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Scarf vs Half lap joint

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Forum topic by MrRon posted 09-13-2011 10:31 PM 4936 views 2 times favorited 24 replies Add to Favorites Watch
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MrRon

2928 posts in 1964 days


09-13-2011 10:31 PM

Which joint is stronger?


24 replies so far

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chrisstef

11334 posts in 1727 days


#1 posted 09-13-2011 10:41 PM

The one with the most glue. ;)

-- "there aren’t many hand tools as awe-inspiring as the #8 jointer. I mean, it just reeks of cast iron heft and hubris" - Smitty

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ShaneA

5411 posts in 1319 days


#2 posted 09-13-2011 11:50 PM

Half lap should be easier, and i agree with cr1 about the end grain, so I would go with the half lap.

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Viktor

448 posts in 2139 days


#3 posted 09-13-2011 11:52 PM

Scarf joint is stronger if you have slope ratio ~ 1:8 – 1:10 or more. You should also have contact surfaces as a single plane (in your picture it is more of a combianation of half-lap and scarf).
Old discussion on the topic here: http://lumberjocks.com/topics/14772 (there were more posts in that thread, but some members left and their replies were removed)
Scarf jount is used to extend boards in boat building and sometimes in bows to connect two limbs.

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NBeener

4806 posts in 1894 days


#4 posted 09-14-2011 12:00 AM

Either way … could you peg it, in the application you’re looking at ?

A little more mechanical strength … wouldn’t hurt.

-- -- Neil

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MrRon

2928 posts in 1964 days


#5 posted 09-14-2011 12:26 AM

The glue area is almost the same in each joint. So far, it appears the half lap is easier to make. The scarf joint would require me to cut it freehand on the bandsaw and make the final fit by hand. The half lap joint would let me cut it in one operation on a table saw with just a little sanding touch-up. The scarf joint looks nicer, but I can’t see any strength advantage of one over the other.

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fussy

980 posts in 1771 days


#6 posted 09-14-2011 03:41 AM

The half-lap is easier, hence less chance for error. Risk of poor fit eliminates any reason for trying a scarf, especially since glue areas are almost equal. Do the half lap and be done. What’x it for?

Steve

-- Steve in KY. 44 years so far with my lovely bride. Think I'll keep her.

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Flyin636

57 posts in 1214 days


#7 posted 09-14-2011 04:50 AM

Rebuilt a Pitts special Bi-plane wing 15 or so years ago here at the shop.It was involved in a wreck….a newb in a trainer ran into this guy’s Pitts while it was parked,busted one of the wings in half.The factory supplied all engineering data(we supplied materials).......it called out for a splice on the main pcs that ran parallel with wing….the pces that all the ribs attatched to.It spec’d a 1:12 scarf.636

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mtenterprises

832 posts in 1413 days


#8 posted 09-14-2011 05:00 AM

When I was building kayaks with my older Boy Scouts I built a 7:1 jig for use on the table saw. Fast, easy and consistant. The same could be built for the band saw. Oh and the the more (#) : 1 ratio the stronger the joint. So if you had a 12:1 ratio thats 12” linear to 1” thickness how long would it take you to cut out a lap joint of the same ratio. With the jig I built it was one quick slice, less than a minute. If I can find it I’ll post a picture.
MIKE

-- See pictures on Flickr - http://www.flickr.com/photos/44216106@N07/ And visit my Facebook page - facebook.com/MTEnterprises

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Grandpa

3193 posts in 1396 days


#9 posted 09-14-2011 05:05 AM

The joint with the best fit is the strongest. The joint with the most edge grain is the strongest. The lap joint has both of those things going for it.

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jerrells

860 posts in 1605 days


#10 posted 09-14-2011 02:30 PM

The scarf joint is (theoratically) stronger becuase of the tangential forces. The fact that the forces are applied at an angle to each other makes it a stronger joint. However, that is good in lab practices but, as others have said, fit and proper glue surfaces come into play, also. Also, depends on use. What and how are the forces applied. Lots to consider here. As NBenner said – PEG IT!

-- Just learning the craft my Lord and Savior Jesus Christ practiced.

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shipwright

5222 posts in 1518 days


#11 posted 09-14-2011 05:15 PM

I read many years ago that the American Plywood Institute (or similar wording) had concluded that an 8:1 scarf was as strong as the rest of the wood, given the glue was as good.
I have used that scarf for many years on boats that took a huge amount of abuse and have never seen one fail. Mine are made like this: http://lumberjocks.com/shipwright/blog/25384
They were the hull planking joints on the Harbour Ferries http://lumberjocks.com/projects/53302
Read the figures in that post about the abuse the boats took.

A lap joint in that service would likely fail before you even got the hull assembled.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fiberglass trees. http://prmdesigns.com/

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reggiek

2240 posts in 1990 days


#12 posted 09-14-2011 06:15 PM

I would also add that each joint has it’s best use application. Depending on where the stress and movement is concentrated would be the deciding factor. As said above, the scarf is better when the forces are not perpendicular….when they are perpendicular that is what the half lap is most useful for. Just a thought.

-- Woodworking.....My small slice of heaven!

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MrRon

2928 posts in 1964 days


#13 posted 09-14-2011 06:16 PM

Thank you all for your input. I am building a 1/8 scale model and I need the splice for a built-up frame. There will be two of these frames and they will have to support about 50 #, but I have cross bracing in place, much like a truss. The upper chord of the truss is the one receiving the scarf/half lap joint. I think I will go with the half lap.

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Viktor

448 posts in 2139 days


#14 posted 09-14-2011 06:49 PM

The answer will depend on application. For bending half-lap joint is weaker and here is why.

In half-lap part of the joint is end grain, which is weak, although not inessential. If the beam is bent the failure will occur either at f1 (if L/h is large) or f2 (if L/h is small). Note that it will not occur along the glue line. Bond along the grain with a regular carpenter’s glue is stronger than surrounding wood. The wood will split somewhere parallel to the glue surface.

Effectively a half lap joint (A) is almost identical to a solid beam (B) undercut by half in two places.

Scarf joint ( C ) will be weak if L/h is small and will fail along the glue line. However at L/h >10 it essentially becomes a long grain bond without the thickness of the beam being compromised. Failure in such joint will occur somewhere in the middle of the beam, but not along the glue line. Fracture (f3) will go through both halves. Scarf jointed beam in this case will not be much different from a solid beam.

This is why scarf joint is preferred method in boat and airplane building as was pointed out previously. In a vertical post subject to compression half-lap might be preferred.

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MrRon

2928 posts in 1964 days


#15 posted 09-14-2011 11:57 PM

Thanks Viktor. The joint shown in my original sketch really wasn’t a scarf joint at all, but a modified half lap. The true scarf joint has no end grain. It’s all parallel grain and the longer L/H is, the better. I learned something valuable today. Even at 76 one can still learn.

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