Thread: 150 mm blast gates

With another 38º day I can't do much in the old or new shed at home - eventually I will get aircon but that's still waiting for the sparky.

Fortunately I can do stuff in the air con workshop at work so I decided to work on my DC system andhave a crack at making some 150 mm blast gates. After pouring over Bill Pentz site for the last 4 weeks I decided to go for female fitting blast gates that will sit right up near the Y take off from the main duct running down the centre of the shed. It's not to high above my head and it will be easy for me to reach up and open/close the gate. The gate up near the Y is supposed to be superior than having gates right down at the machine.

Unlike conventional blast gates which fit inside the ducting material the female fit retains the full pipe width.

The basis of the gate is a threaded pipe coupling, its threaded because I got the fittings from Pipeco and they only stock the threaded couplings and they are cheaper than non-threaded couplings I can get from anyone else. Sorry about the ordinary photos - only had my phone camera with me.

The threaded coupling is machined in half on a lathe so that one half has the internal lip flush with the cut.

The lip is important in preventing any pipe connected to the gate from penetrating its way through the blast gate opening, and in maintaining a smooth air transition at the gate. Now since the other half has no lip I machined off a 3 mm annulus from a piece of 150 mm pipe and solvent glued the annulus in place (see picture later on) to create an equivalent lip on the other side.

Now the blast gate surround. I cut 200 x 200 mm squares of 4.8 mm thick PVC and stacked them together and drilled 3 , 4.5 mm holes diam holes on the two of the opposing sides. I then screwed a pair of PVC squares through the holes using 3/16 machine screws to a makeshift faceplate made of 18 mm MDF and a 10 mm diameter bolt.

The holes for the two coupling halves are then cut with the lathe.


To help stop the face plate from turning at the bolt I added a piece of cloth backed sandpaper between the bolt head and the MDF, I also added another at the back in between the nut and the MDF.

To stop the faceplate from flexing under the pressure of the cutting tool I added a piece of 150 mm pipe between the back of the faceplate and the lathe chuck.


Doing this on a lathe means one can creep up on the 150mm opening and stop and test fit the coupling half.


Now there is a solvent glue up stage. Here you can also see how I have added that second lip to the lipless half. All that excess surface solvent is sanded of so there is a smooth internal surface.
.

Next machining the opening in the gate. The gate is made from 175 x 400 mm x 3 mm thick PVC. The length is required because it is a through gate which makes it much easier to clean any dust out of the groove the gate slide in. Now because the gate is so long it was not possible to cut this on the late. If I was at home I would have cut this hole with a router in a router table, but there is no router table at work but they do have this lovely old school Bridgeport mill and I love using it, especially with the rotary table.


The nice thing about this setup is I can once again creep up on the required size.
I reckon I could machine a pair of these at the same time without a problem.

Two small 12 mm wide x 3 mm thick PC strips then hold the two halves of the Gate apart so the gate can then slide thru as required.

In this picture you can see the fit between the two lips and the gate opening. The method used of paired machining keeps the openings well aligned and the design has no grooves for dust to get trapped in or to disrupt air flow. I will sand everything smooth once it's all complete


I was quite surprised how well the MDF held the machine screws - I guess if they tear out I will have to go for through screws.


Here are the almost complete two gates I made this afternoon, this included the time to make the jigs and a couple of visits from the FUF.


The two halves and PVC spacer strips are all held together with 3/16 screws and will be all sealed together with silicone rather than PVC solvent so they can be taken apart if required.

I will add a couple of piece of MDF to the ends of the gate to act as handles and stops so the gate opening aligns accurately with just the flick of an arm. The handles will be removable just in case there is ever a problem.

Now that I have the jigs and method sorted I reckon I can make them in under an hour each - 6 more to go!