DBT85
Established Member
Thanks for the update, I knew here was something I wanted to watch!
DIY Overhead crown guard with dust extraction.
- Thread starter Tugalis
- Start date
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Sachakins
The most wasted of days is one without woodwork
Love the smoothness of the bends in that steel, great job.
MARK.B.
Established Member
That looks very good indeed ndbrown 
a lot of thought and skill went into that setup .
a lot of thought and skill went into that setup .
Jacob
What goes around comes around.
....
If you have two ports on the extractor maybe use them with one 5" pipe for extraction below, then even less will find its way back up the back of the blade?
Another way is to use something like a zero clearance insert - so even less can find its way back up.
Also maybe look at improving the ducting below the table.
The "rigidity" issue isn't about it simply staying in place, it's more about what happens when someone accidentally nudges it, drops the end of length of timber on it, forgets to tighten it. That's when you get the "flying suva guard" phenomenon. The ordinary riving + crown guard is inherently safe and the worst that happens is that it picks up a few cuts and nicks over the years
If you are doing ripping and sheets the ordinary riving knife and crown guard are designed for the job - simple, cheap, unobtrusive, safe. If the sled is the problem modify the sled to allow use of an ordinary crown guard ?
Because with a proper set up very little dust comes back up. For most users hardly any at all.
If you have two ports on the extractor maybe use them with one 5" pipe for extraction below, then even less will find its way back up the back of the blade?
Another way is to use something like a zero clearance insert - so even less can find its way back up.
Also maybe look at improving the ducting below the table.
The "rigidity" issue isn't about it simply staying in place, it's more about what happens when someone accidentally nudges it, drops the end of length of timber on it, forgets to tighten it. That's when you get the "flying suva guard" phenomenon. The ordinary riving + crown guard is inherently safe and the worst that happens is that it picks up a few cuts and nicks over the years
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DBT85
Established Member
With the amount of material material they are putting through the tools even with a "proper" setup, that much adds up. I've had a tablesaw covered in MDF even with a 3hp extractor hooked directly to the back of it directly to the blade shroud and a zero clearance insert and I do in a year what these folks are putting through in a couple of days. Most saws only come with a 100mm port on the back which is barely big enough for the flow rates the manufacturer demands in the literature.
Jacob
What goes around comes around.
100mm seems plenty to me with a 1.5kw extractor and a 12" saw blade, including cutting MDF. Sometimes gets choked on the thicknesser if planing full width hard wood but otherwise perfect.
My extractor (Axminster ADE 2200) has two 100mm ports but I only every use one. Using both means halving the power of each - could that be making things worse and not better?
The main thing about the ordinary crown guard is how well and how close it fits. The small extractor port placed exactly where the up-draught from the blade lifts dust, fixed with only three bolts to a rigid plate, all within a few inches of the blade. It also moves with the blade and with the machine if you are inclined to shift it, without any adjustment.
Compare this to the DIY devices - loosely suspended from yards away on a wobbly frame work with cable ties and wires. Heath Robinson springs to mind!
PS our OP says his design is not for blade guarding but for dust extraction. Why not just have the flexible hose hanging down from the ceiling, with the open end in the vicinity of the blade but far enough away to avoid contact? Just needs a long enough hose and some string. Sash cord for preference!
You can buy a dust hood from Axminster and fasten that on the end.
The point is - if it isn't rock solid it's safer hanging loose at a slight distance and can only get knocked out of the way rather than caught up in the blade.
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DBT85
Established Member
While I can't say for sure, I'd have thought that if using 1x 100mm got you say 10 units of suck in that one hose, using both at once might get you only 7.5 units on each as the extractor may well have a 125 or 150 inlet, so more overall suck but less in one area. Most of these type extractors are not actually sized correctly from what I can gather. My 3hp one has probably the same 300mm impeller as a 2hp or a 1.5kw so while the motor is indeed more powerful it doesn't get close to stretching its legs as the impeller isn't big enough. I'll give mine a go using a normal vac on the guard though would rather have just one machine running at once. A 4hp table saw, 3hp LPHV extractor and then another 2hp HPLV vac might be getting a bit silly. My inclination would probably be to fashion something that uses the standard riving knife as it's support while still using a 100mm port so the best of both worlds, though as mentioned that doesn't work unless you redesign those kind of sleds which I don't even have.
