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Post by Lord Nibbo on Oct 19, 2007 12:42:56 GMT
Fed up of having to remove the riving knife every time I wanted to use my tenoning jig I decided to cut the knife off just below the level of the blade. But the riving knife holds the guard in place and the guard is also designed to suck up any stray sawdust above the table, not that the suction was any good anyway  So I decided to do something about it, here's what I done. I cut of the riving knife just below the blade height and added this frame to hold the guard.   In raised position. If I need clearance for using my tenoning jig it really easy to remove one bolt in the struts of the frame and swing the cover out of the way. This pic shows how high it will lift without removing the bolt.  While I was at it I thought about the rubbish suction that the guard/cover generates so using a small piece of plastic cut from an old white spirit bottle is pushed into the pipe end to create a ventury effect. It works but I'm going to experiment with different sizes to see what is best to add suction to the blade cover.  I also added a zero clearance kerf slot.  Finally I though I'd stop any dust blowing around the workshop by filling the gaps between the feet on the base. So using just some scrap MDF I did this.  No fixings they just sit on the floor.  I got the box tube and flat bar out of an offcuts bin at a local farm engineering company, cost £5  The pipe is from an old knackered Dyson The paint I already had, I probably used about half a can so that cost was about £3.... so in all I've spent about £8 Ah! forgot £2.50 for the gate spring.... |
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Post by engineerone on Oct 19, 2007 13:43:31 GMT
that looks very good and professional mate. however could you stop being so profligate with your money   ;D how well does the dust stop at the bottom work. although i have mounted my dewalt 744 on a base i still find dust in the strangest places  interesting idea about the top extraction. paul  |
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Post by Lord Nibbo on Oct 19, 2007 14:55:51 GMT
How well does the dust stop at the bottom work. paul The dust is still there but totally out of sight ;D the main reason was to stop air entering the cabinet easily from the bottom and force it to suck more from the top. What I didn't take a pic of was the duct tape below the iron top sealing the air gap to the cabinet, now the only place where air can be sucked in is the large curved slot where the blade height adjusting wheel is, I might just try squeezing in some foam rubber there but some air has got to get in or the motor could get overheated. |
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Post by Scrit on Oct 19, 2007 19:05:45 GMT
A very inmteresting and useful alteration. I might just try squeezing in some foam rubber there but some air has got to get in or the motor could get overheated. Bear in mind that in order to extract air/dust you'll need to allow a certain amount to enter below the worktable so blocking off all the potential inlets may actually reduce the efficiency of your extraction Scrit |
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Post by engineerone on Oct 19, 2007 21:05:31 GMT
we have had a discussion of this kind in another place, and it was suggested that if you have extraction above and below the blade, then one must be less strong than the other. i wonder however if you could use extraction only at the top, but much more strong, whilst having the lower as just a dump. then the air flow up through past the blade should move more of the dust up into the collector. scrit does this make sense, and would that improve air flow and dust collection?? paul |
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noel
Junior Member
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Post by noel on Oct 19, 2007 21:21:48 GMT
I use an overhead guard wth extraction and mostly only keep the blast gate for the top open. After use open the cabinet blast gate and clear out the sawdust that's gathered in the cabinet.
As Scrit rightly mentions don't stave the cabinet of air and toast your extractor although I see there's plenty of space around the blade adjustment wheel. Is the ZC screwed down or are the screws just used for leveling (or both)?
BTW, how do you find the Jessem mitre gauge?
