Thread: New DC setup for lathe - multiple questions

You can reduce the pressure loss of the inlet and hence improved flow rate by using a Bellmouth hood (see this thread https://www.woodworkforums.com/showth...ell+Mouth+hood) The stuff on Bell Mouth begins at post 29

As well as marginally improving flow rate the hood does a really important thing in significantly increasing the air speed in front of the hood so the hood can grab more dust at source. All other types of hoods grab a lot of air from the side of the hood and so are not as effective.

Originally Posted by BobL

You can reduce the pressure loss of the inlet and hence improved flow rate by using a Bellmouth hood (see this thread https://www.woodworkforums.com/showth...ell+Mouth+hood) The stuff on Bell Mouth begins at post 29

As well as marginally improving flow rate the hood does a really important thing in significantly increasing the air speed in front of the hood so the hood can grab more dust at source. All other types of hoods grab a lot of air from the side of the hood and so are not as effective.

BobL,

I studied the bell mouth thread a number of times and hope I understood the principles - it is good as:
- it stops the clean air from the sides of the inlet being sucked in
- it has a nice 'curve' which helps with flow but is not a large area (like a gulp) which is worse than useless
Based on these and the main principle - get the outlet as close to the source as possible I have 'done' below
up close to the DC inlet.jpg
I sanded the inside edges and have it on some solid wood with hinges. As you can see, for the bowl I am currently turning part of it was inside the pipe - I do not think you can get much closer than this

For the inside of my bowls I have a separate outlet and was very happy with this (this was my second attempt) as it even picked up the shavings. Sorry for the crap photo. If there is actually any interest I will take some better photos (during the day) but I only got it working at dusk.
inside bowl close up.jpg

I'd still be keen to know what other people have found with noise and flexi and also the noise at the cyclone itself. I am trying to make sure I have things setup correctly.

Also, in regards to fine dust, I put four small take-away containers filled with water in the corners of the shed to see if anything dropped in them. So far (a few hours after doing some turning and sanding) I did not see any settled dust. I may be an optimist but I am pretty pleased so far

cheers

Mick

Originally Posted by mick59wests

BobL,

I studied the bell mouth thread a number of times and hope I understood the principles - it is good as:
- it stops the clean air from the sides of the inlet being sucked in
- it has a nice 'curve' which helps with flow but is not a large area (like a gulp) which is worse than useless
Based on these and the main principle - get the outlet as close to the source as possible I have 'done' below
up close to the DC inlet.jpg
I sanded the inside edges and have it on some solid wood with hinges. As you can see, for the bowl I am currently turning part of it was inside the pipe - I do not think you can get much closer than this

That's WAAAAAAAAAAy too close.
What you are doing there is reducing the cross sectional are of the inlet and hence the CFMs which means more fine dust will be escaping into the room.
That's why the bell mouth hood works so well.

Mick, with respect to the bell mouth openings. Search 'bellmouth velocity stacks'. They are large and trumpet shaped to allow the air to move into the opening without turbulance and restrictions. The same applies to the air inlet to your duct openings. You want that same trumpet shape to get the air and the dust with it into the dusty.

The flexible ducting makes noise because the inside isn't smooth. All you can do is minimize the amount you use and look for some that has the smoothest inner wall, which usually is more costly.

Pete

Originally Posted by BobL

That's WAAAAAAAAAAy too close.
What you are doing there is reducing the cross sectional are of the inlet and hence the CFMs which means more fine dust will be escaping into the room.
That's why the bell mouth hood works so well.

BobL,
I have read the previous attached post many times (and did so again). Even in one of your posts you state:

'BOTTOM LINE: If you are worried about invisibles get the work piece as close as practically possible to the duct. There will be a trade off between trapping chips using a hood and getting the ducting as close as possible to the workpiece.

Now, what happens when a work piece is inserted in the way.
What I expected to happen was that the work piece would block the air flow and reduce air speed but not so, read on'.

