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Make a board longer - scarf joint?

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Forum topic by greatview posted 02-19-2010 07:53 PM 8119 views 0 times favorited 28 replies Add to Favorites Watch
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greatview

69 posts in 1877 days


02-19-2010 07:53 PM

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.)

-- Tom, New London, NH


28 replies so far

View farmerjohn's profile

farmerjohn

5 posts in 1738 days


#1 posted 02-19-2010 08:19 PM

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.”

-- These are not mistakes...they are hand-crafted kindling!

View Cory's profile

Cory

724 posts in 2139 days


#2 posted 02-19-2010 08:36 PM

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!

-- The secret to getting ahead is getting started.

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richgreer

4524 posts in 1794 days


#3 posted 02-19-2010 09:13 PM

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.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View scrabby's profile

scrabby

42 posts in 1906 days


#4 posted 02-19-2010 09:27 PM

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.

-- Jim, North Vancouver, BC

View JJohnston's profile

JJohnston

1587 posts in 2011 days


#5 posted 02-19-2010 09:31 PM

No, all those boat builders don’t have boards subject to simple bending.

<——Also a civil engineer

-- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger

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greatview

69 posts in 1877 days


#6 posted 02-19-2010 09:37 PM

I’m going to give a finger joint a try. Grizzly has a reasonably priced bit (C1682 $69.95) and I think I’ll find a variety of other uses for such a bit.

-- Tom, New London, NH

View scrabby's profile

scrabby

42 posts in 1906 days


#7 posted 02-19-2010 09:40 PM

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.

-- Jim, North Vancouver, BC

View Viktor's profile

Viktor

448 posts in 2138 days


#8 posted 02-19-2010 09:58 PM

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.

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Viktor

448 posts in 2138 days


#9 posted 02-19-2010 10:18 PM

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.

View richgreer's profile

richgreer

4524 posts in 1794 days


#10 posted 02-19-2010 10:31 PM

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.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View scrabby's profile

scrabby

42 posts in 1906 days


#11 posted 02-19-2010 10:39 PM

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.

-- Jim, North Vancouver, BC

View farmerjohn's profile

farmerjohn

5 posts in 1738 days


#12 posted 02-19-2010 10:40 PM

Rich, you need this http://kalecoauto.com/index.php?main_page=product_info&cPath=2&products_id=45

-- These are not mistakes...they are hand-crafted kindling!

View Viktor's profile

Viktor

448 posts in 2138 days


#13 posted 02-19-2010 11:04 PM

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?

View scrabby's profile

scrabby

42 posts in 1906 days


#14 posted 02-19-2010 11:42 PM

Lesson on scarf joints learned here! Thanks guys.

Jim, aka Scrabby, aka Scrubby

-- Jim, North Vancouver, BC

View JJohnston's profile

JJohnston

1587 posts in 2011 days


#15 posted 02-20-2010 12:04 AM

Got pictures of that scarf joint jig?

-- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger

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