From my experience with that flexible ducting you'd have to fix the end to get it to hang anywhere near where you actually want it. If you just hold one end up and let gravity do the work the hose won't get close to where you want it to be. You could just gaffa tape a couple of weights to the one end but I'm not sure the risk of that dropping off onto your spinning blade would get a pass!
Jacob
What goes around comes around.
I've never used a sled - I don't know but surely one could be designed for use with a conventional riving knife + crown guard? They are DIY after all.
DBT85
Established Member
Normally the part that precludes that is the "fence" at the far end which is there to hold the sled together at that end. You could make one without that but I don't know how well it would maintain the kerf width on the whole sled. At a guess it might just be something that was dreamed up on the slightly nuttier side of the atlantic because nobody knows what a guard is and just gets made again and again without any evolution? I mean it can be as basic as a sheet of ply, 2 bits of 2x3 and something for the runners so since lots of people (including me) have and do use a saw with no guard, little thought is given to how something so simple can be improved.
I'm sure @Steve Maskery has such a sled.
Inspector
Nyuck, Nyuck, Nyuck!
Jacob I will assume your DC has a 150mm inlet split into two 100mm ports in the form of a "Y". Firstly the 150mm can flow three times as much as the 100mm. The 150mm being able to flow roughly 2,000L/hr and the 100mm flows roughly 680L/hr. Running just one 100mm port won't flow any more air even if there is a 4kw DC pulling the air through it. It doesn't harm the DC because it doesn't have to work as hard and actually draws less power but you have loads of capacity unused. Better would be to run a 150mm duct to the bottom of the saw if that is the only place you want to remove the dust from but the cabinet will need openings in it totalling at least two times the area of the 150mm leaving. Another minor point is those "Y" splits provide turbulent air into the inlet of the impeller reducing its efficiency. Better is to have a few feet of 150mm duct to let the air flowing to the impeller smooth itself out. Since you are happy with your 100mm going to the cabinet having second hose with a bell mouth hood hanging to the right and above the crown guard out of the way will help to pick up the fine (invisible to the eye) dust that is escaping from the crown guard. Localized air scrubbing.
I was taught that the saw blade should be about 6mm to 10mm above the material being cut so there would be less exposed blade. When a blade is set that way the chips and dust tend to be thrown forward rather than up so when a LPHV guard is made, irrespective of how it is mounted to the saw, if the hose is at the forward end of the guard (closest to your tummy) the dust velocity slows down a bit to be picked up. A HPLV setup with a narrow guard benefits more if the hose is forward but manufactures place them at the high point to help with visibility of the blade. Sometimes compromises need to be made and if the utmost in safety and dust collection were provided a saw would come with several configurations of quick attaching guards/riving knives but most everyone is loath to spend money and convincing people to take a little extra time change guards to best suit the work is a tough job.
DBT85 your 3hp machine flows more air than a 2hp even though the impellers are the same size because they usually come with twice as much filter area. Putting extra filters on the 2hp DC will work the motor too hard by drawing too much power to keep up so it burns them out. Thats why the 2hp have a 1 bag/cartridge and the 3hp have the 2 bag/cartridges.
Pete
DBT85
Established Member
Very interesting Pete, I couldn't and honestly can't imagine making the filtration airflow too good would cause a problem. I was aware that restricting input airflow would cause less power to be used as its essentially spinning in a vacuum though.
IIRC my 3hp/2200w motor was only drawing about 1500w when completely unrestricted which I figured was due to the impeller not being large enough to have to move enough air. I believe I saw something from Pence suggesting a 16"/400mm impeller being better suited to that size motor. In fact I seem to recall seeing Mattias Wandel using a bit of wood cut to length on the end of the shaft to determine how large he could go as the amperage will be higher with no shroud and input/output restriction than at an other.
So what you are suggesting is that replacing the crappy bags with a proper filter that's too large could in some way cause the motor to overheat?
Inspector
Nyuck, Nyuck, Nyuck!
If there is nothing restricting the airflow out of the DC and nothing restricting what goes in, the motor is working the hardest. The amperage can exceed the rating and cause overheating if run like that for a period of time. Clear Vue recommends not running their cyclones without ducting and filters for more that a quick test because the motor can overheat. If there is ducting to the DC the added resistance to airflow unloads the motor and the same if you add filters back. If you take a 2hp DC and add lots more filtration or vent directly outside it may overheat if there is minimal ducting on the impeller side. So monitor the amperage drawn. Take off all the filtration goodies off the impeller and the same on the inlet on your DC and measure it. You should find it drawing more than the 1500w you get when the stuff is on it. This is the way they do the flow rating for the sales brochure. If you cover the inlet it will be working the least as the air is just going in circles and will draw less than the 1500w.
