# DIY Overhead crown guard with dust extraction.



## Tugalis (21 Mar 2021)

This is a project that I have needed to make but put off for around 18 months now. In our old workshop we had a huge gas furnace heater so never had an issue with opening the roller door when carrying out a lot of timber cutting. Now we rely on a wood stove the heat isn't as instant so we ended up keeping the door shut when working on big projects which wasn't the best idea. My chest was not pleased with this stupidity and so the need for an overhead guard was born...

To buy a 'turn key' option I was looking at between £500 and £1500 which is extortionate for what it is, the table saw only cost £900 new! I had a chat with some people on Facebook and a few other forums, to come up with some ideas and this is my design. Its been made from a mixture of 18mm Hardwood Faced plywood which has been laminated in places for strength and 3mm clear Acrylic for the guard. There were a few issues along the way but for the most part it was a fun build, one big one being an issue with the timber dropping due to the use of a single hinge on the outer arm but after a bit of a play I figured something out. The fix was a section from a centenary wire kit which was bolted to the ceiling joists.

The unit is hinged on both joins and so can move however I have a couple of stop blocks screwed in to keep it rigid. The saw is placed in front of the roller door so I want the option to swing the arm out of the way if I need to move the saw but its not something which will be done every day so thought this was the best option. I also might add a slider on it so I can move the guard left to right but haven't decided just yet. 

The "paddle" arm moves up and down, it has a small 6mm slot cut into it and the bolts are secured using 50mm washers and butterfly nuts. Its not the most pretty of designs but it works well and its quick which is what we need.

I used a mixture of 4" spiral steel ductwork and 4" flexi pipe. It work pretty well, the flexi duct is old so its yellowed which I'm not super pleased with. Might end up replacing it at some point but as its so expensive it can just stay as it is at the moment. We don't have any issues with air leaking and that's the main thing.

The unit works as it should and has vastly improved the dust collection in the shop without limiting my options like a normal guard. We do alot of work on sleds so I need the ability to be able to use a guard that isn't connected to the riving knife.

FYI - I did use the shop laser cutter to cut the plywood and acrylic for the actual guard but everything else was built by hand. In all fairness, this could have easily been made with a jig saw or scroll saw and a drill if you really wanted to and lets be honest, who wouldn't use the tools they have at their disposal!

I would be genuinely interested in what you guys think and if there is anything I missed or could have done better. The hive mind is awesome for this sort of project so feel free to ask any questions!

I created a couple videos on Youtube detailing the process if anyone fancies having a look, just tap the link in my signature.

****EDIT****

Just adding this edit in here to deal with any issues, next time I will make sure to add all info in....trying to post things on a Sunday afternoon and I forgot to include some of the details. 

Acrylic - Yes I have used 3mm acrylic as this is what I had on hand and able to run through the laser machine. I will be upgrading this to Polycarbonate but for the time being the main issue is dust extraction, not guarding the blade. 

Rigidity - The whole boom arm does not move, at all. In terms of the guard, there is approximately 10-15mm flex either side and I think Im going to deal with this by changing the butterfly nuts to DIY wooden knobs which will allow me to add in plenty of clamping force. When the butterfly nuts are tight, there is no lateral movement

HVLP vs HPLV - I spent a long time researching CFM, reading Bill Pentzs ideas and speaking to HVAC companies when designing my original duct work system for the workshop. We run a Felder AF16 and 6"/150mm metal spiral ductwork on the other side of the shop so I would like to think I understand a fair bit. For this side we just have the Charnwood 2hp extractor which has 5" port reduced down to two 4" ports. There is a 4" hose which goes to the base of the saw and the other port is plumbed into the overhead guard. 

Hose Placement - The issue we are having is when cutting long boards we get the tiniest of hits at the back of the cutting kerf which is throwing dust into the air and into my space. The placement of the hose is such so that as the dust flies up it should get sucked directly into the hose port on the top of the guard. 

Riving Knife Guard - We do a huge amount of sheet timber cutting, alot of this is using custom built sleds and as such a riving knife guard wont work. We need to be able to keep the top of the riving knife free to allow this. I dont really know why they keep the ports so small on the top of the guards, I would think its easier to manufacture and ship a flat guard then it is a wide one with a 4" port. Felder uses an 80mm port on their overhead guard. 

