Failed/broken scarf joint

Hi All

 

Hi Paul
Sorry your scarf broke.
Jeff.

 

Try this approach. You don't have to be so careful about positioning and pressure while gluing.
In actual fact, if you use epoxy in a scarfe joint, too much pressure will starve the joint and lead to failure.

 

Welcome,

Did you post a picture of a failed scarf joint a few days ago?

If so:
It looked to be wood failure adjacent to glue, so not glue starved.

Ply looked constructed of 3 plys, very thin surface veneers over very thick core. I suspect core is of a softer species than surfaces. This is common in cheaper ply. Better plywood has more plys and is of one species only.

Some possible alternatives to try:
-Better plywood
-angling scarf so it is not perpendicular to axis of bend
-increase scarf ratio (12:1)
-skinning panel with fiberglass before flexing

Tough learning curve.
Good luck

Edit
In most plywood, the axis od greater strength is aligned with the grain of surface veneer. However, in thick core 3ply, the stronger direction is determined by the core"s grain, you might need to rotate plywood and use several scarves

 

"-angling scarf so it is not perpendicular to axis of bend"

How does that work ?

 

Come on Rwatson you are pulling our leg. You know very well that it is possible to do a scarf joint that is not at right angles to the direction of bend. To be sure, butt to butt is the usual method but it is more than possible to do a scarf joint that is on a diagonal.

I suspect that the problem has already been identified: Inferior ply wood. Using Titebond adhesive is a little bit chancy as well.

 

One of the great features of this forum is faulty information is called out. Thank you Rwatson. I should have said
Not PARALLEL to axis of bend.


If the colored tapes represent scarf joints. Ignoring the difference in widths;
The black one (parallel to axis of bend) would have the most strain due to flexing into the can shape.
The green would have little if any strain.
The blue, some strain but not as much as the black

Another strategy for reducing the strain on a scarf, would be to place it in the portion of the panel which is least curved.

 

Gotcha. I understand. Good visual aid.

That green one would be a bugger though, it would waste soooo much sheet material.

 

Um, had never seriously considered it on plywood due to the high material wastage. I would like to test it and see if anything less than say 20d had any real benefit, and if cut at a higher angle, just ended up with the pointy bits being more subject to breakout under pressure.

The only time I remember ever seeing an angled scarfe would be the cardboard on a toilet roll.

 

The only time I remember ever seeing an angled scarfe would be the cardboard on a toilet roll.[/QUOTE]

Paper tubes employ half-lap joints

Remember scarfing is a way to join wood so that smaller pieces act as larger onrs. Never much waste if used out of need.

Thinking outside boxes. A diagonal scarf turns standard sheet into 5x6 with very little waiste.

As to the green example
Crowned coach roof 7x18 feet. Several joints perpendicular to toll of crown. Could be simple butts if over framing, or scarfed regardless of under framing.

 

Exactly. That' s how common true angled scarfes are, even Dudley Dixes devloped ply hulls don't use them.

Yeah, but IS there any need ? I dont think a very high pressure scarfe would behave better when cut diagonally, and may break at the pointy end even worse. As for turning it into 5 x 6 - you are going to have to show me and example where that is useful.

I didn't think I expressed any problems with the "green" scarfe.