Thread: Additional extraction for a Kapex - makes a difference

This is one of the best ones I've seen for a SCMS.
https://www.woodworkforums.com/showth...=187025&page=8

Originally Posted by BobL

This is one of the best ones I've seen for a SCMS.
https://www.woodworkforums.com/showth...=187025&page=8

AHA!

YES!

Thanks very much for that link Bob - it's indirectly given me the solution to easily get two intakes either side. Now I just can't fit a shroud onto the bench, and I also think the bulk would be rather inconvenient even if I could, as my saw would require a rather large one to allow for tilting and pivoting (and bigger would mean less effective too). Furthermore it's already hard enough to get a strong light source onto the cut with all the arms and things of the Kapex.

However, I noted Glenn's use of two flexible pipes and I suddenly realised this was the solution:
Glue pairs of REMs onto the saw base (or similar) in the various positions that they will be needed, and a couple more onto the end of each pipe. That way I can very quickly move the inlets around to the required position. As they will be right behind the blade (making three inlets in all, including the saw's) I should be able to capture damn near all of the dust at source, which means I should also be getting the vast majority of invisible dust (I think and hope). That will also get rid of the very bulky pipe which has been a bit of a PITA that I have had to live with.

I'll have to use smaller intakes to be able to get them in the right position, but that's a trade-off to get closer to the source, and so I would expect that to compensate admirably.

The reason a cowling is used is more for fine dust as the air speed coming off the saw blade is much higher than the air speed generated by the DC inside the ducting. The cowling slows the air coming off the saw down so the DC can grab it.

nevertheless if you don't have the space there's little else that can be done.
However, if you plan to use a naked flexy end, one thing that can be done would be to add a small bell mouth hood onto the end. For 4" ducting the outer diameter of the hood should be a minimum of 150 mm but even one 125mm would be better than nothing.

Cheers Bob. Yes, I had in mind one of those rectangular to round sections on the end of the flexy, but whatever it is will be determined by whatever i can get closest to the end of the cut. Hear what you say about the air speed of blade verses DE, but I think one thing working in my favour will be that the air from the blade should be more or less blowing straight at the intakes.

Originally Posted by FenceFurniture

Cheers Bob. Yes, I had in mind one of those rectangular to round sections on the end of the flexy, but whatever it is will be determined by whatever i can get closest to the end of the cut. Hear what you say about the air speed of blade verses DE, but I think one thing working in my favour will be that the air from the blade should be more or less blowing straight at the intakes.

What happens with a relatively narrow fast moving jet of sawdust moving through still air is that it drags air in from the sides of the jet and sets up turbulence that quickly spreads the lighter contents of the jet.
The heavier chips will stay in the jet but the fine dust will move with the turbulent air.
In addition the fine dust that is generated is warm so it will tend to rise away from the jet.
So the final result is a jet of visible sawdust (red fan pattern) and an lighter invisible plume of fine dust (blue fan)
The ideal location for a duct to pick up chips is thus different from the ideal location that will pick up fine dust.

Originally Posted by BobL

So the final result is a jet of visible sawdust (red fan pattern) and an lighter invisible plume of fine dust (blue fan)
The ideal location for a duct to pick up chips is thus different from the ideal location that will pick up fine dust.

I see. Presuming your angles are just indicative, the top blue line is reasonably similar to the angle from the blade to the dust capture hood of the Kapex itself. It comes down lower in use of course, but it's reasonably wide and deep so would still cover that angle more or less. However, it sounds like the gas dust would spray outwards (R & L) as well. That being the case then if there's room I could perhaps mount the rectangular intake vertically rather then horizontally (and perhaps with an overhang by cutting a slope), but again it will depend upon what will fit.

Originally Posted by FenceFurniture

I see. Presuming your angles are just indicative.

Yep

, the top blue line is reasonably similar to the angle from the blade to the dust capture hood of the Kapex itself. It comes down lower in use of course, but it's reasonably wide and deep so would still cover that angle more or less. However, it sounds like the gas dust would spray outwards (R & L) as well..