Guard Height/Location - Its worth noting that the guard is never meant to be within more than 25mm of the cutting surface which is why there are no brushes or a sloped lead in. This is by design as with a lot of the thinner boards we cut, there is a tendency to warp and this would cause us further issues when cutting if the timber hits the guard. As for the location and hitting the saw blade, unless there is a catastrophic failure with the boom arm, the guard shouldn't be able to come within 25mm of the blade. Again, I understand that we need to plan for the worst case which is why we will be upgrading the guard to polycarbonate and we always use glasses when cutting, regardless of a guard being in place or not.


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## MARK.B. (21 Mar 2021)

Any improvement verses no guard at all has to be a good thing in time you might make a little tweak here and there but from what I can see you have made a great addition to your shop


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## Tugalis (21 Mar 2021)

Thanks Mark, I think you are right. We always used the riving knife but I think this will make work a lot safer, it only takes a millisecond to lose a finger!


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## MARK.B. (21 Mar 2021)

100% it will help keep your pinkies where they belong


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## EddyCurrent (21 Mar 2021)

Fully agree with you on the rather high price of those units.
I'm not keen on the metal screws, nylon may have been better.


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## DBT85 (21 Mar 2021)

I've been thinking about doing similar for mine so this is an interesting build to me. I have been wondering whether I might try and build something with no metal like screws near the blade just in case it deflected or dropped. Maybe over worrying.

Would be nice to have a proper volume of air being sucked off the top though. Ghe stock guard in the Sip 01446 has a 20mm air port!

I'll give the vids a watch later though! Very nice.


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## Tugalis (21 Mar 2021)

Mark - For sure, dont want to lose them lol.

Eddy - Yeah I dont understand the price of them. I think it must be something to do with the insurance liability maybe? Good shout, it never even occurred to me to use nylon screws to be honest. Ill bare that in mind for the next build or see if I can find some. 

DBT - I think thats a fair enough concern to be honest. It didnt even cross my mind so I might change them out moving forward. The amount of air being moved is ridiculous. It can lift up a 5kg weight out right and move a 10kg weight so its got some suck!


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## Jacob (21 Mar 2021)

Whole thing looks a bit flimsy! It needs to be solidly engineered - remember Steve Maskery's "flying Suva" guard?
You can get your fingers under it but you couldn't get a push stick near it - should be the other way around if anything.
Not easy to adjust.
You don't need so much extraction off the top - nearly all the shavings go down, which is where your LPHV extraction should be - from under the table.
A crown guard on the riving knife is a much, much better option and most designs have an extraction port for high pressure low volume dust extraction, which yours does not have.
Back to the drawing board in my opinion!
Good try though.
PS is "acrylic" strong enough? Steve's "flying Suva" guard caught the blade after a little nudge and got yanked round and shattered/exploded, injuring his finger in the process. Could have been worse!
To be honest - I think your design is potentially dangerous.


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## Inspector (21 Mar 2021)

You stated you used acrylic in the construction. I hope you were using it in a generic way and that you are actually using polycarbonate (Lexan is a trade name) rather than Perspex / Plexiglass. Perspex shatters where Lexan does not. That is why Lexan is used in face shields and machine guarding. Much safer product for a saw guard.

Moving the dust hose to the front half of the guard will pick up more dust than from the present location nearer the back. Also if it is set tight to the wood or floating on it having the back end open allows makeup air into the guard. If you use it above the wood a little the air is coming in from all sides.

Those observations aside, a good overall guard.

Pete


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## DBT85 (21 Mar 2021)

@Jacob I don't think I've ever seen anyone with both hplv and hvlp on a table saw before though I can understand the reasoning. Do you run yours in such a fashion?


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## Jacob (21 Mar 2021)

DBT85 said:


> @Jacob I don't think I've ever seen anyone with both hplv and hvlp on a table saw before though I can understand the reasoning. Do you run yours in such a fashion?


No I don't but I suppose I might use the top port if I was cutting a lot of MDF or similarly dusty stuff. As it is all the extraction is LPHV from below. This is a version of my machine. You can see the top extractor port on the crown guard. It's towards the back because the fine dust tends to come back up with the blade, with the coarser stuff safely extracted below - duct out of sight on the back.
On Tugalis's machine nearly all the dust will go down just the same, with only a little emerging back above the table.







