PVC duct reducing question

[Chris Merriam]

Could someone please please help me? I'm stuck on the seemingly simple matter of cutting some Vs in 6in pvc so I can taper it down to 5in for mating to 5in flex hose.

I feel dumb for not being able to figure it out. I read Bill Pentz's duct section and he says to cut some long Vs to taper the pipe down.

So I get out my coping and hack saws and start cutting Vs. I tried Vs as you're staring through the pipe, Vs as you look down on it, Vs pointed towards the pipe opening, and some pointed towards the middle of the pipe.

Nothing I do seems to work. I feel like there is some fundamental mechanical principal I'm missing here. I started thinking about the circumference of a circle and figured I had to remove about 3 inches to get from 6in down to 5in - but still got nowhere.

Could someone please offer me a tip before I run out of my expensive 6in pvc pipe? :)

Thanks so much!

[Chris Padilla]

http://www.sawmillcreek.org/showthread.php?t=42700 This isn't exactly what you are looking for, Chris, but may be a bit of a start.

6" PVC has a circumfrence that is 6/5 or 20% bigger than a 5" circle (the size of your flex). Actually, the pipe has a thickness and it is more like a diameter of 6.25"--so more like 25% bigger than a 5" circle. This translates to nearly 4" of pipe removal from your V cuts (6.25" diameter circle has a circumfrence of 19.6", 5" diameter is 15.7").

So lets say you make 4 V cuts of some length x. A the top of the V, because you plan 4, you will need to make the V 1" wide. Now the longer x is (the height of the V), the easier it will be to close the V to get the 5" diameter at the end of the pipe.

In theory, it should work fine but you really need to make MANY V cuts to make the taper go smoothly. Using a square, I'd mark the pipe 12" from the end and make a mark all around the whole pipe. Next, divide up the pipe into, say, 8 sections and draw line from the end of the pipe to the mark 12" away. Now place a mark 1/4" away on either side of the line at the end of the pipe. Draw from this mark to the 12" mark, on both sides. You should now have a V that is 1/2" wide at the top, and the two sides of the V meeting at the mark 12" away. Do this 7 more times. Now cut out the Vs.

I have not read Bill's site regarding this but this is, in my mind, how I would go about doing this. If you are doing this, I'm not sure what your issue is. What exactly do you mean that it isn't working? What is happening?

[Dave Harker]

Stick a short length of 6" metal pipe into the PVC, then make your cuts in the metal pipe, to reduce it to 5". In other words, have a metal transition between your 6" pvc and 5" pvc.

[tod evans]

why make this so dificult? cut holes that are sized for the o.d. of the pipe in two pieces of mdf for collars, caulk the pipe to mdf joints then screw the two pieces together. the 5" is flowing into the 6" so the necessity of a tapered reducer is pretty much eliminated, sure there will be some turbulence at the joint but given the velocity of the air/dust passing the joint it`s very unlikely that you`ll get a clog..02 tod

[Julio Navarro]

Chris, I have a suggestion, its a long shot but it may work.

First a few observations:

1.) PVC can be shaped by heating it.
2.) it can be shaped by sanding, cutting and milling.

I used to work for a vinyl railing manufacturer and saw how they made some pretty nifty stuff with the stuff.

having a 6" pvc pipe and needing to reduce it to 5" I would take a section of PVC that is about 2 to 3 inches long and cut it so it is a "c" then remove anough of the pvc material so that when you pres on it so that the ends touch it will fit inside the 6" pvc snuggly. then check to see how much material you will need to remove so that the 5" pipe will fit inside the peice you just put in the 6" pipe.

Chances are you will have to remove some but you wont have to actually remove it.

If you heat the "C" piece it will mold into the 5" section. put the "c" piece in a toaster oven and heat it to 400 for about 15 min or so till its like rubber, using gloves take it out and place it inside the 6" pipe then shove the 5" piece shifting it until it slide into the "C" let the "C" cool a bit then remove the 5" pipe. You can now shape the "C" so that it is a smooth transition from the 6" I.D. to the 5" O.D.

This may not make any sense, not sure I ve explained it well. Ive done similar things with PVC. I'll try to do it and see if it works.

