Plywood Scarph Joint Ratio

[Meanz Beanz]

Just enquiring what the best ratio for joining thin plywood is 6mm & 9mm. I just read that 12:1 is about right, that makes the 9mm joint 108mm wide. To be honest thats pushing the limit of the material I have for this little project. What is the lowest ratio you can use and retain full strength?

Cheers
Mbz

[lewisboats]

8:1

steve

[Meanz Beanz]

Thanks

MBz

[PAR]

I've never had a failure with 8:1 scarfs, though if the plywood is being tortured, or will receive considerable radius in the location of scarf a higher ratio will help mitigate a flat spot. I use 12:1 in these locations.

[Meanz Beanz]

Thanks PAR,

I think I will go 10:1, I have the material to cover that, the maths is easy (& I am lazy) when making up a jig to do the job.

Cheers
MBz

[PAR]

A simple scarf jig I have on my work bench. Just some 2 by stock, bolted to act as a guide at the correct angle. In this case 10:1 on 1/4" plywood. Clamp a piece of wood behind it to hold the plywood down during scarffing.

[Meanz Beanz]

Thanks PAR,

I was considering this setup as I have the gear to do it. What do you think?

Cheers
MBz

[PAR]

I've used that type of setup, in fact I made a fancy one with adjustable ramps and all. I found the set up a pain in the butt, clamping the stock troublesome and it was slow to do. I could do a hand scarf faster then the time it took to set up the jig, the router and clamp things tight enough.

Unless I have many scarfs to do that day, I usually rough them in with a power plane, then finish with a belt sander, hand plane and sharp chisel.

[Meanz Beanz]

I think I will waste sometime and try for accuracy over speed, I'm not bad with my hands but have not got the experience joining timber this way so I'm not confident I can follow your lead.

Cheers
MBz

[marshmat]

If you have one, the power plane does a very nice job. It's quick, efficient and gets you pretty close to the final desired shape. (PAR might have the skill to free-hand it like this, but I know I can't!) The router jig you've drawn looks like it would work, if you're careful to ensure the plywood is securely and evenly clamped (not an easy task with wider sheets).
I did a 2m x 5m MDF plug freehand with a router and power planes last year- it worked, but it's not an experience I'd care to repeat even with skilled help. You can do a lot with remarkably simple equipment, once you understand how it behaves.

[Meanz Beanz]

Yeah I have seen the setup for doing it with a power plane and a belt sander... still pondering that one!

Thanks

MBz

[PAR]

I don't use any type of jig when I'm doing scarfs by hand. The power plane just knocks the bulk of the material off quickly. The belt sander continues this, but to a finer degree, then the chisel (must be sharp or you're wasting time) hones it down to a finished product.

Working with plywood, the veneer glue lines are a great assistant in keeping things beveled the way you want. If doing solid lumber then a simple angle gauge will suffice. Yes, you'll get better the more you do, just like about everything else. I'll bet the first time you spelled your name in the snow, it was pretty sloppy, but after years of practice, you can dot the I's and cross the T's reasonably cleanly.

[ancient kayaker]

I routinely use a 6:1 ratio for scarfing cedar strips, and find the joints are as strong as the wood. I think the old 12:1 rule dates from the days before decent marine glues when the joint was nailed together.

So far I have used butt blocks for ply edge-to-edge joints in my boats but a few months ago I butt joined several test pieces of 3 mm and 4 mm ply using epoxy, no butt block. I found the joint strength was between 55% and 80% of the wood strength.

The longer the joint the stronger it is but once it's as strong as the wood is there any point in making it longer? Assuming the joint strength is proportional to its length my test results suggest the ratio can be as low as 2:1.

However, results might vary for different woods, glues, saturation etc. A ratio of 6:1 provides a 3:1 safety margin which should accommodate variations.

[the1much]

no wonder i hate wood,,,,,too much damned math hehe :D

[PAR]

Plywood scarfs without failure at 8:1 in my experience and this is an industry (plywood) standard.

Strip planking (depending on which type) typically doesn't need to be scarfed, nor planked out of especially good wood (usually), so the comparison is weak to say the least.

Longer scarf ratios are required on materials that will receive some bend or load, the more so the longer the scarf slope.

With the exception of very few, most of the adhesives used in the industry easily exceed the physical properties of most of the woods we use. The longer the glue line in a scarf the less affected by stress risers and "hard point" situations, which can be problematic, especially in curved wooden elements. It also provides sufficient surface area to make the joint strong enough.

The 3:1 safety margin on 6:1 scarfs has me completely stumped as to meaning. Lets take a piece of .25" plywood and apply a 6:1 scarf. This is a sloped joint 1.5" deep, which clearly isn't strong enough to duplicate other areas on the plywood panel in strength. So, where's the safety margin?

I wouldn't use 6:1 on plywood, unless (actually, my minimum is 8:1) the resulting joined panel had very little load or bend. I move up to 10:1 on highly bent or loaded pieces.

[Meanz Beanz]

I did the first one today using the router jig. I did 10:1 just cause the maths is dead easy with a pencil in the shop... and heck whats 20mm or so more ply wood and glue. The jig was a stuff around to setup so that it cuts right, but once set up cutting the joints took very little time and its accurate.

Cheers
MBz

[PAR]

Yep, that's my issue with the jigs MBz. The setup time can be longer then the actual tool in hand time, sometimes much longer. Unless a bunch of planking scarfs (or something) are necessary, I'll stick with wood butcher's friend, a chisel and a power plane. I've preformed this "race" a few times with kids in the shop that think jigs are cool and hand tools for old timers. They set up the jig and I cut the scarf before they load the stock or start the tool.

