Thread: SCMS Dust Extraction

Correction ... Measure results is what I meant to say

Originally Posted by Glenn.Visca

Other than eyeballing this Bob, is there a way I can test results ? (short of expensive equipment)

Not so much the dust pickup, but the flow rates ?

A U tube manometer (pressure gauge) can tell you indirectly what the air flow is.
The lower the pressure the greater the flow and VV.

One end of the U tube has to be inside the air flow and the other end outside.
BUT
Once you choose a pressure sensing point inside the air flow you should not change it otherwise any comparisons might not be valid.
So I would pick a point somewhere downstream in between the impeller and the first junction upstream.
Drill a small hole in the duct and drill and tap a 5mm thread into the hole.
Now get a black retic drip feed tap and cut the end off one of the threaded end so it is flush to the duct wall when it is threaded into the duct - It will be worth doing this with a scrap piece of duct to make sure you have it right
Then glue the tap in place with loctite or superglue.this can then be used to test the pressure in the duct.

Now attach one end of the U tube to that and leave the other out in the open.

Start by checking the static pressure of the DC by completely blocking all air entry points. You should see something like 8" of water column (WC) pressure difference.
That is the maximum pressure (or vacuum) you can get - max pressure is at zero flow.

Now measure the pressure with all ports open - this is the pressure at max flow.
Try partially blocking a part with your hand and watch the pressure change.

Now as you add machines and alter ducting lengths, junctions, and change port sizes and openings in machines you will have a better idea of what the flow is doing.

Originally Posted by BobL

A triangular arc or piece of pie shaped box could be the way to go.
The 150 mm pipe at the apex of the pie and the 4 ducts coming in alongside each other along the outer edge

Is this something like what you meant Bob ? I haven't drawn in the "cover" on the front of the chimney for illustrative purposes.

If it is, I would probably look to:
* Round over the entry of the 150mm into the chimney on the router table.
* Attempt to block the "dead space" in the chimney with some type of filler panels

SCMS Dust Hood v3.jpg

Or ... have I completely missed the point ? D

I guess the next question to ask is - do I want to build this - or am I better off just to divert the existing duct around and come in from underneath (which I had always planned to do - and have made allowances for...)

Originally Posted by Glenn.Visca

Is this something like what you meant Bob ? I haven't drawn in the "cover" on the front of the chimney for illustrative purposes.

If it is, I would probably look to:
* Round over the entry of the 150mm into the chimney on the router table.
* Attempt to block the "dead space" in the chimney with some type of filler panels

SCMS Dust Hood v3.jpg

Or ... have I completely missed the point ? D

I guess the next question to ask is - do I want to build this - or am I better off just to divert the existing duct around and come in from underneath (which I had always planned to do - and have made allowances for...)

It's only worth bothering using the 4 x 100 mm collection if the 100 openings are located at the sides of the shroud.

Something more like this. the black lines represent flexy or ducting.

According to BP static calc a 1m length of flexy has a static pressure loss of 0.63" of WC
BUT
AS there are 4 flexies in parallel the pressure loss is 1/4 of this or 0.16" of WC which is pretty close to 1 m of 6" ducting which has a P loss of 0.13" of WC
The Pie shaped collector will add some resistance.

The advantage of the 4 x 4" ducts is that it would deliver 4 high speed air intakes closer to the chip source.
These 4" ports would work better with an individual bell mouth hood.
Fine dust collection would be equivalent to a 6" port.

Personally I would only build this if the 6" duct poked down well inside the hood and with a bell mouth on it was not grabbing enough chips.

Wow ! I really did misinterpret your original suggestion Bob ! Sorry about that. I hope you don't mind continued questions.

Its as much an experiment as it is a practical need to improve what I already have.

Can I offer an alternative ?

The 150mm duct at the top of the shroud does a good job of picking up the dust that comes straight out of the chute on the saw.

When I lower the top entry duct so it protrudes lower than the shroud roof, the dust impacts against the duct, and I can see it flying to other parts of the shroud. I expect (but cannot prove) the heavier chips would therefore fly and settle on the table and consequently not get picked up.

As an alternative:

1 x 150mm duct at top of shroud.
2 x 90mm from outside in accordance with your diagram (1 each side located low down).

This would represent an area about 14% less than 2 x 150, and about 3% less than 4 x 100.

I had a dig through my plumbing bits tonight and see that I have 2 x 45 degree elbows I can insert into the side of the shroud, and 2 x 90 elbows to get the duct vertical. I might even be able to get away just 1 of the junctions and then some flexi.

