Thread: 100mm Hole in Bandsaw Door

Originally Posted by BobL

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.

Originally Posted by Mini

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.

I posted this mod to my BS for improving DE a while back https://www.woodworkforums.com/f27/du...e-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) 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 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....
Attachment 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


Pete

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.


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.

Originally Posted by BobL

BTW I see you still have the cross plates in the upper port.


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, You didn't happen to do a flow test before and after with your BS? just for interest?


Pete

Originally Posted by pjt

Yep, I have looked at them and thought they can go but just haven't got there yet, 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

btm cabinet dust extraction.JPG
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) 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 prior to this mod, albeit only taped in position


Pete