You got'a make them humble or they'll get uppity on 'ya. Good for you and your sucess.

[Meanz Beanz]

LOL... In my case its just substituting time for skill, thats a trade I need to make :D

[Meanz Beanz]

Par,

If you had to patch a plywood boat, say a 200mm square patch. What sort of angle would you attempt to scarf ?

Given the limitations of such a situation I would be tempted to stick a router in the hole with the flattest angle I can find, but say 45% and then epoxy glue a patch in, with maybe a thinner backing plate glued in place.

What does a pro do?

This is purely hypothetical.

Cheers
MBz

[PAR]

I'd use a grinder with 16 or 24 grit and back the surrounding surfaces off about 8 or 10:1 then dry fit a patch with similar tapered edges. A few times around and it'll be close enough for wood butcher's friend to make nice. Fill, paint and the owner will never know. Damn, I'm usually the owner . . . A 45 degree angle will crack out pretty quick if any stress is on it.

[the1much]

square is a GOOD point too for the patch ( even though i think round would even be harder to fit,,,,,i hate wood ;) ) because when it comes time to paint it,,no matter WHAT you do,,, you'll ALWAYS see the round patch,,,,even a small drill hole will show. ,,hehe......sorry Beanzy and Par ,,,hehe ;) ,,only thing i know bout wood ,, is to burn it in the winter to stay warm,, and i was feeling a little left out..,,,:sob:,,,,:sigh:,,,,so,,,,i only knew bout ,,,welll,,, :(
hahaha :D

[Meanz Beanz]

OK, thanks PAR, I was wondering if you'd still go for the 8:1 ratio.

Jim... ya boil it and eat it... try some :p

Cheers
MBz

[PAR]

Yep, I'd still feather back at least 8:1 on plywood planking. It doesn't have to be particularly precise when using epoxy. In fact, it's better if it isn't. Jim, my repairs don't show, even years later. Just use enough goo and no crisp edges on the affected area.

[lazeyjack]

something for you Beanz
not mine, but am close to starting a build that has many joints
here is the link there are some nice pics
http://westsystem.com/ewmag/18/plywood.html
Plywood basics




By J. R. Watson

Since so many projects in Epoxyworks incorporate plywood, we felt it might be valuable to discuss briefly the types of plywood and some construction methods best suited to it. It's easy to understand why people like plywood and choose it for so many projects: it is readily available, comes in convenient sheets (typically 4'×8'), is pretty light for its stiffness and strength (1/8" plywood weighs about 11 lb per 32 sq ft panel), and is a bargain when compared to the price of many composite panels. However, plywood also has its weak points. There are limits regarding shape development because plywood can be compounded (bent in two directions at once) very little. In addition, plywood contains end grain on all its edges, which makes it susceptible to moisture migration and damage. So while we've had good success in a diversity of projects using plywood, it's important to consider these guidelines.

Understand plywood grades
All plywood is graded based on the quality of the veneers and the size and number of permissible voids and patches. Grades A-C are the most common. To be suitable for marine construction, panels must be made with waterproof glue and marked as exterior plywood (EXT) or marine plywood.

As a general rule for boat projects, use marine-grade plywood because the wood used to make it is of higher quality with smaller voids and fewer patches. Inner veneers are also of higher quality, grade B or better. Marine plywood is therefore more stable, has less potential for checking, and the overall strength and stiffness are better than exterior plywood. When you consider the overall cost of a project, the price for marine grade plywood is well worth it. While AC exterior plywood could be used on "low cost" boats, filling all the knots and voids may cost more in time and materials than if you had bought better grade plywood. Aircraft grade plywood is also very high quality and might be appropriate in specialized applications, such as a cored panel wing mast. Some builders have used door skins (3mm thick) when making a laminate of multiple layers, but you need to make sure they are glued with exterior grade glue and are of adequate quality. You should not use plywood underlayment for boat building because of its poor quality.



U.S. marine grade plywood must meet U.S. Products Standard PS 1-74 (applies only to softwood plywood).

Species: Douglas fir No 1 or western larch. Glue: waterproof. Face and back veneers A-A, A-B, B-B smooth cut and free of knots, pitch pockets and open splits and other open defects. Inner plies B or better. Repairs: no more than 9 repairs allowed on A faces in a 4×8 sheet.

Marine grade for tropical hardwoods must meet British Standard Specification BS 1088.

Species: untreated tropical hardwood veneers with suitable level of resistance to fungal attack. Glue: waterproof glue. Face veneers free of knots and other than solid pin knots no more than 6 in any 1 square foot area. Core veneers shall be the same as those for face veneers. No gaps in faces. In core, not more than one gap no wider than .5mm. Face veneers if three-ply construction shall not be thinner than " of the total thickness.

Aircraft grade plywood meets all specifications of MIL-P-6070, which calls for shear testing after immersion in boiling water for three hours. It is made of imported African mahogany or American birch veneer laminated in a hot press to a hardwood core of poplar or basswood with waterproof glue.

Check the plies
Plywood typically comes in an odd number of plies in order to achieve balanced construction. The number of veneers on each side of the core layer must be equal in number (and thickness) or the panel may warp. The plies should be parallel to one another with alternating layers set at 90° or warping can occur. When you buy plywood, lay the sheet down on the ground. If it's curled or twisted and doesn't lie flat, don't buy it. Select plywood where the total thickness of the two outer plies equals the thickness of the core. Avoid 'cigar box' stock where a thick core is sheathed with thin face veneers.

