A simpler scarf for some applications?

[Norman Bernstein]

After thinkng about a posting which talked about scarf joints for mastmaking, it occurred to me that, in situations where the purpose of the scarf was simply to extend the length of a board, and where there would be no bending (as in a mast), the following joint would be a lot simpler to make:

http://www.marisystems.com/ellipticat/simplescarf.jpg

This joint can be very easily made, using a router, offers maximum surface mating with parallel grain, etc. Can anyone tell me why this wouldn't be an easier, and possibly superior, joint for making a mast? (I'm presuming, of course, that the mast was made up of a lamination of several boards, with these joints positionally staggered).

[Bob Smalser]

Look at all the short grain made by the router and think through where it will fail in shear and in tension.

End grain is useless as a gluing surface...and your only long grain glued is short grain that'll break in shear and in tension.

Sorry, but it's not a joint to use for anything done in solid wood.

[ 03-03-2005, 08:22 PM: Message edited by: Bob Smalser ]

[MJC]

Hi, welcome aboard.

First of all, masts bend and flex a lot!

Your idea has a fundamental flaw, visually push those two pieces together and you should see it.

But, the best test would be to make one of your joints, the problem with your idea should then be readily apparent, then bend it until it breaks. Tell us where it broke.

Scarf joints are really strong! Several hundred years of effort haven't found a better way. Scarf joints aren't hard to make at all. Try it, make a scarf joint, then bend it the same amount as in the test of your joint. It didn't break, did it?

[ 03-03-2005, 10:23 AM: Message edited by: MJC ]

[Norman Bernstein]

I've made lots of scarf joints... even built a neat jig to be able to make perfect tapered scarfs using a router. The only failure I've ever had was probably due to excessive clamping pressure, which may have caused too much squeeze out (I was using West epoxy). The only reason I was thinking of alternatives is because 1) tapered scarfs are slower to make, and 2) in an application where the board is just one of several which would be laminated together, the loss of strength would be less of an issue.

One more thing: by 'not bending', what I meant was that the scarfed joint wouldn't need to be bent at installation, as it would be if I were making gunwales. Certainly, masts do bend!

[Bruce Hooke]

I see a few problems:

1. The boards will end up offset by half the thickness of the boards, which is NOT desirable for most purposes. This is, I think, the fundamental and unavoidably fatal problem with this joint.

2. If the overlap were made as long as a "proper" scarf then the functional surface area (side grain to side grain gluing surface) would be the same as in a "proper" scarf so in tension this joint might actually do fairly well. HOWEVER, when you start bending this joint you will have problems because viewed as a bending member the step at the end will open up when that face is in tension (i.e., when it is on the convex side of the bend) providing a ready failure point.

3. I really do not see anything in this joint that is an improvement over a simple lap joint and in fact a simple lap joint might be stronger (but still not as strong as a proper scarf). All those steps up and down are doing nothing useful from a structural perspective.

[Bruce Hooke]

You do raise a good point that in a hollow mast, because the boards are arranged around the edges of the mast, the individual boards do not actually bend that much -- they are mostly in tension or compression. However, on the tension side you do need a scarf that does not have a vertical cut (relative to the board's face) that can open up under tension.

All of that said, for this joint to have any potential you need to figure out a way to get the suurfaces of the boards being joined to line up. As drawn they do not...

[willmarsh3]

I built this scarfing jig shown here that works with my electric hand planer

web page (http://www.willmarsh3.net/el/elver033104.html)

It is shown second picture down on the left hand side. I clamped a 1x4 plank in place and used the sloping guide rails to guide the planer. Each plank took about 5-10 min to make a good quality 12:1 scarf.
A similar one could be built to use with a router. I liked the hand planer better since the blades last longer than the router bit and the router bit tends to drift up or down in the collet sometimes. (cheap router)
Also shown is my tablesaw scarf jig which turns out to only be good for thin strips.
Hope this helps.
Will.

[Keith Wilson]

Norm, even if it worked as well (and I don't think it will), it's quite a lot harder to make than a normal scarf, at least in smaller sizes. You have to set up the router, and a jig, and get the spacing and depth right - very Norm Abrams. Just sharpen your plane and go at it, it'll be a lot easier.

[Bruce Hooke]

In Norm's defense, in a production situation with a bunch of scarfs to make up (which would not be uncommon in making a set of spars for a boat), the overall time to make this joint could be quite a bit faster than for a traditional scarf because the setup would only have to be done once. I think it would be easier to do with a dado blade than with a router, but that partly depends on the setup available. However, all of this is a moot point because I don't think this scarf is suitable for the reasons already discussed.

[Tom Lathrop]

Come on guys,

This is a terrible idea. Harder to make, flawed in finished form and fatally flawed in performance.

The kindest thing would be to tell Norm to learn to make a proper scarf joint.

In teaching plywood boatbuilding to beginners, I find that they are almost always afraid of the assumed difficulty in making scarf joints. That is, until they are convinced to try one. After that, they are converts.

[Bruce Hooke]

Tom,

I think maybe you overlooked the point that Norm knows how to make a traditional scarf and has made them. So, this does not appear to be an issue of being "scared" of scarf-making in the traditional way...

- Bruce

[Norman Bernstein]

Thanks for the defense, Bruce... I have indeed made 'traditional' scarf joints before, so it has nothing whatsoever to do with being 'afraid' of them.

Also, my apologies for the incorrect drawing... properly drawn, the suggested scheme would mesh perfectly. I'm a bit too lazy to redraw, but you get the idea.....