Had a quick google - nearly all the crown guards have a small top extractor port the same table saw crown guards - Google Search


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## Doug71 (21 Mar 2021)

DBT85 said:


> @Jacob I don't think I've ever seen anyone with both hplv and hvlp on a table saw before though I can understand the reasoning. Do you run yours in such a fashion?



I do, well kind of.

I have a Record DX 4000 on the bottom of my saw (think it's actually classed as hplv but it takes a 4" hose). When I split it to go to crown guard the extraction really seemed to suffer, I had a spare shop vac kicking around so put that to the crown guard instead. They both turn on together with remote switches, works really well.


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## Doug71 (21 Mar 2021)

I need an overhead crown guard but my saw is in the middle of the workshop so it will have to be freestanding. 

I'm kicking myself because there was an Axi one on ebay local to me for £75 a while ago but I didn't have a saw to warrant it at the time  

I quite like Mike Farringtons version, love his laid back presentation style. 



TBH I will probably end up buying the £300 Axi one as it will cost me more in time and materials to make one.


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## Jacob (21 Mar 2021)

Doug71 said:


> I need an overhead crown guard but my saw is in the middle of the workshop so it will have to be freestanding.
> 
> I'm kicking myself because there was an Axi one on ebay local to me for £75 a while ago but I didn't have a saw to warrant it at the time
> 
> ...



Dear oh dear another dodgy flimsy gadget! I didn't realise there was a fashion!
Same very basic mistake; LPHV extractor above where it won't shift anything - it's really needed below.
Not to mention the overall flimsiness.
Here's Steve's job The importance of practising what one preaches different design but the same problem - flimsy construction and wrong materials. 
I don't know why there's a prejudice against riving knife crown guards - they are very safe and sensible, not least because they move with the saw adjustment and stay permanently in position.


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## DBT85 (21 Mar 2021)

Jacob said:


> Dear oh dear another dodgy flimsy gadget! I didn't realise there was a fashion!
> Same very basic mistake; LPHV extractor above where it won't shift anything - it's really needed below.
> Not to mention the overall flimsiness


I thought the whole point of lphv was that it was able to bring in enough volume to catch all the finer dust that you said comes back above the top? 

If I'm honest I'd not put much stock in the small crown guard ports being there because you're supposed to use hplv and more likely just because companies aren't thinking about it properly. The machines don't mention it in any literature and in fact just ask for a large volume.

I have a suspicion that all the saw stops that actually have the guard and extraction fitted (hahaha no but really some do!) have 50 or 60mm ports designed to be used with lphv.


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## DBT85 (21 Mar 2021)

Doug71 said:


> I do, well kind of.
> 
> I have a Record DX 4000 on the bottom of my saw (think it's actually classed as hplv but it takes a 4" hose). When I split it to go to crown guard the extraction really seemed to suffer, I had a spare shop vac kicking around so put that to the crown guard instead. They both turn on together with remote switches, works really well.


Same hplv as me then there Doug. Where did you find the extraction suffered when only using the dx4000? Above ghe table or under the table?


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## Jacob (21 Mar 2021)

DBT85 said:


> ......
> If I'm honest I'd not put much stock in the small crown guard ports being there because you're supposed to use hplv ......


You are right the crown guard port is not a lot of use for the simple reason that nearly all the sawdust goes down under the table, which is where the extraction is needed. Only a little fine dust finds its way back above the table.


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## Doug71 (21 Mar 2021)

DBT85 said:


> Same hplv as me then there Doug. Where did you find the extraction suffered when only using the dx4000? Above ghe table or under the table?



I was getting more dust than I expected above the table. On my old saw I had a LPHV to the bottom and shop vac to the top and was happy with it.

When I got a new saw and the Record extractor I hoped I would be able to split the hose and use it for both. I split the 4" hose with a Y then reduced the side going to the crown guard down to 60mm, I guess it might have been better if I took it all the way to crown guard at 100mm then reduced it.

Like I say I had a shop vac I wasn't using so I just ran that to the crown guard and left the Record just doing the bottom of the saw and get good results again.