Most plywood under 1/4" (6mm) is three ply construction. One quarter inch plywood and thicker is available in 5 or more plies. More plies are better because the panel is more dimensionally stable, less likely to warp and stiffer than three-ply.

Know the wood species
Most marine grade plywood is made from tropical hardwoods, like African mahogany, sapele, utile, and okoume. Some wood species are more durable than others. Coating surfaces (especially exposed end grain) with epoxy greatly protects less durable woods from the harmful effects of moisture. While okoume is considered a non-durable species, we have had good long-term results with it. Much of this is the result of thorough coating with WEST SYSTEM® epoxy and careful maintenance of the finished boat, including ventilation and prompt repairs.

The biggest challenge with Douglas fir plywood is the propensity of its outer (face) veneer to check, that is, split along the grain. This is especially true when the outer surface forms a curve perpendicular to the face grain. Covering fir plywood with 6 oz fiberglass fabric is mandatory to contain the checking.

Limitations of pressure-treated plywood
Many people ask about the value of pressure treated plywood. Often, pressure treated plywood is heavily warped. This is primarily the result of the high temperature and rapid moisture take up during processing. Some plywood delaminates during the process. In addition, some pressure treating methods incorporate water repellant materials. These materials can easily jeopardize bonding so you won't be able to glue or paint the plywood successfully. Generally speaking, pressure treated plywood may be acceptable for house projects but not for boats.

Select the right plywood for the project
With the many types of plywood available, it is important to choose and combine the types and thicknesses of plywood best suited to your project. For the kind of projects typically described in Epoxyworks, strength, durability and weight are primary considerations. So we usually choose somewhat specialized plywood to meet the particular requirements of the project. For example, on a wing mast I built twenty years ago, I was after light weight and flexibility. So, I selected plywood that would allow me to bend the laminate to a tight radius foil section: aircraft grade 1/16" birch on the mast's exterior, and 1/64" (3 ply) on the interior, with a honeycomb core. On a current project, I've chosen a 9 mm teak-faced plywood to replace the live well and wet locker hatches on my Mako Angler. I wanted the teak finish and flat, hinged panels. So I chose a plywood with a teak face, stiffness and dimensional stability. The point is to select carefully the best plywood for your project.

Building your own plywood
Some folks opt to build their own plywood by laying up multiple layers of sliced or peeled veneer. You need a suitable flat surface and a method of applying pressure until the adhesive that joins the layers has cured. Building your own plywood rarely saves money and the panels are often heavy. However, you can tailor your choice of a face veneer and it is possible to make some really beautiful panels. You also can make the panels whatever size you want. When you laminate a hull, as in cold-molded construction, you are actually making plywood to a specific shape, that of a boat hull (see Laminated construction gives lasting value).

Joining plywood panels
You can join plywood panels in a number of ways if you need sizes greater than standard ones. Scarf joints are the best approach for strength and weight. (If you order 16' long panels, offered by some plywood suppliers, the panels are simply scarfed together, but you will pay a premium both for the material and the shipping.) Gougeon Brothers, Inc. does offer a custom attachment (875 Scarffer™) that fits onto a circular saw. This tool makes scarfing plywood panels of 3/8" thickness or less quick and accurate. You can also fashion scarfs with a plane and/or grinder. The rule is to create a bevel 8 times the thickness of the plywood. Panels can also be joined with butt blocks, but this has the potential for exposing end grain at the butt. We like to dish out the butted area similar to a scarf and bond in fiberglass cloth.

Construction methods with plywood
Plywood is ideal for several construction methods. The most common is "stitch and glue" construction where panels are cut to a designed shape and temporarily joined with wire or plastic ties. Then you apply epoxy fillets and fiberglass tape to permanently join the structure in a rigid shape. (See Building prams) Plywood has also been used successfully to build lightweight lapstrake boats. Tom Hill described this in Ultra Light Boatbuilding (available through International Marine Publications, phone 207-236-4837).

Probably the most extreme shapes can be gotten via "tortured," also known as developed plywood construction, where the plywood is actually compounded. This construction method is limited to thin plywood and hulls with modest shapes, most typically multihull craft. Designing your own plywood boat used to be very challenging because if you introduced excessive compounding, you could have trouble laying out the plywood to the shape. With stitch and glue construction, it is extremely difficult to predict panel shapes. Now, however, some excellent software is available so you can get accurate offsets for the true shapes of panels. Two that we have used are Plyboats, 714-840-7284, www.plyboats.com and New Wave Systems, Inc., 401-423-1852, www.newavesys.com.

Epoxyworks 18 / Fall 2001

[Meanz Beanz]

Thanks LJ... I will explore. :D

[Meanz Beanz]

Lessons learned....

1. Make sure that the jig setup is totally clean prior to putting ply down. The smallest particle can have quite an effect.

2. Although the router is very accurate take the joint to within about 1 mm of the final desired cut and take the rest back with a sanding machine. This gives better control over the outcome.

3. Clamp the long end of the join and set the short end up against a backstop. This makes it easy to drop and clamp the short end in place without having to worry too much about placement, the backstop pretty well assures it lands where you dry fitted it.

