Thread: SCMS Dust Extraction

I wondered how that table was used !!! Now I get it !!!

Just to confirm Bob, if I run your formula - I get a value of 1073.971CFM on w.c. of 7" in a 6" duct.

If I can get back to my 7 7/8" - 1143CFM

hey !! That's not bad ! its only half the original specifications by the manufacturer of 2300CFM!!

Actually - I was hoping I could land somewhere around the 1100 CFM range when I bought the thing after reading all the material on this forum - so I think I am pretty happy so far.

Originally Posted by Glenn.Visca

I wondered how that table was used !!! Now I get it !!!

I love a EUREKA moment in Dust extraction - YAY

Just to confirm Bob, if I run your formula - I get a value of 1073.971CFM on w.c. of 7" in a 6" duct.

If I can get back to my 7 7/8" - 1143CFM[/QUOTE]
Yep and Yep

hey !! That's not bad ! its only half the original specifications by the manufacturer of 2300CFM!!

Yep, the manufacturers specification is typically for the impeller alone with the air flow measured in the middle of the opening and no allowance for edge friction plus a bit of fudge factor.

Actually - I was hoping I could land somewhere around the 1100 CFM range when I bought the thing after reading all the material on this forum - so I think I am pretty happy so far.

The elephant in the corner will be the restriction on flow cause by machine ports - so get yer angle grinder tuned up!!!!

Originally Posted by BobL

The elephant in the corner will be the restriction on flow cause by machine ports - so get yer angle grinder tuned up!!!!

Other than my recently acquired Jet thicknesser - all my machines are sans DC ports ... so the world is my oyster. Actually - not sure whether having no DC ports is a good thing or a bad thing.

The thicknesser has this plastic thingemy of a bolt on shroud that I think I will have to replace. It currently has a 4" port in it, and is attached to the thicknesser by tapped thread into the cast head.

Cheers - and thanks for all the advice Bob.

Originally Posted by Glenn.Visca

Other than my recently acquired Jet thicknesser - all my machines are sans DC ports ... so the world is my oyster. Actually - not sure whether having no DC ports is a good thing or a bad thing.

The thicknesser has this plastic thingemy of a bolt on shroud that I think I will have to replace. It currently has a 4" port in it, and is attached to the thicknesser by tapped thread into the cast head.

Cheers - and thanks for all the advice Bob.

No worries and for dust port advice have a look here.
https://www.woodworkforums.com/f200/improving-machine-cabinet-dust-ports-187936

Just playing around with some concepts here.

Assuming I can manufacture it, does this type of arrangement get somewhere close to satisfying a tapered duct entry for my SCMS hood ?

SCMS Dust Hood.jpg

The tapered duct is 400w x 200d at the base - tapering up to 170w x 170w at the top, over a height of 400mm (all are very approx. outside measurements). I am restricted in the height of the transition by the location of the entry to the duct main.

I would cut a round hole in the top of the chimney, insert a PVC POP and glue with polyurethane, then round over on the router with a 1/2inch round over bit (well - that's my theory anyway). This would then attach to the main duct 45degree with a slip joint sealed with some trusty duct tape.

The base of the chimney could also be rounded over.

The area of the chimney at the base would be 3.31 times the area of the 150mm duct. Too much ? To little ?

Originally Posted by Glenn.Visca

Just playing around with some concepts here.

Assuming I can manufacture it, does this type of arrangement get somewhere close to satisfying a tapered duct entry for my SCMS hood ?

SCMS Dust Hood.jpg

It will be OK for fine dust but not so good on chips because the air speed is to low near the bottom of the box.

If you putting the exit to the box up high because the box is up against a wall then you have no choice but if you have the room behind then I would get the suck point down lower and then use the higher air speed in the duct to lift the sawdust vertically.

One way to test out which position which will collect the most chips would be to get a large cardboard box about the size of the hood you want to make, and cut some of the side off so that the box fits in behind the SCMS. Then make a range of test cuts and watch where the sawdust flies off too and strikes the box and put ducting at those points.

My scms is definitely located gainstt a wall, and I originally I thought about running the duct down under the bench and then back up in a big "U" shape.

Although, I suspect the losses by all the 90degree bends outweigh the benefit.

Through a small redesign I have been able to drop the height of the hood by 250mm, and therefore the ducting, now having the duct entry 400mm above the table surface.

The little dust port is pretty good at directing much of the dust straight into the port. I can add a little 45degree fitting to give better control.

It's the "spray" and times when the saw is not set to 90 degrees that appersi the most challenging to solve.

When creating a tapered chimney, is it as simple as creating a rectangular section of equivalent size to a 250mm diameter circular duct ? Or does the area have to be larger due to other factors ?

Glenn,

I only have a drop saw, so dust collection is much easier for me.

However, I think you will find that the size of the opening at the front of your hood will largely determine the air velocity. I managed to make my opening that lets air in quite small (about the same size as a 6" pipe) and dust/chip collection is very good. It is much more difficult with a SCMS, but it may be worth looking at ways to shroud/curtain the front off to get the air moving faster where the dust is.