If you do try remember to use thick leather gloves and a large wrench or vice grip to help manipulate the hot PVC and make sure you have a work surface that wont melt. Belive me when PVC gets rubbery it is HOT but you can do anything with it you want. When it cools it is hard as ever.

Hope this helps.

[tod evans]

pics of pipe collars.....

46741

[Dan Lyman]

Hi,
What I do is get a metal 6" to 5" reducer.. It slides perfectly into the 6"PVC pipe (ASTM2729 Sewer & Drain) and then slightly crimp the 5" metal end and slide the flex hose over it. Sometimes I don't need to crimpat all, depends on the flex hose.

This has worked out real well for me. You can also do this with 6" -> 4" reducers or whatever size you need.

Dan

[Chris Merriam]

Thanks for the responses.

Chris P., when I said it's not working, I tried cutting a V or two, then putting a band clamp on to see how much taper I got. The best I got was 1/4 inch, but I had a giant bulge down my V.

I like your suggestion about 8 Vs each 12in long. I'll try that tonight. I guess I was confused because reading other descriptions it sounded like you just cut a simple V and it'll taper down. In reality, you need to cut multiple deep Vs.

I'll expore some of these other suggestions if I can't get this to work. I've done a little PVC bending, so Julio's idea certainly seems do-able.

Whatever I do, I would prefer to keep a tapered transistion. I installed all the ductwork trying to maintain optimum flow and would hate to abandon that right at the end.

Thanks again for the help!

[Chris Merriam]

Dan that's a great tip too. I'll try the zero cost route first then look at buying some metal reducers.

Thanks

[Julio Navarro]

Hi,
What I do is get a metal 6" to 5" reducer.. It slides perfectly into the 6"PVC pipe (ASTM2729 Sewer & Drain) and then slightly crimp the 5" metal end and slide the flex hose over it. Sometimes I don't need to crimpat all, depends on the flex hose.

This has worked out real well for me. You can also do this with 6" -> 4" reducers or whatever size you need.

Dan

Thats a good idea. less work I think.

[tod evans]

sorry folks! i was way off base here, i didn`t read the original post well enough to glean "flex" from it i was talking about ridgid to ridgid..tod

[Dick Strauss]

Do they make a rubber 6" to 5" reducer for cast iron that might fit? I've seen rubber reducers in the plumbing aisle in HD/Lowes that might work...just a thought.


Good luck,
Dick

[glenn bradley]

I don't know about the larger sizes but I use a rubber 4" to 3" reducer successfully for one machine. I had to add a quick disconnect to the end of the 4" flex hose to make a good connection to the reducer. The reducer then slipped right on the the 3" port. I don't even need to clamp them.

[Ben Grunow]

Could also cut a 5" (OD) hole in a 6" pipe cap.

[Justin Dreier]

Chris - Good question. In my case, I have 6" pvc that needed to be connected to 5" flex. I found that Dan Lyman's suggestion of using an HVAC 6" to 5" reducer works well. The 6" side slips right inside the 6" PVC (a little caulk to seal it) and the 5" flex slips tighly over the HVAC 5" end. There are several reducers of different sizes found easily at Lowes or HD.

The other option, which I also used is found on this link:

http://gallery.clearvuecyclones.com/Daves-new-shop-and-cyclone/PB130041

Through these two solutions, I've gone from 6" PVC to 5" flex, from 6" PVC to 6" flex and from 6" PVC to 5" flex. This is one of the tougher things to figure out as a DIY'r. Go with the PVC mod and the HVAC reducers.

DCG

Dan Lyman

[Chris Merriam]

Well, I tried out Chris' idea of cutting 8 triangles out. I didn't have enough scrap pipe to make 12in long cuts, so I went with 9in deep. Each cut was roughly 1/2in.

I got a nice clean taper, but it actually went too far. Coincidentally, I removed 4 3/8 from the diameter, and my new tapered ID ended up being 4 3/8, pretty weird.

Here's some pictures:

46788

46789

46790

Now I'm concerned about the giant gaps I'd have to fill. I'm going to try to start trimming the end off the pipe to see how short I can get it and still have it taper ok. So right now those cuts are 9in deep, maybe I can get down to 5in or something.

Depending on how that goes, I may just follow Justin and Dan's advice and go buy a 6x5 reducer.