4. A 10:1 join feels damn solid.... :D (as I am sure that a 8:1 does!)

Cheers
MBz

[Meanz Beanz]

A simple scarf jig I have on my work bench. Just some 2 by stock, bolted to act as a guide at the correct angle. In this case 10:1 on 1/4" plywood. Clamp a piece of wood behind it to hold the plywood down during scarffing.

This might become a feature of my next work bench... plans for the new shed are very grand!

:D

[lazeyjack]

well we can all plan, I have not got a shed, now, last scarfing did was on some chairs, two pieces 40x40 box one on top one under, held the joint, , but slid all over the place Miss my workshop
are you using an end mill bit, in that router? and wherever you live do you have square drive screws? you don't see them in Au but in NZ we use nothing but, they stay in the driver at anything but down angles, adn dont slip, bestest invention since sliced bread Best high ten ss seems come from USA, many tai screws like cheese

[Landlubber]

Scarfing plywood is easiest with a 3 or 4 inch power plane, simply keep the glue lines parallel and evenly spaced as the wood is constructed. Hand finished with a standard jack plane , sharpened correctly of course, and the job is finished long before any jig is set up and played with.

Even when we do a pallet of ply scarfs, the power plane is the tool of choice, not a jig. We do about 4 or 5 sheets at a time staggered stacks, and hit with the planer.

Please understand, the jig is very accurate, no doubt and it has its uses, but in reality the epoxy takes up and needs to be there to create the strongest joint

[PAR]

Lubber, that's exactly what I've found. After hundreds of scarfs, the cleanest, fastest and most likely to fit in tight places (such as doing plank scarfs on the boat) is a plane, power if you have the room to swing it, hand if not. I've done many scrafs on lapstrake planking, while I'm hanging planks. No jig works in this application, just a sharp hand plane and chisel.

On the bench, you have some options, but unless you're plowing through a dozen similar dimension scarfs, in one session, the jigs seem to take too long to set up, are bulky and require clever clamping arrangements to the stock doesn't cup, move or other wise ruin your day.

The circular saw jig I show above is the only one I use most of the time, unless I need a precision scarf in a bit of bright work. There's no set up, just clamp the work and slide the saw down the guide. It was originally intended for full length (8') scarfs, when I was planking up a big boat and needed to cover large areas of hull with full (or nearly so) sheets of plywood. It works for short scarfs too.

[ancient kayaker]

Nice jig MB. The simplest ideas are the bast.

Lazeyjack, a bit of history. The square drive screws are Robertson head screws invented about 100 years ago by a guy of that name (in Canada). Its main competitor, the Philips, was designed for US factory workers so the screwdriver bit would cam out of the screw head before stripping the thread. The English WW2 Spitfire fighter used Robertsons.

Par: good point about gap filling properties of epoxy. What do you do for precision scarfs? I haven't done a decent scarf yet but I am ambitious! A plane works OK but the ply takes the edge off the blade pretty quick. On the scarffer jig, I have a problem using circular saws for scarfing due to edge tearout but that might have been cheap ply. It wasn't a cheap blade though - blade cost more than the saw.

[lazeyjack]

Nice jig MB. The simplest ideas are the bast.

Lazeyjack, a bit of history. The square drive screws are Robertson head screws invented about 100 years ago by a guy of that name (in Canada). Its main competitor, the Philips, was designed for US factory workers so the screwdriver bit would cam out of the screw head before stripping the thread. The English WW2 Spitfire fighter used Robertsons.

Par: good point about gap filling properties of epoxy. What do you do for precision scarfs? I haven't done a decent scarf yet but I am ambitious! A plane works OK but the ply takes the edge off the blade pretty quick. On the scarffer jig, I have a problem using circular saws for scarfing due to edge tearout but that might have been cheap ply. It wasn't a cheap blade though - blade cost more than the saw.


thanks for that, there is a lot of rubbish coming out of the East Bought some(bronze) sq drives, they certainly were not quality bronze which can a t times be stronger tensile than ss(ni Al bronze) yes you can put a lot of torque on a sq drive, but no prob if you use a hand driver or limit the torque on your drill, they are far easier to use, no comparison to Phillips, Pozi, slot, and personally would never go back, neither would anybody I know in the industry Lay you saw over, that may help

[PAR]

The adhesive of choice, when I don't have to worry about things is resorcinol. It's temperature sensitive and requires considerable clamping pressure, but it's bullet proof when cured. On bright work, not in contact with the water I use aliphatic resin and when tolerating intermittent contact, plastic resin or TiteBond III. Continuous contact is epoxy, if encapsulated or resorcinol if not.

[the1much]

ya know,,,,,people like you Par p!ss me off!!,,,;)
ya know what gonna happen????? ,,, im gonna go make people think im smart,, and go ask for resorcinol and say,,,,hey,, you guys got any of that (ree-sok-I-nul) ,,,their gonna look at me like i poo'd in my pants!!,,hahaha :D:P
and just wait till i ask for that otha stuff!! ;)


:EDIT: ,,,,for some reason i can pronounce "tight-bond" pretty good :D

[ancient kayaker]

The adhesive of choice, when I don't have to worry about things is resorcinol. It's temperature sensitive and requires considerable clamping pressure, but it's bullet proof when cured. On bright work, not in contact with the water I use aliphatic resin and when tolerating intermittent contact, plastic resin or TiteBond III. Continuous contact is epoxy, if encapsulated or resorcinol if not.

Isn't resorcinol the adhesive in marine ply? That's a pretty good advert. Can be difficult to find though. I find ply adhesive gums up power tool bits on occasion, especially hole saws which tend to run hot.

