Thread: Blast gate gallery

No worries will do. I’ll have to laminate some 3mm stuff up to 6mm because the 6mm sheets were over $200 [emoji45]

Hi Bobl

A good range of blast gates. I'm curious to know why you "build" the hole"? The gates I use...

Blast Gate from YW-1000 Kit 7 websize.jpg

...either block the flow or not, they do not "make a hole" per se.

Basically those gates sit inside the ducting which reduces the diameter of the pipe. Bobs gates are built around the pipe to keep a consistent diameter and improve airflow.

Originally Posted by MrFixIt

Hi Bobl

A good range of blast gates. I'm curious to know why you "build" the hole"? The gates I use...

Blast Gate from YW-1000 Kit 7 websize.jpg

These gates have 4 significant design faults.

1) They have an ID of only 90 mm.
While this is only a 10% reduction in radius, it's a 19 % reduction in cross section area, but the actual flow reduction at the same pressure will be closer to 25%!
If you have several of these types of gates in series along a ducting run the problem gets increasingly worse.

2) When the gate valve is open the groove the slider rides in is left unoccupied.
This grooves induces further turbulence in air further reducing the air flow see next diagram.
Bgateprobs.jpg

3) The exposed groove collects crap making the slider harder and harder to close the gate.

4) The tapered ends do not sit flush inside 100 mm duct - instead they sit proud above the inside wall of the ducting, this is a different factor to 1) and also adds to decreased flow.
The picture below shows a cross section of the black gates and I have exaggerated the taped to show their effect
The curved arrows marked "T" show turbulence points
Screen Shot 2018-03-22 at 3.59.05 pm.png

A better blast gate has the following properties; as bueller says, the gate connectors fit Over rather than Inside ducting, and no slider groove or gap is left exposed or vacant instead the slide fills the groove. Note how the PVC ducting is inside the gate body, there are no protrusions into, or gaps/slots in the ducting, this reduces the turbulence and improves the air flow
Screen Shot 2018-03-22 at 4.00.16 pm.png

A well made blast gate will have less than 1% transmission loss depending on how close fitting everything is.

Originally Posted by Flintlock

That's what I thought but on the plan for B & F there is no 'e' shown.
Ducting update.

Whoops - I'll get these right one day!

Try these - please let me know if there is still a problem - I am almost blind to these diagrams by now.
Screen Shot 2018-03-22 at 4.20.01 pm.png

Screen Shot 2018-03-22 at 4.20.14 pm.png

Originally Posted by bueller

No worries will do. I’ll have to laminate some 3mm stuff up to 6mm because the 6mm sheets were over $200 [emoji45]

That's ridiculous isn't it. Plastics prices are related to to the price of oil, problem is when oil goes up the price goes up but when oil goes down the price of plastics doesn't!

Have you tried this bloke on Gum Tree?
https://www.gumtree.com.au/s-ad/arma...ene/1170629668

I could not believe my luck when I found a heap of 6mm white PVC in a skip at work about 10 years ago. I gave a heap of it away and used some for various projects but only had a bit left when I was making the first of my 6" gates.

Cheers for the Gumtree link mate, I’ll give him a shout!

Thanks bob. All good now. I have my templates cut out now and just waiting to pick up a sheet of MDF

Started work on my gates today! Cut a pipe connector in half and then sanded it down flush with the lip on the inside of the pipe. Then I cut out about 6 flanges (of 10 attempts) and now I’m able to cut them to a precise press fit every time. Those circle cutting jigs are awesome once you figure out their weaknesses and work around them. And because they leave a nice circle cutout left over I have more material cut out ready for 100mm flanges. Tomorrow I’ll work on the sliders and putting everything together!

One thing I’m wondering about is fixing all this PVC together. I bought some PVC primer and cement which I’ll try but I’ve always wanted to try my hand at plastic welding so I might give that a go if I can get setup relatively cheaply. Only other thing I could think of was some epoxy but seems like it could be messy.

Originally Posted by bueller

One thing I’m wondering about is fixing all this PVC together. I bought some PVC primer and cement which I’ll try but I’ve always wanted to try my hand at plastic welding so I might give that a go if I can get setup relatively cheaply. Only other thing I could think of was some epoxy but seems like it could be messy.

I've done quite a bit of plastic welding but it was many years ago so am somewhat out of practice but I could probably still "show you the basics". The reason I have not done it on my gates is because it takes a lot of skill to weld plastic without any ripple or buckling of a flat surface and bear in mind that section of a gate needs to be kept dead flat.

When we made a bunch of gates at the mens shed out of Al our TIG welding guru (40 years of experience) could not weld a 3mm thick Al ring to the Al body of a Blast gate and keep the weld dead flat. At least with Al it's usually possible to flatten some of the heat buckles out but that's not going to happen with PVC.

To weld PVC you need a variable heat control heat gun and a fine (~6mm diameter) nozzle. You also need some PVC welding rod which melts at a slightly higher temp than regular PVC. To practice you can get away with some 3 x 3 or x 4 mm strips of regular PVC.

All of my PVC gates just use regular PVC cement and some are now getting on for 7 years old and have not shown any signs of weakening.

Cheers Bob! I’ll stick with PVC cement for the gates and try out the plastic welding when I make a dust hood for my table saw. Thanks for the tips, they’ll come in handy.

To further strengthen the joint on the blast gate I usually cement in a support ring like this.
Its just a Short piece of PVC cut through so it slides over and glues in place
RockePVCrs.jpg

Or home made flange like this.
The flange is epoxied in place and also held in place by screws.
SliderAl12s.jpg

The flange is made using a similar mould and process as the BMHs.

Yep I’m planning on using two or three of the discs I made today stacked together and cemented to the coupler. Then I’ll cement those to the blast gate. Should be pretty strong, the rings are a very tight fit around the coupler and stacking them makes for a very substantial joint.