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View Full Version : How do you cut scarf joints in planks?



joliette
10-30-2011, 07:47 AM
I recently found a great way to cut scarf joints to make frame repairs using a 20 grit disk on a grinder - very quick and accurate on a 1 1/2" x 1 1/4" oak frame which would've been difficult to access with hand saws, chisels and planes ... although there was a lot of dust!

I'd now like to find an equally quick and accurate way to cut scarf joints in planking. I'm making repairs so the 8:1 scarf joints need to be cut in situ between adjacent planks. I've tried using hand planes but the scarfs do take a long time to cut in the 7/8" planks and then there's the problem of the adjacent plank edge which touches the body of the plane and prevents it from shaving the edge of the plank.

So, how do you do it? Anybody have any quick and easy methods to share?

TerryLL
10-30-2011, 08:10 AM
... I've tried using hand planes but the scarfs do take a long time to cut in the 7/8" planks and then there's the problem of the adjacent plank edge which touches the body of the plane and prevents it from shaving the edge of the plank...



A rabbet plane will cut out those corners at the edge of the plank.

http://www.tooltrolley.co.uk/images/Toolbank/STA112092.JPG

JoshuaIII
10-30-2011, 08:12 AM
I am not a big fan of scarfing planks, it's not really a good long term solution. The feather edge always end up sticking out of the paint and if the glue fail it's hard to caulk and fix...

Why not just use a traditional butt block?!

Ian McColgin
10-30-2011, 09:03 AM
I, on the otherhand, love scarfed in repairs. The essential tools are a japanese style pull saw with rip blade and a good slick - big chisel. Actually, I like at least two slicks - one meant to be tapped on with the mallet and the other, finer blade, with a really big handle for getting both hands on it for finishing the bevel. Close enough to essential that I should have said three tools - a nice low angle plane for finishing the bevel on the part to be inserted.

First I whack out all the rot. One year I had a friend who'd been through a nasty breakup. Gave her a chisel and told her to think of gouging out ----'s eyes. Worked a charm.

Second I regularize the shape of the hole, whether all the way through the plank or part way. I make some sort of rectangle with the sides that will be normal to the surface with the grain and the ends to be beveled normal to the grain. This shape represents the inside dimension of the new piece.

Third, after marking where the outer part of the bevels will end, I use the pull saw to kerf the slope, cutting down so the deep end just clears the inside surface and the shallow end just misses the end line. I like to have kerfs about 1/2" apart.

Once kerfed, I take the shorthandled slick and mallet and whack off the wood down to the bottom of the kerfs. Then I take the long handled slick to finish and flatten the bevel. I don't really worry if it's exactly 8:1 or if the two ends are exactly the same slope. I just measure with my bevel gauge and transfer that to the stock for the new bit. Again with the kerfing and short slick but for this bit it's often easier to clean up with a plane. When it's about right I dry fit. Sometimes I put a bit of chalk on the bevels on the hull and sometimes I use carbon paper but whatever, have something in there for the dry fit so that when you whack the new bit it transfers the marks of any high spots. Just keep planing a little at a time till you have good even contact and glue it in.

I make these to be a bit thicker than the planking, standing proud, and plane them down after the epoxy is cured. It helps to put masking tape on the hull at the end of the bevels so that once you've planed down a good ways you can peel the excess glue off. The glue line is harder than the wood so if you do this process with a sander, you'll end up with a high spot on the glue line. Shape with edge tools and you'll get it fair.

This process takes me less time than most pros can make a jig to do the same job with a router, but if you have more than three or four to do and if you're good with routers and jig making (which I am not) then it pays to go that way.

G'luck

wizbang 13
10-30-2011, 09:16 AM
Combination of sawzall with a long bayonet blade, power planer, and that disc sander.
Sometimes the disc and backing pad go on "upside down" . dangerous , but effective.
Same story with the sawzall blade.
The problem though, is accuracy. Or, the idea that you need accuracy. If you know how to use epoxy, you do not need the accuracy. I would venture that most epoxy failures are due to ( other than being mixed wrong) accuracy of joints. I am a freaking lousy carpenter, yet I have amazing success with epoxy. Many folks with greater joiner skills than I have a history of "glue failures".

mnegley
10-30-2011, 12:44 PM
If the plank is flat at the point where you are making the scarf I would use a router on a beveled guide fixture. Gougeon Brothers (West Systems) describes this in their book, THE GOUGEON BROTHERS ON BOAT CONSTRUCTION on pages 73-74 and even shows the jig they use. You would have to use a pattern bit to stay beween the planks on each side. Once the jig is made it is a quick solution and you will get perfectly matching scarfs.