I use Titebond III a lot, it's faster than epoxy and as strong and water cleanup is nice. No alternative to epoxy for filling the gaps though.

[lazeyjack]

there is a huge amount of usefull info on glue here and many other things, can not copy and past e from it
http://books.google.co.uk/books?id=I_xMJ5QY23oC&pg=PA36&lpg=PA36&dq=resorcinol+glue&source=web&ots=2i5_0d4_RL&sig=b1rZ0Xs-1mnamR55nkgxbmKVwiU&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA47,M1

[PAR]

Dap makes a version of resorcinol (there's two different types)

http://www.jamestowndistributors.com/userportal/show_product.do?pid=2060&PTECH

TiteBond III is classified a type I waterproof adhesive, but it just barely passes this test and isn't a structural adhesive, just a glue. It is the best formulation of it's type currently going, but still lives within the limitations of it's chemistry and physical abilities. It doesn't match up favorably with epoxy, in any category of comparison. I use TiteBond III for many things, unless it will be subject to high loading, wet wood or very damp conditions. It does have slight gap filling properties, but a 1/16" is about all it can be trusted to do. I've also tested it with silica and/or milled fibers, but peel strength was compromised.

Marine and exterior grades of Plywood use phenol-formaldehyde adhesives, which are similar chemically to resorcinol.

[Meanz Beanz]

Just on the square drive screws, I love them, great to use! I have never seen SS ones on offer in Oz, who supplies these things?

Cheers
Mbz

[lazeyjack]

Just on the square drive screws, I love them, great to use! I have never seen SS ones on offer in Oz, who supplies these things?

Cheers
Mbz

beanz, how the hell do I know, cant find anything here, but you are revealing your whereabouts!

[Meanz Beanz]

Thats no secret... Melbourne, Australia.

[the1much]

hmmmm,,,,, i thought you was in london ??? hahahahaa :D:P

[Boston]

`ya there is kinda a knack to fixing things so they dont show
I think glazing is kinda key if you are after nice invisible fix
I kinda ended up the fix it so it doesn't show guy in the shop were I worked
buy oh my
have I ever seen some
" I tried this myself and can you kinda clean it up a little ? "
some perfectly competent folks just done have the knack
kinda like me and spelling
B

[ancient kayaker]

Dap makes a version of resorcinol (there's two different types)

http://www.jamestowndistributors.com/userportal/show_product.do?pid=2060&PTECH

TiteBond III is classified a type I waterproof adhesive, but it just barely passes this test and isn't a structural adhesive, just a glue. It is the best formulation of it's type currently going, but still lives within the limitations of it's chemistry and physical abilities. It doesn't match up favorably with epoxy, in any category of comparison. I use TiteBond III for many things, unless it will be subject to high loading, wet wood or very damp conditions. It does have slight gap filling properties, but a 1/16" is about all it can be trusted to do. I've also tested it with silica and/or milled fibers, but peel strength was compromised.

Marine and exterior grades of Plywood use phenol-formaldehyde adhesives, which are similar chemically to resorcinol.

Thanks for the info.

According to the manufacture Titebond III does not fill gaps and needs a few psi clamping pressure to ensure it penetrates the grain and develops full strength which on hardwoods is around 4,000 psi in shear. Probably the tensile strength is similar, that agrees with tests I've done, but I can't find any data from responsible authorities. That's close to most woods tensile strength with the grain. I have tested T-III's water resistance, it did not soften like most water-based glues but tends to pull out of the grain so the bond weakens in tension if not in shear. Epoxy soaks in further and does better. I think the problem is the speed with which T-III sets. In a thin layer on dry wood it starts to firm up in seconds as the wood pulls the water out from the glue which initiates the setting process, so it does not have much time to penetrate the grain. I get better water-resistance if I dilute it 10% or moisten the wood before applying the glue. Strangely, T-III recovers its bond strength after soaking if it allowed to dry.

Epoxy, which sets in its own good time, is still to be preferred for continuous immersion.

I did some math on the scarf ratio business; assuming the glue strength is less than the woods (assumption 1) under bending stress for uniform material (assumption 2) then the strength of the joint is proportional to the square of its length. Assuming the ratio is high enough that the joint is more or less parallel to the grain (assumption 3) the wood will break adjacent to the the joint face when the stress reaches its cross grain tensile strength unless the wood breaks someplace else because the stress has already reached its with the grain tensile strength. Therefore the scarf ratio should be greater than:

square root (with grain strength / cross grain strength)

As an example, for dry white pine for which these strengths are typically 3,600 and 300 psi the ratio should be at least sqrt (12) or 3.46, say 4:1.

Of course, the use of ply invalidates assumption 2. Nonetheless, I tried 4:1 scarfs on marine ply; it was a bugger to clamp, slid all over the place, a longer scarf would not have that problem. When I broke the first samples the wood broke, not the glue, in 3 out of 4 cases. However, those samples had the joint parallel with the grain of the face plies. When the joint was normal to the face grain the glue failed starting at the face veneer. I suspect the 4:1 ratio invalidates assumption 3, which would mean that the joint in the face ply was more or less across the grain which would likely invalidate assumption 1 as well. With all 3 assumptions invalid all bets are off. Sigh.

Conclusion? If you can control the sliding problem you can get away with a 4:1 scarf if the joint is in the same direction as the face ply, otherwide use a longer joint.