Also my saw is designed to take a 5" hose at the bottom so I thought I had better have as much extraction at the bottom as possible.


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## ndbrown (22 Mar 2021)

I am seeing good results on mine with a hvlp set up 100mm below and 50mm above connected to the same outlet. I roughed mine out in plastic pipe and plywood to try the concept and then manufactured it in stainless and aluminium. After a year of use, very happy with the performance


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## Tugalis (22 Mar 2021)

Thanks for all of the replies. I have bunched them all together below, let me know if I have forgotten anything.. Great job by the ndbrown, that pipework is something else!

Replies - 

Acrylic - Yes I have used 3mm acrylic as this is what I had on hand and able to run through the laser machine. I will be upgrading this to Polycarbonate but for the time being the main issue is dust extraction, not guarding the blade. 

Rigidity - The whole boom arm does not move, at all. In terms of the guard, there is approximately 10-15mm flex either side and I think Im going to deal with this by changing the butterfly nuts to DIY wooden knobs which will allow me to add in plenty of clamping force. When the butterfly nuts are tight, there is no lateral movement

HVLP vs HPLV - I spent a long time researching CFM, reading Bill Pentzs ideas and speaking to HVAC companies when designing my original duct work system for the workshop. We run a Felder AF16 and 6"/150mm metal spiral ductwork on the other side of the shop so I would like to think I understand a fair bit. For this side we just have the Charnwood 2hp extractor which has 5" port reduced down to two 4" ports. There is a 4" hose which goes to the base of the saw and the other port is plumbed into the overhead guard. 

Hose Placement - The issue we are having is when cutting long boards we get the tiniest of hits at the back of the cutting kerf which is throwing dust into the air and into my space. The placement of the hose is such so that as the dust flies up it should get sucked directly into the hose port on the top of the guard. 

Riving Knife Guard - We do a huge amount of sheet timber cutting, alot of this is using custom built sleds and as such a riving knife guard wont work. We need to be able to keep the top of the riving knife free to allow this. I dont really know why they keep the ports so small on the top of the guards, I would think its easier to manufacture and ship a flat guard then it is a wide one with a 4" port. Felder uses an 80mm port on their overhead guard. 

Guard Height/Location - Its worth noting that the guard is never meant to be within more than 25mm of the cutting surface which is why there are no brushes or a sloped lead in. This is by design as with a lot of the thinner boards we cut, there is a tendency to warp and this would cause us further issues when cutting if the timber hits the guard. As for the location and hitting the saw blade, unless there is a catastrophic failure with the boom arm, the guard shouldn't be able to come within 25mm of the blade. Again, I understand that we need to plan for the worst case which is why we will be upgrading the guard to polycarbonate and we always use glasses when cutting, regardless of a guard being in place or not.


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## DBT85 (22 Mar 2021)

Thanks for the update, I knew here was something I wanted to watch!


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## Sachakins (22 Mar 2021)

ndbrown said:


> I am seeing good results on mine with a hvlp set up 100mm below and 50mm above connected to the same outlet. I roughed mine out in plastic pipe and plywood to try the concept and then manufactured it in stainless and aluminium. After a year of use, very happy with the performance
> View attachment 106481
> 
> View attachment 106482
> View attachment 106483


Love the smoothness of the bends in that steel, great job.


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## MARK.B. (22 Mar 2021)

That looks very good indeed ndbrown a lot of thought and skill went into that setup .


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## Jacob (22 Mar 2021)

....


Tugalis said:


> ...........
> 
> Riving Knife Guard - We do a huge amount of sheet timber cutting, alot of this is using custom built sleds and as such a riving knife guard wont work. We need to be able to keep the top of the riving knife free to allow this.


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 ?


> I don't really know why they keep the ports so small on the top of the guards,


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 (22 Mar 2021)

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.


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## Jacob (23 Mar 2021)

DBT85 said:


> 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.


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 (23 Mar 2021)

Jacob said:


> 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?



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.



Jacob said:


> 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!



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!


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## Jacob (23 Mar 2021)

DBT85 said:


> 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!


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.


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## DBT85 (23 Mar 2021)

Jacob said:


> 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.


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.


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## Inspector (23 Mar 2021)

Jacob said:


> 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?



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


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## DBT85 (23 Mar 2021)

Inspector said:


> 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


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?