Dan McCosh
10-30-2011, 04:27 PM
I think I may use the system described above. A wedge on each side, a long router cutter, and the router attached to a short extension that straddles the plank in situ.

Ian McColgin
10-30-2011, 04:49 PM
Paul G has a good point. I am always amazed at how well people who use the tools a lot can make machine tools behave. I've a friend in the cabinet trade who's father was a boat builder who can make a flat bevel with either a disc or belt sander quick as can be. In face, with those machines he's at least three minutes faster than I am with my saw and slicks. So if you're a product of the long apprenticeship and in the trade and good with the tool and concerned about production speed, by all means use the machine tools. But if you resist the "This Old House" and all the other hour length long ads for machine tools and want to just get the job done, it's really hard to beat hand tools. Frankly it takes more skill and wasted wood on the learning curve to use power tools.

Carvel
10-30-2011, 06:15 PM
A method one of my former classmates at IYRS uses is to cut off the bad end right at the frame and make saw kerfs to within 1/32 along the scarf line - then use a grinder with a low grit (25)flapper wheel to take down material using the saw kerfs as a depth guide- then plane the rest with a low angle jack and/or large shoulder rabbet plane (or better yet a 10 1/2 carriage rabbet). It helps to secure some protective material on the planks on either side but the flapper wheel does give you some precision control along the long edges and you can get a precise feather edge with the frame backing the plank.

As far as the appropriateness of scarfs goes the preferred method these days is to do hide-a-butts (two scarfs into a third butt scarffed on both sides). If you follow the manufacturers guidelines, thickened epoxy will be more resilient than a butt... in most cases.

Dan McCosh
10-30-2011, 06:38 PM
The tricky part is not cutting the scarf, but clamping it. I like to land it on a frame, so the fastenings draw it together and reinforce it.

Ian McColgin
10-30-2011, 07:06 PM
I never figured out how to use a grinder to bevel in situ on a hull the space for a new piece. Even when the plank is wider than the diameter of the grinding disc, you still have to make the inside landing flat without damaging frames or other stuff inside the hull and it's hard for me to see how that's done when a radius of the grinding disc must of necessity go past the end of the bevel.

Ian McColgin
10-30-2011, 07:16 PM
Paul, I see how you use a disc grinder to make bevels on a plank on a bench. How exactly do you do it on the ends where it's to fit, on the hull, with frames and perhaps a ceiling inside that you don't want to damage?

A router on a jig, sure. A grinder -- I don't see it.

joliette
10-31-2011, 05:03 AM
[QUOTE=Carvel;3180681]A method one of my former classmates at IYRS uses is to cut off the bad end right at the frame and make saw kerfs to within 1/32 along the scarf line - then use a grinder with a low grit (25)flapper wheel to take down material using the saw kerfs as a depth guide- then plane the rest with a low angle jack and/or large shoulder rabbet plane (or better yet a 10 1/2 carriage rabbet). It helps to secure some protective material on the planks on either side but the flapper wheel does give you some precision control along the long edges and you can get a precise feather edge with the frame backing the plank. QUOTE]

Thank you to everyone for those amazing ideas! I'd thought about trying to make up some kind of router jig but figured that I'd get into trouble trying to use the router against the vertical side of the hull. I'd thought about starting from the edge of the frame but hadn't thought about putting in some saw kerfs to use as a guide. I think I will try Carvel's idea - saw kerfs, grinder, and then finish with block plane and chisel. I'll be using thickened epoxy so there'll be some room for error!

I'm intrigued about the hide-a-butts as I'm having a problem picturing what you mean. I've put diamond graving pieces in over the top of butt joins before, fastening through the graving piece into the butt block. Not quite as good as a scarf but much stronger than a standard butt. Does the hide-a-butt involve a scarfed graving piece over the join and is it fastened through into a butt block?