I know your measurements started at 50mm but the closer they got to the workpiece the better they always got and you did not get any reduction in airflow with the work piece in the way so I am lost to see why having the workpiece partially in the duct would be anything but a good idea.

Also, in relation to the bell mouth, I have tapered the inside of the melamine outlet (basically by sanding) and I was expecting the melamine frame (although not round like a bell mouth) would also stop clean air from behind the duct getting in. It certainly seems to work well - after a couple of hours on the lathe yesterday (including more sanding than most people would need to do) there is no dust this morning in any of my 4 water jars I have setup in the corners of the shed

So I am a little confused

thanks

Mick

Good to hear the water jar system is working.

Originally Posted by mick59wests

.
.
.
I know your measurements started at 50mm but the closer they got to the workpiece the better they always got and you did not get any reduction in airflow with the work piece in the way so I am lost to see why having the workpiece partially in the duct would be anything but a good idea. k

Sure, but all the measurements I did in comparing the bell mouth with naked ducts were done with nothing in the way of the air flow. Putting the workpiece partially in the ducting MUST reduce the flow substantially and force even more air to be taken from the sides than from the front of the work piece which is where the dust is being generated. The bell mouth maintains a higher air speed further from the hood allowing the duct opening to be further back from the work piece which enables the duct to breathe better.

The curve on the entry edge is the most significant aspect of the bell mouth. Tapering the inlet does not add much to the flow rate.

Pete and BobL (and others),

OK, I have done some further research on using a bellmouth and it seems that my overall design could be improved , however.......

I could not work out what is the 'optimal' design and / or the most practical one. BobLs bellmouth looked like it was very short between the end of the actual pipe and the flange while another article had the length of the bellmouth (from the pipe to the flange) the same as the diameter of the pipe and not as tight a curve as the one BobL used. It also had the inside diameter at the entry being around twice the diameter of the pipe.

I'd be very interested if others have made their own, especially for 6" PVC

Still learning

thanks

Mick

Originally Posted by mick59wests

Pete and BobL (and others),

OK, I have done some further research on using a bellmouth and it seems that my overall design could be improved , however.......

I could not work out what is the 'optimal' design and / or the most practical one. BobLs bellmouth looked like it was very short between the end of the actual pipe and the flange while another article had the length of the bellmouth (from the pipe to the flange) the same as the diameter of the pipe and not as tight a curve as the one BobL used. It also had the inside diameter at the entry being around twice the diameter of the pipe.

I'd be very interested if others have made their own, especially for 6" PVC

Still learning

thanks

Mick

The majority of of the benefits of a bell mouth hood are obtained by making the radius of the edges of the entry port one half of the radius of the dust.
The also need to be as smooth as possible
So if you have a 6" PVC, the radius is 3" so half that is 1.5". Further refinements, especially in the length, result in only marginal improvements.

Originally Posted by BobL

The majority of of the benefits of a bell mouth hood are obtained by making the radius of the edges of the entry port one half of the radius of the dust.
The also need to be as smooth as possible
So if you have a 6" PVC, the radius is 3" so half that is 1.5". Further refinements, especially in the length, result in only marginal improvements.

BobL,
I read your reply a few times but am still not confident I know what I am doing. So with my modern technology drawing tool I have posted below what I think this means, but still have questions.bellmouth.jpg
sorry for the crap picture - this tool has worked OK in the past but hopefully if you open the diagram you can see what I think. To put my hand on my heart and be honest, I am hoping for a diagram with dimensions and then I will set about making it.

cheers

Mick

Your diagram is correct for the simple "radius of pipe"/2 [R_pipe/2)implementation

There are two lengths to consider
- the length of any taper that is part of and follows the curved entry edge
- the length of the duct before introducing another change in the ducting

The first one depends on the equation of model used on the curved entry.
The simple R_pipe/2 has no additional length.
If a more complex mathematical curve is followed the optimum length is determined by experiment and practicalities