Bill Pentz recommends a 14" impeller for a 3hp and a 15" on a 5hp. The CV 1800 has a 15" impeller for 6" ducting and the CV Max has a 16" impeller but with a bigger inlet for 8" ducting. Both use the same 5hp motor.
Pete
Bill Pentz recommends a 14" impeller for a 3hp and a 15" on a 5hp. The CV 1800 has a 15" impeller for 6" ducting and the CV Max has a 16" impeller but with a bigger inlet for 8" ducting. Both use the same 5hp motor.
Pete
Jacob
What goes around comes around.
Are you sure about the motor power? My understanding of induction motors is that unloaded they draw least power and the current goes up as the load is increased. Biggest current draw being start-up against the inertia of the motor, with the current rapidly dropping once up to speed.
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Inspector
Nyuck, Nyuck, Nyuck!
If a blower is attached and you close off the inlet you draw the least power because the impeller is cavitating. Give it unrestricted airflow and it does the most work, therefore draws the most power. Put a meter on your DC and see for yourself.
Pete
Pete
DBT85
Established Member
Apologies @Tugalis we apear to have done the usual and gone off piste a little.
I shall return with readings in a few days.
Apologies Pete maybe I wasn't clear. When I said mine was drawing 1500w unrestricted that was the the inlet shroud completely removed (so now a 305mm inlet) and nothing at all connected to the end of the snail. As soon as I connected the inlet shroud back on, covering the impeller and restricting it back down to 150mm the power draw dropped as expected as less air was able to get in the way of the blades turning and that number only drops further as more hoses get added. It's a little tricker to measure now that its on a 16A supply but I have a clip around that I can find a way of rigging up. A new class M filter is on its way for this machine to replace the bags from a company you suggested back in some other deep dark thread.
I shall return with readings in a few days.
Jacob
What goes around comes around.
So - the more work it does the less energy it uses? Have you discovered a perpetual motion machine?
DBT85
Established Member
Me Jacob? Not at all.
Hornbeam
Established Member
I think there are pros and cons to both an independent overhead guard and the crown guard fitted to the riving knife. I have had both.
The guard fitted to the riving knife is more foolproof. Its always there unless you make a concious decision to remove it. The riving knife is generally higher than the top of the blade so you cant do stopped cuts
With the independent overhead guard, the mechanism has to be solid and the adjustment spot on so there is no possibility of it flexing/catching the blade. Also the crown guard is also part of the kickback protection so it has to be strong enough and set close enough to the workpiece to stop the back of the blade being able to lift the timber up (in the event of stresses causing the cut to close. It is very easy to be complacent with this type of guard by not setting it correctly for different cuts, The guard also has to be wide enough to allow for angled cuts as it doesnttilt with the riving knife/sawblade Main benefit is you can do partial cuts like tennon shoulders etc.
The guard fitted to the riving knife is more foolproof. Its always there unless you make a concious decision to remove it. The riving knife is generally higher than the top of the blade so you cant do stopped cuts
With the independent overhead guard, the mechanism has to be solid and the adjustment spot on so there is no possibility of it flexing/catching the blade. Also the crown guard is also part of the kickback protection so it has to be strong enough and set close enough to the workpiece to stop the back of the blade being able to lift the timber up (in the event of stresses causing the cut to close. It is very easy to be complacent with this type of guard by not setting it correctly for different cuts, The guard also has to be wide enough to allow for angled cuts as it doesnttilt with the riving knife/sawblade Main benefit is you can do partial cuts like tennon shoulders etc.
Jacob
What goes around comes around.
Well yes. The remotely suspended overhead guard needs to be very substantial, unless it is suspended at a height just as a dust extractor, out of harms way.
And yes - an ordinary sturdy crown guard is good for stopping the work lifting, set close to the surface but narrow enough to not be in the way.
Jacob
What goes around comes around.
If a loaded motor uses less energy then set it to drive a generator and get free energy!
I had a look at Bill Pentz's site but he appears to be another over enthusiastic enthusiast, a bit like a sharpening nutter - masses of dubious experiments and observations generating a huge fog of vague mis-information.