Ian McColgin
10-31-2011, 09:31 AM
The hide-a-butt is for places where in the repair you're doing a long plank area with two or more planks and it's inconvenient to either scarf into one long piece or to use a regular butt block on the inside because of stuff in the way, lack of access, or whatever. In the hide-a-butt (there is a mention in a WB article on Gannon&Benjamine who first advocated it) the butts of the two planks are the one vertical seam you see from the outside. In other words, the bevels on the planks face in. In normal scarfing in a short bit of replacement plank, the bevel on the plank that's already on the hull faces out.

I've yet to use the hide-a-butt since even when I had a long bit to replace, it was easier for me just to scarf the pieces. I suspect that the hide-a-butt is more useful on larger boats than I happen to deal with.

Carvel
10-31-2011, 09:43 AM
Basically yes except that the block is part of the plank. The hideabutt is basically a double ended scarf, the two planks are scarfed with the feather edges abutting each other on the inner face and a third graving peice tapered out from the center in both directions at the same slope and then glued and screwed into place. For a picture see WB 185:53 where Andy Giblin gives a description of its use.

Todd D
11-01-2011, 07:56 AM
When I reframed Tortuga I had 3 spots where plank ends landed on frames. The planks ends were like swiss cheese from all the fasteners so I decided to cut them out and scarfe in a new bit of wood to join the two planks. I used the scarfe method on the first joint. I used a combination of electric plane, chisels and course sand paper to cut the scarfs the first time. I set the locations of the scarfs up so that they were centered on frames. Because I didn't really trust the scarfs I also put in butt blocks. On the other two spots I didn't use the scarfs. I just put in a piece of new plank with butt blocks.

joliette
11-03-2011, 01:39 PM
Well, I've just made a couple of scarf joints! I did the saw kerfs and then used a grinder - using the bottom of the saw kerfs as a guide - before finishing flat with a block plane. I can see the advantage of having a rabbet plane to get in close to the edge. I will put one of those on my shopping list. The grinder did catch the edges of the adjacent planks but I'm not too concerned about that as I'm planning to spline the topsides.

wizbang 13
11-03-2011, 01:43 PM
Consider the "tooth" that is needed for epoxy.
A hand plane, gives the opposite of tooth.

Ian McColgin
11-03-2011, 01:56 PM
You really don't need 'tooth' with endgrain. Don't need it much anywhere else either. Edge gluing I've done seems about equally strong whether the surface was roughed a little with sandpaper or was planed smooth. Edge gluing I've done where the 'tooth' was provided by the saw has been a less exact fit and required more thickener in the glue, making a fatter glue line. It seems strong so long as the wood is not subject to bending, but makes too brittle a glue line - at least with structural fillers I've used - to be good in structures subject to flexing like spars, oars and such.

Carvel
11-03-2011, 02:32 PM
I'll second the lack of necessity for tooth- usually I just lightly scuff the surface with low grit paper. If the joint has to flex (for long battens) I've been using a 50/50 mix of west 406 and 407 in hard wood and 30/70 in soft.
I forgot to mention that I use a power planer for the hogging phase- its easier to use especially if you're trying to curtail collateral damage to adjoing planks- the added benefit is that the blade runs to a rabbet edge. Though a pp isn't for the faint of heart...

wizbang 13
11-03-2011, 02:37 PM
Carvel, If you are using 407 in a structural joint, your boat is dangerous.
Ian, I respect you and your experience too much to hit back.
We disagree on "gap" .

JimConlin
11-03-2011, 02:59 PM
Carvel, If you are using 407 in a structural joint, your boat is dangerous.
Ian, I respect you and your experience too much to hit back.
We disagree on "gap" .

Maybe not.
The adhesive only needs to be as strong as the wood being joined.
Many softwoods used for planking have tensile strength parallel to the grain of well under 10,000 psi (70MPa).
I wouldn't be surprised if a cabosil & microballoon bog was about the same.

