Thread: Sawstop - Festool Overhead Dust Collection

Good to see they used at least 100 mm ducting but here are 3 things I can see that suggest the designers have not studied the air and dust flow over/around/away from a circular blade.

These are common mistakes on OH guards.

1) The majority of dust is made at the front of the blade, and combined with the rotational direction of the blade this further concentrates the chips and the dust in front of the blade. To optimise fine and coarse dust extraction the ducting port should be as close as possible to the source, so this is where the ducting should be located. Unfortunately this blocks the operators view so the best one can do is at the top of the front.

2) To sweep away the dust requires maximum air flow. However, to prevent sawdust escaping from under the sides and front of the guard the guard should be as close as possible to the TS top. This leaves only the back of the guard as a possible air inlet - in fact it can be almost completely open as this is a low pressure region anyway. This sets up a natural flow path assisted bey the DC extraction with the air easily passing from back to top front for dust removal

3) The front slope of the guard has to be opposite to what they used. This is because a guard that slopes upwards away from operator still scatters chips downwards and the air being dragged around by the blade is peeled off the blade by the throat plate towards the operator as show by the brown arrows in the Std Design diagram

In contrast a guard that slopes downwards away from the operator scatters chips upwards and if the duct is right there the duct will more readily grab the dust and the chips. See optimised Design diagram

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I experimented and tested a number of the "Std design" lay outs because I believed that the std design would have less turbulence. After much testing I found that both designs are just as turbulent and gave up some view for a vastly improved chip collection of the optimised design.

DomAu's. Lappa's and John Samuels guards also have the slope of the front the same way.
A good test for chip suction is how big a piece of wood can be sucked up from the front of the blade.

That is pretty much a copy of the Excalibur Guard that has been around a couple dozen years give or take. General International had them for a while but they are no more as far as I know and there have been others making variations for some time too. Maybe SawStop bought the design or copied it and painted it black. If I remember correctly the pipe is 3" inside and is usually hooked up to a 4" pipe/hose. The Excalibur version used to cost $400+ so I'm curious to know how much these will cost.

Pete

Originally Posted by QC Inspector

That is pretty much a copy of the Excalibur Guard The Excalibur version used to cost $400+ so I'm curious to know how much these will cost.

Pete

Well, assuming that both Sawstop and Festool have had a hand in it.... hmmm I don't know... $10,000,000? ��

Originally Posted by QC Inspector

The Excalibur version used to cost $400+ so I'm curious to know how much these will cost.

Pete

They are $399US.see New Options | SawStop. So ++ in Oz.

On a side note is the dust collection on Shark Guard any good?

Well that makes it $500Can for me so I'll cheap out and make something similar with improvements more along the lines of what Bob posted for my SawStop.

Pete

Their shroud is different to that which @BobL drew, in that the skirt appears to sit close to the deck, with the exception of a large mouth at the front, promoting maximum airflow right where the most dust is created. I'd be interested to know if anyone has tested this setup before, as it, together with the angled hose attachment appears to promote a rather good flow across the front and top of the blade.

WP_20170925_08_15_16_Pro.jpg

Lance

Originally Posted by Xfigio

WP_20170925_08_15_16_Pro.jpg

That drawing is not correct - take another look .
What looks like a large opening at the front is just a handle / ramp to help push the hood upwards when it comes into contact with the wood.
The guard itself is behind that handle and has a sloping transparent panel

sawstop-dust-collection.jpg


I've tried your design and it did not work at all for chips.
As I describe above the slope direction continues to allow the chips coming off the blade to move in a forward direction.
The other problem is that collection behind the blade using an open front and closed back means the air stream has to overcome the high air speeds the blade generates in a forward direction.
Its much better to have the air stream into the DC port working with the forward direction of the air and dust from the blade.
Front collection also puts the collection as close as is practicable to the source of the dust which is essential in dust collection.

The chip speeds coming off the front of the blade are essentially the same as the blade speed ~60 m/s whereas the air speed near a 100 mm duct is well below this and the flow from 100 mm duct on the other side of the blade simply cannot slow the chips down enough before they escape out of the range of the air flow.

Originally Posted by Xfigio

Their shroud is different to that which @BobL drew, in that the skirt appears to sit close to the deck, with the exception of a large mouth at the front, promoting maximum airflow right where the most dust is created. I'd be interested to know if anyone has tested this setup before, as it, together with the angled hose attachment appears to promote a rather good flow across the front and top of the blade.

WP_20170925_08_15_16_Pro.jpg

Lance

I think this doesn't work well in practise because you're unlikely to have enough airflow velocity in from the front to stop dust and particularly larger particles and chips that are firing forward at the velocity of the blade teeth.

To get more velocity you have to reduce the inlet area, but then you reduce volume significantly and velocity only partially as you'll just have more airflow through the larger port in the cabiney instead (assuming a typical cabinet/overhead split setup where the static pressure will balance between the branches).

The other thing with that design is that you'll have air coming in from both front and back as the guard lifts up as you feed a piece in, particularly with shorter cross cuts.

This is my DIY setup with a forward hose position. It works very well. You can see larger particles hitting the front rubber fingers and then getting sucked away. Fine dust is pulled straight out.

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Originally Posted by DomAU

The other thing with that design is that you'll have air coming in from both front and back as the guard lifts up as you feed a piece in, particularly with shorter cross cuts.

That is a really important point as more air, like water and electricity, will flow through the path pf least resistance.

I tried to find the graph below in the dust forum which I posted in 2012 and see it has unfortunately been wiped so here it is again.
What it shows is the air speeds as a function of distance from duct openings - the relationship is roughy inverse square, i.e. at twice the distance the air speed drops by a factor of 4.

OK this is for a naked or open ducts but it still shows why it is important to get the pick up point as close as possible to the source of the chips - hoods that are > 300 mm away from chips are unlikely to collect many chips.

Screen Shot 2017-09-25 at 7.03.50 am.png

BTW the data for the bell mouth might not look all that much better but this data is for my old turned wooden BMH the new PVC BMHs are better than this.

Originally Posted by BobL

That drawing is not correct - take another look .
What looks like a large opening at the front is just a handle / ramp to help push the hood upwards when it comes into contact with the wood.

Ah huh! It is indeed. Thanks for correcting my miss-conception.