100mm Hole in Bandsaw Door

[Chris Parks]

I need to cut a 100mm hole in the steel door of my Jet 18" bandsaw to let air in so that the dust extraction actually has a chance to work. I can take the door off but the only method I would have to do it would be the gas ax (goodbye paint, hello distortion) or drilling a line of holes around the diameter of the circle and then attacking it with a file which I think is not going to look terrific when finished. Any other suggestions for a do it your self job or do I just someone to do it with the proper tools. I have hole saws but I doubt as good as they are if the teeth are hard enough to do this job.

[Metmachmad]

A 100mm holesaw should do the trick, What thickness is the material to be cut?

[matthew_g]

A 100mm holesaw should do the trick, What thickness is the material to be cut?

:aro-u:+1..Just be sure to use a Bi-Metal hole saw..

[markharrison]

Dremel? I don't know about this saw in particular but perhaps a better solution would be a shroud around the lower blade guides with an additional pick up?

[pjt]

Hi Chris,
Line of holes then cleanup with a half round file would be the way I would do it given minimal tools, holesaw would be next but needs to be a quality saw, then need to get your speed down sloooooow and good tourque as to drive a 100mm holesaw takes some grunt and hold the panel ridgid, clamped down or sqeezed between sheets of ply (or such) you would probably get away with a not so ridgid setup if the hole was right in the corner if near the edge folds, large holes in sheet can also be done in a radial arm drill or mill but would need a suitable made up tool for the job, if you don't want to mess about I'd take it to the sheetmetal blokes and they'd nip it out, might have to find one that will do it cold if you don't want to mess up the paint.


Pete

[issatree]

Hi Chris,
I don't quite get why you need a 100mm hole.
What about say 3 x 25mm. Holes, as surly that would be enough.
With a 100mm. hole, how could the D/E do it's job properly?

[Greg Q]

I have only ever sealed holes and added shrouding to improve extraction. Your plan seems counterproductive to me.

Greg

[Chris Parks]

I will see if I can find a holesaw, the panel is most probably only 1.5mm thick or thereabouts. As to why I need to do it, this saw suffers the same disease as most, no make up air when the door is closed. It has a 100mm DE adapter just under the blade but when the door is closed there is no equivalent hole to allow air into the cavity and no air in = no air out. I am going to put it straight opposite the existing hole just under the table and the air will travel at high speed straight across and capture the dust when it first enters the cabinet still attached to the blade or that is the theory. Most bandsaws have no allowance for the make up air and this is the reason most DE is not effective on them and everyone tries to use external pickups. I did the same thing on my TS and it works really well keeping the cabinet fairly clear. The other approach would be to let the dust fall and put the pick up at the bottom and leave the original pick up to allow the air to enter the cabinet but I am not sure if that will work as once the dust has fallen it might start to build up in pockets in the cabinet, that will most probably either way but we will see. Thanks for the suggestions.

[tea lady]

:hmm: As issatree says. You don't really need a 100 mm hole, just the equivalent in holes. A whole bunch of little holes in a nice design perhaps. :cool:

[Chris Parks]

TL, as good as that sounds I think a big hole will work better. Yes, small holes will let the air in but it will be "random", the best word I can think of. What I am after is a gale for want of a better word blowing straight across the blade to capture as much of the debris as possible. Admittedly this is a bit of an experiment to see if the idea works and you could be right but we all have to start somewhere. I should get an effect in the cabinet like this link shows happening with a mitre saw, actually pulling the air and debris to the pick up.....
Chop Saw Dust Collection with a Clearvue Cyclone - YouTube (http://www.youtube.com/watch?v=8AIITfs51PE&eurl=http%3A%2F%2Fwww.youtube.com%2Fembed%2F8AIITfs51PE&feature=player_embedded)

[A Duke]

I was also thinking why round? Was thinking of cutting a hole with a metal cutting blade in a jig saw to fit one of those plastic louvre type inserts for cupboard doors,but that dose not seem to fit in with your thinking. I would still think of some type of guard as the size and position of your intended hole could be inviting disaster if some little hands should wonder into your shed.
Regards

[unixbigot]

What about an air- or drill- powered nibbler?

