one side of the box is left open which also suits the fact that the saw throws the sawdust in one direction - towards the back of the box.

In the case of bandsaw, If all you worry about is the fine dust then a honking big duct with a big air flow (no box needed) above the BS will suffice. However this setup will pick up no chips because the air speed needed to pick the chips up will be too slow at the point of generation. The result is the chips will fill up the BS cabinets, scatter all over the table and build up on the floor so some attempt to pick up chips is needed. Just like a table saw needs a two point extraction setup (above and below the table) a band saw needs the same and, plus the (lower) cabinet need to be kept clear.

How this is done is up to the operator but some practicalities apply. To pick up chips from the table, and the obstructions around the blade under the table, a high air speed is more efficient. The easiest way to do this is to use multiple pick up points using narrower ducting. If you still have high air speed with multiple 150 mm ducts then go for it but bear in mind that 150 mm ducting will significantly clutter the work space (I find even 100 mm ducting above the BS table a nuisance).

In terms of the fine dust control, as long as you have ~1000 cfm being extracted in the vicinity of the machine this will be sufficient.
3 x 100 mm ducts is similar to 1 x 150 mm duct.

On a TS a 100mm duct above the table provides sufficient flow and air speed to collect chips, and a 150 mm duct under the table (where high speed is not essential) are usually used. The sum of the flow from both of these will be enough to pick up the fine dust.

Although I don't worry too much about chips as they don't represent an immediate health threat it is still a good idea to pick up the chips as they are formed or asap thereafter. Leaving chips on the floor to be walked over is a sure way of making fine dust. I leave my flexies loosely connected to my machines so I can quickly pick up stray chips, although I am not good at doing this on a regular basis.

Originally Posted by LuckyDuck

This is very useful to know. I assume that the TS, SCMS, Thicknesser/Planer and Lathe are different propositions, and we need as much airflow and volume as possible for them?

We always want max airflow ... both volume and velocity. That's why to do a great job we need a minimum of 6" ducts and the magic figures of about 1,000 and 4,000. The other trick is to build great ports/shrouds. I cannot overemphasise the critical nature of hoods and shrouds. Not even a monster cyclone with 10" ducts and a mountain of air will help much if ports and hoods are poor. It is here ... ports, hoods, shrouds ... where a band saw is a bit different from most machines. But let's back up a minute ...

What grabs fine dust is velocity. From memory, we need at least 50 fps to grab very fine dust, assuming it is not very high velocity dust. At each port/hood we aim to create a large negative pressure bubble that will grab the dust. So, our negative pressure bubble is only effective whilst the velocity is above 50 fps. As the bubble grows in size, the velocity at its edges drops very rapidly. A smoke test shows this readily. The higher the volume, the bigger the bubble with a velocity of 50 fps. The magic figures of 1,000 and 4,000 do two things for us. First they keep the air in the ducts moving fast enough to avoid settling out and blockages. Second, they give us a negative pressure bubble about 18" in diameter (where the velocity is at least 50 fps) at the end of a bell-mouth. Again, the higher the volume, the bigger is the negative pressure bubble (and the better it is at grabbing bigger particles). BUT ... in the case of a band saw, if you can keep the under-table flexy or port very close to the blade, you don't need such a big negative pressure bubble, and a 4" port will do the job ... but you do need to be pretty close.

Originally Posted by LuckyDuck

This is a revelation to me, and I appreciate you taking the time to explain your position. From all I've read -- and I've read extensively on this forum, and the Bill Pentz site, etc. -- the message has come home to me that larger diameter ducting produces better dust extraction, as long as the air volume (e.g. 1000CFM) is maintained. From what you're saying, however, it is actually better to split the ducting into smaller (e.g. 4") sizes. The reason why I am surprised -- not doubting you -- but surprised, is that so many posts "poo-poo" the multiple 4" ducts and strongly recommend combining the ducting into one bigger duct of 6". Is it just that the BS is a different machine with different extraction requirements, or what have I missed?

Generally, the notion of one 6" port being better than three 4" ports is true ... but not always. Consider my drum sander. It had one 4" port, which I enlarged to a single 6" port. I considered using three 4" ports, but thankfully BobL alerted me to the potential issue. Three 4" ports would have been an easier fix for me, but it likely would have created dead zones between the ports as these ports fought each other for air. Where this additional air disturbance and the "fighting for air" between ports is likely, multiple ports are probably not the best solution.

The band saw is a very different animal. The three ports on my machines cannot fight each other for air. So long as you get good airflow through each of them (and with your monster, you will), three ports is a superior solution. If you look at BP's site, his sketch shows two ports for a band saw. One from the lower cabinet and one directly under the table ... see http://billpentz.com/woodworking/cyc...cfm#dust_hoods BobL showed us how to add a third, over-table port to scavenge stray dust and to keep the air around our faces clean.

