Question about scarfing plywood

[Longbow]

I'm sure this has probably been covered, but I couldn't find the answer I was looking for. When scarfing plywood in the regular way it seems like you would be glueing endgrain to endgrain, but from what I've read it's said to be very strong. I saw a picture recently where someone made basically a half-lap joint and glued it with epoxy and it broke before the surrounding wood. I have heard of a step scarf, but I'm not sure that is refering to what I have in mind. I thought that using my router I could make a series of cuts and basically cut a stair step pattern into my wood and then glue it. It seems like it would be quicker than trying to perfect an angled scarf and would be glueing long grain to long grain. I'm assuming that since no one seems to do it this way that there is some problem with this technique. Any thoughts?

[Jay Greer]

I have an old Skill power plane that I modified to make step scarfs. There is a guide on one side that allows the plane to drop only through the first veneer. That guide then rides on the face of the sheet preventing the blades from cutting deeper. The other side of the plane has a fence guide that keeps the plane aligned along the lower edge of the scarf. It is also a depth stop that prevents the plane from cutting into the first veneer on the bottom of the sheet. Depth is controled by setting the sheet over several other sheets so that there is a gap between the working sheet and the under supporting sheets.
The scarf is bothe clean and automatic. There are no ragged shim edges to deal with for alignment during glue up. Sorry, the plane is not where I can take a picture of it easily.
Jay

[CundysHarbor]

I guess you run the modified plane across the sheet. Very interesting approach; I think I will give it a try.
Dave

[Rational Root]

Now that just sounds perfect.


I have an old Skill power plane that I modified to make step scarfs. There is a guide on one side that allows the plane to drop only through the first veneer. That guide then rides on the face of the sheet preventing the blades from cutting deeper. The other side of the plane has a fence guide that keeps the plane aligned along the lower edge of the scarf. It is also a depth stop that prevents the plane from cutting into the first veneer on the bottom of the sheet. Depth is controled by setting the sheet over several other sheets so that there is a gap between the working sheet and the under supporting sheets.
The scarf is bothe clean and automatic. There are no ragged shim edges to deal with for alignment during glue up. Sorry, the plane is not where I can take a picture of it easily.
Jay

[Ian McColgin]

Step scarfs in plywood are in most applications a dreadful mistake. A single step makes the plywood half as strong as it was. More steps are better but still are weaker than a proper beveled scarf and also make a hard spot where the ply will not bend fairly. It is quite easy to plane (hand plane being faster and more accurate than a power plane) an 8:1 or 12:1 bevel in at least two sheets to be mated.

Easiest to make up a scarfing and gluing frame outdoors with uprights pounded into the dirt and a framework to support the plys both for planing and for gluing. Normally this means a working surface about 12' long if joining just two pieces, more like 20' if three and so on, and 4' wide but have a rail down one long side to push the sheets against for alignment.

Rack up the edges to be beveled, staggerd back. So if 1/2" ply and 12:1, the edge of the top sheet is 6" back from the edge of the lower, with the edge of the lower firmly over a supporting end beam of your frame. Clamp all down, mark a line on the top sheet 6" back from it's edge and plane till across the two plys you have a nice foot long slope. THe ply lines will help you see how to make it flat.

A nice touch is to put just the very slightest step at each end rather than have a feather edge that leaves a wider glue line and makes smooth fairing harder. THe easiest way is after the big bevel is made for two planks, seperate them and pass the plane carefully so sink the bevel of each just a smidge - barely 1/64". Even though the step down to the new bevel level is not quite perpendicular to the ply, being perpendicular to the bevel instead, this will not matter. Your test fit will show how much of each feather edge to trim back.

When gluing remember that epoxy is slippery so get the planks positioned and well clamped before putting pressure on the joint itself. I like just laying a board across it (remember plastic on both sides so glue squeeze out won't be a problem) and putting some weights atop that.

One can make jigs like the nifty unit in an back number of our favorite periodical, but if you're just making one boat, and hand plane and outdoor rack are actually less total work, and ever so much more virtuous.

[gert]

This is a stepped scarph; the "shoulder" and thus the "heel" measures less than 1/32" deep.
http://farm4.static.flickr.com/3574/3323794436_dcd838f877.jpg
the result:
http://farm4.static.flickr.com/3537/3323033475_9d3495bb58.jpg
And don't do whole sheets; it's a waste of expensive material and way harder to do a 4' scarph compared to an 8". Just nest your parts & you'll get 95% yeild instead of 50.

[Jay Greer]

Step scarfs in plywood are in most applications a dreadful mistake. A single step makes the plywood half as strong as it was. More steps are better but still are weaker than a proper beveled scarf and also make a hard spot where the ply will not bend fairly. It is quite easy to plane (hand plane being faster and more accurate than a power plane) an 8:1 or 12:1 bevel in at least two sheets to be mated.

