Thread: Improving machine cabinet dust ports

Originally Posted by legin

Cheers Bob
When I get back from work next week I will restore it to 4" and see if it makes a difference. I can put a reducer on it and try it with 4"

That will be interesting to see but it's not exactly the same as before.
Try it with a 6" elbow if you have one available

I ended up using the old chute and moulded the 6" pipe on to it. It then goes into a wide 90 and a short run of flexi up to another 90 and then down the main trunk.
I did think about making the chute go straight up from the top plate but I like the rollers for my stock when I am thicknessing.
I didnt even think of a phone app decibel reader. Ill get onto that.

Maybe have a read of this - I know its about sound enclosures but there's stuff in there about general sound measurements and sound pressure level apps that you might find useful
DRAFT: FAQ - Dust Extraction (Practical Aspects)

Hi

The sound waves travel along the ducting just like the noise in a cars exhaust pipe. There is a science to the propagation of sound in a pipe, but I don't know enough about it. I can tell you though that you can make the noise louder or quieter by adjusting the length of the pipe. If you can "calculate" (figure out) the right length, you could make the noise through the pipe virtually disappear.

For those of you interested in dust collection, this link has some very good information...
Dust Collection Research - Dust Collection Basics

Originally Posted by MrFixIt

Hi

The sound waves travel along the ducting just like the noise in a cars exhaust pipe. There is a science to the propagation of sound in a pipe, but I don't know enough about it. I can tell you though that you can make the noise louder or quieter by adjusting the length of the pipe. If you can "calculate" (figure out) the right length, you could make the noise through the pipe virtually disappear.

The resonant frequency of a pipe depends on whether it's open or closed.
Pipes closed at one end will have a resonant frequency of ( N v)/(4 L)
Pipes open at both ends have a resonant frequency of ( N v)/(2 L)
Where N is an integer, v is the speed of sound and L is the length of the pipe.

The primary frequency of something like a thicknesser is the vibration made by the cutter blades making contact with the wood and also as the blades push pulses of air past the dust port that is probably why is setting up resonances in the ducting.
A 3000 rpm cutter head with 3 blades should have at least one note (the fundamental frequency) in the ~9000/60 = 150 Hz range but a thicknesser will generate many harmonics of this note.
A Fourier analyser App will be able to sample and extract the fundamental and the harmonic frequencies in the noise and suggest what can be done about it.
Its not that easy to deal with because there are probably lots of harmonics and suppressing one sound can lead to enhancement of others.

I've made about 40 PVC BMH in the last month or so and am streamlining the process to make things easier.

One thing that takes up time is setting up the piece of PVC so that it goes onto the former straight and stays straight during the forming process.
What I was using to push the PVC pipe onto the former was a disc of Al with a pilot hole in it, through which the point of a live centre could just pass.
Then prior to heating, to centre the pipe I would apply some pressure with the tailstock and manually spin the PVC and tap the outer edge of the PVC pipe near the Al disc until it spun true.
However sometimes the PVC would slip during the push and lead to the BMH forming off centre.



To help avoid this problem I decided to make some push plates that would keep the pipe centred all through he process.

I had heaps of Al disc shapes laying around from cutting out the holes in Al Blast gate bodies so I could make a dedicated pushers for the 100 mm and 150 mm BMHs
I turned up 2 live centre inserts for the lathe live centre and attached them to the centres of the Al pushers
Pushers.jpg

Each pusher plate consists of two plates - The smaller one just fits inside the PVC and acts as a centring guide while the larger one is just over OD of the PVC and does the pushing.
Pusher1.jpg

here it shows how ir rids on the live centre.
Pusher3.jpg

Pusher2.jpg

I used Al because I had so much of it laying around but these pushers could equally be made from MDF.
After making them I realised the two pushers could have also been made into one as a series of different sized discs, or turned out of MDF as turned steps.
I might well combine the two plates into one as it will be dead easy to do in my case.

Originally Posted by BobL

The resonant frequency of a pipe depends on whether it's open or closed.
Pipes closed at one end will have a resonant frequency of ( N v)/(4 L)
Pipes open at both ends have a resonant frequency of ( N v)/(2 L)
Where N is an integer, v is the speed of sound and L is the length of the pipe.

The primary frequency of something like a thicknesser is the vibration made by the cutter blades making contact with the wood and also as the blades push pulses of air past the dust port that is probably why is setting up resonances in the ducting.
A 3000 rpm cutter head with 3 blades should have at least one note (the fundamental frequency) in the ~9000/60 = 150 Hz range but a thicknesser will generate many harmonics of this note.
A Fourier analyser App will be able to sample and extract the fundamental and the harmonic frequencies in the noise and suggest what can be done about it.
Its not that easy to deal with because there are probably lots of harmonics and suppressing one sound can lead to enhancement of others.

Hi Bobl

While I appreciate what you say, I don't think the problem is the resonant frequency of the pipe per se, I think it's more of an "amplification" factor, somewhat like a "manual" megaphone i.e. not electronic.

