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

[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.