Stretching your plywood...just 1 technique

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Re: Stretching your plywood...just 1 technique

Postby MickinOz » Tue Dec 24, 2019 5:33 pm

I've fiddled with the photo in MS paint to show the approximate glue line.
The glued panel is 6 inches wide (~150mm)
I laid it over the edge of the bench this morning and couldn't flex the joint putting my (not inconsiderable :( ) upper body weight into it.
Since there has been no commentary saying this is a really bad technique, I think this is how my 17mm (11/16) wall panels will be extended.
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Re: Stretching your plywood...just 1 technique

Postby twisted lines » Wed Feb 12, 2020 12:50 pm

Looks good Mick :thumbsup:
I searched a little after you did your's, and found this and may try this self aligning trick next time :FNP
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Re: Stretching your plywood...just 1 technique

Postby MickinOz » Fri Mar 06, 2020 3:07 pm

Good Lawd. How the heck do they cut that?
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Re: Stretching your plywood...just 1 technique

Postby twisted lines » Sat Mar 07, 2020 11:38 am

MickinOz wrote:Good Lawd. How the heck do they cut that?



Push a button :lol:
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Re: Stretching your plywood...just 1 technique

Postby MassiveD » Fri Dec 01, 2023 2:10 am

Some random notes on scarfing. The main technique for joining panels end to end:

The slope of the scarf is 1-8 so 1/4" material would have a 2 inch scarf. However, this is for general use (much of which would be at higher loads than teardrops). Scarfs can be 1-20, or they can be less than 1-8. As a general rule, assuming the wood will be significantly loaded (not usually the case here) there isn't much point in scarfing over or under the grade of the wood itself. I am not saying this is an accurate example. But if one was scarfing a 2x4 there would not be a huge benefit to scarfing it 1-8 if the wood itself ran out at 1-6, or was full of knots.

The most common means of scarfing plywood is to use something similar to the Gougeon scarfer. This a a bracket attached to a circular saw that scarfs ply in one cut, at 8-1. It can't do ply thicker than 3/8" Your choices above that are to finish the scarf by hand, relying on the partial bevel, or to make a jig that cuts at 1-6. I have used the 1-6 with complete satisfaction. There are lots of other ways to make scarfs. A pro favourite is to step them off (so with 1/4" make a staircase with 2 inch run to the steps). Then grind it down. You can tell when you are done by all the straight lines from the revealed ply levels. This works well when one has a lot of sheets to cut.

Scarfing is used to make a full strengths joint in the wood, but also to maintain the flex of the ply. It also can minimize the print through in the final finish, though that can be a fail. When making boats they sometime need to use ply bent into extremely small radii. This puts huge stress on joints. Wood failure is often rated relative to allowable deflection. So floor joists are sold as 1-240, 1-360, etc... But something like the sidewall of teardrop may be a flat panel so flex won't come into it. They are often overbuilt to make them easier to assemble, or achieve a stiff floor, or make them warmer or quieter. So again, the scarf is probably not loaded.

A failure mode in a scarf, often results when a bend causes a rupture in the top, outside ply. On thinner ply this can often be as little as .6mm thick. This is part of the reason why glassing is often substituted. Even light glass can often replace the strength that a properly aligned .6mm of wood would have. Superior panels like BS 1088 (in most cases the BS is truer than the 1088) still only have 1mm plies. So one does not need to achieve all that much to make durable panels where glass is also used.

Before panicking about the scarf, consider the layout. I remember getting upset I was required to add a second scarf to a particular part, thinking maybe the designer could have designed it within the sheet lengths a little better. But when I got around to it, it wasn't a full width scarf, it was around 6" as the panel was tapering out at that point. So consider where the scarf will be cut, and try to minimize excess work that way.

A lot of the stuff written about joining ply is based on hugely loaded structure like racing sailboats, and aircraft. Be sensible about how extreme you get assembling structures for boxes on wheels. A pal of mine just finished a 46 foot catamaran that was made of 1/2" plywood, and joined together with butt blocks. A 30 foot racing tri might use 1/4" ply. So again, be reasonable in your scantlings for a trailer.

An excellent alternative to scarfs are butt blocks. These are pieces of plywood attached across the joint. So a 4x8 sheet might be joined to another with 4-6 inches of plywood glued to one side. If these joints are to be loaded, it is very important that the wood within the glued surfaces have the same grain orientation. Working with ply when making boats, aircraft, and some slippery trailers, will often generate a lot of long thin pieces of ply, from curved cut waste, that would be ideal for butt blocks other than that the strips have the wrong grain orientation. There are some workarounds for this, but generally, it is better to simply align the grain. The trick with butt blocks, in something like a trailer is to figure out where to put them where they will not be unsightly. They can look great but they can also look amateurish. Putting them where they will transition bulkhead loads can look professional. But having them stick into your shoulder at the bunk, not so much. Butt Blocks tend to leave very easily concealed seams on the outside of the surface.

Puzzle joints are an excellent method, but mostly exist to calm fears about joining plywood panels that newbies might feel. I did try making my own with a pattern, and it worked, but I was unprepared for the huge noise and dust that even my small joint required, in the making. They don't have good flow so the panels do not flex properly, if that matters. I think a lot of the attraction is how easy they make assembling boats, and I think people look forward to the satisfaction that comes of assembling parts with technology they remember from their childhoods. They can also fix the parts to some extent, minimizing the need for tools a punter may not have. I have seen a few cases where they were used artistically.

https://guillemot-kayaks.com/sites/defa ... k=Scu7sebb
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