Making a scarf joint, HOW TO?

[Ira Matheny]

I have the need for some longer pieces of wood. Therefore I have started trying to make scarf joints. I have had some success, some less than successful result.

What I am doing is make cold bent fly fishing nets. I start with various wood strips cut 0.065" [1/16"] T x 1.5" W and about 6' L. Then glue 7 or 8 strips together and form or bend over a well made pattern, using lots of clamps. The radius of the form is about 4" at the tightest. SUCCESS.

But I have some very beautiful exotic woods that are significantly less than minimum length required. I have tried to glue up using a 'Scarf Joint'. My trials have been using a 12 degree scarf on both of the two pieces of wood. Using Tite Bond PolyU, pre misting the wood which is about 8 to 10 % mc. I glue with wood strip that are 0.125" thick, then sanding down in the wide belt sander to 0.065" finish. About 65% success when I glue and bend as described above.

What can I do to improve my results. [Steam is not and option] Should I use a more tapered longer joint [8 or 10 degree taper]? Or a less tapered shorter joint [16 to 20 degree]?. Obviously, non waterproof glue is not an option.

[Richard Venturelli]

Not sure what your problem is. If the glue is not holding and you did write about exotic woods. Some Exotic's have lots of oils in them you may need to get some special epoxy for oily wood. I think there is a post on Sawmill for that problem if that is your problem

[Richard Coers]

Most exotics are very brittle. Have you ever seen how guitar sides are made? Luthiers put a torch inside a pipe and flex the wood over that hot pipe. It relaxes the lignin in the wood. Might be worth an experiment, but the glue likely will not like the heat. Another option would be to dampen the wood and put a wet washcloth on it, then iron with a hot clothes iron. That should make the wood more flexible.

[Jeff Duncan]

So in general I think that the more surface area you have the better. If your gluing up 1/8" thick strips your going to need to really make the angle looooong in order to get enough surface area. However your trying to bend them into shape after gluing and sanding down..... that's going to be tough! My guess is we may be able to throw very general ideas at you, but your probably going to have to do a fair amount of experimenting to find what actually works..... if anything! Maybe try switching to epoxy for a glue, try cleaning the joints with solvent first etc..

good luck,
JeffD

[Ira Matheny]

Thanks Jeff D,
I have used Tite Bond 2, 3, Polyurethane, and epoxy. Epoxy seems to work slightly better that PVA for the strength of the joint.
And, yes, I am of the belief the longer scarf joint is preferred for strength. However, the glue in the longer scarf joint shows in the finished project, and is therefore undesirable. Of course, I am referring the only the outer [and inner] laminations of the 'hoop' of the fly fishing net.

[andy bessette]

Scarph joints are generally specified by ratio, not angle; for example 12:1.

You have not bothered to mention the radius about which you are bending. There is certainly a minimum bend radius for each species and thickness. Also stock should be chosen for straightness of grain.

Try WEST epoxy for glue; apply precisely as recommended by Gougeon Brothers.

[Jacob Reverb]

I'd try a longer scarf and epoxy...possibly the West System "Gflex" epoxy, which seems to have more "ductility"/toughness than their normal 105/206. It's quite a bit more expensive than 105/206, but as with nearly any epoxy, you can mix/dilute the resins/hardeners to "stretch," so long as you use the right proportions of each. Good luck.

[andy bessette]

...as with nearly any epoxy, you can mix/dilute the resins/hardeners...

WEST 105 resin and 206 hardener must be mixed at 5:1 ratio. Never dilute it or change the mixing ratio. And never add anything to it that isn't specifically recommended by the formulator.

[Tom Bender]

You might not have good clamping of the joints. Uneven pressure will build stress into the joint, as will high clamping force. This could cause a blowup in the sander. Clamp gently and evenly. Vacuum clamping could fix it or clamping between layers of foam might be easier.

Bending with heat may help but it puts the glue at risk. Hot melt glue could work.

When bending, a continuous outer ply would be best, but you may not have that length of material available. Try to have no joints in the sharpest part of the bend on the outside ply.

[Carl Beckett]

Can you describe the failure mode?

It sounds like you are making thin strips via scarf joint, which then get laminated while bending around a radius. When you say it is unsuccessful, in what way?