[Bryan]

Trepanning tool. The kind that goes in a drill. Like a flycutter with a drill in the centre.

[BobL]

I agree that BS are poorly ventilated and they suffer because they just don't get enough air.

What is critical is the air flow is as high as possible at the source of the dust making but it does not matter how the air gets there, ie large or small holes. This will still generate the "gale" that is needed at the blade.

However, if a number of 25 mm holes or a 100 mm hole with metal louvres across are used the number of holes needed to generate the air flow required will have to be about double the cross sectional area of one 100 mm dia hole.


The other approach would be to let the dust fall and put the pick up at the bottom and leave the original pick up to allow the air to enter the cabinet but I am not sure if that will work as once the dust has fallen it might start to build up in pockets in the cabinet, that will most probably either way but we will see.
My BS has a port just under the table and another in the base of the cabinet and it seems to work OK (ie dust does not pool) but it still could use more input air supply. My plan is to cut a square hole in the bottom door and add a large set of metal louvres so that more air is made available to both ports.

[tea lady]

You can get metal cutting blades for jigsaws:shrug:

Still think lots of little holes would work fine. :p

[Dave J]

If it was me I would use a jig saw, rough it out first then do the final cut. I have even used hacksaw blades ground down to suit the jigsaw in the past when I have ran out of metal blades or fine tooth wood blades and taken it real easy. It will wreck the blade but will do the job if you keep it cool.

Without a jigsaw I would drill a series of holes just short of the line and chisel between them on a solid surface, then grab the 4 inch angle grinder with a slightly worn disk and clean it up with that. Having a slightly worn 4 inch wheel (say 3 1/2 inch) to go into the hole will leave a nice round hole with minimum effort.

Both these tools above can be bought for $30-$100 and if you go the $30 one it would be cheaper than a good hole saw that size.

If I was to file it, I would use a half round file after a series of holes and as long as you where careful it would come out looking like it was done with a hole saw. How much work you do with the file will depend on the size holes you drill, so if you drill with a 10mm drill bit you will have more work than drilling with a 3mm drill bit, but the drilling will take long.

Dave

Edit
You could also use a round grinding stone in the drill press or the hand drill if your careful after drilling a series of holes. You may have a few laying around, but if not that can be bought for a couple of dollars and usually have a 6mm- 1/4 inch shank on them.

[Fuzzie]

There should also be an OH&S consideration to putting a 100mm hole in the bottom door. A pattern of say 1/4" drill holes in a 100mm circle would give a reasonable air intake, but also give a grill effect to keep things out of contact with the moving blade/wheel. You could also just cut a rough hole with a hacksaw blade in a jigsaw and screw a formed grill over it it and not worry too much about the rough job underneath.

You may have to consider screwing a patch over the hole in any case if you find it doesn't achieve what you want.

[krisfarm]

Mini,
I recently had to cut a large hole in a 13mm thick aluminium panel that I could not remove to enable me to use machinery on. I made up this double headed centre punch to use with 3/16" drill. By spacing the punches to leave a small piece of material between holes -0.020" it made the job easy. You could do a neat job by scribing an inner circle to just have the drilled holes clear of your finished size and then centre punching along the line, it leaves little material to file away and you end up with a nice round hole. I set one punch 0.025" longer than the other and this way you can drop the long leg into the first hole and keep your punch vertical to punch the second.
Bob

[BobL]

I just rummaged around in my scraps bin and found a couple of pieces of 200 x 175 mm mesh with 1400 , 4.3 mm diam holes stamped out of it. I'm going to cut a hole in the door and bolt the mesh onto the door to cover the hole. This should give me x 3 the X-sectional area which should hopefully be enough flow. Anyway can test the flow with a Pitot tube air speed measuring device. I'd do it right now except its 40º in the shed so it will have to wait a while till it cools down.

[Chris Parks]

Yes, a guard would be necessary to keep things safe. I hadn't thought of a jigsaw so I can work on that idea as well. I am off to the shed to put the finishing touches to my new mitre saw dust hood complete with integral blast gate. I must learn how to post pictures as I have a lot of interesting stuff going on at the moment.