Yep

That being the case then if there's room I could perhaps mount the rectangular intake vertically rather then horizontally (and perhaps with an overhang by cutting a slope), but again it will depend upon what will fit.

Yep

Just cutting a 200mm piece of MDF now I took notice of the swirling dust. I didn't have the big extractor connected so it showed it how it is with just the in-saw extraction. The dust curled around like smoke as it came off the blade into the rubber chute, but it certainly grabbed a very great deal of it. Kinda did a 360° swirl. MDF was a good example to see it on because all the dust is very fine.

Originally Posted by FenceFurniture

Just cutting a 200mm piece of MDF now I took notice of the swirling dust. I didn't have the big extractor connected so it showed it how it is with just the in-saw extraction. The dust curled around like smoke as it came off the blade into the rubber chute, but it certainly grabbed a very great deal of it. Kinda did a 360° swirl. MDF was a good example to see it on because all the dust is very fine.

If you can see dust it's still relatively large particles. The fine dust below about 10 microns cannot be seen with the naked eye.

Interesting experience today. I almost always have the Kapex on speed 2, mainly to prevent any potential burning but also to keep noise down a bit - it cuts a beautiful surface, so that'll do.

However, I had to cut some very large profile Douglas Fir a few times - 190x70mm so I cranked up the speed to 4. There was hardly any dust at all left behind, so I'm thinking that blade speed will flick the dust into the chute more efficiently. Can't really draw a firm conclusion from today because there were a number of variations from the cuts that I photographed earlier (species, profile size, blade speed). Certainly the chute was sitting right down on top of the cut because of the 70mm thickness.

I suspect that timber species (and how dry it is) will also play a roll in dust collection. I get the impression that Cypress dust is quite clumpy and may not collect all that well. This Cypress has been a floor for 60 years so it should be pretty damn dry (although coming out of a club, it may have had a torrid time with beer spills ).

Anyway, in the quite near future I'll have to cut more of the same Cypress profile (90x19) as photographed, so I'll fool around with blade speed to see what happens and report back.

Just as an aside, the Kapex on speed 4 (goes up to 6) absolutely howled through the 190x70 DF and left a beautifully smooth end grain. I could also hear the electronics kick in to keep the blade speed up when it got to full load (about 2/3 through the profile).

It's a helluva saw!

Originally Posted by BobL

If you can see dust it's still relatively large particles.

Yes, understood Bob - just saying it's very fine dust from MDF compared to real timber.

Originally Posted by FenceFurniture

Yes, understood Bob - just saying it's very fine dust from MDF compared to real timber.

This could be another case of, if it can't be see it isn't there?

There have been quite a few studies of the amount of dust generated, and the particle size distributions produced by woodworking MDF in comparison to other timber.

It's a very complex picture but some of the general findings are;
- it's highly operator dependent, an operator that drags a saw slowly through a piece of timber, or sands wood more lightly, makes more fine dust
- Not unexpectedly, for sanding it's grit size size dependent, higher grits - more fine dust, for sawing its tooth count and RPM dependent.
- Hardness of the wood plays a major role , hardwoods generally produce more finer particles than softwoods and MDF is sort of in between.
- It's also wood fibre character dependent

If you compare MDF to pine then probably what you are seeing is real but comparing MDF to something harder may be misleading simply because the increased amount of fine dust from the hardwood is now largely invisible, so it might look like MDF has more fine dust but that won't be the case.

One issue with MDF dust in a shed is that it decomposes under moist conditions more rapidly over time, so a layer of MDF dust in the shed gets converted into finer and finer dust.

All this is why OHS dust exposure levels for soft and hardwoods are different with hardwoods being about 5 times lower than softwoods. MDF is currently ln the hardwood range and set to go go lower but not so much because of the dust levels but more because of the chemicals it contains.

Originally Posted by BobL

This could be another case of, if it can't be see it isn't there?

No way Man - I'm para (sort of) about invisible dust! To be clear, what I was saying about the very fine MDF dust is that because it is finer than most real wood dust (visually) it gave me the opportunity to see how very fine but still visible dust swirls around - that's all.