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## Inspector (23 Mar 2021)

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


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## Jacob (23 Mar 2021)

Inspector said:


> 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


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 (23 Mar 2021)

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


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## DBT85 (23 Mar 2021)

Apologies @Tugalis we apear to have done the usual and gone off piste a little.



Inspector said:


> 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


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.


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## Jacob (23 Mar 2021)

So - the more work it does the less energy it uses? Have you discovered a perpetual motion machine?


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## DBT85 (23 Mar 2021)

Jacob said:


> So - the more work it does the less energy it uses? Have you discovered a perpetual motion machine?


Me Jacob? Not at all.


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## Hornbeam (23 Mar 2021)

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.


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## Jacob (24 Mar 2021)

Hornbeam said:


> 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.


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.


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## Jacob (24 Mar 2021)

DBT85 said:


> Me Jacob? Not at all.


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.


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## DBT85 (24 Mar 2021)

Jacob said:


> If a loaded motor uses less energy then set it to drive a generator and get free energy!


I'm not sure I get you Jacob?

If you cover the inlet to an impeller the load goes down, not up. The impeller is at that point spinning in a vacuum (as any air in the impeller housing is pretty much blown out of the outlet) with no pesky air getting in the way of the blades creating resistance.


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## Jacob (24 Mar 2021)

DBT85 said:


> I'm not sure I get you Jacob?
> 
> If you cover the inlet to an impeller the load goes down, not up. The impeller is at that point spinning in a vacuum (as any air in the impeller housing is pretty much blown out of the outlet) with no pesky air getting in the way of the blades creating resistance.


If there was a vacuum in the housing (there wouldn't be but there might be lower pressure) it would be working _against_ the flow of air being impelled out and the impeller would encounter more resistance than if the thing was open both ends. If you put your hand over the inlet or outlet of a vacuum cleaner you can hear the motor using more power and speeding up. So much so that if you left it blocked the motor might overheat and/or the current blow the fuse.
It's very basic physics. If it's Bill Pentz's theory then he's probably wrong about everything else as well! Which is a relief as I wasn't looking forward to wading through his dense and long-winded output and I'll forego the pleasure!


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## DBT85 (24 Mar 2021)

Jacob said:


> If there was a vacuum in the housing (there wouldn't be but there might be lower pressure) it would be working _against_ the flow of air being impelled out and the impeller would encounter more resistance than if the thing was open both ends. If you put your hand over the inlet or outlet of a vacuum cleaner you can hear the motor using more power and speeding up. So much so that if you left it blocked the motor might overheat and/or the current blow the fuse.
> It's very basic physics. If it's Bill Pentz's theory then he's probably wrong about everything else as well! Which is a relief as I wasn't looking forward to wading through his dense and long-winded output and I'll forego the pleasure!


I do not believe you are correct Jacob. Once covered, the housing has expelled any air inside and the pressure is lower, if the pressure is lower there is less resistance to the blades and it is that resistance that increases the load. The more air getting to the impeller that harder it has to work to move that air out of the way.

I am happy to be proved wrong, however. I'll provide my own numbers by the weekend for anyone curious.

Here is Mattias Wandel showing you the current draw on a centrifugal blower (as used in these extractors) dropping when the inlet is covered, from 2:30 on the video. The fan also speeds up which is more noticeable if you wind back a bit. Only in that case becase the impellor was so large that the motor couldn't spin it at the correct speed I believe.



I do not know if the same applies to ordinary vacuums, though the issue with those is that with blocked airflow the motor can no longer be cooled and so will heat up. Not the case on extractors as the motor is not cooled by the air it is moving.

I'll try and take my own readings of both for the purposes of science though maybe what appears basic physics isn;t quite that straightforward after all.


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## Doug71 (24 Mar 2021)

As I understand when you block the end of a vac hose with your hand it sounds like it's working harder but it's not, it's just spinning faster and freely because it isn't having to move any air. This can cause them to burn out because they are not designed to spin that fast plus there is no air cooling the motor. Some of the better vacs like Festool have a bypass built in so the motor is still cooled even when the hose is blocked.