[Avery]

I may be totally and utterly wrong here - it wouldn't be the first time, BUT

most of the dust from a bandsaw goes straight down through the table with the band. The dust ports on the bandsaws that i am familiar with (that's only 2), are located in the bottom of the cabinet under the table. If you put a great big hole or lots of little holes in the front door, surely you will be sucking air (and little dust) from the front to the back of the cabinet. Wouldn't it be a better idea to seal up the cabinet and make sure the only air input is somewhere just under the table to create maximum suck around the cutting area.

I would also guess that by creating a great big inlet hole, the air velocity will be very low and not musc dust will be sucked in. OTOH if the input whole is too small there will not be enough total air movement to move the dust.

There must be some sort of optimum input hole size to move the most dust most effectively.

[Chris Parks]

I may be totally and utterly wrong here - it wouldn't be the first time, BUT

most of the dust from a bandsaw goes straight down through the table with the band. The dust ports on the bandsaws that i am familiar with (that's only 2), are located in the bottom of the cabinet under the table. If you put a great big hole or lots of little holes in the front door, surely you will be sucking air (and little dust) from the front to the back of the cabinet. Wouldn't it be a better idea to seal up the cabinet and make sure the only air input is somewhere just under the table to create maximum suck around the cutting area.

I would also guess that by creating a great big inlet hole, the air velocity will be very low and not musc dust will be sucked in. OTOH if the input whole is too small there will not be enough total air movement to move the dust.

There must be some sort of optimum input hole size to move the most dust most effectively.

I must learn how to do multi quotes! My BS has the DE outlet hard up under the table where the blade enters the cabinet. If I put the hole exactly opposite or I am thinking slightly lower and the same size I will get air rushing across the cabinet into the exhaust hole. Both holes need to be the same size as there is no reason for them to be different sizes to do any different would upset the apple cart so to speak. The inlet air flow should be supplied at the same volume and speed as the exhaust flow or near enough. In fact a sharp edged hole will lower the inlet air speed and cause unwanted turbulence but that is getting very picky and anal about the whole thing. If there was a supplementary air supply like large gaps etc they would have to be taken into consideration as well. My TS has a 100mm inlet and a 150mm outlet and if there were not gaps around the top of the cabinet to supply the extra air the cabinet would experience a low pressure and a lack of air volume into the DE.

It used to be that everyone thought that their tools lacked good design in DE extraction, bandsaws being one of the big offenders, but what they mostly lack is a good supply of inlet air and the ability of the DE to extract at a good volume and air speed. Have a look at most BS's and you have to ask yourself how any designer thought the DE was going to work. Most DE outlets on cabinet based tools only look pretty, they don't do much unfortunately because of this issue. The door on my BS even has a rubber seal around it to prevent as little air as possible getting in, now that is highly intelligent!

[Avery]

OK. It seems like you really understand what you want to achieve.

I hope once you have found the best way to cut the hole(s), you will be able to come back and tell us the actual results achieved - there are plenty of bandsaw owners out here that would be interested.

Sorry that I could not be of any help in actually answering your original question.

[Chris Parks]

Avery, when people ask questions everyone learns, that is the way I look at it. There are a lot of people out there wondering why their tools don't have good DE and unwittingly they are doing exactly the wrong thing by sealing up all the holes. Thanks for your contribution to the thread and it might help you think about your BS and DE.

[Avery]

I guess the nest thing that I just don't understand is

Why don't the manufacturers do the job properly in the first place?

A bit of a silly question, I know.

If you look at what is going on in the USA right now with table saws, where perhaps manufacturers/sellers will be compelled to include expensive "saw stop" technology to reduce injury hazards, I would have thought excellent DE would have been high on the list of "THINGS TO DO TO PREVENT LAW SUITS". If it is as simple as getting the inlet/outlet ports in the right place during the design phase, why haven't they done it already? I mean, the modification that you propose would cost nothing if it were included in the manufacturing specs.

I guess a cut off finger is perhaps more obvious and immediate than a few years of dust damage.

[BobL]

Before I cut a hole in my door I thought I had better undertake some measurements.

