Plywood Scarph Joint Ratio

[Boston]

funny
we call it the eraser

course I was upside down one day cutting something
blade slipped and came at me
broke a tooth on that one (one of mine not the saw's )
hurt but could have been worse

its actually the only time a blade ever touched me
other than that
count em kids
ten fingers
ten toes
1/2 a tooth down
but Ill take it
:-)

[the1much]

ive only used guards when my grampy made me,,,,,hehe,,,,and on scarfing hehe,,,couldnt ya make a jig for a table saw?,,,making it may take a while,,,but if ya use clamps,,,and a jig that goes down the "slide" of the table saw,,,,jus thinkin,,hehe

[Boston]

two or three scarfs later and your a pro
belt sander
chisel
Im even guilty of side swiping with a skill saw
to get the bulk out
you do the same thing on a log home with a chain saw to cope out the joints
kinda freaks people out when I do it that way
but it saves a lot of chiseling

[Meanz Beanz]

My way might be slow to set up but the joint is so well cut that you can hardly see it from the side... Now someone is gunna tell me its better rougher cause you can get more epoxy in the joint.... well, a preemptive to you, I like tight looking joints!... no Jim not that kind

[Boston]

yerp
and if you pick your grain carefully enough you wont even see that

[the1much]

Built a scarfing gjg!

[ancient kayaker]

Originally Posted by Meanz Beanz

My way might be slow to set up but the joint is so well cut that you can hardly see it from the side... Now someone is gunna tell me its better rougher cause you can get more epoxy in the joint.... well, a preemptive to you, I like tight looking joints!... no Jim not that kind

I modified your jig design to suit my own applications, it works great. I am halfway through a canoe, first time I built one with scarf joints. I am using 8:1 on the strips for the gunnels, which are laminated, but I used 4:1 on the planks (not the bottom one) that are reinforced with chine logs.

The scarfed planks (3 mm) handled well, no breakages despite being very long and springy. Half were joined using epoxy and the rest used Titebond III. The joint quality improved from adequate to virtually invisible as I got better and acquired a feel for the method. I have always used butt blocks before so it was a big move for me. I am tempted to go for bright finish.

[Meanz Beanz]

Cool

Although I can't take credit, I copied the idea from someone else... and so it goes.

Cheers
Mbz

[ThomD]

The angle of scarphs should always be related to the underlying quality of the wood, and the degree to which it will be heavily loaded in use. So with wooden stringers, if you had less than perfect grain alignment, a 12-1 scarph would be overkill since the adjacent wood will certainly fail first. With aircraft grade the 12-1 might be underselling. For an arrow where run-out is as near zero as possible, scarphs should be avoided.

As mentioned load is also a factor, the simple minded way of assessing that is the degree to which the material will flex in use. In hulls that is likely to be near zero, in most cases, but for a spar it could be significant, and for an archery bow it would be an extreme application.

Plywood presents a number of challenges in assessing structure. For one thing, many of the preferred options are tropical woods with no easily visible grain, since there is no winter at the equator. What matters most for ply is getting the scarps accurately cut and properly aligned. This is way more important than the angle. Minor errors can mean that end grain is bonded to side grain and resulting panels can be very fragile.

Early in my career, before I built my vacuum jig, I made a scarph in 1/8" ply that had a bit of a curve to the edge. As a result the middle of the panel did not bond end grain to end grain, and when the hull side was cut out of the panel all that remained were the middle sections that were poorly bonded.

As I carried the panel to the assembly area, it folded like a greeting card, but somehow stayed together. I quickly bonded a stringer to it and folded it up into the deck jig. Once it was folded in place I smeared some epoxy over the inside joint for a flat cove, and applied some 4 oz bias glass. Later the outside of the panel got 4 oz boatcloth. Still in use over 20 years later.

That is the one good thing about tortured ply: If it survives assembly it will normally survive in use!

[magwas]

I am preparing to do my first scarf.
I have made the setup based on this page: http://oneoceankayaks.com/stitchglue.../scarfjig2.htm
I have several questions though:

Which is better, to precut the plywood to approximately to the size of the strakes and scarf them, or glue whole sheets together?

I am not very good at handwork. I am happy with the precision of the jig (given my abilities), but It seems to cut somewhat uneven lines (tried with thin cupboard back panel). What should I check on the jig, and/or be careful with when cutting?

I figured that if the cut will be too uneven, I could go through it with a sander trying to make the lines straight and paralell. Is it right? What is the deviation which a scarf will tolerate?

I am using 4mm okume plywood.

[ancient kayaker]

Nice jig. I tried using a circular saw years ago but found there was too much tearout for a really neat joint, at least on Okoume ply.

I got the cleanest joint using a router, but the setup time was a problem and the blade width is limited.

I tend to use a simple hand plane for a small job, moving down to a low angle block plane for the finish cuts. The advantage of the plane is, it holds the ply down, since the thinner plywood sheets tend to curl. My work with a plane improved considerably after I discovered how to get it really sharp ...

[Landlubber]

get it really sharp ...


the be all and end all of woodworking......any sharp tool really must be, it makes all the difference. Learn to sharpen then relearn how to work with wood all over again.

[ancient kayaker]

I used to believe all that stuff about being able to shave with the blade was just that, stuff. Until I got a superfine Japanese water stone for the finishing touch, with a Veritas honing guide. All part of a good education ...

[magwas]

Originally Posted by ancient kayaker

Nice jig. I tried using a circular saw years ago but found there was too much tearout for a really neat joint, at least on Okoume ply.

I got the cleanest joint using a router, but the setup time was a problem and the blade width is limited.

I tend to use a simple hand plane for a small job, moving down to a low angle block plane for the finish cuts. The advantage of the plane is, it holds the ply down, since the thinner plywood sheets tend to curl. My work with a plane improved considerably after I discovered how to get it really sharp ...

Thank you for the help. I have a block plane inherited from my grandfather. Sometimes I use it, but I thought that no one else does such things nowadays in the age of electric planes and grinders...

[ancient kayaker]

Power tools are nice but they have limitations. My planes get used far more than the other hand tools. A boat-building buddy does my thickness planing for me on his floor-standing power plane. A hand-held power plane is fast but inaccurate so I finish with a smooth plane. My low-angle block planes get a lot of use - having 2 saves having to re-sharpen in the middle of a job. A low angle plane gives a cleaner edge to the scarf and is great for cross-grain work. Good quality planes are expensive these days; I used to look for old tools in antique stores but unfortunately they have become collector’s items.

I don’t use a regular grinder much, but I have a power-drive water stone that is nice for initial sharpening. A sheet of waterproof abrasive paper on a piece of glass works well: soak it, clean the glass and roll the paper onto the glass to get out bubbles, start with coarse grades and progress to finer abrasives, finish with #1500. There are books written and a lot to learn about sharpening; try a google search for more information if you need it.

Getting back to your questions in post #70:

I tried using a sander to cut a scarf but I got very variable results: the first time was brilliant: I don’t even want to talk about the second time!

I have had the same problem as you did, uneven lines on a large sheet when thin ply curled. I use 3 mm and 4 mm ply mostly. I have no trouble at all with wide scarf joints on thicker ply like 6 mm, the thinner stuff is more troublesome.

I prefer to cut the strakes from the plywood sheet first, then scarf them. I keep the scarfs near the end of the strakes if I can then precut the center section accurately and cut the end bits oversize so I can trim them after scarfing. That reduces both waste and effort for me. For my boats that I build the least stressed part of the planks is usually near the ends.

The harder woods like birch are easier to work with. I find okoume tends to tear out at the edge of the scarf cut, trimming with a very sharp low-angle plane cleans that up.