Shop-made PVC wye
Building a custom fitting
Published 15 Oct 2010
In assembling the ductwork under my router/extension table, I needed two real "Y" fittings. The wye fittings you can buy are a straight run with a branch off to one side, whereas what I needed was for the duct to actually split in two at 45° angles. I could have used a normal wye, but I was cramped for space and those would have taken too much space, so I needed a real Y fitting.
I searched all over the internet looking for 6" PVC S&D Y-fittings and found nothing. Even my go-to sources like McMaster-Carr don't have them. So I decided to make my own by whacking up a pair of 45° elbows and gluing them back together.
The tricky part of doing this is the fact that the resulting fitting actually has to, you know, fit. It can't be too big or you have a massive air leak. If its too small, your duct won't fit into it. It has to be just right.
Therefore it has to be cut exactly right. You can't just split the 45° elbows in the middle. The saw kerf will remove a little bit from each side, so you end up with a little less than half the fitting on each side and the resulting part is slightly too small. Which is officially a Bad Thing™
The way to attack this is to make a jig that will hold the uncut elbow and allow you precisely align the blade so that the entire kerf is on the waste side, leaving you with exactly half an elbow. These two drawings illustrate what we're trying to accomplish. The size of the blade kerf is exaggerated for clarity.
To make the process accurate and repeatable, I made a simple jig out of ¾" plywood. It originally was a leftover part from one of my shop-made blast gates, so the hole is slightly oversized. I found this didn't affect the accuracy of the jig, since I drilled a hole in one side and used a drywall screw to pin the elbow against the opposite side.
I also carefully measured and marked the point straight out from the center of the opening. I used this mark to align the seam of the elbow, assuring the elbow was pointing 90° away from the cut line.
Once the elbow is securely locked in place, adjust the bandsaw fence so the kerf falls exactly to one side of the centerline, as shown in the drawings above. Test cuts will help set this correctly, making sure you account for the blade tooth set. Once you make the first cut through the jig, you'll be able to use it to set the position much quicker in the future. Also make sure your blade guide is raised enough to clear the much-higher outside edge of the elbow.
Once you have everything lined up, you're ready to split the fitting. Note that for safety, you should always use a bandsaw blade that has three to four teeth in the material at all times. With the thin PVC you'll be sawing through, this calls for a very fine-toothed blade. Check the thickness of the wall, keeping in mind that you'll be cuting at a 45° angle which has the effect of making the material thicker. Go slowly and let the blade work it's own way through the PVC, don't force it or you may deflect the blade. Check out the video for a demonstration.
Once you have one side split, do the other. Keep in mind that you don't turn the second elbow the other way, face it the same way so the blade kerf is also on the right side of the centerline.
Once the elbows are split, we need to join the two halves. For gluing PVC to anything, including itself, a polyurethane glue works very well. It foams up and expands as it cures, filling any voids and making an airtight seal.
Although the glue works very well, a duct (worm drive) clamp is cheap insurance. I like these from Lowes, they have an extra-long slotted section that allows more adjustability than the ones from Home Depot. Run it out so that it has plenty of room for the two halves. Also, check the elbow halves for any protrusions like the one shown here, just trim them off so they don't get in the way of the clamp.
DO A TEST FIT!!! You don't want to have pieces all covered with glue only to discover you've miscut and they don't quite fit or the clamp's too tight or whatever.
Also, get a piece of wax paper to protect your work surface from drips, runs, and squeeze-out.
Polyurethane glues require moisture to activate and cure. Use a spray bottle to wet one side of the elbow. On the other side, lay down a generous bead of glue. Put the two halves into the clamp, one bottom corner in each side to form a V shape, then lower the halves down and bring them together. Slide the clamp up to the upper part of the sleeve and tighten it. Then use a clamp to draw the top area together.
You'll see some immediate squeeze-out. On the outside it really doesn't matter, but the inside will interfere with the pipe sliding in. Just wipe it off a few minutes after clamping everything together, then check back after another ten minutes to do it again. The glue foams and expands, so you may get some more come out. It's fairly easy to trim off later, but wiping it now is much simpler.
After letting the glue cure (I let it set overnight, check your glue for recommendations) remove the top clamp but leave the duct clamp as permanent reinforcement. Trim any additional squeeze-out on the inside and do the same on the outside if you feel like it.
One other issue needs to be dealt with. When you split the elbows, the cut goes through one side of the collar where the duct slides in. So if you slide in duct from each side, they'll run into each other before seating fully. So we need to trim off a corner of each side.
Start by inserting a piece of duct into one side. Push it down until just before it enters the other side, then mark the position of the bottom of the duct in the elbow. Measure the distance from the bottom of the collar to this mark, this tells you how far up the duct the cut needs to go. Mark this distance on the duct and make a 45D cut to this mark. I used a miter saw, but that's dangerous and I don't recommend it. Every time I do it the cutoff falls into the blade and gets thrown across the shop. Someday, I'll find a less-stupid way to make this cut.
Once you've sliced off this overlap area, the duct will seat fully. That's it, you're all set!