LumberJocks Woodworking Forum banner
  • Please post in our Community Feedback thread for help with the new forum software! If you are having trouble logging in, please Contact Us for assistance.
1 - 20 of 31 Posts

·
Registered
Joined
·
190 Posts
Without going into too much detail, I need to lengthen some boards by about 8 inches or so (currently about 6 feet long). Due to their use, I must add in the middle rather than either end. The boards are about 8 inches wide by about 1 inch thick and the material is probably mahogany. The appearance is not too important as they are not usually visible. I'm wondering what the best joint might be.

(If you really want the details, I'm converting a full size bed into a queen size and the rails and ends are covered by a skirt. I've made a new headboard and now need to lengthen the members that I'd like to keep. I don't really want to spend the money on new stock.)
 

·
Registered
Joined
·
6 Posts
Probably some type of bisquit or dowel in combination with dovetail or fingerjoint might be strongest. Trim is finger-jointed all the time, but not subject to same stresses. Best to withstand the weight in my opinion (as limited as it surely is) would be matching L-shaped overlaps, doweled, with a threaded rod vertically through the joint Have you considered metal frame rails instead? Might be free with the new mattress set. Jujst kidding! Another alternative would be to start with new stock for the complete side rails and recreate the pockets for the attaching hardware to be transferred.

Reminds me of a college friend who was student teaching shop and the student who came up to him after being instructed to re-cut a board that came up short for a project and said "I've cut this board twice more and it's still too short."
 

·
Registered
Joined
·
807 Posts
Scarf joints have little, if any, structural support. They're great when you've got a long run of trim, but I wouldn't trust that type of joint for a bed rail. I agree with socalwood, a finger joint would be far superior. You'll need as much glue area as possible.

Good luck!
 

·
Registered
Joined
·
4,787 Posts
Many opinions here - I would drill holes, using a doweling jig, into the ends of the board. I would probably go about 3 inches deep in each end. Then insert oak dowels and glue. I would use the biggest dowel that will fit and I would probably put 4 in. I think this would give you more structural support and any other method suggested so far. Warning - Dowels are often not exactly the size they say they are - especially the larger dowels. Check this out before you have glue all over the place.
 

·
Registered
Joined
·
42 Posts
I would agree, as a civil-structural engineer, with cory and socal that fingerjoints are better than the proposed scarf joint.

There are tensile stresses incurred by the bottom third of the bed rail during typical loading (i.e. bending). I would recommend dowels, with at least six 3/8" dowels, at least 3" long, and four of them should be in the bottom half of the joint for the same reason that a concrete beam has more longitudinal reinforcing steel on the bottom where the tension occurs.
 

·
Registered
Joined
·
42 Posts
Exactly…thanks JJ - extending a board for a boat or trim is a different loading condition. There are clearly different methods to solve this challenge, I was just offering an opinion based on basic principles.
 

·
Registered
Joined
·
519 Posts
I actually used scarf joint to extend a bed rail. It works just fine. The joint must be shallow as Dave suggested (at least 1:6 or more).
There are tensile force and shear force normal and parallel to the scarf joint governed by two equations. Depending on the material and adhesive you could determine an angle that provides sufficient strength. At a shallow enough angle, strength of the joint continues to increase and (in wood) failure will occur outside of the joint.
Finger joint (with triangular fingers) and scarf joint are IDENTICAL in strength if the glue area is equal (think of finger joint as a folded up scarf joint). Scarf joint though is easier to make, no special tools needed. Needless to say the faces have to be precisely flat. The down side is more waste (boards must overlap more).
Dowel joint would be greatly inferior to either of the above as effective cross section of the beam is diminished to the cross sections of the dowels.
Hope this helps.
 

·
Registered
Joined
·
519 Posts
Scrubby,
Wooden beam should NOT be compared with reinforced concrete beam. Concrete works on compression and steel on tension. Wooden dowels in a wooden beam create homogeneous(!) beam. Steel dowels in wooden beams is a better comparison. The beauty of wood is that it glues really well creating joint that is stronger than the surrounding material.
Just a note: Old English longbows sometimes were made of two pieces joint in the middle (at the handle) with a simple W-shaped finger joint and hide glue made of fish bones.
 

·
Registered
Joined
·
4,787 Posts
True story - - I was once building a deck. My wife told me she was going on to town and asked if I needed anything. I told her that some of my boards were coming up a little short and I asked if she could stop into the hardware to pick up a board stretcher. I told her that if she just asked a sales person, they would know what she met. She did it. There was a distinct chill in our household that evening.

True story 2 - - On another day, my wife was taking her car to the dealer for routine service. I told her to make sure they changed the air in the tires. She actually asked the service manager to make sure they changed the air in the tires. I'm a terrible person.

Yes - we are still married - 38 years.
 

·
Registered
Joined
·
42 Posts
I'm with you Viktor, and appreciate the input and positive discussion. I'm an engineer by training and career, and a budding woodworker at home with a great interest in learning and improving (which is why I read this site at work, I suppose).

Further, I agree with you that the scarf joint would be an elegant and strong joint, but it seems to me it would also take the highest (vs. fingerjointed or dowelled butt joint) level of skill to execute, given the need for jointed, well-fitted faces in order to create a thin, strong glue interface.
 

·
Registered
Joined
·
519 Posts
I agree, Scrubby. It is not easy, at least with hand tools, which is what I did. I clamped two boards together with an overlap and cut them together at shallow angle with a hand saw. I thought that the angles on two pieces would match. LOL!... And then there was sanding, and sanding, and more sanding. It is hard to maintain an even edge on a board cut at very shallow angle. At the end the joint was very much visible, but the bed survived kids jumping on it. Now, if you want to go with dowels, I would use steel dowels (more of them at the bottom half as you suggested). The problem is how you glue steel into wood. Epoxy?
 

·
Registered
Joined
·
471 Posts
I always thought a scarf joint was strongest if the joint planes were perpendicular to the stress load (same with a finger joint, which is fine for a compression load, but usually not suitable for a lateral load.) The way most so far have described the scarf joint is with the load parallel to the joint faces. That said, splice plates in joists are usually 8 times the width of the board in length, so I guess a scarf joint over 1:8 ratio would be the same. I don't think a finger joint would hold up without some dowel support.

If a scarf joint is used, it would be better to use a "non-creeping" glue, such as a plastic resin.

JM2CW

Go
 

·
Registered
Joined
·
109 Posts
If you can spare the WIDTH, cut the 1×8 from South west corner to North east corner ( diag the whole length)

Now slide the two triangles past each other till you have the length you want and reglue it back together…add biscuts if you wish.

Your glue joint is now long grain to long grain and after some trimming the board is as long as you want it ( within reason).
 

·
Registered
Joined
·
11,472 Posts
"Why don't more people do it?" Probably because we aren't all Engineers. We're "Woodworkers" and "Destructive Testing", whatever that is, is not our thing. We BUILD things. We don't Destroy things.
 

·
Registered
Joined
·
4,787 Posts
This conversation has been quite educational and beneficial for me. I've known what a scarf joint is and I always wondered how people cut them accurately. I never thought of the router approach mentioned above.

I wonder - would a large lap joint be just as strong as a scarf joint and probably easier to cut? I would cut the lap joint on my table saw with a dado.
 
1 - 20 of 31 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top