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Porthole Linings

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Anonymous

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Hi,

I am currently fitting out a narrowboat and one task I have to do is make nine ash porthole linings. These are approx 15" dia and 3" deep. Any ideas how to go about making them?

Thank, Colin
 

Scrit

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Assuming that you don't have a vacuum bag press you can still laminate-up your rings around a wooden form - the form should be wide enough to make three or four rings at a time.. This can be made up from 3/4in F/E hardwood plywood. Start with a single layer of ash veneer then add layers of 3GL birch plywood (1.5mm thick BTW). You should aim for 3 to 5 layers of birch plywood wound round the form in a single pass and use a waterproof PVA (D3) type glue or better still a UF or RF-type glue (eg. Extramite). To keep everything in place whilst you are veneering it up it may be an idea to use an air pinner (18g or smaller) to hold the outside edges of the stock - you are nailing the edges to the form, but it doesn't matter as the form is sacrificial. The job can be done with tacks, but then you will find that you need to remove tacks from the previous pass each time you go round. There is also no reason why you shouldn't have multiple rows of tacks, say 3-1/2inches apart around the circumference of the tube you are forming. As you go round you may also need to use a small batten held by a couple of G-cramps to stop things unrolling whilst you wind the stock onto the form. Try to keep everything tight as you apply it and hold it all together with band cramps whilst the glue is setting (these can be as simple as bicycle inner tubes with Spanish windlasses). When it has set bandsaw or use the table saw to split the whole into rings, discarding the ends and any sections from the middle which you have nailed through (which are full of nails in any case). To accomplish this will need a V-bottomed cradle rather like an engineers V-block to keep the work square to the blade.

Hope that points you in the right direction

Scrit
 

Alf

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I've never done this, and I wouldn't have a clue if it would be suitable for a porthole, but could you make a mitred frame which you then cut into a ring using a router and trammel bar? Hmm, maybe not 3" deep though, depending on what you mean by "deep" of course. I suppose you could do several and glue them together if you meant 3" thick, staggering the joints would be helpful for strength too. FWIW (not much, and if you don't get the mitred frame idea I'm going to have to resort to pencil, paper and scanner!)

Cheers, Alf
 

Scrit

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Biggest problem, Alf, is that the longest router bits don't give you much more than 2 inches.... yes, I know that you can get special long bits, but they cost a heck of a lot and my experience is that they are remarkably prone to bending (and breaking). It is also certainly a lot easier to rout around the outside of a circle than the inside, even with a pin router.

If you laminate up tubes from 3GL birch plywood they are remarkably strong at 3 to 5 layers. The technique I quoted is in effect the manual version of the veneer bag lamination I use for some of the stuff I make, so I know from experience that it should work, although I "tack weld" laminations to each other with a wood welder (RF glue curer) these days for speed. As ever choosing the correct glue and leaving it to cure long enough is essential, as is covering the form with a release compound (e.g. waxed paper) - sorry, forgot to mention that.

Scrit
 

Alf

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Scrit,

Yep, even as I was typing I saw the flaw. Just a possibility if lamination wasn't an option though perhaps.

Cheers, Alf
 

Scrit

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Of course, if it were possible to persuade the client that birch is very similar to ash (well, they ARE both light - or what about woodgraining which was very popular in traditional narrowboats....), or that a painted finish was what was really required, nthe whole task would be a lot simpler and cheaper. Sadly my technique still involves routing the elements of a round form and is quite a bit of work, too.

Scrit
 

Signal

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Alf, Scrit

Just been on my uncles boat and checked the portholes
their brass... no only kidding.

Alf seem closest with her method.

They appear to be mitred sections which have been spun to a circle
by some method, hmm how about turning them?

Signal
 

Scrit

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Signal

To turn them would be like inside or end turning on a metalworking lathe. The only type of woodworking lathe I know which will accomplish this repeatably is a patternmaker's lathe (the type with a carraige like an engineer's/metalworking lathe). I reckon that turning a straight sided ring like that would be quite difficult with a bowl turning lathe as lathe tools are not normally designed for this (you need a turning tool shaped rather like a hockey stick to achieve a parallel sided hole like this and the danger of digging in on inside turning is very much more than outside turning).

If you can live with the end grain and don't want to go the lamination route, why not go with Alf's suggestion of three routed rings glued together. You can always reduce the amount of end grain by coopering a hexagon or the like together. It is a simpler approach than the laminated method I would use.

I've just had a thought, did you mean port hole linings, as in a sort of spacer to go between the (steel?) outer wall of the boat and the inner wall, or did you mean a porthole frame, to hold the glass?

Scrit
 

Signal

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Scrit,

yeah turning them would be a pain, just chucking another idea
in the pot. :eek:

As I said, Alfs idea looked closest to what I saw, from what I could see looked like a hexagon which had then been cleaned up with a router. Unfortunately my Uncle is in Crete at the moment so I cant ask him for further details.

The bits I saw where the pieces you described, spacer between out and
inner wall of the boat, about 3 inches thick from memory. Hope this is what was meant

Signal
 
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Anonymous

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How about mitred frame and round em off with a band saw as 3" is a fair depth for a router
 
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Anonymous

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Hi,

Many thanks for your helpful ideas. I have seen some portholes linings made from 4 sections. I had assumed that these had been cut from 1" thick ash board on a bandsaw, jointed together and then routed somehow and a rebate then formed on the inside to take 3" ash veneered ply. However, your comments have now provided me with a couple of alternatives.

regards, Colin
 
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Anonymous

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Hi,

Many thanks for your helpful ideas. I have seen some portholes linings made from 4 sections. I had assumed that these had been cut from 1" thick ash board on a bandsaw, jointed together and then routed somehow and a rebate then formed on the inside to take 3" ash veneered ply. However, your comments have now provided me with a couple of alternatives.

regards, Colin
 

Waka

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Hi

I have just made a porthole liner for my narrow boat. It was done on my lathe and I guess start to finish is about 1 1/2 hours. I used oak as my choice of wood and they turned out quite nice.

The dimensions are 15" by 2" deep. If you don't have a lathe then not sure how to make them.

You can always buy them but \i think you would be paying about £50 each whereas for the cost of wood that comes down to about £8 per liner and you still have the inside portion to use again.

Need any help on this gove me call. 01305 835665

Waka
 

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