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## Jacob (24 Mar 2021)

Doug71 said:


> As I understand when you block the end of a vac hose with your hand it sounds like it's working harder but it's not, it's just spinning faster and freely because it isn't having to move any air. This can cause them to burn out because they are not designed to spin that fast plus there is no air cooling the motor. Some of the better vacs like Festool have a bypass built in so the motor is still cooled even when the hose is blocked.


Just checked - I am right _and_ wrong! 
A brushed motor speeds up under no load and vice versa. 
An induction motor doesn't - stays close to design speed 1500 or 3000 rpm most common. More load takes more power.
Most dust extractors are induction motors, well mine is anyway (bigger motor but cheaper and quieter more reliable). Most vacuum cleaners (and hand electric tools) are brushed motors (smaller and lighter) so I was wrong about that. Oh well thats what being on a forum is all about!


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## Sideways (24 Mar 2021)

Just to confirm, here's the current draw on my vacuum running with nothing attached





And despite the increase in speed / noise, the current goes DOWN when you block the hose. (at least until it overheats from lack of cooling).


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## DBT85 (24 Mar 2021)

Jacob said:


> Just checked - I am right _and_ wrong!
> A brushed motor speeds up under no load and vice versa.
> An induction motor doesn't - stays close to design speed 1500 or 3000 rpm most common. More load takes more power.
> Most dust extractors are induction motors, well mine is anyway (bigger motor but cheaper and quieter more reliable). Most vacuum cleaners (and hand electric tools) are brushed motors (smaller and lighter) so I was wrong about that. Oh well thats what being on a forum is all about!



More load on an induction does take more power. Never in question. Just a matter of what "load" is.

This is what I like about a forum, a spirited discussion about something interesting (to us).


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## JimmyStartrite (24 Mar 2021)

You can buy that as original

SAW GUARD AND RIVING KNIFE - Machine Spares


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## Inspector (24 Mar 2021)

Jacob said:


> 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.



Jacob you are one that will never understand dust and it's dangers because you don't want to make the effort to learn and that's okay for you. Telling everyone far and wide that Bill's information is wrong is a danger to newer woodworkers. You're "knowledge" of dust collection is rooted in the distant past and because you have always been doing things a certain way you are not open to learn that it may be wrong and can be improved. What Bill said on his site about fine dust a couple decades ago is being realized in todays regulations. The EU standards for dust are among the most stringent in the world now and are magnitudes lower than allowed when you got your start. When Bill came out with his information he got dumped on by lots like yourself including duct collection manufactures. Funny thing is that many have copied his cyclone because it is that good. The only company that makes his cyclone and is decent enough to pay him a royalty is Clear Vue. The rest just poached his work. His work has been verified and confirmed by others and it isn't, as you believe, "masses of dubious experiments and observations generating a huge fog of vague mis-information."

Someday you will be in a bed and you can proudly say "I never.... cough.... bothered with.... cough hack.... fine dust collection.... because it gasp hack cough cough.... isn't problem....harrk cough.... and isn't needed. Cough hack gob spit gasp.... where's my oxygen?"

Pete, a nutter.


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## Jacob (24 Mar 2021)

Inspector said:


> Jacob you are one that will never understand dust and it's dangers because you don't want to make the effort to learn and that's okay for you. Telling everyone far and wide that Bill's information is wrong is a danger to newer woodworkers. You're "knowledge" of dust collection is rooted in the distant past and because you have always been doing things a certain way you are not open to learn that it may be wrong and can be improved. What Bill said on his site about fine dust a couple decades ago is being realized in todays regulations. The EU standards for dust are among the most stringent in the world now and are magnitudes lower than allowed when you got your start. When Bill came out with his information he got dumped on by lots like yourself including duct collection manufactures. Funny thing is that many have copied his cyclone because it is that good. The only company that makes his cyclone and is decent enough to pay him a royalty is Clear Vue. The rest just poached his work. His work has been verified and confirmed by others and it isn't, as you believe, "masses of dubious experiments and observations generating a huge fog of vague mis-information."
> 
> Someday you will be in a bed and you can proudly say "I never.... cough.... bothered with.... cough hack.... fine dust collection.... because it gasp hack cough cough.... isn't problem....harrk cough.... and isn't needed. Cough hack gob spit gasp.... where's my oxygen?"
> 
> Pete, a nutter.


