Spar Installation

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Spar Installation

Postby Capebuild » Wed Oct 13, 2021 6:35 am

I'm getting ready to install the headliner today and thinking about the next step of installing the spars.
Do the spars get attached by screwing through the outer side wall surface into the spar's end grain, or is it better to create pocket holes in the spar ends and screw into the side wall surface?

As I write this I'm thinking maybe the pocket hole direction might be best, but looking to hear how others have done it.

Thanks

John
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Re: Spar Installation

Postby western traveler » Wed Oct 13, 2021 7:12 am

John,
Moving right along. I screwed down into the walls. It was how Steve Fredricks suggested and I saw it in a photo in Tony’s build journal. Of course with my woody build it was the obvious thing for me to hide the heads of the screws.
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Re: Spar Installation

Postby halfdome, Danny » Wed Oct 13, 2021 7:32 am

John, since you’re covering the exterior with aluminum the strongest method would be to glue and screw from the exterior.
Pocket screws are great but you’ll have more pulling power going from the outside.
The general rule is the fasteners should be twice the length of the thickness of the material you’re going through.
I Bondo the recessed screw holes.
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Re: Spar Installation

Postby Capebuild » Wed Oct 13, 2021 4:55 pm

Thanks so much for your thoughts, Ralph and Danny. I appreciate your help. Screwing the spars in from the outside wall makes sense.
I did get the headliner installed today. No easy feat.
Will show some photos in my build album.

Thanks again.

John
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Re: Spar Installation

Postby DrewsBrews » Thu Oct 14, 2021 7:02 am

I find pocket screws to be somewhat difficult to use as typically Im using the screw as the clamp for gluing.. But since the screw is going in at an angle it tends to move the piece unless it is being pulled in towards a rabbit joint/cleat. So often needs extra temporary clamping to try and minimize the movement. By that point my laziness really starts taking over.

Im going to PMF so filling in recessed screw heads isn't an issue for me. Im screwing through the 3/4" wall into the spar end grain. I did initially have some problems with 2.5" screws stripping out the end grain so switched to 3".. Predrilling to a depth just before the tip starts narrowing so all the threads have a chance to bite.
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Re: Spar Installation

Postby noseoil » Thu Oct 14, 2021 8:47 am

I don't own a pocket jig & don't have much use for one myself. When I started building cabinets early in the dark ages, they didn't exist & weren't available anyway. I see too many people rely on them for structural fastenings without understanding the strength of materials they're using. A proper rabbet joint with fasteners from two directions & glue will always be a stronger joint than a "butt joint" with fasteners from two directions at an angle. Granted, in some applications it is strong enough for a pocket type of joint, once it's reinforced with another material supporting the joint from a different plane, but I just don't see a need for them in my work.

As a case in point, face frames are made with pocket screws now by a lot of people, but a simple dowel joint with glue & pins will work quite well. Once the face frame is fastened to the cabinet with nails & glue, the joint is reinforced by the body of the cabinet. With a simple doweling machine & pencil marks from the layout stick to mark pieces, it's much faster to build than using a pocket jig. Will post a picture of my setup for face frames & attach it later today.

Through-fastening from outside the walls with screws is a better joint than trying to put pocket screws in at an angle from the inside & into a relatively thin piece of plywood. YMMV!
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Re: Spar Installation

Postby western traveler » Thu Oct 14, 2021 10:03 am

Gee, I feel like I need to comment that I was NOT speaking of the use of pocket screws or jig.

I used coated deck screws and predrilled with a tapered bit and countersink to match the size of my screws along with PL premium. I think dowels would have been another option.

What I seek is a immovable joint that will stay in place over time.

My thinking as a first time builder was the shear power of a heavier shanked screw through the 2” poplar I used sunk deep into the multiple layers of the 3/4” ply bedded in PL afforded better side to side resistance to pressure, than a countersunk screw through 1/4” ply which depends on the screw head to not pull through the plywood. (Dang, that is a long sentence).

I then wrapped fiberglass cloth over the joint and roof skin.

I know most of you have way more real life experience in building these torsion boxes than me. It was just my thinking at the time.
Last edited by western traveler on Thu Oct 14, 2021 10:06 am, edited 1 time in total.
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Re: Spar Installation

Postby Pmullen503 » Thu Oct 14, 2021 10:05 am

noseoil wrote:......... A proper rabbet joint with fasteners from two directions & glue will always be a stronger joint than a "butt joint" with fasteners from two directions at an angle. Granted, in some applications it is strong enough for a pocket type of joint, once it's reinforced with another material supporting the joint from a different plane, but I just don't see a need for them in my work.

..........With a simple doweling machine & pencil marks from the layout stick to mark pieces, it's much faster to build than using a pocket jig. ..........


I have to agree.

A rabbit in the side walls would support the spar; glue and screw would merely hold the spar in the rabbit. I would prefer a dowel in a drilled hole with glue if a rabbit is not feasible. The problem with a screw in end grain is that it has a tendency to split the spar.

Gluing the spar to the roof gives a lot of support to the spar so you are not relying on the screw in the end grain to carry the weight alone. It's probably an entirely adequate way to do it. I think I would still remove the screw after the glue has dried and replace it with a hardwood dowel.
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Re: Spar Installation

Postby halfdome, Danny » Thu Oct 14, 2021 10:07 am

Noseoil, I know first hand what you posted since I served part of my apprenticeship making doweled face frames all day long.
I still have my Ritter pneumatic horizontal boring machine, something I'll never part with as screwing face frame mullions on uppers doesn't hold as well as two 3/8" x 2" dowels at each end.
I also have a pocket screw jig and used it on many commercial jobs and my customers loved the results.
There's a time and situation for all methods :thumbsup: .
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Re: Spar Installation

Postby DrewsBrews » Thu Oct 14, 2021 10:42 am

Capebuild - I looked back at your build. looks like you have created a rabbit with the inner wall skin.

I figure a pocket screw (from outside angling inward) could work out in that case for reasonable glue clamping?
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