Thread: Improving machine cabinet dust ports

Originally Posted by NeilS

I have been thinking that an oval bellmouth, like the one below, would be a good under table solution in terms of fit and optimisation for bandsaws.

Good point for discussion Neil and I'm glad you raised it.

It's easy to understand why one might think that oval collection might help that but here's what happens.

Let's say you have a piece of 150 mm duct (not BMH, just the duct) and it is squeezed into a 170 x 130 mm shape , i.e. perimeter stays about same and the area drops by about 2%
This probably won't do that much but it's much more than you think it will be.

Now let's squeeze it a little more, 192 - 100 mm , it will have the same perimeter as the original duct but the area now drops by 15%.
If a 150 mm round duct nominally can pull a max of 1250 cfm - then it sounds reasonable to think that the flow should be 15% less or ~1050 CFM - still good right?
Unfortunately this is not the case because the loss of flow will be dominated by the smaller dimension i.e. the 100 mm dimension, which the increase larger dimension just does not compensate for.

A 100 mm round duct nominally can pull a max of 425 CFM.
Then a 192 x 100 mm duct is going to be limited to a flow closer to the 425 cfm than to the expected 1050 cfm based on area alone i.e. a serious loss of flow.
This is why oval pipe and oval openings are not worth chasing.

Now the same thing will happened to oval BMHs even if the hood is partially blocked by parts of the machine. In that case its better to move to BMH back a little like this.
In cases like this the machinery will usually direct the flow irrespective of it the hood is oval or not
16inchfront.jpg

When the sawdust is primarily dust rather than chips its better to optimise total air flow than it is is to try and direct the flow because this will just end up reducing the flow.
A circular BMH will have the max flow possible from a point in space and the flow will be taken from well out in front of the hood as compared to a naked duct opening that drags significant amounts of (often not so dusty air) from the side and even behind the hood.

When chip collection is needed then shaping the flow and hence air speed may be of some value.

Bob
Looking at he bottom of the table with all the casting ribs and such, what kind of improvement to the airflow into the bell mouth would we get if it were filled with bog or covered with sheet metal or thin plywood?

Pete

Originally Posted by QC Inspector

Bob
Looking at he bottom of the table with all the casting ribs and such, what kind of improvement to the airflow into the bell mouth would we get if it were filled with bog or covered with sheet metal or thin plywood?

Pete

Good point, it will certainly add to the turbulence, but it is the same for whatever collection port is used, so its a second order effect.
If it was my machine I would use a thin sheet of metal but this is a mens shed machine and we have much more important things to do dust collection wise.
Probably get to that eventually in a second or third iteration of improvements.

I was selfishly thinking of my own bandsaw.

Pete

I found this Australian manufacturer that custom makes PVC plumbing: Custom Moulding

Perhaps we can ask them to make two things for us.

-- 150mm bell mouthed ends
-- an adapter for 100mm and 150mm hose to fit onto regular cheap PVC pipe.

The last will let us finally connect flexible hoses to freely available commodity pipes!

Ill write to them.

Originally Posted by woodPixel

I found this Australian manufacturer that custom makes PVC plumbing: Custom Moulding

Perhaps we can ask them to make two things for us.

-- 150mm bell mouthed ends
-- an adapter for 100mm and 150mm hose to fit onto regular cheap PVC pipe.

The last will let us finally connect flexible hoses to freely available commodity pipes!

Ill write to them.

Somehow I seriously doubt there are sufficient guaranteed volume sales, and give the high cost of production setup, unit costs will end up pretty exxy.

Besides, if you have access to a WW lathe they are both solved problems and the BMH costs mo more than the length of pipe needed to make the hood, they simply won't be able to compete with that.

For you and I, certainly, but for 4000 others without lathes perhaps not.

Ive written explaining our problem plus a proposed distribution model.

Its such a prevalent problem it needs addressing. It's not going to go away, especially as your work is increasing both interest and awe ness (concern)

Originally Posted by woodPixel

For you and I, certainly, but for 4000 others without lathes perhaps not.