OK could be wrong! It's just all those pages of rambling stuff - he needs an editor.
Heart and lungs OK - I do take dust seriously. But dust generation for me is very intermittent - even more so now I'm retired. I've got a big extractor and a Triton vax/extractor with all the filters
Bigger problem has been hearing - I also wear ear muffs, but both my parents went deafer than me but neither of them worked in a noisy environment.
I also have all my fingers, thanks to two push sticks mainly!
As far as I know Pentz's cyclone is not a new idea - it's been in use with variations for a long time, particularly in grinding mills for flour etc. So I guess he couldn't patent it - he could only copyright his particular designs to some extent.


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## Inspector (24 Mar 2021)

Bill took the agricultural cyclones and added the inlet guide ramp and optimized the size and lengths for smaller woodworking shops. He may have made enough changes to qualify for a patent but you would have to take out worldwide patents and be prepared to defend them in court. Not practical for a pensioner to do for a product with a limited market.

Yes it is a long winded site but it was the norm 20 years ago when people had an attention span. It is repetitive because many, possibly like yourself, hop in for a short read of a specific subject and the important repeated information may get taken in so the info is in context. There is a cyclone design spreadsheet free for anyone to use that wants to make their own cyclone that prints out the cut patterns. It sizes it to match the impeller and motor power, the 3hp being a bigger diameter than the 5hp for instance. I'm sure Bill would not object to someone helping him upgrade/modernize/edit his site but I doubt anyone is lining up to do it so it will remain for the most part as is.

Don't get me wrong I respect your knowledge and abilities but you are known to spout off your opinions as fact when maybe you should be asking questions and doing a little research.

I'm done now as we have got off the topic that started this thread.

Pete


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## Jacob (24 Mar 2021)

Inspector said:


> Bill took the agricultural cyclones and added the inlet guide ramp and optimized the size and lengths for smaller woodworking shops. He may have made enough changes to qualify for a patent but you would have to take out worldwide patents and be prepared to defend them in court. Not practical for a pensioner to do for a product with a limited market.
> 
> Yes it is a long winded site but it was the norm 20 years ago when people had an attention span. It is repetitive because many, possibly like yourself, hop in for a short read of a specific subject and the important repeated information may get taken in so the info is in context. There is a cyclone design spreadsheet free for anyone to use that wants to make their own cyclone that prints out the cut patterns. It sizes it to match the impeller and motor power, the 3hp being a bigger diameter than the 5hp for instance. I'm sure Bill would not object to someone helping him upgrade/modernize/edit his site but I doubt anyone is lining up to do it so it will remain for the most part as is.
> 
> ...


All right all right I'm reading Mr Pentz!
Just hit on why a reduced pipe is not good branching into a larger pipe diameter - the reduced pipe reduces air flow and you get pile ups in the larger pipe. Thank you mr Pentz; its obvious when you think about it! That's why the small crown guard port needs it's own HPLV extraction. That'll do me for one session! Took a few thousand words to get that far


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## Woodmatt (13 Apr 2021)

Doug71 said:


> I do, well kind of.
> 
> I have a Record DX 4000 on the bottom of my saw (think it's actually classed as hplv but it takes a 4" hose). When I split it to go to crown guard the extraction really seemed to suffer, I had a spare shop vac kicking around so put that to the crown guard instead. They both turn on together with remote switches, works really well.


Thats exactly the set up I have and it works pretty well but I'm never sure if each extractor is working against the other


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## Woodmatt (13 Apr 2021)

DBT85 said:


> @Jacob I don't think I've ever seen anyone with both hplv and hvlp on a table saw before though I can understand the reasoning. Do you run yours in such a fashion?


I do and it works pretty well


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## Sideways (13 Apr 2021)

Good to know  

I have HVLP under the table on mine but I'm convinced by the theory and I'm (slowly) building a custom crown guard that will use my shop vac to create a concentrated suction at the front edge of the blade where it normally spits right out at you !


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## Jacob (14 Apr 2021)

Woodmatt said:


> ..... I'm never sure if each extractor is working against the other


I doubt the top extractor would have any effect on the bulk extraction going on below the table. Too far away, with a gap and the saw insert pate between them. The top extractor just catches the finer dust which the bottom has failed to pick up.


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