I do remember reading somewhere (a long time ago) that making a pipe longer or shorter can dramatically alter the sound output. The length variation may only be very small, but because of the sinusoidal action of the sound (waves) the pipe may corrupt the action and therefore modify the sound output. I do understand that the harmonics to which you refer will effectively create multiple frequencies, so it would be difficult to eliminate all of the noise. However, the new ducting must have made a substantial difference to the noise (loudness) of the thicknesser, so it is feasible that adjustments to the length could assist in reducing this "loudness".

A similar "theme" is the use of tuned ports (usually tubular) in loudspeakers. The air exiting the port can reinforce the sound of the speaker (normally the bass speaker) or if the size of the port is incorrect, diminish the sound. The speaker and the port would be out of phase.

The sound exiting at the end of the tube (red in the image) would be different if the tube was shortened to the blue line, as would the sound if the tube was shortened to the black line. This is of course (on my part) making a broad assumption the the sound is more or less a constant frequency, not a sound with many harmonics. I still believe that the change in pipe length could improve (i.e.quieten) the "amplification" of the noise in this situation.

Sound in tube.jpg

Peter, what you are describing is a change in a resonant condition which produces a change in volume - many years ago I did some physics research on this so I have an idea of what that does.

A non-electrically amplified megaphone does not increase the loudness of a sound - all it does is collect some of the sound that is escaping behind and to the side of a sound source and reflects more of it in a forwards direction - in other words it changes its directionality. I don't think this is what legin is experiencing. If it was this he would hear different sound volumes in different parts of his shed

However your discussion has lead me to think that it may be something closely related
The 150 mm pipe is bigger all round so it has more volume and surface area so any vibrations that get into the pipe itself and make it vibrate will make more air outside the pipe vibrate just like the vibrating strings make the body of a guitar vibrate and produce sound.

Legin, just checking, have you got a hard connect to the thicknesser? If so try some flexy between the ducting and machine .
Also try adding some more support brackets to the ducting to reduce the chance of the pipe flexing.
Even hanging some weights off it at strategic points might help.

This reminds me of a million $ machine we had in a lab that was very sensitive to external vibrations. E
ven someone walking around the lab could upset it and slamming a door sent the thing crazy.
A temporary solution was provided by placing a lead brick on a specific part of the machine which damped out 99% of the vibrations.

Originally Posted by BobL

Peter, what you are describing is a change in a resonant condition which produces a change in volume - many years ago I did some physics research on this so I have an idea of what that does.

A non-electrically amplified megaphone does not increase the loudness of a sound - all it does is collect some of the sound that is escaping behind and to the side of a sound source and reflects more of it in a forwards direction - in other words it changes its directionality. I don't think this is what legin is experiencing. If it was this he would hear different sound volumes in different parts of his shed

However your discussion has lead me to think that it may be something closely related
The 150 mm pipe is bigger all round so it has more volume and surface area so any vibrations that get into the pipe itself and make it vibrate will make more air outside the pipe vibrate just like the vibrating strings make the body of a guitar vibrate and produce sound.

Legin, just checking, have you got a hard connect to the thicknesser? If so try some flexy between the ducting and machine .
Also try adding some more support brackets to the ducting to reduce the chance of the pipe flexing.
Even hanging some weights off it at strategic points might help.

This reminds me of a million $ machine we had in a lab that was very sensitive to external vibrations. E
ven someone walking around the lab could upset it and slamming a door sent the thing crazy.
A temporary solution was provided by placing a lead brick on a specific part of the machine which damped out 99% of the vibrations.

Hi Bobl

I am enjoying this discourse. I always love a challenging topic )

When I did physics, that was at school about 53 years ago ) So I don't remember that much about specific sound properties etc, though I hope I remember enough about the basics )

I disagree with your statement on the megaphone though. If the megaphone did not amplify sound then the original (and now ancient) horn based gramophone would not exist. More recently folded horn speakers would not exist. Many years ago at a hifi show (when they used to have them), the Sheraton Perth hotel was the hosting venue. The Klipschorn speakers displayed there were amazingly efficient, this was due to the horn loading Check here for details The presenters were no allowed to use more that 15 watts to drive the speakers for fear of damaging the hotel structure. Though I am digressing a little here, the potential for an expanded pipe could be cause for an increase in the loudness.

Also, while I understand your comments about the resonance of the pipe, this really pertains to a pipe that allows movement in the pipe or its mounting. I imagine that the ducting pipe in question would be mounted in such a way to preclude such resonance, if not additional or improved mounting should be investigated.

Rigid coupling of the ducting to the tool is not so likely to cause an increase in loudness of the tool, partly because of the rigid coupling. Additional noise, different to the noise of the tool may be apparent, due to the noise created by vibrations of the pipe itself (rattling), not necessarily due to the frequencies generated by the tool. Though it is possible that the harmonics created by the tool could influence the vibrations of the pipe, such vibrations could easily be eliminated by additional mounting points or even repositioning the existing mounting points.

