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Making Strong Wood Splices

Making Strong Wood Splices

I was sitting under a shade tent at a giant-scale fun fly recently when someone asked me where I got the longer-than-standard-size materials I use to build my big airplanes. I didn’t know what the fellow meant. He told me that his local hobby shop carried only 36-inch lengths of balsa and spruce, and that 48-inch-long sticks were available only by special order. I said that I regularly splice my stringers and spars and think nothing of it. “But how do you do it safely?” he asked.

These tips show some common construction techniques and how wood splicing should be done.

 Strong Splices

When you want to turn two pieces of wood into one, you can’t simply glue them together. Some cutting and fitting is necessary so that the splice will bear as much stress as the original wood pieces. To accomplish this, you have to increase the gluing area for the splice by cutting the two mating surfaces in a diagonal line.

The weakest, least supported joint you can make is a simple butt joint. Gluing two pieces of wood together end to end provides very little surface area for the glue. Overlapping the two pieces is a much stronger way to join them, but if you want to use the part as a stringer or a spar, the overlap isn’t practical because the pieces are not attached in a straight line. A diagonal splice keeps both pieces in alignment. As a general rule, I make the length of a splice at least six times the thickness of the material being glued together—roughly 3 inches across for a 1/2-inch-square spar.

To make the two pieces match precisely, I first tack-glue the two parts on top of each other with a few drops of thick CA while making sure the edges of each stick are flush. I then draw a straight diagonal line at the ends to be joined (see photos). I use my band saw to make the cut, and I make certain it is square to the top edge. I then sand the cut surfaces (still glued together) smooth with a belt sander. When the two parts are separated, the two angled surfaces match perfectly.

To keep the two pieces aligned when I glue them together, I use a simple, wooden alignment jig. The jig is made with a flat base and two guide pieces (rails) glued on top that form a space between them that’s the same as the thickness of the pieces being glued together. For the jig shown here, I used pieces cut from an old yardstick to act as rails. I also use a piece of Great Planes’ Plans Protector material to prevent the parts from being glued to the jig when the adhesive oozes out of the joint.

To form the glue joint, I place one piece in the jig and spray it with a light mist of kicker. I then apply CA to the second piece and slide it into the jig and up against the first piece. After the glue has set, I use a sanding block to sand the face of the joint smooth. I then turn the part over and sand the opposite face smooth as well. That’s it. No magic—just a very strong joint.

1 Start by   tack-gluing the two pieces to be spliced together with CA and kicker. Use   just a couple of drops of glue.
2 On the ends of   the pieces, draw a diagonal line that’s about six times as long as the pieces   are thick.
3 Cut the   diagonal splice with a band saw, and make sure the cut is square to the top   edge.
4 Sand the cut   edges smooth with a belt sander.
5 Here, the two longerons are ready to be glued together.
6 I use this   simple jig to keep the pieces in alignment while the glue dries.
7 The finished   splice.
8 For additional strength, I   positioned the splice against the plywood side sheeting.
9 Here you see the splice   positioned close to a cluster junction where other fuselage members join.
10 By increasing the gluing area   that holds the joints together, these 1/32-inch gusset plates add strength to   an already strong fuselage structure.
11 This is a close-up of an   internal gusset plate used to strengthen a lower fuselage longeron. These are used when you want a flush outer model surface.splice11


Here’s a sheeting joint using similar thinking, from my Balsa USA Fokker Dr.1 Triplane. All according to the instructions.

IMG_0010 (5)


Updated: December 9, 2015 — 2:56 PM
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Add a Comment
  1. Thank You For the building tips. Love the joints look plenty strong will us the in my upcomming build. Stick build get away from the ply builds.

  2. Great piece Gerry! I habe used s[plicing b-4 but the 6 times rule was a new one. Thanks!!

  3. Standard aircraft practice is a 1:15 ratio per FAA Advisory Circular 43.13-1B. Here is the link to the appropriate section (the pertinent part starts at page 1-16): http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2001.pdf

    You can obviously use a shorter slope as you describe, but since end grain to end grain joints are not as strong as face grain joints, I would use a minimum of 10:1 in critical locations. Gusset plates definitely help too. One thing you need to guard against is creating a splice that is much stiffer than the adjacent areas, as you will end up with a stress concentration and that is where a failure is likely to occur.

    1. thanks for the comment Dennis. Seems a bit overkill for RC models to use a 10:1 slope. That almost 4 inches for a 3/8 inch square longeron or spar. I have be using my ratio for over 20 years and never have ever had a joint fail.