At the moment, making bell mouth hoods is a little difficult as my lathe is happily located on my Dad's bench, as I havent yet built the bench in my workshop to put it on . But I can get some form of radiused entry with a 1/2 round over bit on the router table.

BTW and FWIW, I did some SP tests tonight which now compliment the test done the other day.

So:

Max DC SP 7 7/8"
(Measured with test pipe direct to DC inlet).

System measurements:
(Measured via retic fitting tapped about 2 feet up stream from impeller).

Max System SP 7"
Min system SP 2"
(Duct system open at 3 points ... Which is all I have at the moment ... )

SCMS port zero projection 3.5"
(I.e. flush to inside surface of shroud roof)
SCMS port 35mm projection 3.5"
(Projection meaning duct entry 35mm lower than shroud roof).

SCMS port 90mm projection 3.5"

However, on the 90mm projection, as the arm of the saw was pushed back to its farthest point, we saw a loss of flow of somewhere around 1/2" w.c.

So clearly, that is something to avoid.

Cheers !

Originally Posted by Glenn.Visca

As an alternative:
1 x 150mm duct at top of shroud.
2 x 90mm from outside in accordance with your diagram (1 each side located low down).
This would represent an area about 14% less than 2 x 150, and about 3% less than 4 x 100.

Sounds doable - I'd suck it and see what happens

I had a dig through my plumbing bits tonight and see that I have 2 x 45 degree elbows I can insert into the side of the shroud, and 2 x 90 elbows to get the duct vertical. I might even be able to get away just 1 of the junctions and then some flexi.

At the moment, making bell mouth hoods is a little difficult as my lathe is happily located on my Dad's bench, as I havent yet built the bench in my workshop to put it on . But I can get some form of radiused entry with a 1/2 round over bit on the router table.

Better than nothing

Good to see you trying to come to grips with the manometer.
The results they produce don't always make sense but some information is always better than nothing.

Have started making my 2 x 150 plus 2 x 100 junction.

Plan:


Actual:


Junction pieces from 16mm MDF and front and rear wall from 3mm MDF.

A blast gate will be integrated into the top.

Now we wait for the polyurethane glue to dry on each of the ducted pieces before rounding over the insides.

Couldn't really make it any taller as I wouldn't have been able to reach the blast gate on top. The side walls are at 22.5degrees which I hope smooths the airflow in the junction.

The junction is around 350mm tall in the body (not including duct pieces).

I am considering a perspex face cover for giggles ... But not quite sure whether its worth the buggerising around.

Thanks for looking.

Looks Good

Junction sans face cover. Getting a little late to have machinery running.

My sphincter got the better of me rounding over the ducts on the router table with a bearing guide bit. The individual pieces were just too small to get a good hold on, and, having lost the last joint on my middle finger left hand to a router 13 years ago, I was just not prepared to front up for a repeat performance.

So, into the vice, rasp and file and round over by hand (thank you son ... Good job).

I also purchased a clamp amp meter today and did a quick test on the D.C. with a single 150mm port open. She pulled 4.5A, rated at 9.6A.

I will check tomorrow with all open and all closed.

Thanks for looking.



Compares to

With rounding over those small pieces I just Tek screw them to longer pieces and clamp them to the bench and do the routing from on top - you really need to keep all your fingers.

Anyway I reckon it looks SCHMIK.

A transparent cover would be most illuminating - especially if you could fix short (2"?) streamers (e.g. coloured, string, wool?) to the back of the junction box - we might be able to see where the turbulence is?

Alright ...

I will head out tomorrow and get some polycarb from bunnies ... But purely for the sake of science.

How does the 4.5amps sound for a single duct opening bob ?

Originally Posted by Glenn.Visca

Alright ...

I will head out tomorrow and get some polycarb from bunnies ... But purely for the sake of science.

How does the 4.5amps sound for a single duct opening bob ?

Before commenting it would be good to know current it draws with nothing in front of the impeller and then with the impeller completely blocked.

Here are a couple of videos Bob.

https://www.youtube.com/watch?v=pI5Y...e_gdata_player


https://www.youtube.com/watch?v=wrbe...e_gdata_player


I will post some observations soon.

Good work Glen.

It sure seems to be working well and I would be pleased with what you have achieved.

It's not rocket science.
A good sign is obviously where the streamers continuously hold the same forward (desired) position.
When the streamers flick back and forth thats a sign of turbulence, but over all it doesn't look too bad.

About the only thing I would suggest for anyone else making one is to reduce the width of the shoulder near the exit from the junction box to the 150 mm duct.