Here is my setup showing one port in ttheeh cabinet just underneath the table and the other at the bottom of the cabinet.
http://www.woodworkforums.com/attachment.php?attachmentid=192552&stc=1&d=1325075900
To measure air speed accurately a Test Duct (straight length of ducting with a hole drilled in the side) is placed in line with the duct as shown in the photo. The hole enables a air speed meter probe to be inserted into the air flow. When I measured the flow through the other port the Test Duct was transferred to the other port.

Here is the view from the other side of the cabinet with the door open so one can see the ports.
http://www.woodworkforums.com/attachment.php?attachmentid=192554&stc=1&d=1325075900
The effectiveness of the lower port (B) is demonstrated by the small amount of dust in the cabinet

With all door closed the air flow through
A was 244 cuft/min
B was 300 cuft/min

With the door open
A was 280 cuft/min
B was 600 cuft/min

In other words poor - this BS definitely needs attention.

What is surprising is that even opening the cabinet door fully wide open does SFA for A but does help B.
With the door closed I would expect A and B to some extent fight each other for air as there are few other openings to allow air into the cabinet.

What is also surprising is how little air A collects but if one looks closely at the inside of the port one can see why, ie there is stuff all room for air to get into the area of the port.
http://www.woodworkforums.com/attachment.php?attachmentid=192555&stc=1&d=1325076848
However, instead of putting holes in the door to assist this port it is probably better to put holes in the cabinet casing above the port since this is where dust seems to be pooling. The question is how to do this without compromising the strength of the cabinet and table

[Chris Parks]

Bob, mine only has one exhaust port so I only have half the problem you have. :U

[joe greiner]

Before committing major surgery, I'd consider some less heroic efforts. As noted, total area is more important than shape.

1. Modify the latch so that it leaves the lower door ajar, but not loose enough to self-open.

and/or,
2. Replace the door hinges with spacers between the frame and the hinge, or between the hinge and the door; or both.

Properly devised, these modifications can be made with the "UnDo" option, more or less.

Cheers,
Joe

[markharrison]

Have you thought about a box shroud around the trunions and the lower blade guide assembly and bypassing the existing upper duct? Collecting the dust before it enters the lower cabinet works better for me.

[Chris Parks]

Properly devised, these modifications can be made with the "UnDo" option, more or less.

Cheers,
Joe

Joe, why would I want to undo the mod? As Bob has found through measurement bandsaws or at least his and I know mine needs more air. As for playing with the door mine has an inhibitor switch on the door to prevent its operation while the door is open. I guess I could bypass that but I don't think that it is wise to do so. I cut a bloody big hole in my TS cabinet for exactly the same reason as the BS with no regrets and a big lift in DE performance. My approach to something like this, it is only a tool and if it does not work properly it gets fixed. I also need to show people what can be done by themselves in their own workshop to get better results from their DE. I understand that others most probably have a reluctance to cut holes in their machines and so be it but I do not, but what I do not want is for it to be ugly and look butchered. I am starting to think if I can put an adapter in that goes from the cabinet door up to the underneath of the table and picks up the dust that collects there like Bob shows in his photo. I get my cake and eat it too if that could be made to work. Maybe I won't rush into it for a few days!

[Chris Parks]

Have you thought about a box shroud around the trunions and the lower blade guide assembly and bypassing the existing upper duct? Collecting the dust before it enters the lower cabinet works better for me.

Mark, I guess that is because of all the problems that have been outlined here. Have a look and see if that is the case. It can also depend on the capabilities of the DE system which is ignored by a lot of people.

[BobL]

Have you thought about a box shroud around the trunions and the lower blade guide assembly and bypassing the existing upper duct? Collecting the dust before it enters the lower cabinet works better for me.

I think this is the way I will go but it is pretty obvious the other duct could benefit from some extra venting in the door way. I can understand manufacturers not wishing to put a box shroud around the trunions and the lower blade guide assembly because that would interfere with the table tilt. However, I cannot remember the last time if ever I have used the tilt on this BS.

[markharrison]

My thoughts are if you are trying to help your customers improve dust collection, the method that is easiest to reproduce would be the most desirable.

This example in the attached photo is not mine. Mine is far more primitive!

It comes from the Sauer and Steiner blog here (http://sauerandsteiner.blogspot.com/). No surgery involved.

As I said, this is reversible if it doesn't work to your satisfaction.

