Introduction: Birdsmouth Joint for Wood Cylinder Build.
This instructable will be focusing on the build of the wood cone / cylinder which could just as easily be used to build a hand drum or a downspout or an architectural column or a hollow spar. In this particular case its for a cyclone shop vac project.
There are already some very good instructables on how to build the cyclone shop vac . In the final step of this instructable, I will point out a few of the details that I did differently from others.
This instructable assumes that you already have a solid skill set using the table saw. That you know how to keep your hands and body away from blade and the line of fire - That you wear appropriate eye and ear protection - That you use the proper guides and attachments etc - AND that you never rush.
Step 1: Layout
While considering a cyclone separator build for a shop vac, I coincidentally came across an old clipping I had saved on birdsmouth joinery used by boat builders. It looked like a relatively simple solution.
The dimensions I chose are fairly arbitrary, based on comparing several models and picking the numbers that suited my aesthetic judgement. I'm not about to spend the time to work out the physics of the thing. Bill Pentz at http://billpentz.com/woodworking/cyclone/ discusses this is more detail than I'm ready to study.
Broken Board has a nice instructable on a wooden separator that he made https://www.instructables.com/id/Wooden-Cyclone-Seperator-Shop-Vac/
Birdsmouth versus mitered edge? I don't know the answer to this. The fitted joint created by the birdsmouth versus the straight glue edge of a mitered edge should be a stronger joint, because there's more surface area and it's glued on 2 planes.
I had also assumed that it would be easier to hold together while gluing, but I had problems, and I didn't try it both ways, so I don't know. A birdsmouth joint involves fitting the 90 degree edge of one stave into an angled cut of the next board. A typical birdsmouth has 8 staves. The 8-stave birdsmouth has one 90 degree edge and the other edge has 2 opposing 45 degree cuts. Since the roundness of the piece increases by increasing number of segments, I doubled the number of staves to 16, then the inside entry cut was figured at 22.5 degrees instead of 45 ( 22.5 X16 = 360 degrees)
I did much of my figuring directly laying it out on the work. Shooting for approximately 20" height top inner diameter of 12" bottom inner diameter of 6"
The following exercise shows how to divide a circle into 16 segmented arcs with a compass. To calculate the actual math would require dividing the circumference pi X diameter by 16, but then taking that number and marking it out, but would be more prone to fractional error. In the end, I did not use this layout. Rather, I glued the pieces up as described further on. Then I traced the pieces to cut out. I wasn't sure that my calculations would be accurate. What I should have done before the glue up was to take the finished dimensions of each slat and multiplied by 16 to check the circumference. Having that wooden piece to clamp or screw to, would have made the glue-up much easier. But more about that in step 3
Step 2: Cuts
This went quickly and easily. I took scrap 1"X wood and cut 8 blanks to 4 1/2" X 20 " long. They are cut in half along a taper so that each piece has a 3" and 1 1/2" end. When they are ripped to their taper there will be 16 staves.
The tapered jig / push-stick rides along the tablesaw fence and has that exact dimension.
Set the table saw blade at 22.5 degrees There are 2 cuts and it's best to work out the deptht of each cut on a scrap. Line all the boards in the same direction so you don't mix up the cuts. I made the first cut with the board flat (wide end parallel to saw table.) The fence is just shy of the saw blade.
The second cut is made with the board riding on edge (90 degrees to the first cut) with the fence approx. 1/16" away. I did this for 2 reasons - the board will have something to ride on the table at the end of the cut and it leaves slightly thicker and stronger bit of wood .
Step 3: Glue Up
On my 3rd attempt at the glue up, I broke the process down and glued 2 staves together at a time. Then when they're all paired off, I glued pairs of 2 staves to another 2 so that i had 4 sets of 4. At the point, I could bungee the 4 sets together and trace the top wide circle and cut that out. ( I guessed that the drum tapered at 15 degrees and set my saber saw and cut it out) Nothing fancy. On the top outer stave of each set of 4, I drilled a countersink hole, so that in the final glue up, I could screw either edge as I brought the piece together - 8 screws in all.
This method worked fine, though not necessarily the best. approach. Still, the only real downside was that it took several days waiting for the glue to cure between pairings.
After it's all glued, the outside has a jagged edge. Also the top and bottom are uneven. I trimmed the top of the stave flush with the disk. For the outer perimeter jagged edges - If I had remembered where I last left my hand plane, it would have been easy enough, but in the winter, I keep my tools in the unheated workshop and do the actual work in my basement. Typically, at the end of a job, if it's too cold - well - some things get misplaced till spring. Anyway a sharp chisel did a reasonable job quickly enough. I left it as you see in the final pictures.
Step 4: Failures
Learning by my failures?
My initial attempt at gluing it was a disaster. I lined up the pieces spread the glue, had the whole thing together and I as I was trying to tweak the top and bottom alignments, the whole thing collapsed. Well that meant I had to wash all the drying glue off and then after it all dried, I scraped off what I had missed. This method might have worked if I had lured an accomplice to assist me.
On my 2nd try I thought I might be able to use some hoops that matched the approximate diameters. I only spread the glue as I went, one piece at a time, but I soon realized that the hoops I chose would not hold it all together the way I imagined and I quickly shifted to my 3rd and successful method.
If I had felt confident about what the final dimension of the top disk was to be, I could have fastened or clamped to that as I went along.
Step 5: Details for Vacuum Chamber
A lot of people use a joint compound bucket, but from many years of experience I felt that is not an ideal solution, because prying that top off to empty the bucket? What a nuisance. I already had the blue bucket you see missing it's lid. The bucket has 2 clamp handles, which makes it easy to open and dump.
To stabilize the Bottom of the bucket I cut a square of plywood. Using my bandsaw, I cut through and around a traced circle, making it slightly smaller than the bucket bottom. Pushed the bucket into the base. You can see the entry cut spread wide by this action. It sits nice and snug.
The lid of the bucket is also the connection for the bottom of the cone. Traced it, cut it and used 4 screws from the underside to hold it together along with caulking.
After I attached the vacuum and tried it out, the suction was poor so I added extra caulking and weatherstripping and it worked pretty much as advertised.