Bendability of Robbins Ply for Rolled-in Decks

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chris
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by chris »

It would be stiffer to bend though.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by JimC »

chris wrote:It would be stiffer to bend though.
Agreed. Everything's a compromise?

Chris, do you think your steaming technique gave sufficient time for the heat (and moisture?) to penetrate to the middle of the wood? The references I've seen suggest that the primary effect of steamed timber is that the cells on the inner side of the curve compress, but that those on the outside do not elongate significantly, so its necessary for the steaming to affect the wood right through the middle so that cells in the middle layer of the timber are just as flexible as those on the inside.

I think I've seen a reference somewhere that Uffa used to immerse wood in boiling water rather than use steam, but either way the temperature will be much the same I suppose, its not as if the steam will be superheated!
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by chris »

It's a technique that is used in musical instruments sometimes where you are dealing with fairly thin timber say upto 5 or 6 mm. It does force steam into the timber rather well as it is in effect under pressure. I have steamed ribs this way quite successfully both for boats and harps!
Violin sides are usually bent with dry heat ie no steam, over a heated former or shaped iron.

I have read that for the cell walls to become soft and slide or reshape the temperature needs to be above 130 degrees an as soon as the timber comes out of the steam and drops below that problems can happen. Certainly there must be a gradation of layers of cells moving rathe than just the top or bottom layer. I think the problem with ply is twofold, first you have layers of glue remaining rigid and forming a seal , and secondly layers with grain at right angles wanting to do different things.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Obscured by clouds »

Hmmm there seems to be a fixation on using expensive ply to do something it does'nt really want to do ie bend round something.

No-one has yet commented on or answered my query as to why 3 [or more] layers of boat building veneer cannot be applied around what is a relatively simple former. got to be cheaper than elite at a £100 a sheet or whatever it is this year.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Rupert »

I assume that they are trying to avoid cold moulding, especially if the original was made from ply. Mind, an ugly layer of thin ply (to give the rigidity) followd by a pretty veneer would look OK.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Obscured by clouds »

an ugly layer of thin ply (to give the rigidity) followed by a pretty veneer would look OK
door panel?
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by JimC »

chris wrote: first you have layers of glue remaining rigid and forming a seal
And a seal, presumably, against moisture getting through, should that be an issue. I'll know more about this next month I expect! Not altogether looking forward to this as its way out of my comfort zone...

------

Noting the suggestion about laminating up a plywood from veneer, it does occur to me that with these decks appearing to be a constant section/radius it ought to be possible to make an mdf/batten jig fairly readily, and prefabricate a piece of prebent plywood from veneers off the boat and then cut it to size once it has cured. I would guess it would be rather less trouble to laminate the veneers off the boat rather than on, especially as if you make the jig suitably oversize you could staple the veneers down in the area to be discarded and save a lot of trouble removing staples and filling holes.
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Ed
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Ed »

Nothing has been discounted or ignored....

everything is still being considered. :-)

We have certainly thought about the concept of cold moulding it. Would be like a Fairey Fin or Europe and would look very pretty.

I am sure we will talk more on this. As rupe says pulling some nice kyaha veneers down over some cheaper thin ply might be a good compromise....but....there are a couple of issues.

To keep it all consistent, you would either have to carefully choose some ply that matches....or do the fore and aft decks the same way. One would be hard and the other I am not so sure about.

Then, as rupe says....the idea is as much as possible to keep it original looking. And originally it has a high quality very red 3 mahogany ply board, that looks much like the Super Elite Plus, but this is only available in 4mm...and well....that was why I was asking.

I like JimC's idea of either moulding it, or partially bending it over a jig before getting it into the boat. That is an interesting thought.

I totally agree with Chris's reservations about steaming ply. I am sure as you say, the problem is due to the glue lines and the differing grain directions.

I think I will have to dig through my bits of wood and see what i have got to test with.

Less than I had though....