[Chris Parks]

Think about it a bit more Mark. If you added a vent to the cabinet and the air drawn through it also drew air from a box like that or a simpler dust pick up you get two for one. You vent the cabinet, pick up the under table dust using the same vent and it all goes through the cabinet into the DE. Sounds good to me, I like simple.

[BobL]

Unfortunately putting a vent in the door is not going to help remove dust from the vicinity of the conventional under table port because very little air seems to travel through that port, even with the door open. Maybe it's better on other saws but the pathways to that port are just too constrictive on my setup.

If a vent is placed in the door air will also preferentially travel through pathway 2 and less will travel through pathway 1 which is what is really required.

http://www.woodworkforums.com/attachment.php?attachmentid=192558&stc=1&d=1325082705

As Mark (and others have said) a better place seems to be just to locate a naked hose at position 4

[Chris Parks]

Unfortunately putting a vent in the door is not going to help remove dust from the vicinity of the conventional under table port because very little air travels through that port even with the door open.

If a vent is placed in the door air will also preferentially travel through pathway 2 and less will travel through pathway 1 which is what is really required.

http://www.woodworkforums.com/attachment.php?attachmentid=192558&stc=1&d=1325082705

As Mark (and others have said) a better place seems to be just to locate a naked hose at position 4

That is not what I meant at all. Put the vent in the door and lead an adapter, dust pick up call it what you will from the door vent up to under the table to where the dust is shown in your photo. The air is drawn from under the table as it is now a dust pick extraction point, through the cabinet into the DE exhaust.

[BobL]

That is not what I meant at all. Put the vent in the door and lead an adapter, dust pick up call it what you will from the door vent up to under the table to where the dust is shown in your photo. The air is drawn from under the table as it is now a dust pick extraction point, through the cabinet into the DE exhaust.

Maybe your port is differently designed but I just established that cabinet port A is a dog that only conducts about 300 cfm even if the door is completely left open.
http://www.woodworkforums.com/attachments/f65/192554d1325075900-100mm-hole-bandsaw-door-bothports.jpg

For BSs with both ports A and B, port A is best completely closed since A and B will just fight each other for similar air.

For BSs without a port B, installing one into the cabinet is probably the first thing to do. Putting a grill in the door so this port can breathe more easily seems like it will help.

For all BSs, the most useful ducting is probably placed directly to the underside of the table without any box around it so it can draw enough air. Routing this ducting through the lower cabinet is only going to reduce the total air flow from that area.

I will perform some testing of my grill material to see how much it slows the air flow down.

[markharrison]

If you only get 300 CFM with the door open, that's the maximum you could get with an additional air inlet.

The other box shroud I am intending to build for my saw will be open at the opposite end to the DC collection point. I'm not sure if Conrad Sauer's is. It's hard to be sure from that photo but I expect it probably is. He reports that while it isn't ideal that you have to do this in the first place, it does work.

Anyway, it's your saw and you should feel free to do as you please.

[BobL]

If you only get 300 CFM with the door open, that's the maximum you could get with an additional air inlet.
Yep I agree with that (see my previous posts), but the flow through the port in the bottom part of the cabinet increases from 300 to 600 cfm by opening the door open so a grill in the door still makes sense for that port.


The other box shroud I am intending to build for my saw will be open at the opposite end to the DC collection point. I'm not sure if Conrad Sauer's is. It's hard to be sure from that photo but I expect it probably is. He reports that while it isn't ideal that you have to do this in the first place, it does work.

Thats a good idea - I'll see if I can do a similar thing and measure the air flow.

[Chris Parks]

Maybe your port is differently designed but I just established that cabinet port A is a dog that only conducts about 300 cfm even if the door is completely left open.

Yes, my port is a different design so your contention has no relevance. Actually what counts in the real world is whether it works or not.

[Stustoys]

Yep I agree with that (see my previous posts), but the flow through the port in the bottom part of the cabinet increases from 300 to 600 cfm by opening the door open so a grill in the door still makes sense for that port.

The airflow figures sound a little strange. One thing that might happen by adding a hole in the door to get the 600cfm into B is that the flow into A will drop even further. What about adding a plate/box to the door so more air can get into port A? Something not completely unlike this.