Whilst obsessing in this healthy manner, I decided to modify the scarf joint per attached sketch. On the outside it's a furniture quality joint, virtually invisible, but hardly productive use of my time ...

Dear Abby: my family tells me to get a life. What does this mean? Sleepless in the boathouse

Dear Sleepless: you wouldn't understand. Keep taking the medication.

[lazeyjack]

beanz you should just get ANZOR fastening from Auckland to freight em, they come in 3 drives, 1,2,3,, you could become a reseller

I found thsi today a woodworkers forum, 1,2, 3 bits are readily available from trade tools, if you cant find a decent high tensile bit then I will post one down I have a spare number 1, hand driver(I think) somewhere too
http://www.woodworkforums.ubeaut.com.au/showthread.php?t=72830
Optimark
Senior Member

Join Date: Oct 2004
Location: Melbourne, Australia.

Default New square drive screws in Oz
Today I was visiting my local Mitre 10 in Moorabbin, when I saw something that surprised me. Apart from organising some timber, I was also looking for some outdoor furniture screws, either stainless steel, or coated with something for weather resistance.

What surprised me was the array of square drive screws available, something I hadn’t seen before in a national chain hardware. There were plastic tubs of them galore.

I asked the staff about these. Their reply was that it’s a new line and it’s a product that they think will take off. At the cash register they had a large plastic covered display of drill bits designed to take the various sizes of many drive systems, including the 3mm and 1.5mm square drives in your cordless drill.

I have been using the Robertson square drive screws from Canada, for quite some time now, I believe they are quite alright and have been slowly moving towards them for more jobs.

This is an interesting development for screwing in Australia, but possibly a bad thing for the small business that imports the Robertson screws!

Mick
Last edited by Optimark; 19th May 2008 at 12:09 AM. Reason: Punctuation

[the1much]

ya lazy,,,, them ausies need all the help screwing they can get hehe :D

[Meanz Beanz]

beanz you should just get ANZOR fastening from Auckland to freight em, they come in 3 drives, 1,2,3,, you could become a reseller

I found thsi today a woodworkers forum, 1,2, 3 bits are readily available from trade tools, if you cant find a decent high tensile bit then I will post one down I have a spare number 1, hand driver(I think) somewhere too
http://www.woodworkforums.ubeaut.com.au/showthread.php?t=72830
Optimark
Senior Member

Join Date: Oct 2004
Location: Melbourne, Australia.

Default New square drive screws in Oz
Today I was visiting my local Mitre 10 in Moorabbin, when I saw something that surprised me. Apart from organising some timber, I was also looking for some outdoor furniture screws, either stainless steel, or coated with something for weather resistance.

What surprised me was the array of square drive screws available, something I hadn’t seen before in a national chain hardware. There were plastic tubs of them galore.

I asked the staff about these. Their reply was that it’s a new line and it’s a product that they think will take off. At the cash register they had a large plastic covered display of drill bits designed to take the various sizes of many drive systems, including the 3mm and 1.5mm square drives in your cordless drill.

I have been using the Robertson square drive screws from Canada, for quite some time now, I believe they are quite alright and have been slowly moving towards them for more jobs.

This is an interesting development for screwing in Australia, but possibly a bad thing for the small business that imports the Robertson screws!

Mick
Last edited by Optimark; 19th May 2008 at 12:09 AM. Reason: Punctuation

Thanks LJ

I will snoop around :D

[Meanz Beanz]

ya lazy,,,, them ausies need all the help screwing they can get hehe :D

Screwing squares is fun, you old reprobate you... :D

[ancient kayaker]

It's good to know that Australian screwing is catching up with Canadian. There's no hope for US or Euro of course. Robertson's also come in SS and brass, maybe bronze too so pester your retailer.

[Boston]

found this little jewel floating around in cyberspace
polyurethanes vs recorsinol

ABSTRACT
One-part polyurethane wood adhesives comprise a new class of general purpose consumer products. Manufacturers' claims of waterproof bonds brought many inquiries to the Forest Products Laboratory (FPL) from users constructing aircraft, boats, lawn furniture, and other laminated materials for outdoor use.

Although FPL has technical information on several types of polyurethane and isocyanate-based adhesives for wood, no information was available on this new class of adhesives. Four commercial polyure-thane adhesives, along with a resorcinol-formaldehyde adhesive to represent a standard of performance, were subjected to a series of industry-accepted tests that assess varying levels of bond strength and durability.

In bonds to yellow birch and Douglas-fir, the polyurethanes did not differ significantly from each other in their performance; as a group, though, their dry shear strengths showed that they were significantly stronger than the resorcinol. Dry wood failures by the polyurethanes were high and did not differ significantly from the resorcinol.

After three water-saturating procedures, wet shear strengths of polyurethanes and the resorcinol were statistically comparable. Wet wood failures, however, were very low among polyurethanes, which is a sharp contrast to the high wood failure by the resorcinol.