Originally Posted by LuckyDuck

Would you say that your 3-line solution is better at chip collection, in that it is the most efficient at getting the visible chips that collect on top of the timber being cut, and collect in the bottom of the cabinet? May I clarify that you believe your solution is also better for capturing the invisible dust too? The reason why I ask, is that just about every other post from the gurus on this forum advocates a 6" port (with bell mouth) as the most efficient way of capturing that finest dust. Please understand that I am not doubting you, just asking for clarification! Thank you.

I reckon dust collection on my band saw is brilliant for both dust and chips ... but the chips are tiny, even with a re-saw blade. I guess that's why we call it sawdust. I happily cut MDF on my small band saw, certain that the dust collection system is grabbing the evil MDF dust it produces. I don't have a particle counter, but if I cut slivers of camphor laurel from the end of a 25 mmX 25 mm stick, the slivers are grabbed by the overhead port and whizzed off to the cyclone. More importantly, until I withdraw the wood from the air-stream, I cannot smell the camphor laurel. By my reckoning, if the odour of the camphor can't escape, neither can the very fine dust. I hope that is true, but until I can convince BobL to visit my shop with his instruments, I must be content with that test. I used the same test with my linisher and some other machines. If I can't smell the camphor until I remove it from the air-stream, I'm a happy chappie.

The very fine dust is invisible, but if it is escaping then over time it will build up and become visible. So, I deliberately do not clean down certain surfaces ... I allow them to gather dust. Then I look at it. If it is grey, I ignore it. If it is brown, I have fine wood dust leaking somewhere. I get no collection of fine brown dust on the cabinets etc close to the band saw. I am confident I am getting the overwhelming majority of the very fine dust. Failure to do that would be total failure. It is the fine dust that damages our health.

Originally Posted by LuckyDuck

All bandsaws appear to be, more or less, the same construction. Mine seems a bit tighter up under the table than some, as the cabinet is only just as wide as it needs to be. It would be impossible to introduce a decent sized flex side-on as you do, as the trunions and associated steel supporting the trunnions are too bulky and in the way. I like Bob's idea of the "bed-pan" design, coming around either side of the trunions. I will have to "play"!

You are on the right bus. Sit and look at it a while. A solution will appear. You need to get the equivalent of a 4" duct as close to the blade as possible. I generally rig up a test prototype with sticky tape wire and cardboard ... and spit. Then I do a smoke test to see how big the negative air pressure bubble is. Can you bring your flexy in from the opposite side to mine? I actually considered that. Another approach is to heat and squash a PVC pipe, and cut a shape into it that allows you to get the inlet where you want it.

Have fun!

John

Chips need velocity and fine dust needs flow.

To pick up chips from a surface, of to grab chips with momentum heading away from a blade, an air stream with high velocity is needed. If the chips manage to pass through a high speed air stream and settle out, much higher air speeds are needed to pick these up. OTOH fine dust is like a gas, within cm of it being formed it collides with gas molecules and starts to slow down and even a gentle breeze can drive it around. However because it is usually warm it rises and disperses into a work area so lots of volume flow is needed to control this dust.

To give you an idea of how slow and low an air speed can affect fine dust consider the following
Although drilling generates lots of chips, provided it is done at optimum speeds it does not generate a lot of fine dust.
While testing this I discovered that the small fan inside the VFD attached to the left hand side of my DP strongly effected the dispersion of small amount of fine dust being generated during the drilling process.
The VFD fan is ~50cm away from the bit. While drilling, the dust detector only picks up wood dust on the side of the bit where the VFD is located - there was nothing above background dust on the other side.
The speed and volume of flow generated by this tiny little fan is enough to pull the fine dust away from the source.
Of course if huge volumes of fine dust are generated more flow is needed, but it does not need to be at high speed.
I should measure the air speed at the source to get an idea of what is required.

Ideally you would have both high velocity and high flow but as well as being potentially dangerous this is not possible for home workshops so we manipulate Speed V flow.
A vacuum cleaner is low flow but high speed and is good at grabbing chips at point sources and (tool dependent).
A DC is low speed but high volume and is good for machinery that disperses dust during its operation

Hi Bob and John:

You guys are very generous with your time and advice. Thank you!

I hear you with regard to ducting size being a nuisance Bob. I will try either 100 or 120 ducting over the top of my bandsaw, and hopefully will be able to live with the inconvenience -- I'm certainly VERY motivated at the moment because I'm so sick of choking on dust!