Stuart

p.s. sorrry for the rubbish drawing but I dont even know what the door looks like

[BobL]

The airflow figures sound a little strange. One thing that might happen by adding a hole in the door to get the 600cfm into B is that the flow into A will drop even further.
You must be looking at the figures the wrong way. Opening the door increases A from 244 to 280 cfm so adding holes in the door should still increase the flow at A.


What about adding a plate/box to the door so more air can get into port A? Something not completely unlike this.
Port A can is so poorly designed to begin with that it only pull 280 cfm with the cabinet door wide open so adding plates or holes is a waste of time . I'm going to try disconnecting port A and used that flexy directly underneath the table.

[Stustoys]

Opening the door increases A from 244 to 280 cfm so adding holes in the door should still increase the flow at A.
Yes but by adding a hole mostly over port A only you may increase the flow in port A to more than 280. Adding it were its going to go into port B may decrease flow in port A to less than 244.



Port A can is so poorly designed to begin with that it only pull 280 cfm with the cabinet door wide open so adding plates or holes is a waste of time . I'm going to try disconnecting port A and used that flexy directly underneath the table.
Whats the flow in port A with port B blocked?(granted, depending on how your system is set up it may not make much difference)

Stuart

[BobL]

Yes but by adding a hole mostly over port A only you may increase the flow in port A to more than 280. Adding it were its going to go into port B may decrease flow in port A to less than 244.

Short of drilling large holes or cutting metal away in the cabinet (not the door) to open up access for air to the area around port A there is no greater access of air to port A than leaving the door open. There is no way deflectors or cutting holes in just the door etc will increase the flow above 280 cfm


Whats the flow in port A with port B blocked?(granted, depending on how your system is set up it may not make much difference)
Its the same, the limit is port A (and to a lesser extent the fact that flexy is being used)

Anyway this doesn't solve Mini's problem because he has a differently designed port A.

However the following info is likely to be useful all concerned.
It's around 40º in the shed right now but I managed to get into the shed and do some quick measurements.

In these pics you can see the grill material I'm planning to use in the door way to improve the flow for port B.

The grill is held in turn in front of a 150 and 100 mm intakes connected to 100 mm flexy and the air flow measured with and without the grill. The grill itself is 200 x 175 mm and is 58% transparent.

http://www.woodworkforums.com/attachment.php?attachmentid=192610&stc=1&d=1325129678
Placing the grill in front of the 100 mm intake like above reduces the airflow by 27%

http://www.woodworkforums.com/attachment.php?attachmentid=192611&stc=1&d=1325129678
Placing the grill in front of the 150 mm intake like above reduces the airflow by only 7%.

My plan is to use the full 200 x 175 grill in the bottom door of the cabinet to improve the flow in port B. The question now is where in the door is the best place to put the grill ?

[Chris Parks]

Bob, no way would I use a grill like that. I would build a stand off guard that covered the opening and had at least the intake area of the hole but more would not hurt. A stand off guard drawing through four sides would not have a huge gap between it and the cabinet and given a large overlap on the hole would be effective as a finger guard. I will dig the aenometer out and do some tests when I get it all going but I don't think I have a flow problem as the port draws entirely differently from Bob's BS.

[BobL]

Bob, no way would I use a grill like that. I would build a stand off guard that covered the opening and had at least the intake area of the hole but more would not hurt.

The test was not intended to demonstrate an actual working grill but to demonstrate that using a series of smaller openings in a grill requires that significantly MORE than the intake area of the ducting hole should be used. The 150 mm diam of that particular grid represents 20% more air transmission area than the total cross section of a 100 mm duct but it still reduces the flow by 7%. Given that we should try and get all the flow we can, my recommendation is to use at least 2 times the area of holes of the 100 mm duct.

[Chris Parks]

The test was not intended to demonstrate an actual working grill but to demonstrate that using a series of openings in a grill requires that MORE than the intake area of the ducting hole should be used. The 150 mm diam of that particular grid represents 20% more air transmission area than the total cross section of a 100 mm duct but it still reduces the flow by 7%. Given that we should try and get all the flow we can my recommendation is to use at least 2 times the area of the 100 mm duct.