A moderately severe delamination test indicated varying levels of water resistance among the polyurethanes, but the resorcinol was completely resistant to delamination. A very severe cyclic delamination test caused severe delamination of polyurethane bonds. However, a recently discovered hy-droxymethylated resorcinol (HMR) coupling agent dramatically increased delamination resistance of polyurethane adhesives. In a test of resistance to deformation under static loads, polyurethane bonds withstood extreme exposures of temperature and relative humidity for 60 days without deformation.

http://www.fpl.fs.fed.us/documnts/pdf1998/vick98b.pdf

enjoy

[lazeyjack]

and very usefull on all but steel, keep the guard at this angle, chase alloy welds, get rid surplus anything, , once you have this in your toolbox you will never live without they are essential for ally boatbuilding for weld prep
Best with air tool, as airtools stall if you do something silly
available in 5 and 4 inch tungsten, I have never lost a tooth, about one hundred NZ dollars using dead flat you can skim surfaces Excellent for digging out glue lines,

[ancient kayaker]

found this little jewel floating around in cyberspace
polyurethanes vs recorsinol

ABSTRACT
One-part polyurethane wood adhesives comprise a new class of general purpose consumer products. Manufacturers' claims of waterproof bonds brought many inquiries to the Forest Products Laboratory (FPL) from users constructing aircraft, boats, lawn furniture, and other laminated materials for outdoor use.

Although FPL has technical information on several types of polyurethane and isocyanate-based adhesives for wood, no information was available on this new class of adhesives. Four commercial polyure-thane adhesives, along with a resorcinol-formaldehyde adhesive to represent a standard of performance, were subjected to a series of industry-accepted tests that assess varying levels of bond strength and durability.

In bonds to yellow birch and Douglas-fir, the polyurethanes did not differ significantly from each other in their performance; as a group, though, their dry shear strengths showed that they were significantly stronger than the resorcinol. Dry wood failures by the polyurethanes were high and did not differ significantly from the resorcinol.

After three water-saturating procedures, wet shear strengths of polyurethanes and the resorcinol were statistically comparable. Wet wood failures, however, were very low among polyurethanes, which is a sharp contrast to the high wood failure by the resorcinol.

A moderately severe delamination test indicated varying levels of water resistance among the polyurethanes, but the resorcinol was completely resistant to delamination. A very severe cyclic delamination test caused severe delamination of polyurethane bonds. However, a recently discovered hy-droxymethylated resorcinol (HMR) coupling agent dramatically increased delamination resistance of polyurethane adhesives. In a test of resistance to deformation under static loads, polyurethane bonds withstood extreme exposures of temperature and relative humidity for 60 days without deformation.

http://www.fpl.fs.fed.us/documnts/pdf1998/vick98b.pdf

enjoy

I think Gorilla Glue is a one part polyurethane, these glues are gap filling because they expand into the joint but lose strength if they are made to expand too much. Nice to work with and cleanup is a snap as the fully expanded glue is just a weak foam, but it costs more than epoxy ...

Titebond also has a polyurethane glue, haven't tried it. Titebond II and III are PVA type glues, I understand.

[Landlubber]

ahhh Lazy, that is the little sucker i was talking about on another posting regarding cutting curves....a most useful tool indeed, but not for the feint hearted....used fearlessly, it is simply superb for all sorts of roughing off.
Ya got ta luv it!

[the1much]

wut ta hell is it called?,,,i want a few,,hehehe,,,dat looks FUN,hehehe :D:D;)

[Landlubber]

Much,

It is just a 4" angle grinder with a carbide tipped 32T blade on it.

[the1much]

thanks man,,,,,i aint never seen 1 of those blades used,,,,and it looks as much fun as a "saws-all".

[Landlubber]

I have never had so much fun since the pigs ate my little brother!

[ancient kayaker]

Nice looking blade, vicious little sucker in the wrong hands. It's about the same size as I use (85 mm) for cutting ply panels, but that's in an electric panel saw, very precise but the guard is a pain, always in the way. Maybe I should take it off and go unprotected, provided I don't catch something. I don't have air but I'm starting to wish I had, more power to weight and better manners than electric.

I checked out the saws-all (sawzall?); nice idea but a carbide blade will go through most stuff. My thin-kerf finish blade acidentally went through a couple of 1/4 SS bolts, happened so fast that it was done before I realised what I was doing. I was pissed off as that balade is only used for boat building normally but it still cuts as well as ever. Mind you, it cost more than the hand-held electric saw that it's mounted in. The blades the thing.

I was the little brother in our family but we didn't have pigs, just turkeys. The bastards used to chase me. But I digress ... don't we all; what does all this have to do with scarf joints anyway?

"Dances with Turkeys"

[the1much]

ya,,,the swazall ( reciprocating saw,, i think its real name) will go through EVERYTHING,,hehe :D
maybe thats why everyone at work started yelling and running whenever they saw me with 1,,hehe ;)
if Landyz, and lazeyz tells me i HAVE to have a guard,,,,,im guessing its time for a new grinder anywayz hehe ;)
and oh ya,,, scarf joints,,,,, my advice,,,,,Pay PAR to cut em fer me ,,ehhe < see Beanzy,, i stayed on topic hehe :D:D;)

[lazeyjack]

ya,,,the swazall ( reciprocating saw,, i think its real name) will go through EVERYTHING,,hehe :D
maybe thats why everyone at work started yelling and running whenever they saw me with 1,,hehe ;)
if Landyz, and lazeyz tells me i HAVE to have a guard,,,,,im guessing its time for a new grinder anywayz hehe ;)
and oh ya,,, scarf joints,,,,, my advice,,,,,Pay PAR to cut em fer me ,,ehhe < see Beanzy,, i stayed on topic hehe :D:D;)
we call it a meet axe, in my early years I had some big chunks cut out of me, , maybe a few screws missing

[Boston]

ya I took all the guard's off my saws early on in my shipwrights world
you just need to watch when you set em down so they dont go for a run across the floor
and keep the rookies away from em

one of the turkey's

[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 :D

[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... :D Now someone is gunna tell me its better rougher cause you can get more epoxy in the joint.... well, a preemptive :p to you, I like tight looking joints!... no Jim not that kind :p

[Boston]

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

[the1much]

http://www.boatdesign.net/forums/showthread.php?t=23697

[ancient kayaker]

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

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 :D

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/plyshophtm/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]

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.