You've both convinced me that the three port solution is best for the bandsaw. Yes, it is more trouble with branches and duct/flex going everywhere, but I like using the bandsaw a lot (as opposed to the tablesaw which I try to use as little as possible); its just a nice machine to work on. A very easy solution for the bandsaw for me would be to introduce 3x120 ducts, which is just larger in cross section to a 200mm vertical. The reason why this is easy for me is that the machine already has a 120mm port at the base of the cabinet, and I think I can stuff a 120mm port between the trunnions under table, and the third 120 could come over the top as you both have on your saws.

Question, for the port in the base of the cabinet, I assume you have a grill/vent somewhere up high on the door of the cabinet, so as to avoid flow restrictions? If this assumption is correct, do you put this as close as possible to the underneath of the table, say right in front of the lower blade guides? Or would this encourage the lower cabinet duct to compete for air with the under table port? Thanks.

Another question, Bob you mention using your flex which is loosely attached to your machines to pick up stray chips off the floor. Do you have a stationary "floor sweep"? I assume not as this would entail "sweeping" and therefore stir up dust. I know what you mean about vigilance! Keeping up with best practices is always hard work...

John, thank you for reminding me about the problem of various ports "fighting for air". I want to make sure I avoid that, and is the basis for my question about grill/vent location on the bandsaw. Thanks.

Your reference to "grey" versus "brown" dust on undisturbed surfaces must be like an impossible dream to most members here -- as it is to me. However, I've read enough of your posts about how you have modified your machines over time to know that it can be done. I'm inspired!

Once my dusty is up and running, I'll play with different sized flex as you have suggested. Sounds like a good idea!

Thanks guys.

[QUOTE=LuckyDuck;1754798] You've both convinced me that the three port solution is best for the bandsaw. Yes, it is more trouble with branches and duct/flex going everywhere, but I like using the bandsaw a lot (as opposed to the tablesaw which I try to use as little as possible); its just a nice machine to work on. A very easy solution for the bandsaw for me would be to introduce 3x120 ducts, which is just larger in cross section to a 200mm vertical. The reason why this is easy for me is that the machine already has a 120mm port at the base of the cabinet, and I think I can stuff a 120mm port between the trunnions under table, and the third 120 could come over the top as you both have on your saws.[QUOTE]

3 x 120mm = 200 mm in cross sectional area, but not in terms of flow

Lets take a look at this diagram. I'm going to work with 10" of WC (X - Axis )just because its easier but if your DC does of does not meet this the argument still applies
The blue mark points to the flow a 120 mm pipe will generate, take the top left hand corner of the blue mark across to the Y-Axis and I get just under 700 CFM
The red mark indicates what the 8" should draw, ~2600 CFM, but 3 x 700 = 2100 so you are starving the 8" duct.
Working the other way, divide 2600 by 3 and you get ~860 CFM - to get that at 10" of WC you need to be using a 130 mm pipe.
Its always better to use the "sum of the smaller ducting flow rates" that exceeds the capacity of the system. This reduces the back pressure and improves the flow.

Now I realise that we have to live within the practicalities of the ducting we can get so if that's what you have already or is all you can get, so be it.

[QUOTE]Question, for the port in the base of the cabinet, I assume you have a grill/vent somewhere up high on the door of the cabinet, so as to avoid flow restrictions? If this assumption is correct, do you put this as close as possible to the underneath of the table, say right in front of the lower blade guides? Or would this encourage the lower cabinet duct to compete for air with the under table port? Thanks.[QUOTE]

This is mine. It has a very small connection with the area under the table where the blade passes through so i doesn't compete with that or the flexy connected to the RHS port that does practically nothing, it is so restricted it has less than 100 cfm through it.
The Flexy on the ground is the one I jam up under the table.

Another question, Bob you mention using your flex which is loosely attached to your machines to pick up stray chips off the floor. Do you have a stationary "floor sweep"? I assume not as this would entail "sweeping" and therefore stir up dust. I know what you mean about vigilance! Keeping up with best practices is always hard work...

I don'y have a floor sweep - I have been thinking about getting one but it doesn't help with all the nooks and crannies and ledges etc.

Originally Posted by LuckyDuck

Question, for the port in the base of the cabinet, I assume you have a grill/vent somewhere up high on the door of the cabinet, so as to avoid flow restrictions? If this assumption is correct, do you put this as close as possible to the underneath of the table, say right in front of the lower blade guides? Or would this encourage the lower cabinet duct to compete for air with the under table port? Thanks.

I took the easy way out and rigged the door to both lower cabinets so they stay open about an inch to let air in.

Cheerio!

John