Ive written explaining our problem plus a proposed distribution model.

Its such a prevalent problem it needs addressing. It's not going to go away, especially as your work is increasing both interest and awe ness (concern)

Thanks WP.

I'll be interested in their response.

Meanwhile, I'm about to kick off another BMH production run of these for the mens shed so if any Perth members want one or two I will consider expressions of interest (pick up only)
This offer lasts for one week.
PM me if you are interested.

Just cross linking this thread showing more dust ports sporting BMHs at the mens shed in case folks haven't seen it.

I wrote to Reln and it had me think about a few things.

I've asked them for two things that interest me.

The first is an adapter to allow us to connect 100/150mm DWV to its same sized counterpart in hose. Essentially a collar that will let the flexi fit into the inside.

Second is the making of BMH's.

With the adapter it seems to me that EVERYTHING in both plumbing and woodwork uses the word "nominal" as some sort of joke. It means, close-to, perhaps-about, sort-of, near-to or our-system of measurement - absolutely nothing is exact. Given this hideous non-standardisation, what would be the best way to have an ultra low cost connector?

Just a sleeve that the grey flexi fits into, or one it fits over. Seems the "into" option would be best to preserve the maximum possible cross section, but the "onto" would be best for fit given the whole "nominal" issue of sizes. (i.e. one can only guarantee the internal diameter of the hose.... but even that is a joke in my experience).

Any wisdom?

edit: something like these adapters would be nice!

Originally Posted by woodPixel

I wrote to Reln and it had me think about a few things.
I've asked them for two things that interest me.
The first is an adapter to allow us to connect 100/150mm DWV to its same sized counterpart in hose. Essentially a collar that will let the flexi fit into the inside.

I've posted about this MANY times.

Standard DWV Threaded coupler and end cap.

The reasons I use this method are
- At the machine end I don't always fix the coupler onto the machine dust port or BMH - I leave it as a push fit so it can be quickly disconnected and used as a vacuum hose to clean up afterwards.
- As you say, the flexy hose sizes are all over the place - the hole in the end cap is turned out to suit the specific size of the hose - you won't get that in a custom moulded product
- the screw fitting is very positive, seals well and can be removed easily enough, no buggerizing around with gaffer tape or screws.

Here's a 100 mm one that connects to the BMH on my BS
$4.67 from Pipeline. ($12.90 from bunnings)
IMG_1922.jpg

Heres a 150 mm one that connects the flex between the gate and the BMH on the lathe.
Even this end is just push fit - I should screw this one down.
$13.52 for the coupler and the cap from Pipeonline
Flexjoin.jpg

I'll bet they won't be able to custom make them for anywhere near those prices.

$4.67!... crikey. They wanted 38 for it here! Ill buy from Pipeonline then....

edit: Here is pipeonline DWV FITTINGS - DWV ACCESS COUPLING and even my local bunnings is $32.

Originally Posted by woodPixel

$4.67!... crikey. They wanted 38 for it here! Ill buy from Pipeonline then....

It really shocks me how much price variation there is on something as simple as PVC fittings. Very glad I've got someone local who doesn't charge like a wounded bull.

I have been a Pipeonline line customer for about 10 years and have sent them a lot of business via recommending WA mens sheds get quotes from them.
Of the shed that have gone there I don't think any of them have got their stuff elsewhere.
Our own mens shed has spent over $2k on PVC ducting for their DC system and about $500 on pipe and fittings for their compressed air reticulation system so they have done well out of this.
BTW I haven't got anything personally forgoing this.

While the mining boom was on and everyone else prices went berko they were one of the few businesses that did not hike up prices to stupid levels. Their prices do go up and down a bit due to price changes in PVC feedstock but otherwise they have not change that much over that time. I remember when they only the two owners working for them and now the have about 10 people working there. The store is always busy and the new young assistants working there have a very good service mentality.