[Landlubber]

"I used to look for old tools in antique stores but unfortunately they have become collector’s items."

....so true, there is no new tools like the better quality older ones, but as you say, the bloody collectors make even the simplest tools expensive for those that wish to replace their tools...it is a pain we have to bear I guess if we want quality....

....still, it is cheaper and better to buy used tools than the new ones anyhow.

[ancient kayaker]

[QUOTE=Landlubber;357564... still, it is cheaper and better to buy used tools than the new ones anyhow.[/QUOTE]

-very true, thanks to the Chinese! I am just glad my job wasn't making tools. I had a workshop fire three years ago and lost some treasured old friends. I had to replace almost every tool. I was amazed how little it cost: even the insurance adjuster was surprised! The price I paid for a right angle electric drill, variable speed and reversible was about the same as I paid for a basic two-speed drill in 1973.

However, when it comes to quality hand tools of the unpowered persuasion, , the Chinese don't (yet) deliver the quality needed. I got a lovely set of Japanese chisels at the same time to replace some ancient ones. Not cheap, but they take, and keep and incredible edge. I miss those beautifully shaped wood handles, the sense of history (some were well over 100 years old) and the patina, though.

There are some nice tools out there to be had if you have enough money. I salivate over the Veritas hand tools every time I visit my nearest Lee Valley store, but I would need a mortgage to fully equip myself with those!

[ancient kayaker]

This is from my post #13 way back in 2008:

[QUOTE=ancient kayaker;216217]... So far I have used butt blocks for ply edge-to-edge joints in my boats but a few months ago I butt joined several test pieces of 3 mm and 4 mm ply using epoxy, no butt block. I found the joint strength was between 55% and 80% of the wood strength. ... QUOTE]

That was BS1088 so for the 3mm ply at least, the veneers would have been all the same thickness. I did some math on these results, based on strength being proportional to the square of the thickness. I assume that only the face veneers, which had grain parallel to the joint, were glued successfully, the end-grain joint of the core veneer had zero strength, and the wood strength across the grain is about 1/6th of its strength with the grain. I don't have data for Okoume so I averaged data for several other species.

The joint strength is equal to the outer veneers’ strength or (3^2 - 1^2)/6 = 1.33 (arbitrary units, with-grain strength set to 1)
The uncut ply strength equals (3^2 - 1^2)/6 + 1 = 2.33

This would predict a joint strength about 1.33/2.33 = 57%.

That supports the low-end results I got by experiment. The high-end results indicate the glue adhesion to the core veneer was partially successful, about 50% of the wood strength with the grain.

Now if I do the same math for the case where the face veneer grain is across the joint, assuming the glue performance is 50% on the face veneers since I was able to add glue when it was absorbed by the end grain, I get joint strength equal to (3^2 - 1^2) x 0.5 + 1/6 = 4.17 compared with uncut ply (3^2-1^2) + 1/6 = 8.1666: this leads to 51% ply strength. Again, this is similar to what I got by experiment.

For the 4 mm ply with 1 mm face veneers and 2 mm core, the results should be:
face veneer grain parallel to joint: joint strength/ply strength =
worst case: {(4^2 - 2^2)/6} / {(4^2 - 2^2)/6 + 2^2} = 33% (I did not reproduce this in practice)
best case: {(4^2 - 2^2)/6 + 2^2 x 0.5} / {(4^2 - 2^2)/6 + 2^2} = 67%
face veneer grain normal to joint: joint strength/ply strength =
{(4^2 - 2^2) x 0.5} / {(4^2 - 2^2) + 2^2/6} = 47%

Once more, reasonably close to experimental results.

Now I have a theoretical basis as well as practical results, I can predict that the joint will be as strong as the wood for scarf angles finer than sqrt (1/0.33) = 1.732. This would be difficult to clamp of course, and absolutely intolerant of scarf cutting errors, so it is impractical.

If a finer angle is used, any slight curve along the length of the cut gets divided by the slope. For example, if my scarf cut has a 1 mm curve error along its length between the 2 joined pieces, that would likely result in a weak spot where the glue, even if it filled the gap, would be brittle. In contrast, in a 1:8 scarf with the same error the clamping would close the joint leaving a 0.125 mm mismatch in the thickness of the face veneer at the joint, which could be sanded off without too much impact on joint strength.

Nonetheless, for a joint that can easily be perfectly cut, such as a thin batten, the angle can be coarser than current best practice indicates. I have a jig that enables me cut accurately and a clamping jig which allows me to use very coarse angles if I wish to, as it prevents the pieces sliding apart. Although I routinely use 8:1 I have on occasion used 4:1 to save on material and the joints were fine, but I took care to ensure the end grain is saturated with glue before clamping.

As Par and others noted much earlier in the thread, finer angles are advisable if the plank is to be bent sharply, and particularly if it is to be twisted. I have not done experiments twisting scarfed joints.

[Boston]

I really need to draw up that old router guide we used to have
all you do is shove a board in the end clamp it down and plow it out
thing weighed a ton but you could do perfect scarfs as fast as you could run a router

cheers
B