Carvel
11-03-2011, 11:31 PM
If you read the post I was referring to items that need lots of flex like my heavy battens (sheer/profile lines and long waterlines) and this is a mix as Jim states. (BTW: the softwood percentage should be reversed): with 50% or more 406 present there's no chance of failure, assuming the joints are fair and aren't starved. As for my scarffed planking they're steam bent first and then scarfed in place with just 406 since I'm not demanding any flex at that point in the process. The simple fact is that for a piece of wood to behave like wood after glue up it must be allowed to have some flex, 406 gives you some, enough for spars and the like, but for things that need more give on a more freqent basis a little 407 gives you some internal "lubrication" if you will, moreover West specifically endorese 407/406 mixes for structural filleting and the only thing they specifically don't recommended for 407 is hardware bonding. So maybe you'd like to back off the slander....

boattruck
11-04-2011, 12:22 AM
J. We use a router loaded with a long straight cutter, an over sized rectangular base attached to the router, and a jig that is essentially two matching wedges that match your desired ratio,in our case we use 12/1, these wedges fastened to a 1/4 piece of ply to create a rectangular jig,( we have a half dozen of different sizes to accommodate unique situations but they are mostly maybe 12" wide and maybe 18" or 20" long ) that we then cut a 'window ' out of the middle, this contraption is screwed to the surrounding planks with a few zip screws, depending on the curve of the boat where you are working, you may need to use a few small wedges to get the whole shooting match to stay stable and oriented. then it is a simple job of snarling away till you have a perfect scarf, work around the old fastening if you value that expensive cutter., and also leave a tiny sliver of the plank at the top and bottom seam, so you don't mangle your caulk seam, this last bit goes away with one or two swipes of your sharp chisel. If you have more than one to do you will beat the chisel , grinder, rabbit plane guy in both speed and quality... Cheers, Steve/BT

dennisfdunn
02-07-2012, 12:20 PM
I wonder if someone might comment on the problem of glue squeeze out inadvertently bonding the repair to the adjacent planks? Thanks.

-Denny

JimConlin
02-07-2012, 01:30 PM
I wonder if someone might comment on the problem of glue squeeze out inadvertently bonding the repair to the adjacent planks? Thanks.

-Denny
Slip a piece of poly film where you don't want bonding. 3 or 4 mil poly is strong enough to pull out after cure and thin enough to be a negligible gap.

Carvel
02-07-2012, 01:38 PM
Denny, the best prophylactic is wide clear scotch packing tape and don't be shy about using it. Clean out the seams and slide the tape in between- remember to tape on the inside and below. After the stuff kicks you just pull the tape. I've also used wedges of pine coated in wax or tape to lift the planking up and away from the hull surface. Hope this helps.

dennisfdunn
02-07-2012, 03:46 PM
Thanks for the tips; however, I don't quite see how packing tape would work where you're scarfing in a repair in situ. If I tape the plank edges, how do I later get the tape out?
My situation is complicated somewhat in that the boat is double planked. In addition to protecting the adjacent plank edges, I probably don't want to bond the inner plank to the outer - rather some form of sealer should be used. Right?

JimConlin
02-07-2012, 05:28 PM
Versus packing tape, polyethylene film also doesn't bond, is lower in friction is tougher and stretchier, so it can be maybe pulled out of a simple epoxy joint without disassembly if it isn't clamped. I can visualize the geometry of the joint. If the film is to be beside and under a plank scarf, you're gonna have to take the plank up to get a release film out.

I avoid waxing things that will be near wood parts lest the parts get contaminated so paint or glue won't stick.
For composites work, Partall #2 paste wax is a godsend.

RFNK
02-07-2012, 08:25 PM
Having read most of this, I think I'd go with Stephane's suggestion and use butt joints and butt blocks! In fact, as I'm looking at replacing a heap of planking on my Twister, that's what I'll be doing.
Rick

Carvel
02-07-2012, 11:59 PM
Well I was imagining a single planking but unfastened at the scarf (for clamp access) as a repair rather than a hide-a-butt or scarf in place- if the seams are flat and clean I've found tape stiffer than film to shove through the seam and more resistent to movement. Jim is correct that it requires a bit more finesse with a blade to remove. Personally for double planking I'd be doing them on a bench and taking extra time on my spiling.

dennisfdunn
02-08-2012, 10:36 AM
Thanks again for all the tips. I agree that removing the plank would be best, but this would be difficult in my case. The inner planking is fastened to the outer planking from the inside and some of these fasteners are hidden by the bilge stringers. I can't remove the bilge stringers without removing bulkheads, which means removing the deck, etc. So I think I'm forced to make any repairs in place. To see the kind of repairs that I need to make, please visit
www.cse.psu.edu/~dunn/Alita
and click on the Plank Damage link.

-Denny