Plywood Scarph Joint Ratio

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

 

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.

 

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
and just wait till i ask for that otha stuff!!


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

 

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.

 

glues and more

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=...X&oi=book_result&resnum=10&ct=result#PPA47,M1

 

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.

 

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!

 

Thats no secret... Melbourne, Australia.

 

hmmmm,,,,, i thought you was in london ??? hahahahaa

 

`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

 

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.

 

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

 

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