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paulm
New Member
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Post by paulm on Oct 20, 2007 8:02:43 GMT
You probably won't want to hear this after you've done all that work LN, but what I did to sort the same problem for using tenon jigs etc was just to elongate the slot in the bottom of the riving knife blade so it was 10mm or so closer to the top of the blade. That was enough to let the top of the riving knife sit just below the top of the saw blade, still take the guard and dust extraction, and still allow the blade and knife to lower below the table surface so I can cover with a sheet of thin mdf for extra work surface when not in use. Works great ! Cheers, Paul.  |
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Post by Lord Nibbo on Oct 20, 2007 11:55:50 GMT
I use an overhead guard wth extraction and mostly only keep the blast gate for the top open. After use open the cabinet blast gate and clear out the sawdust that's gathered in the cabinet. As Scrit rightly mentions don't stave the cabinet of air and toast your extractor although I see there's plenty of space around the blade adjustment wheel. Is the ZC screwed down or are the screws just used for leveling (or both)? BTW, how do you find the Jessem mitre gauge? It's screwed down, no adjustment. I made it slightly thick then planed it down on my jointer machine taking off fractions at a time until it was only thou's too thick, I finished it exact level with a pad sander. Re: Jessem... I know there are many fans here of the osbourne range and Incra and I'm not going to knock them, they do the job so why knock them The Jessem is in another league, it's not within thous of an inch it spot on you just cant get any more accurate than that. OK it is expensive kit for cutting an angle and using the locking pins are daunting at first but you soon get used how to do it and that is using it before reading the instructions ;D The bar that fits in the table slot is adjustable to take up any slack similar to the osbourne and incra but this model has four all made of bronze. The fence also extends out far further than I need infact with out looking I can't remember how far but I would guess 800mm- 900mm. Another plus it is very quick & easy to calibrate the measure from the blade when you are using the stop. All in all I would give it 10+ out of 10. |
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niki
New Member
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Post by niki on Oct 20, 2007 14:28:50 GMT
Hi Lord Nibbo I think that to create a real "Venturi effect", you have to make a real Venturi. When the air is traveling along a smooth surface or in a tube with constant diameter, on the immediate surface, the air velocity is near zero...one layer above, the velocity increases, the next layer will have higher velocity and so on till, the air velocity equals the "free stream velocity" that is at the center of the tube. This layer, that the air velocity changes from zero to the "free stream velocity" is called "Boundary layer" and as you can see on the drawing below, the flow is laminar at the beginning (near the leading edge) and becomes more and more turbulent down the stream. This turbulent layer causes drag or "Loss of momentum"...it's undesirable but also unavoidable.  What I learnt from the boundary layer is, to use short, as straight as possible and smooth surface tubes (the flexible hoses that we are using, adds even more to the turbulent layer and causes even more drag and "loss of momentum" or what is called "Duct losses") To create a lower pressure, we must accelerate the air...Venturi did it by reducing the tube diameter gradually, using a profile (that reminds the airplane wing profile...for some reason  . The law is "what comes in - must come out" so, for the same air mass to pass through this "profile", the air must increase its velocity...increasing the velocity causes (among other thing) the pressure to drop (this pressure drop, creates the "Lift" force on the airplane wing to keep the airplane in the air but...the real reason that keeps the airplane in the air is...the Captain's salary...trust me - I know ).   I think that it will be very difficult to make "home made" three dimensional Venturi not mentioning that you have to design the camber (or profile) so the air flow will be laminar and not turbulent. If you feel some suction at the opening that you made for the overhead guard, I think that it's because your dust extraction is strong enough and it's capable to draw more air than the 100mm (4") tube limit. I made some experiment and pushed a 50mm tube into the 100mm tube with the opening at the center of the 100mm tube, where the flow is pure "free stream flow velocity". The 100mm tube cross-section area is 7850²mm and the 50mm tube is 1962²mm so the ratio is around 1:4 or 75% of the air is coming from the 100mm tube and 25% from the 50mm tube I think that I got twofold effect... 