      1. The FAA circular addresses full size aircraft materials, mainly sitka spruce. Spruce is much stronger and more stiff than balsa and would take much more force to break than balsa thus
        imparting more force on the joint. Therefore the joint must be at least as strong as the wood itself.

        The balsa joint should also be as strong as the wood itself but would probably not require the same
        surface area as spruce to develop the required strength. Hope this makes sense.

  4. When making your initial cut, you will find that, if you put the two pieces one above the other, aligned in the same manner in which they will be glued, they will fit together perfectly, regardless of any fluctuations in the cut. In fact, a “wavy” cut can be made to increase the gluing surface area and they will fit perfectly to one another. Use the thinnest and narrowest scroll blade you can find for the best results.

    1. Excellent point Roland. thanks for the comment

      1. aaha so thats why my bandsaw insists on cutting wavy lines when I try to make straight cuts Hmm?

        1. Flyoz. Hey I wrote this article a while back, but it is assumed that you keep your bandsaw properly adjusted. I just went over several tools in my shop and I had to spend a little time replacing the blade, guide blocks and replacing and oiling the thrust bearings, and generally adjusting the bandsaw’s tracking. Good tools require good maintenance. it all shows up in the quality of the cut.

  5. Been cutting and gluing sticks since 1959. Always done what the plans showed. Seen how you made these spices. Don’t have the band saw but did use a hacksaw blade. Sanded the balsa and spruce 1/4sq stringers with a sanding block. Glued and clamped, sanded the splice smooth. Looked better than the splice joint on the plans.

  6. If you saw the two pieces while glue tacked they should match. Why would yo sand them? wouldn’t this introduce mismatched surfaces?

    1. Not the way I show. basically you are just cleaning up the cut surfaces, not taking any measurable amount of material away. Yes, if you do sand it rounded or off angle it would affect joint quality. You can also just use a sanding block

  7. With lighter smaller models having 1/16 or 1/8 strips that would need to be spliced, as I have done for years, I simply lay one piece over the other, and slice the spliced faces with a razor blade – definitely not an Xacto. This yields perfect faces for next step, gluing.

  8. I always admire your work and over the years your articles have made me a better builder.

    1. Thanks Billy. I appreciate the kind words. I enjoy both building and sharing techniques that I have found useful.

  9. I just stack the sticks without glue tack, and hold them tight. Then I use my disk sander with coarse paper to grind a nice long slope. Goes quick, fits tight. Just need to make sure the sander table is squared up. In aircraft school, we learned a fresh saw cut glues better than a sanded surface for wood, thus I use coarse paper, and in some cases, like spars, i’ll use my xacto and strike across the joint face to rough it up a little more and expose grain better or give some mechanical grip.

    1. I agree Daniel, that’s why with load bearing structures I prefer Epoxy (slow setting) adhesive over fast curing CA glue. For better grain penetration.

  10. Very good intructions!

  11. From section 1-6 of the FAA advisory circular: “Sandpaper must never be
    used to smooth softwood surfaces that are to
    be bonded. ”

    My aircraft mechanic friend and EAA builder reminded me of this years ago. Sanding dust fills the pores and reduces the strength of the bond.

    1. Lyle, you’ll be glad to know that all my sanded glue joints on my 1/3-scale pietenpol’s longerons and spars (Spruce), have never separated. And the model was built 13 years ago. :^)

  12. My Goodness! I can’t believe the knowledge that has been shared in the article and the comments. Wow, thanks for sharing.

  13. A technique I like to is to make a laminate which is used in place of a stick. For instance, a 1/4 sq stick could be made up of (2) 1/8 x 1/4 sticks laminated together. In the case of having to splice strips for longer part, this technique enables you to offset the splices, in essence leaving you with a tongue and groove joint in the finished part. The spices here would be positioned on an unspliced section of the corresponding laminate. Last, I recommend Titebond III for gluing wood since it is seriously strong, is not as brittle as CA and gives you more working time.

    1. Thanks for the comments Rob. I agree offset splices are an excellent technique as well. I also agree with you about the glue, Titebond is great stuff and available everywhere.

  14. Another thing that works quite well is a spline. Do what you did here, but also cut a groove down the center. When you glue the two halves together, insert a thin piece of plywood into the groove. Gives a similar joint as you get with the gusset. Useful in those situations where the splice tends to hang out in the open with no other structural support nearby. My bandsaw leaves a 1/32″ cut, perfect for 1/32″ ply.

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