Perfect "Scarf" joints..... Every time

[lenb]

From Wikipedia:
A scarf joint (also known as a scarph joint) is a method of joining two members end to end in woodworking or metalworking. The scarf joint is used when the material being joined is not available in the length required. It is an alternative to other joints such as the butt joint and the splice joint and is often favored over these in joinery because it yields a barely visible glue line.


While building large scale model airplanes, you'll certainly need to join wood together for spars or longerons. This simple, strong and fast method works every time and anyone can do it.


1) Mark all the pieces you'll need for the segment of the project you are working on.



2) Clamp a straight block of wood to your disc sander table. ( you do have a disc sander, right?)



3) Start the sander and slider the wood stock into the sander until it sands to a point.




4) Repeat the process with the other pieces, do not let the clamped block move,



5) Using a straight edge, push the two pieces together until they're fitted perfectly.



6) Once happy with the fit, glue the joint and allow to dry.



7) Sand the top and bottom of the piece and you're done.

[JimD]

Scarf joint...when the piece of cloth around your neck is not long enough to flap in the breeze from the open cockpit of your biplane...so you tie two pieces together to make a longer scarf.

[lenb]

I figured someone would go with "Scarf Joint" A place where you buy scarfs.... Or Where scarfs hang out. Or the latest in medical Marijuana, Or .........

[Tango Juliet]

I believe I read somewhere that the length of the scarf should be an 8:1 joint for best strength. But that may not be necessary in our models.

[Mark9]

You're correct here's a quote from Wiki: "Where scarfed joints are used in the restoration of vintage aircraft most developed countries will only issue an airworthyness certificate if all such joints have used an angle no less than 1:8." We're not flying in our models, but the joint can fracture more easily the less the steeper the angle.

[SMYK]

Thank you Len for bringing this subject. Fatigue versus stress resistance of glued joint is far less then continuous wood fiber. Do not use on inner part of the spar runs. Make them toward the tips.

Mike

[lenb]

Great info guys and thanks for chiming in. It makes sense the longer you can make the splice the better.

@ Mike, good point and one I should have mentioned, always place the joints as far outboard as possible, and change the location from a lower spar cap scarf to the upper spar cap scarf if possible.

[BryanB]

In full size wooden aircraft spar repairs the "scarf" is cut perpendicular to the grain rather than longitudinal. Like the picture SMYK posted. And typically have reinforcement plates (doublers). The fitment and accuracy of the joint is critical. This is for maximum strength spar repair. I loved this part of A&P school but making a passable spar repair that the FAA would accept and approve is a lot harder than it looks. Without making copies out of my 43.13 here's a good online reference. http://content.aviation-safety-burea...ures4.php#1-39.
This really isn't the same thing as what Len posted but it's good reference.

[lenb]

Thank you Bryan,

I've been traveling for business the last few days and have a ton of time to think about things, I was wondering myself about the exact thing you posted. So the "proper" Scarf joint for a piece of wood like I posted is through the 1/8" thickness, not the 1/4" thickness.

Math-wise, using the rule of thumb 8:1 you would have a 1/2" long scarf with the wood standing on edge when sanded in the example shown.
( 8x 1/8" )

For the record, I know there are some cautions in the full scale world about not sanding a scarf joint too, the theory is it closes the wood pores and prevents the glue from penetrating.

A friend of mine was a boat builder and showed me how he did it one time with a large block plane ( bench plane ) and an angle block of wood as the guide.


Thanks for the input guys

Len

[SMYK]

OMG..... YES! Bryan
Thank you very much for posting this FAA reference/regulation link
I wish to have access to it years ego to explain or support what I know about it
I have been educated extensively in wooden technology based on FAR( British) and JAR22 regulation
Poles got long tradition in building wooden structures mastered in 50ties to 70ties in form of stunning flying gliders like Jaskolka, Mucha, Foka, Kobra, Pirat.
Large scale model industry is absolutely not aware of these issues and most of the kits are absolute lemons if it comes structural correctness
Large scale models should be build like the real counterparts to coupe with much larger flight and ground loads. They are not toys anymore. Safety and reputation of the hobby is in stake
I had have planned to do series of articles covering several aspects of wooden technology, but since my web site was gone in 2015 (Verizon closed it due to discontinued support for web hosting) I simply had my wings cut
Now ... you folks inspired me again
I have just put test front page with the new provider. It is not official yet. Please keep me inspired

https://www.littlesoaringfleet.com

stay tuned
more to come
Mike

By the way
Read comments under my pictures

[Asher Carmichael]

For the record, I know there are some cautions in the full scale world about not sanding a scarf joint too, the theory is it closes the wood pores and prevents the glue from penetrating.

A friend of mine was a boat builder and showed me how he did it one time with a large block plane ( bench plane ) and an angle block of wood as the guide.

While I can’t speak from an A&P perspective, I would surmise that the reason you don’t sand the joint is because modern woodworking adhesives work best when the adjoining wood surfaces are as smooth and slick as possible. Dr Bruce Hoadley explains this in his book “Understanding Wood”. The structural adhesion is not one of mechanical advantage but is molecular. The adhesives actually form a molecular bond with the cell surface and the least amount of damage or abrasion to the cell wall the better. Also, the thinner the glue line the better. In other words, a planed or “sliced” surface is better than an abraded one. I think this may only true for standard wood adhesives such as aliphatic resins(titebond” ,PVA’s, phenolics and the like. Perhaps Gorilla type glues and epoxies are not as vulnerable. Think of it this way; if absorption and rough, open cell structure were desirable, then we should be able to glue butt or end grain joints with abandon because of the openess and texture of end grain. But adhesives don’t work as well on end grain as they do on edge or side grain because the cell wall on side grain presents so much more surface for the molecular bond. The purpose of a scarf joint, or most any joint in woodworking, is to present as much of the length of the cell wall as possible. Again, this may not be the the case with some of the newer adhesives but the fact that you present more gluing area with certain joints has to be a good thing.

[SMYK]

Asher

Another GREAT inspiration

All glues work best at sheering and are calculated and tested for it.
Sheer factor (tested) X surface of the joint ( moderated to the desired load) and we got joint with build in expected safety factor ( usually 150% of the calculated load (aviation only))
Continuity of the wood fibers being wetted in the glue joint is critical. Ruff sanding makes open wounds in capillary structure of organic fibers. These wounds create crevices and spiky local loads due to variable stiffness of local (micro) anchoring in the glue joint. Fatigue of such joint is low. Something is going to give, and micro separation will progress under not necessarily large loads. Usually plain wood fibers brake off next to the much stiffer dried glue joint.
wood is tricky
so is all fibers world
mixing glue in with totally different properties (Resin, CA, aliphatic, PVA...) makes whole new complexity in every case

Mike

[LRFitch]

Guys, Len pointed me to the disc sander method a few years ago, and I have used it many times since. I normally use about a 12:1 angle. I sand both halves of the joint at the same time stacked on top of one another. I have tested several of these joints made with differing wood types, and have always found the glued joint to be stronger than the native adjacent wood. Not saying this testing is proof of much of anything, but it makes me feel good about the joint. If the jointed piece is for a longeron, care must be taken to be sure you don't have the scarf joint near where you need to bend the piece, as the joint is much stiffer (due to the glued joint) than the normal material. Using a steeper (maybe 8:1) joint angle helps with these situations.

Larry