I really look forward to your further input, thank you for the enlightening discussion and a suitable reason to exercise my brain )


BTW, the corner positioned Klipshorn speakers were (still are?) the only speakers able to produce a 30 foot standing wave, because they used the walls of the room as part of the folded horn output. The sound from the speakers traveled along the walls of the room. You did need a big room though. )

For all you audiophile woodworkers out there, you can make your own Klipschorn cabinets from the plans on the net. IIRC you need a 15" woofer (and a big room ) ) The sound is AMAZING!

You make some good points but I think we are getting off track for both the OP and the Dust forum. Maybe post something in a more relevant forum if you want to keep the discussion going.

Originally Posted by BobL

You make some good points but I think we are getting off track for both the OP and the Dust forum. Maybe post something in a more relevant forum if you want to keep the discussion going.

So its ok to make the statements in the post but not to refute the rebuttals?

In case you miss this, Fletty is showing some good stuff in his Shed Thread on things relevant to this thread and forum

Rocker blast gates
Fletty's new shed WIP ....or is it a retrospective??
Drill press/WASP dust extraction.
Fletty's new shed WIP ....or is it a retrospective??
Table saw connection
Fletty's new shed WIP ....or is it a retrospective??

After watching Fletty duct his DP it reminded me I needed to get some ducting down to that part of the shed and put some suck on the DP.

The DP gets mainly used for metal.
I have tested how much wood dust drilling wood generates and it's not that much, especially if you only drill the occasional hole but if you happen to be drilling lots it all adds up. The main high volume dust making activity that I undertake with my drill press is when I use small drum sanders. I don't do it often but when I do, it does make a mess. To extract that dust I have been unhooking one of the 100mm flexies off the nearby bandsaw and attaching a longer (2.5m) piece of flexy and dragging across the grinders stand, over the metal BS and the clamping the naked end of the flexy to the DP table. It sort of works except when I was really hogging out wood with the coarse grit sanding drum.

Oh yeah, I also have a WASP but rarely use it because I have two other belt sanders.

Anyway I wanted something permanent that could be used routinely.

The first thing I did was test the dust output from drum sanding situation with a particle counter and the old "long flexy" system.
The was interesting because it showed that provided I held the end of the flexy close enough to the drum i.e. 100 mm , that was enough to prevent the fine dust from escaping into the shed.
Then I repeated the test with the flexy ~ 150mm from the drum and at higher rpm and then some dust was starting to escape into the shed.
It was not much but it would add up if it was done for long enough.
Some of this was probably because I was using such a long piece of flexy and the other was probably because a naked flexy end is not a very efficient was to grab the dust compared to say a BMH.

Based on this I decide to stay with 100 mm ducting (and add a BMH) and just as well because extending even 100 mm ducting past the AC proved a right PITA.
There is not much room to do this sort of thing along that wall and it requiring two 15º bends and two 90º bends above the ceiling line.

At one point I need to lock down the ducting as it traversed one of the pieces of angle iron that straddles the ceiling on top of which is placed all manner of stuff and this is what I came up with.
There's a similar triangular clip on the other side and the other half of the 4" hose clamp attached to each clip enables the ducting to be well and truly locked down.
IMG_2039.jpg

At the DP the setup looks like this.
For a regular DP this is not the ideal side as it can get in the way of the quill handle.
The problem I have is there's a VFD is on the other side but if you don't have a VFD then that's the side I would use.
IMG_2047.jpg
Fortunately the most likely time I will be using the hood will be with the drum sander when I don't need to use the handles.

The mounting bracket is attached to the main post with a hinged clamp and couple of articulated pieces like this.
IMG_2048.jpg

IMG_2049.jpg

Th post clamp can be adjusted up and down if needed.
The most likely adjustment is horizontal position of the BMH which is done by loosing and tightening locking lever L and/or the right most Hex socket screw in the picture below.
IMG_2050.jpg

Lever L can be loosened and this also allows the BMH and bracket to be completely removed and used as a vacuum cleaner to pick up big chips on/around the DP table.

I've made a couple of short vids showing the visible extraction.
It doesn't look like it but the BMH was >175mm from the drum.
Measurements made with a particle counter show that all fine dust is fully extracted.

This one is at 2000 RPM


This one is at 3000 RPM


There's not a lot of difference between two clips but I was definitely hogging out more wood in the second one.

If you look closely at the second vid you will see some dust flecks coming into the dust stream from the bottom of the image, this is dust being scattered probably off my hands out of screen before being pulled back into the air stream.

Visual movement of dust is not always conclusive but I think it goes some way to show how effective even a small BMH is in this situation.

One more thing - it will need a guard of some kind otherwise a small piece or two is gonna disappear up that orifice.

Another update I completed this week was converting the 150mm belt sander from 100 mm extraction to 150 mm.

Was this
BMHguard.jpg

Now this
IMG_2041.jpg

Those two videos are excellent. What an illuminating visualisation

Im very much looking forward to putting the 6" BMH's into service.