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 Post subject: Wood working tips needed
PostPosted: August 7th, 2018, 9:33 pm 
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I will be building conical midbass horns. Horn is made up of three sections with different flares. Horn is about 3' long. Material is 3/4" Baltic birch.

Need advice on gluing order and securing the sections together. Thinking of gluing the three sections separately and then putting the sections together. Would the horn be strong enough at the joints between the sections as shown?

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horn_front.jpg
horn_front.jpg [ 37.95 KiB | Viewed 634 times ]

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Shashi


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PostPosted: August 7th, 2018, 10:27 pm 
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Depends on how good the dry joint is with respect to gaps. I would use Titebond III glue.


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PostPosted: August 7th, 2018, 10:34 pm 
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Biscuits would be a huge help...


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PostPosted: August 7th, 2018, 11:04 pm 
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I have a biscuit plate joiner if you want to borrow it. I live in Silver Spring. Also if you are near White Flint you might want to drop into the Woodworkers Club and ask for advice. One problem with biscuits would be setting the angle correctly. What could work better would be to use a spline and cut the slot on a tablesaw. Another alternative would be to just butt glue the sections and then use fiberglass tape and resin to make sure they don't come apart.


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PostPosted: August 8th, 2018, 4:20 am 
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I did this several years ago. Make sure you carefully calculate the angles on the cuts, you need to both cut at an angle (a sliding table is essential), and set the blade at an angle for it to work. Layout your cuts base on the interior dimension after fit-up. it is a very difficult assembly to do, and you need to be precise or it will not fit up without gaps. Also, use very slow setting glue to ensure that you have enough time to fit the pieces together. Clamping is another complex issue. And I would not recommend biscuits unless you can precisely set the angle, which will be different section to section. I found the old glue and screw worked the best for me, quick assembly.

I put flanges on each tapered section and bolted them together rather than trying to fit all three sections together. Made for much easier fit up and assembly. This is traditionally done for multi-section straight horn if you look at pictures. Also serves as stiffeners.

Think out every step in advance. Improvisation, especially during gluing and assembly, will likely lead to panic and frustration, especially as the work will move when clamped with any force UNLESS it is fixed in place by screws or dowels. My concern with biscuits are that joints can slide when clamped (and WILL slide considering the angles and wet glue as a lubricant).

Good luck.

David


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PostPosted: August 8th, 2018, 8:27 am 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
I question the need for 3/4 inch stock. This would be quite heavy and support will become an issue. Maybe 3/4 inch for the first section near the driver, then 1/2 inch for the rest.
Biscuits are a good way for joining, but with plywood you could get away with a wide lap joint.
There are many YouTube videos out there for help
Speaking of YouTube videos there are many out there touting that Titebond lol is no stronger thane regular Titebond. What Titebond lll gives you longer work time and is waterproof which does not matter in this use


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PostPosted: August 8th, 2018, 8:41 am 
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I know it wouldn't be DIY, except maybe the design, but I know a millwork shop that can make anything and it will look good and be solid.


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PostPosted: August 8th, 2018, 8:56 am 
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After having built the truncated pyramids for my egg project I can say that you will face at least two problems making that sturcture. The first is setting the proper angles for the cuts. In the pyramids there was not one single 90 degree cut so I purchased a Wixey WR410 8" digital protractor to set the blade angles on my table saw. Don't even think of doing this with a hand saw. With large pieces even a small angular error can lead to large misalignments.

I almost always use biscuits which are great with straight surfaces. Even angled surfaces are not a problem as long as the pieces are of consistent length. When you get a shape such as the pyramids or your horns you wind up with two problems. The first is that clamps tend to slip off the surface. The second is that when pressure is applied, the pieces tend to slip along each other which is the thin direction of the biscuit, offering no resistance to that slip. I think I would go with the screw method suggested by Dave. The entire egg project will be in three issues of audioXpress starting with the September issue. Here are photos of two of the jigs I used when gluing the pyramids. BTW, the egg project was the most difficult woodworking project I ever did and from start to finish including the electrical design took about a year.


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Clamping fixtures_small.jpg
Clamping fixtures_small.jpg [ 331.37 KiB | Viewed 601 times ]
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PostPosted: August 8th, 2018, 9:12 am 
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I would like to add a couple more things.

First (a minor quibble), it the horn profile you are showing appears more like a 3 section approximation of an exponential horn (consisting of 3 conical sections).

Second, you might want to consider expanding flare on only one pair of sides, and keeping the other pair straight. It simplifies the angles in that you can keep the blade at perpendicular to the panel when cutting, rather attempting a compound angle. You will need to set the blade at an angle for the joint between flared sections. It will be much easier to use biscuit joints, and you can actually just rough cut (oversize) the straight flare sides, and trim down with a router after assembly and it will look perfect. You will have to work out the proportions to maintain the correct flare, but it will be A LOT easier. I think this is how Bruce Edgar designed and built his straight midbass horns.

BTW, what are your horn dimensions?


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PostPosted: August 8th, 2018, 10:28 am 
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A thought just popped into my head. I almost always use MDF, not plywood. The screw method works in MDF but it is more difficult to get accurate alignment when drilling pilot holes in plywood because of the differences in layer density and grain direction. You might want to try a hybrid solution with mostly biscuits for alignment and just to keep the panels from sliding during gluing using one or two screws away from the ends to minimize splitting. Pilot holes for those screws would be necessary but you could drill them after the biscuits are in place and the panels joined dry. After the pilot holes are drilled the panels can be separated and the glue added before reassembling the panels. That would give a better chance of registration of the screws and less chance of them screwing up (pun intended) the overall registration. You would still need to clamp while the glue is setting up requiring the jigs in the picture that would keep the clamps from sliding off. They were pieces of 2 X 4 with slots cut in them for the clamps. The material used in kitchen shelves to keep things from sliding was hot glued to the side facing the work to provide additional grip and a clamp was used to fasten the jig to the work.

BTW, if you keep both ends of the horn proportional, that is square on each end or rectangular with the same proportions, some of the angles will be 90 degrees. With the pyramids, to get the required volume and minimize the visual effects the bases were rectangular. However, to mate with the round egg bottoms, the tops were made square and then rounded off. The transition from rectangle to square caused the 85 degree angles where the panels butted together.


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