Originally Posted by John Samuel

t is much more difficult with a SCMS, but it may be worth looking at ways to shroud/curtain the front off to get the air moving faster where the dust is.

I've found the cafe blind in front of my enclosure has reduced the free (albeit visible, I don't know about the invisible) dust and chips to less than 5%.

Originally Posted by Glenn.Visca

My scms is definitely located gainstt a wall, and I originally I thought about running the duct down under the bench and then back up in a big "U" shape.
Although, I suspect the losses by all the 90degree bends outweigh the benefit.

One way around the need for extra bends would be to use more collection ports (on either side) that are combined into the 150 mm duct above the cabinet. This has not been discussed very often but one way to reduce the effect of more bends is to use more ducting - it's not always practical to do this for long runs but it might be for short runs.

Using
2 x 150 mm ducts (one on either side) would drop the collection air speed too far for good chip collection.
2 x 100 mm ducts is too consrtictive and reduce air flow.
3 x 100 mm ducts would approx the air flow of a x 150 mm duct but what is needed is low frictional flow which is why I suggest
4 x 100 mm ducts - the cross sectional area is less than 2 x 150 mm ducts so the air speed will still be high enough for chip collection

The little dust port is pretty good at directing much of the dust straight into the port. I can add a little 45degree fitting to give better control.

The enclosure is more what I would call a shroud, while the port is the start of the tapered air entryway

It's the "spray" and times when the saw is not set to 90 degrees that appersi the most challenging to solve.

Collecting from both sides would assist this problem

When creating a tapered chimney, is it as simple as creating a rectangular section of equivalent size to a 250mm diameter circular duct ? Or does the area have to be larger due to other factors ?

The two major turbulence/friction points are the red lined areas marked F. The topmost one has the higher air speed so will also have the most friction.
The way to have near zero friction is to use a hell mouth hood connection.
With tapered connections, the longer the better but joints should be as smooth as possible.

JS,

Thanks for the reply. You are right .. the combinations of cutting positions with an SCMS give so many permutations. I will definitely have a shot a blanking off dead space on the shroud.

JR,

Cafe blinds ? A bit fancy

Bob,

Many thanks.

Time to start some construction.

Here is a start on my shroud.





The rear is set at 45 degrees to the rear wall, the middle square and the front wings at 22.5 degrees.

I figured a softer angle on the front lead in to the shroud would create less dead air at the left and right widest points, which will be important for mitres.

Good to observe that even without a top, the shroud is already having a positive effect, with dust being visibly pulled into the shroud.

Made my shroud roof tonight, and inserted the duct. Tried several configurations with the walls but couldn't really get anything better than this. 10 test cuts were made on structafloor stock about 4 inches wide, which would seem to generate quite large chips.

Photo 1.

Shroud in place and no dust collector running. A very unpleasant experience.



Photo 2.
The base of the duct flush to the lower face of the shroud roof.



Photo 3.
Duct pushed duct down and added a little extension (from a cut of a larger piece that went pear shaped). The bottom of the duct was just a smidge above the SCMS arm.



While photos 2 and 3 don't appear markedly different to 1, they most definitely are ... as you would expect. The amount of residual dust after 10 cuts through a 4 inch wide piece of stock was significant.

So far I am pleased with the results, but I think it could be made better with additional pickups. It's just a matter of how.

Bob, you mentioned in a prior post using 4 x 100 mm pickups. Anyone seen a 4 x 100 to 1 x 150 junction ?

The only thing I can think of is building a chamber / closed box. Then cut a 6" entry at top, and 4 x 4" at bottom into the shroud.

Originally Posted by Glenn.Visca

While photos 2 and 3 don't appear markedly different to 1, they most definitely are ... as you would expect. The amount of residual dust after 10 cuts through a 4 inch wide piece of stock was significant.

Yep I can see it.
It would be interesting to leave the duct opening at the same place as itr is for Pic #3 and then stick a bell mouth hood (even a small one) on the end of the duct and look at the sawdust pattern.
Like wise a diagonally cut piece of dust with the open face of the cut pipe facing forward would be interesting.

Remember you are looking at visible dust - the best invisible dust comes with max flow and that will be with a bell mouth)

So far I am pleased with the results, but I think it could be made better with additional pickups. It's just a matter of how.
Bob, you mentioned in a prior post using 4 x 100 mm pickups. Anyone seen a 4 x 100 to 1 x 150 junction ?

Never seen one worth using - There is something called a 4 way T junction with the 4 small pipes 9separated by 90º) entering the sides of a larger one.
It would be too turbulent to work usefully for high speed air flow.

The only thing I can think of is building a chamber / closed box. Then cut a 6" entry at top, and 4 x 4" at bottom into the shroud.

The expensive alternative is 3, 150-100 Y adapters.
A box would need to be careful designed otherwise it may introduce too much turbulence.

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