I agree that the joint would be fine, in compression... and probably less acceptable in tension. However, for an interior lamination, I'd bet dollars to donuts that it would be just fine. As for the difficulty of doing it, the trick is to route the slots on both boards simultaneously... all you need is a couple of spacer blocks that are precisely the same width as the router bit... easy to do, against the fence of a router table.

[John of Phoenix]

If it meshes perfectly... it becomes a butt joint or, in this case, a series of butt joints like a cutting board. I think these are your choices: butt, scarf, finger.

http://www.columbia.edu/cu/gsapp/BT/STRUCTI/WEEK3/st3.31_a.jpg

[ 03-03-2005, 02:39 PM: Message edited by: John Teetsel ]

[Tom Lathrop]

Sorry Norm and Bruce,

I just think this is a not a good way to join sheets of plywood.

It is a butt joint, staggered maybe, but still a butt joint. If covered with Fiberglass, it becomes just about the same as a simple glassed butt joint. Since the outer skins, where most of the stress is concentrated, are not scarfed, it can not be very strong in bending.

My story and I'm sticking to it.

[ 03-03-2005, 04:26 PM: Message edited by: Tom Lathrop ]

[Bruce Hooke]

Tom,

Please note that I do not think, and did not say that I thought this scarf was a good idea. I just pointed out that Norm does know how to do a standard scarf so this is not an issue of him being scared off be the perceived difficulty of making a standard scarf.

Edited to add: also, I don't think Norm ever mentioned plywood. I believe he is talking about solid wood.

- Bruce

[ 03-03-2005, 05:28 PM: Message edited by: Bruce Hooke ]

[Bruce Hooke]

Norm,

Is version A in the drawing below what you actually had in mind (show assembled in view B)?

If so, I am not convinced that this is a structural improvement over C, but I am also not convinced that C would be much worse than a standard scarf IF the joint is only loaded in tension. As soon as there is any real bending load then any of these joints becomes VERY problematic. Furthermore, even with just a tension load I am concerned that any of these joints would be more vulnerable to failure than a standard scarf as a result of defects in the wood or simply the grain running in an untoward direction. Also, I think the end-grain cuts at the ends of the joint would be more likely to open up and allow water intrusion and subsequent rot than a standard scarf would be.

http://www.bghooke.com/brucehooke/temp/Norms-Joint.gif

[Tom Lathrop]

Intresting Bruce,

If we assume that the glue line of "C" does not fail and a butt joint has no strength, then the tensile strength will be 50% of the unspliced board. What the tensile strength of "B" is is harder to judge but it looks like it will be less than 50% to me.

In a scarf joint and assuming again that the glue does not fail, the strength will be 100% as strong as the unspliced board.

If the above reasoning is off the mark, it is still hard to credit the stepped joint with anywhere near the strength of a scarf joint in tension or bending.

[Ian McColgin]

Think for a moment how wood splits. If you've lots of cuts across the grain, it is easy to chisel out a trough that runs with the grain. The wood between the cuts about jumps out with each tap.

Which is why the finger joint scarf will not work well. Finger joints work at 90 degrees to this proposed application.

If you do make a mast this way, try not to stand to leeward on a hard beat.

G'luck

[Bruce Hooke]

Tom,

You're right. I was thinking about the strength of the glue joint itself and forgetting to think about the impact of the cuts on the strength of the wood itself. So, the traditional scarf wins out even in tension. The only ceveats worth noting are:

1) This joint would still presumably match the traditional scarf in compression, but so would a straight butt joint!
2) Straight out tension failures are relatively rare because wood is very strong in tension so achieving maximum tensile strength may not be that important.

Still, I think your point is the final nail in this coffin... :D

If someone needs a scarf-type joint that can be made quickly in a production situations a finger joint would seem to hold more potential.

- Bruce

[StevenBauer]

I still like Ken's method:

http://www.imagestation.com/picture/sraid81/p0a86206838a1833cc24385a518ebe544/fafd4aba.jpg

fingerjointed at a 2 to 1 scarf

Steven

[Tom Lathrop]

The finger joint appears to be kin to a scarf that takes up much less length from the board. That it can be cut by machine in a production line makes it very popular with molding and trim manufacturers.

However the finger joint still has a significant amount of butt in it and can not be as strong as a full scarf.

I will be happy to put this one to rest

Now, in big timber construction, such notched joinery is more common since it handles wood movement better and glued joints are rare.

[ssor]

Finger joints in moulding are no more prone to failure than any other part of the stick. I am always impressed with just how well they do. The long scarfed fj combination looks to me to be the best of both worlds. A standard scarf is it, seems to me to be very demanding of accuracy and clamping, while the fj can be glued and driven together with alot less concern for the varibles of clamping.

[Harry Miller]

I think Norm's lying about his last name. :D

[Bruce Hooke]

I am somewhat concerned about finger joints in boatbuilding because, as Tom noted, there is some end grain left in the joint so in high load situations I am not convinced that it would be as strong as a proper scarf. There is a big difference between the loads placed on the lumber used in your average house (which is where I have usually seen finger joints) and the load placed on many parts of a boat. This is why we are a LOT more picky about the wood going into a boat than we are about the wood going into a house.

That said, it does seem like a finger joint might be a reasonably solution to joining pieces of wood that will go into a birdsmouth spar. Without some solid data to rely on, however, I probably would not use it in a spar that would be big enough to really hurt someone or create a dangerous situation if it broke in use.