If a scarf joint laminate... it might fail the glue joint and flare out on the outer laminate at the tip of the scarf where it is very thin? (random guessing here on how its failing). The scarf joints that were on an internal laminate shouldnt fail because these are captured by outer laminate (which I assume you stagger joint location).

So I wonder if, the outer laminate should just be a butt joint? (not sure that really solves though if glue joint failure), or some other joint like a finger joint.

For sure different woods glue differently. And a scarf is some ratio of end grain to edge grain so think about joints that have mostly edge grain for glueup.

As for epoxy, I have used some West systems and others (Masterbond, etc). At the end of the day, for many things, regular 5 min epoxy you get from the hardware store works pretty well.

Any pictures?

[Jacob Reverb]

WEST 105 resin and 206 hardener must be mixed at 5:1 ratio. Never dilute it or change the mixing ratio. And never add anything to it that isn't specifically recommended by the formulator.

The formulator (the boys at Gougeon) are the ones who told me it was fine to do. What I meant by "dilute" is that you can "dilute" a proper mixture of the Gflex with a proper mixture of 105/206 for example if you want something with properties (and cost) somewhere in the middle. This is well-documented and recommended in the West literature.

In fact, according to the guy I spoke to at West, EVERY epoxy/hardener combo is compatable with every OTHER epoxy/hardener combo, regardless of mfgr.

As for "never add anything to the epoxy that isn't specifically recommended by the formulator," I have mixed a pretty wide array of "unsanctioned" fillers and pigments with all kinds of epoxies, and never had a problem. I suspect that if they had to create a list of "approved" fillers and pigments, it would fill a book containing many tens of thousands of pages, and still not be definitive ("Did we forget powdered k-feldspar?"). How many chemical compounds are there in the world?

The only things I've found that you can't mix with epoxy are vinegar and Gojo, which is a good thing since that's what I use to remove epoxy from hands and tools.

[Doug Garson]

Photos of the failure and your bending setup would help, presumably the failure is in the outer lamination. Can you add an outer layer of full length wood (or other material perhaps PVC? or thin metal) without glue to prevent the outer layer from blowing out?

[Lee Schierer]

Have you tried backing up the piece being bent with a steel strap so that the bending force is distributed along the length of the piece? The steel staping used for attaching boxes to pallets should work well.

[andy bessette]

The formulator (the boys at Gougeon) are the ones who told me it was fine to do. What I meant by "dilute" is that you can "dilute" a proper mixture of the Gflex with a proper mixture of 105/206 for example if you want something with properties (and cost) somewhere in the middle. This is well-documented and recommended in the West literature.

This falls under the category of "approved by formulator" but is not "diluting" but rather "combining".

In fact, according to the guy I spoke to at West, EVERY epoxy/hardener combo is compatable with every OTHER epoxy/hardener combo, regardless of mfgr.

According to Gougeon Brothers technical adviser: "While other epoxy brands might be compatible, the properties of such mixes are unknown."

As for "never add anything to the epoxy that isn't specifically recommended by the formulator," I have mixed a pretty wide array of "unsanctioned" fillers and pigments with all kinds of epoxies, and never had a problem...

That would be a sample of precisely ONE.



Anyone wishing to perform such science experiments is of course welcome to try.

[Jacob Reverb]

This falls under the category of "approved by formulator" but is not "diluting" but rather "combining".

FWIW, thinning is also approved by West (would that be considered "diluting"? Gotta check my dictionary...) by up to 10% with acetone.


That would be a sample of precisely ONE.

...in other words, like every other reply that was ever posted in any thread here, ever, thanks.


Anyone wishing to perform such science experiments is of course welcome to try.

Well, thank you!

[Alan Schwabacher]

While epoxy gets dry in a relatively short time (a few minutes to hours, depending on type) most continue to get stronger for about a week. (High temperature cured stuff probably gets to strength faster.)

I have bent scarfed stock (1:8) to make canoe gunwales, always letting the epoxy cure for at least a week before bending. The radii weren't too tight, but the joins remain strong decades later.

I like the old-fashioned method of painting porous surfaces to be joined with unthickened epoxy until no more soaks in, then mixing in microfiber or other thickener and applying that, then clamping the joint.