[Bill Pentz]

Chris Merriam

Could someone please please help me? I'm stuck on the seemingly simple matter of cutting some Vs in 6in PVC so I can taper it down to 5in for mating to 5in flex hose.

I feel dumb for not being able to figure it out. I read Bill Pentz's duct section and he says to cut some long Vs to taper the pipe down.

So I get out my coping and hack saws and start cutting Vs. I tried Vs as you're staring through the pipe, Vs as you look down on it, Vs pointed towards the pipe opening, and some pointed towards the middle of the pipe.

Nothing I do seems to work. I feel like there is some fundamental mechanical principal I'm missing here. I started thinking about the circumference of a circle and figured I had to remove about 3 inches to get from 6in down to 5in - but still got nowhere.

Could someone please offer me a tip before I run out of my expensive 6in pvc pipe?

Thanks so much!

Chris,

First, we normally do not go from 6” duct to 5” flex because in just a few inches of pipe the smaller diameter flex will reduce an 800 CFM to only about 545 CFM. What we normally do is either remake tools with single tool ports to 6” using a flange, or in the case of two port tools use a pair of ports sized for the machine coming off a wye that does the reduction.

In terms of reducing your pipe there are a number of ways that work, and more that do not. Easiest is to use tapered HVAC reducers as was mentioned. Hardest is to try and melt and stretch the PCV.

The easiest way I know to consistently and quickly reduce the PVC size is to cut 8 wedge shaped notches in the pipe, squeeze the ends together, use a heat gun to set that new shape, then seal.

Calculating the wedges is not too tough. Each wedge should be as long as the pipe diameter so for your 6” pipe each wedge will be 6” long. To calculate the width of each wedge we need to know the circumference, meaning distance around the pipe and the circumference of the size we want it to be, subtract the two, then divide that into how many wedge shaped cuts to be made. We should make a wedge for every roughly 2” of finished pipe.

If you measure the pipe ID it will be 6” and the OD will be about 6.25”. Multiply Pi times the diameter to get the circumference around the outside of the pipe so 3.14159*6.25=19.63495” You want to make this fit into a piece of 5” flex which has an interior diameter of just over 5” to allow that hose to slip onto a 5” pipe. I’ve seen some flex hose able to go over as big as a 5.25” pipe. So we want to reduce to 5” which has a circumference of 5*3.14159=15.90796”. So you are going to need to cut out the difference from your pipe which is 19.63495-15.90796=3.926991”. Round that to 4” to give a little more taper to make starting the hose easier. Our target pipe has a near 16” diameter, so dividing by 2 says we need eight wedges. Divide our 4” by 8 wedges and we get ½” wedge sizes.

To lay this out put 8 lines each 6” long on your pipe equally spaced all the way around. On the pipe end mark ¼” on each side of each line to make eight ½” segments. Connect each of those segments to the top of the lines making eight triangles that are ½” wide at the pipe end and 6” long. You will want to cut these on the inside of the lines and remove that material. If you squeeze those pieces together you end up with a 5” pipe. I found using a wooden ring with a tapered hole to squeeze those pieces together and warming the result with a heat gun then letting it cool will “lock” the plastic in shape. A little sealant and you have something that should fit pretty well and put at least 1” of hose on your PVC.

Bill

[Chris Merriam]

Thanks for the reply Bill. I think I ended up doing most of that through Chris' instructions, but it's definitely helpful to see the geometry principles laid out. Instead of cutting 9in deep I'll reduce to 6in. I'll definitely try out my heat gun on the taper as well.

Your point about not going from 6in duct to 5in flex has me at a loss. I was planning on making a 6x5x3.5 reducing wye per instructions on your site using that online template calculator.

The purpose of this post was to learn how to do the 6 to 5 part. I understand your comment about the CFM efficiency. I think I'm ok right? Were you assuming this was just a straight run of pipe?

If not, are you saying it's better to reduce 6in to 5in, then run a little 5in metal pipe until I have to switch to flex hose? I will run the 6in as close as I can to each machine before doing the taper down to 5in. So really the flex will just serve as a little flexibility barrier in case I bump a machine or something.

[Julio Navarro]

Thanks to Bill's post I am going to have to re do a couple of connections to some of my machines. I think I am reducing way to far from the machine.

Thanks again Bill.