What do you say when you come back to find the very nice builders who are plastering a room say: "Oh, hope you don't mind, I pinched one of the bits of scrap ply you had against the wall in your workshop".....and of course it wasn't the half-sheet of 8mm crap-ply from Tragos with a value of few quid, it was a half sheet of 4mm super elite plus......sigh....

thanks again.

The decision isn't made yet so keep the thoughts coming in and I guess we will talk at w/end so will tell you what we decide to do.

cheers

eib
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chris
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by chris »

I had to veneer up a small amount of ply to make a fore- tank in the 14. Yes it was flat sheets and I had just enough mahog veneer in stock. I used the vacuum press and epoxy and it worked well and looks the part. I often get my veneer from http://www.originalmarquetry.co.uk which is in Bristol . Although they specialize in cabinet work they do have a good range including constructional and sawn veneers so finding the right match shouldn't be too hard if you go down that route..
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Obscured by clouds »

that reminds me, I really have to stick some burr oak veneer down onto a table I've restored. should have done it months ago....

whoops thread drift.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Rupert »

Tricky putting a drink down on the table if it is a curved as a Flying Moth side tank.
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by JimC »

Chris, when it comes to steaming and effectiveness thereof, what is the effect of grain alignment? If the effect of the steaming is to enable cells to compress then it just must be related to the grain. I keep coming back to thoughts about the alignment, and this splitting problem... |It really seems to me that if the grain is perpendicular to the bend it must be less prone to splitting than if parallel to the bend, and in the case of ply it means if you align the outer grain perpendicularly the inner layer which is parallel to the bend will have minimal distortion. And then the panel is also stiffer oriented like that. None of that helps if you can't get the wood round the bend of course, but I'm wondering if the steaming might be more effective with the fires perpendicular to the bend which would somewhat reduce the problems caused by the ply being intrinsically stiffer in that direction...
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by chris »

Its compression of the cell wall as well as softening the lignin (sp?) that glues the cells together. The cells are much longer in line with the gran which is why cross grain timber is very easy to snap. ( ask any karate kid which way they have the grain). I'm currently restoring a piano from 1830 and have had to repair the edging veneer round some tight curves round the lid. I had to apply brown paper tape to the veneer to do it without cracking. As this was traditional veneering with animal glue I was using a hot iron any way. I just turned the heat up a bit and the moist glue acts as moisture then bent in situ.

But have just done an experiment with plywood ( see photo) the ply is 3ply 3mm poplar from avon plywoods cost about £17 for a full sheet. It is very light indeed but only graded as water resistant so don't use it for a hull. I used a piece about 7" x2" . The method of steaming is a little eccentric but only OK for small pieces, on the other hand very quick.
1 soaked material for about 1/2 hour
2 wrap in wet paper towel
3 wrap in cling film.
4 Microwave for one -two minutes
5 bend round wine bottle.
6 remove after a minute
7 remove cling film etc
8 tied string round it to hold shape.
9 Microwave for 1/2 minute to dry it,

It has held its shape very well, has not split or delaminated. it very stiff and strong. The tightest curve has a radius of about 3". Grain follows round the curve.

Mind you poplar ply is very easy to bend cold and 3mm would, I'm sure bend into these decks with no trouble. I don't know if a WBP or marine version is available.
bend.jpg
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by Rupert »

That's amazing. Now, where can I find an 8x4 microwave?
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Re: Bendability of Robbins Ply for Rolled-in Decks

Post by JimC »

Rupert wrote:That's amazing. Now, where can I find an 8x4 microwave?
They exist I think you'll find...

http://www.heraldscotland.com/news/home ... -1.1069991
Fitting the latest microwave cooker in your kitchen may be a challenge – the device developed by a small Scots firm is the size of a small car.
Grangemouth-based Advanced Microwave Technologies (AMT) has developed the world’s largest microwave in conjunction with Edinburgh’s Queen Margaret University.
I think personally I'll have a go with fabricating a steam oven with a wallpaper steamer and lots of plastic sheet though. I don't think I've got any bits bigger than about 3'*3' to do...
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