Why use a grill at all? Everyone knows they reduce flow, you wouldn't have to be a brain surgeon to work that out.

[BobL]

Why use a grill at all? Everyone knows they reduce flow, you wouldn't have to be a brain surgeon to work that out.

purely for safety reasons.

My BS uses two doors to enclose the cabinet, one for the upper part of the cabinet and one for the lower part of the cabinet. When both doors are installed the lower one must be closed first so the upper one can close, but only the upper door has a lock out switch on it so I can remove the bottom door completely and the BS will still operate. However I don't think removing the lower door is an answer either.

[pjt]

I posted this mod to my BS for improving DE a while back http://www.woodworkforums.com/f27/dust-collection-mod-door-underside-table-92444/
My BS is the same breed as Bob's, when I looked at my top port it seemed impossible for that port to be very effective at all in terms of airflow into it and Bob's measurements confirm this, to get air into the top port I decided on cutting a piece off the top of the door on the same plane as the angled piece inside the cabinet, this by itself would have improved airflow into the top port, once again as Bob has confirmed from measuring, at this point we could say that by not much and I don't know by how much mine improved (didn't?) (I'm sure it did tho:cool:) when doing comparisons it is important that all downstream and upstream elements remain as unchanged as possible apart from the thing we are actually testing, my upstream pipework is different to Bob's so I may have gained more/less:no::; airflow into the top port than Bob, but that's all food for another post sometime:;
I then added a piece onto the door angled out away from the cabinet to form what becomes a chute which when combined with the top shroud directs air into the top port
My evidence is visual in that after the mod no dust collected on/in/around the lower guide bearings whereas before they would be totally covered, the top shroud collects air from the hole in the table and from around the back of the guides and from a smallish gap all the way around under the table, I wasn't too worried by gaps as if air is to move out via the DC it also has to be able to get into the cabinet and ideally back into the dust stream to pick up dust which by my reckoning is where Chris is going with this additional 100mm hole
With mine I also see a clean line (no dust) just in front of the door, the fugitve dust that falls down in front of the door and lands on the base within 10mmish of the door gets sucked into the cabinet, this tells me I have capacity in my system to allow more air into the cabinet, which by my reckoning is where Bob is going with cutting a hole in his lower door.
Someone mentioned air entereing a pipe, in general anytime a fluid (air) enters a square edged pipe the actual area of airflow is somewhat reduced to smaller than the actual diameter of the pipe, for max airflow into pipe a smooth transition is required, a curved inlet achieves this, something like this....
192689
Another area where this same thing occours is at inlets/outlets in cabinets and anytime we can make a smooth transition into a pipe we get less turbulence/eddy currents which are all detrimental to airflow, I'll get a pic of my lower cabinet if I remember:doh:


Pete

[BobL]

Thanks for posting that link Pete - I remember you had done something like that but could not find it. It's good that the dust is gone from the guide bearings area under the table.

BTW I see you still have the cross plates in the upper port.
http://www.woodworkforums.com/attachments/f27/100460d1238041534-dust-collection-mod-door-underside-table-door-closed.jpg

These plates don't help the air flow in such a small duct so you might want to think about cutting them out as this will improve things a bit more.

[pjt]

BTW I see you still have the cross plates in the upper port.
http://www.woodworkforums.com/attachments/f27/100460d1238041534-dust-collection-mod-door-underside-table-door-closed.jpg

These plates don't help the air flow in such a small duct so you might want to think about cutting them out as this will improve things a bit more.
Yep, I have looked at them and thought they can go but just haven't got there yet:B, You didn't happen to do a flow test before and after with your BS? just for interest?


Pete

[BobL]

Yep, I have looked at them and thought they can go but just haven't got there yet:B, You didn't happen to do a flow test before and after with your BS? just for interest?


Pete

No I didn't but I still have the strips and can put them into a piece of 100 mm pipe and measure the effect

[pjt]

192832
This sort of mod assists in getting the air into the outlet, at this point I have only done one side, (the left side is on the to do:B) the right is probably the most important side tho, the sawdust comes down off the blade and deflected by the metal and straight into the outlet, the triangle on the underside of the metal would have been an area of high turblance and eddy currents:no: prior to this mod, albeit only taped in position:-


Pete