Easiest to make up a scarfing and gluing frame outdoors with uprights pounded into the dirt and a framework to support the plys both for planing and for gluing. Normally this means a working surface about 12' long if joining just two pieces, more like 20' if three and so on, and 4' wide but have a rail down one long side to push the sheets against for alignment.

Rack up the edges to be beveled, staggerd back. So if 1/2" ply and 12:1, the edge of the top sheet is 6" back from the edge of the lower, with the edge of the lower firmly over a supporting end beam of your frame. Clamp all down, mark a line on the top sheet 6" back from it's edge and plane till across the two plys you have a nice foot long slope. THe ply lines will help you see how to make it flat.

A nice touch is to put just the very slightest step at each end rather than have a feather edge that leaves a wider glue line and makes smooth fairing harder. THe easiest way is after the big bevel is made for two planks, seperate them and pass the plane carefully so sink the bevel of each just a smidge - barely 1/64". Even though the step down to the new bevel level is not quite perpendicular to the ply, being perpendicular to the bevel instead, this will not matter. Your test fit will show how much of each feather edge to trim back.

When gluing remember that epoxy is slippery so get the planks positioned and well clamped before putting pressure on the joint itself. I like just laying a board across it (remember plastic on both sides so glue squeeze out won't be a problem) and putting some weights atop that.

One can make jigs like the nifty unit in an back number of our favorite periodical, but if you're just making one boat, and hand plane and outdoor rack are actually less total work, and ever so much more virtuous.
Gosh, does that mean that the 55' catamaran I built forty years ago is going to come apart?
Jay

[Ian McColgin]

Conventional boats have butt blocks all over the place and don't fall apart. There is no reason why a well made plywood boat with step scarfs in suitably places without extreme curves at the joint should have any problem at all. Quite obviously Jay's cat is not a risk from failing scarfs.

Thirty five years ago when I was building Leeward (Chamberlain gunning dory) I thought one had to use machinery, unable to get any of the borrowed saws to which I had access to make a straight bevel, I did some scrap wood tests. The one step simply failed and the three step made a hard spot visually spoiling the curve. In that respect, the three step was like the plywood butt joint that some Bolger designs use quite well in places where the curve is not much.

As regular readers know, my repairs of the Thistle I'm restoring amount to scarfing in four layers to join with a 5 layer 1/4" ply hull and these are essentially step scarfs applied one at a time.

So, if making a fair bend is not an issue for the plank, then a multi-step scarf might be a good choise if you have the tools. If, however, you've not spent your time supporting Yankee Workshop style power tooling, the beveled scarf is fast, clean and absolutely the best, though perhaps at times better than you need.

[Tom Lathrop]

Step scarfs in plywood are in most applications a dreadful mistake. A single step makes the plywood half as strong as it was. More steps are better but still are weaker than a proper beveled scarf and also make a hard spot where the ply will not bend fairly.

That was probably true in the past but an NC machine can make some very good scarfs that are both strong, uniform in bending and much easier to align than the beveled scarfs that you and I still make.

These machines can cut to a thousandth of an inch if desired and the small lip at the edge makes perfect aligninment easy and leaves a very planar surface. Many kits are being made this way today and are working out well.

[Pugwash]

There's a difference between a "stepped scarf" that has been around for a few thousand years and a "stair stepped scarf" in ply wood.

If you do a real scarf you're not really glueing end grain it's more like, or should be, 1:8 grain. Also, only @ 50% is "endgrain". (Uneven # of plys?, I don't have a big enough brain to work out the actual percentage.)

Scarfing is not that difficult, in fact it's easy and quite therapeutic.

Maybe the question you should be asking is "How do I sharpen a hand plane?"

That's a whole 'nother kettle of worms.

:)

[flydog]

I built a 12ft sailing dinghy that required the use of 6:1 scarfs for 1/4 in plywood. I did them with a hand plane and it was not all that time consuming. The scarfs came out well and the joints were stronger than the wood around them. I considered building a router jig, but I figured that the jig would take more time than cutting the scarfs.

[RodB]

The easiest solution for scarfing up to 1/2" ply.... works like a charm... perfect scarfs every time...

http://johnhenryinc.com/

RodB

[Longbow]

Thanks for the info. I sharpened my planes and chisels last night and I'm going to practice on some plywood scraps tomorrow. By the time I get ready to do the real ones I'll hopefully have the technique down.

[cybulski]

i considerd scarf joints, but i was scared of them, so i used used butt joints instead, im not really worried about the flat spots, i was able to position them in good spots, im gonna tape them anyway, and they will build back up to the curve