Thread: 6" PVC pipe puzzle

"The one that's on your machine" is the smart-#### answer.

Two things that occur to me straight off are:

1. You could reinforce the ducting in some way .. maybe even just wrap the outside with duct tape to whatever depth you like .. if you are worried about exploding pvc. Fibreglass - another option.
There may be some way to put in a temporary internal reinforcement.

2. Are you overthinking this? Maybe put on the ol' glasses and face-guard helmet and try cutting some ... see how it goes. You've got other hands haven't you?

Cheers,
Paul McGee

Originally Posted by brucegseidner

I am trying to cut 6" thin wall PVC. We call it ASTM 2729 or sewer&drain PVC. It has a thinner wall than Schedule 40 because it is meant for drain systems and is not pressurized. We use it for dust systems and I am needing to cut wedges of 6" PVC pipe to make a 2xradius 90degree elbow to connect my dust making machines to the dust extraction ducting that goes through the blower into and out the cyclone to keep my lungs and home relatively free of wood dust.

I have been through this problem over the last 9 months in building my own 6" ducting system and in the end I decided that making wedges and glueing or welding the PVC into a long radius 90º bend will end up with the same or maybe worse resistance than 6 x 15º bends that are nice and smooth inside. I have not made a definitive test of this I am just basing this on my experience in moving large amounts of air around ultra-clean air laboratories for the last 30+ years and building segmented bends out of welded HDPE.

Because even the 6 x 15º elbows solution is not cheap (~$80) and my system has ten 90º bends I bought just one set of 15º elbows and then tested a 6 x 15º elbows versus 2 x 45º and the flow difference was <5% so I decided to go mainly with 2 x 45º elbows which cost 1/8 of the 6 x 15º cost of $80. (BTW the difference between 2 x 45 and a single x 90º elbow is another 5%)

As long as the Bill Pentz 1000 cfm and 4000 cfm criteria is met for each machine I figure I am OK and I am getting about 1100 cfm and 5500 fpm at most machineswith my setup.

Originally Posted by brucegseidner

Cutting PVC is weird science. It is strange material. It can seem pretty robust and then seem quite fragile. I want to use my band saw to cut precise angles that will allow the segments to be welded or glued into place to make the long radius elbows I need. These elbows are very costly to buy commercially. I imagine that if my wood shop made money this would be an operating expense. But, my shop only costs money and is a hobby.

So, if you wanted to cut 6" PVC pipe using a miter sled to get the 84degree and 82 degree angles that when glued up made a 2xradius elbow pipe, what band saw blade would a sensible fellow use?

one with lots of teeth. I'm thinking of the maxim "keep at least three teeth in the cut"

You can get knife (band-knife, not band saw) blades for bandsaws - used for cutting leather and PU foam - maybe one of them would work.

Or to think differently, maybe make a nice radiused mold and make some from fiberglass and epoxy!

Why not just use some 6" flexi-hose?

Originally Posted by elanjacobs

Why not just use some 6" flexi-hose?

Depending on air speed 6" flexi double radius bend will have the about the same resistance as a 90º smooth wall short radius bend.

Originally Posted by BobL

Depending on air speed 6" flexi will have the about the same resistance as a 90º smooth wall short radius bend.

I thought there'd be a simple answer like that

I guess what I was going for was: will it make a significant difference in the greater scheme?

Originally Posted by elanjacobs

I thought there'd be a simple answer like that

I guess what I was going for was: will it make a significant difference in the greater scheme?

It depends on the raw flow rate of your DC/Cyclone system, length of ducting and how many 90º bends you need to make.

the finest cut blade that you have ,

instead of cutting wedges , cut small "v" sections from the pipe and flex the pipe around ,
then you will have a pipe that is curved but with out several joins .

Jeff

Originally Posted by vk4

the finest cut blade that you have ,

instead of cutting wedges , cut small "v" sections from the pipe and flex the pipe around ,
then you will have a pipe that is curved but with out several joins .

Jeff

That's a good idea but there are several problems with it, the main one is that it will constrict the pipe.
The other is that because the bend has to be bent in one go it will make it harder to clean up and smooth the internal joints. When a bend is made up one wedge at a time the internal joints can be cleaned up more easily.

If a North American can be permitted to swing,

Make a core of wax with end fittings embedded, smooth as you like. Lay up resin and fiberglass to form the wall. When the resin has cured, carve, drill, etc. to remove most of the wax. Heat to melt away the rest, but not hot enough to melt the end fittings.

I've made flexible bellows this way, using silicone sealant as the material.

Cheers,
Joe

Why not heat the pipe and bend it? It took me a while to get the hang of it but it works.

Ross

Originally Posted by Ross

Why not heat the pipe and bend it? It took me a while to get the hang of it but it works.

Ross

How did you do it and do you have a picture of any of the bends

Bob

The first time I saw it done was by a builder working on my house and he used a blow torch on 100mm pastic pipe. It was very fast, the pastic was like rubber till it cooled. He all so enlarged one end so the pipes fitted together.


I use a heat gun and do it slowly, it won't do a tight radius . Think about how metal workers bend shapes and make up a jig using a flat board with a couple of posts to bend the pipe against as you heat the area where you are bending.

It works on 100mm so it should work on 150mm. Get some scrap and give it a go.

Ross