1.The 100mm tube cross-section area is decreased, that causes the air to accelerate and create a low pressure at the opening of the 50mm tube (a kind of "jet pump"). 2.the total cross-section area, remains the same so, it's like I divided the 100mm tube into two, 75% and 25% but the airflow (mass) is the same.   The clear plastic bottle is from my table saw DC that is 100mm but my shop-vac it 50mm and I made it to get smooth transition   The "smoke source" is 50 mm from the opening, no suction...part of the smoke went to the main inlet (arrow)  With the "Jet pump"   Please note that the smoke goes always to the center of the tube where the "free stream velocity" is the highest   I also "sealed" my table saw from every side...I have a hole (too big hole if you ask me) )at the place that the arbor is moving up/down  I cut this 30mm thick board using only the R2D2   Some small amount of dust came out from the top but not something "to be admired of"  I hope that it will help you to design your system and....sorry for the long "story"... Regards niki |
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Post by Lord Nibbo on Oct 20, 2007 15:00:08 GMT
Thanks for the pics Niki very interesting. I did wonder about extending the small pipe connection into the middle of the bigger pipe but I haven't got any small pipe fittings to try it, so the adding of the small piece of plastic is/was experimental but surpizingly it has improved the suction for above the table, if I can get some small fittings I'll experiment further. |
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Post by dom on Oct 20, 2007 17:41:45 GMT
Thanks Niki, I now have a headache ;D
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Post by Scrit on Oct 20, 2007 18:16:03 GMT
does this make sense, and would that improve air flow and dust collection?? Most of the heavier particles tend to drop out on the underside of the saw and that's where the greatest extraction needs to be I'd say You probably won't want to hear this after you've done all that work LN, but what I did to sort the same problem for using tenon jigs etc was just to elongate the slot in the bottom of the riving knife blade so it was 10mm or so closer to the top of the blade. If you have a look at any panel saw such as the Altendorf (like my F45 shown below), Felder, etc with a SUVA-type crown guard (i.e. the sort of guard Lord Nibbo has created) you'll see that the riving knife is lower and ends about 3mm beloe the top of the blade  The current Altendorf SUVA-type crown gurad and riving knife:  Scrit Edit: Sorry for the mis-spelling of your name yer Lordship. It's me old war wound, y'know.... (that and I'm crap at typen) |
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Post by Lord Nibbo on Oct 21, 2007 12:08:19 GMT
This morning I also added some shelves  |
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Post by engineerone on Oct 21, 2007 16:16:32 GMT
niki, forgive me for being thick but where are the two fittings fixed to the saw?? or are the hoses going into this fitting somewhere else??? paul |
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noel
Junior Member
Posts: 52
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Post by noel on Oct 21, 2007 21:57:10 GMT
I use an overhead guard wth extraction and mostly only keep the blast gate for the top open. After use open the cabinet blast gate and clear out the sawdust that's gathered in the cabinet. As Scrit rightly mentions don't stave the cabinet of air and toast your extractor although I see there's plenty of space around the blade adjustment wheel. Is the ZC screwed down or are the screws just used for leveling (or both)? BTW, how do you find the Jessem mitre gauge? It's screwed down, no adjustment. I made it slightly thick then planed it down on my jointer machine taking off fractions at a time until it was only thou's too thick, I finished it exact level with a pad sander. Re: Jessem... I know there are many fans here of the osbourne range and Incra and I'm not going to knock them, they do the job so why knock them The Jessem is in another league, it's not within thous of an inch it spot on you just cant get any more accurate than that. OK it is expensive kit for cutting an angle and using the locking pins are daunting at first but you soon get used how to do it and that is using it before reading the instructions ;D The bar that fits in the table slot is adjustable to take up any slack similar to the osbourne and incra but this model has four all made of bronze. The fence also extends out far further than I need infact with out looking I can't remember how far but I would guess 800mm- 900mm. Another plus it is very quick & easy to calibrate the measure from the blade when you are using the stop. All in all I would give it 10+ out of 10. Thanks for that. Always liked Jessem stuff. I'm not an Inca or Obourne fan, have an JDS Accu-mitre for some years which I find excellent. |
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So I decided to do something about it, here's what I done.
The pipe is from an old knackered Dyson 
;D
interesting idea about the top extraction.
.