The second one depends on what is being done.
All changes to a duct, like a junction, naked outlet/inlet, poor quality blast gate, bend etc, introduces turbulence. When one change is too close to another change the turbulence induced by the first change is compounded by the send change. Ideally changes should be ~10 ducting diameters apart so the flow has time to stabilise. In practice doesn't stabilize as much as we'd like, buty any improvement in stability is better than none. 10 dusting diameters (1.5m !) is difficult to do in practice so ~5 ducting diameters is a more practical goal. Even then it may be impossible to avoid changes e.g. ducting hard up against a wall. The additional value of a Bell Mouth (provided it is not blocked by an object) is that in introduces minimal turbulence and so you can have a change in the ducting (like a right angle) very close to a BMH and not lose too much flow.

Originally Posted by BobL

Your diagram is correct for the simple "radius of pipe"/2 [R_pipe/2)implementation

There are two lengths to consider
- the length of any taper that is part of and follows the curved entry edge
- the length of the duct before introducing another change in the ducting

The first one depends on the equation of model used on the curved entry.
The simple R_pipe/2 has no additional length.
If a more complex mathematical curve is followed the optimum length is determined by experiment and practicalities

The second one depends on what is being done.
All changes to a duct, like a junction, naked outlet/inlet, poor quality blast gate, bend etc, introduces turbulence. When one change is too close to another change the turbulence induced by the first change is compounded by the send change. Ideally changes should be ~10 ducting diameters apart so the flow has time to stabilise. In practice doesn't stabilize as much as we'd like, buty any improvement in stability is better than none. 10 dusting diameters (1.5m !) is difficult to do in practice so ~5 ducting diameters is a more practical goal. Even then it may be impossible to avoid changes e.g. ducting hard up against a wall. The additional value of a Bell Mouth (provided it is not blocked by an object) is that in introduces minimal turbulence and so you can have a change in the ducting (like a right angle) very close to a BMH and not lose too much flow.

BobL,
very much appreciated. This increases my understanding a bit further and I will be looking at design improvements. I also plan to look at how I can capture a lot of the non fine dust at the same time. It may not be an improvement on my lung health but it would be so nice not to have to have woodchips everywhere. I am still looking into the crystal ball for this one . My design for inside of bowls just happened to capture just about everything.
thanks
Mick

I am still to make a bell-mouth..... but the other thing that had been bothering me was just how much 'waste' was not being collected when I did the outside of the bowl between centres. As I had the DC inlet on the opposite side from where I was turning very few shavings were getting into the DC. Also, as it was a bowl that I was turning the cut was often quite a distance from the DC inlet so I also wondered if all of the fine dust was being sucked in.

You may be thinking that from BobL's experiment it seemed the airflow was almost identical in any area around the work piece being turned and was almost only dependent on where the DC inlet was positioned. If so, good thinking , however with a bowl you are further from the inlet. I do not know if it matters: at all, bugger all or enough to make a difference but I figured it could not be a bad thing to have the DC inlet closer to where the contact (cut, scrape, sand) is taking place. Also, based on what I got going for the inside of the bowl it should also capture most of the shavings which I would be most grateful for. So, without further ado here is a new setup I havecloseup new between centre DC setup 2.jpgcloseup new between centre DC setup 1.jpg


It is the flexy on the left that is being used. I have an 'outrigger' on the lathe for turning the inside of bowls which is holding the flexi (it is NOT on the rest I am using for turning). It is 5 to 10cm from the workpiece and it captured almost everything which is very much a nice to have in my opinion - much less cleaning up. It may look a bit crowded in there but I had enough room> think I could improve it further if I used some extra PVC above and only had the flexy coming in from the left. However, I had intended to do some turning again this century so I will see how I go.

I would be grateful for comments from others who are turning, especially bowls. It goes without saying that any reply from BobL is most appreciated.

cheers

Mick