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thomaskennedy

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Hi,

In the midst of making a (reclaimed) mahogany coffee table, i decided that i would have the top made from pine with mitred mahogany edges...

But the problem i am having is i don't want to joint the edges (26mm thick and 85mm wide) straight to the pine (18mm thick)....

I think this because i jointed some edges to a veneered chipboard that were 70mm wide with biscuits, and they have worked loose from people sitting on it :roll:

Any ideas how to strengthen the joint, as i imagine that it will be used as an extra seat as well :roll: :evil:

Ta

Tom
 

Philly

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Tom
What about using splines the whole way round. You could make these from ply and will give a good joint, better than biscuits.
cheers
Philly
 

Waka

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Tom

With the thickness of the wood you could afford to double up on the biscuits. Also if you put them about 1" apart you will end up with a very strong joint.
 

johnelliott

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IMHO, those mitres are going to open up in a period of time, which could be quite short. An open mahogany frame might open at the mitres, around a piece of solid pine it's a virtual certainty.
I would suggest modifying your design to eliminate the mitres
John
 

frank

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thomas why not just put bread board ends on . and some sharp nails showing in the top then see who sits on it . :twisted: :twisted: :twisted:
 

thomaskennedy

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and some sharp nails showing in the top then see who sits on it .
What a great idea.....they'll soon realise that a coffee table is for putting coffee's on and not an extra seat :p

But seriously...

IMHO, those mitres are going to open up in a period of time
Really?..Even on Mahog. Boards that are around 100 years old :shock:

Philly...I was thinking that but won't the thinner pine just snap (depending on the size of the splint of course)....as this is really dense, heavy mahog. :?

Ta

Tom
 

johnelliott

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thomaskennedy":3vxrbiff said:
IMHO, those mitres are going to open up in a period of time
Really?..Even on Mahog. Boards that are around 100 years old :shock:
It's the pine that's going to cause the main problem. It will expand and contract across the grain as the humidity changes. The sideways pressure when it expands will force the weak mitre joints open, because the piece of mahogany going across the grain will have its grain at right angles to the pine
John
 

Midnight

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John's spot on Tom; your pine will expand and contract at totally different rates to the mahogany so your construction will have to allow for this. A breadboard end is by far the best solution in this case, possibly with a razor wire edge banding... ;)
 

thomaskennedy

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:oops: :oops: Duh....Stupid me....

Hmm....Still unsure....what about if i did something like this...



Damn i wish i could draw on a PC :( :cry:

I hope you get the idea, i dunno if it would work, frankly my mind has just about given up with ideas!!

Isn't there a special, or even magical router bit that would solve my problems forever, *sigh* :wink: :roll:

Ta

Tom

ps. Sorry about the delay in reply.....been ill over last few days :(
 

johnelliott

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Unable to see what you're getting at there, Thomas, but regardless of what jointing system you use, the idea of mitreing a frame of mahogany around a piece of pine is NOT going to work.
It's the basic idea you need to change, not the joints
All the above IMHO of course
John
 

frank

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tom fit your ends dry then drill holes in the ends and the tenons ,take off ends and elongate the holes in the tenons fit ends back on and fix with contrasting dowels the wood can then move and not split .

ps dont use glue
 

tim

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Unless I'm mistaken that drawing looks pretty close to a breadboard end.
T
 

Midnight

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Tom...

the basic flaw with the concept isn't so much the jointing method, as the dynamics of the materials; I'll try to explain.

You've been around here long enough to have read a few times that natural wood is a dynamic material; it expands and contracts seasonally in reaction to seasonal changes in humidity. That expansion / contraction is predominantly tangental i.e. it moves a lot more across the grain that it will move along the grain. Now.. try to apply that to your top design..

As I understand it, you want to surround a pine top with wide mahogany edging on all 4 sides, mitering the mahogany at the corners... it's an idea I've had myself more than once...

Now... try to model that tangental movement with your design. The long grain to long grain joints are the strongest, and most stable too as there shouldn't be any dynamic conflicts. However, the long grain to end grain pieces at either end of the top will have a much harder time of it; your jointing will need to allow for some movement, but it's technically possible to do (breadboard end being an example). The worst of the trouble is at the mitred corners. The boards capping the end grain will hardly be moving at all, while their mating halves will be expanding and contracting quite freely. That is, until you make the glue joints at the corners. In time, the seasonal movement will trash the joints due to the unequal rates of expansion / contraction; the joints will weaken through fatigue, leading to their eventual failure.


However... it's not all doom and gloom...

IF.... you concede the point about the mitres, but figure that the look of the wrap around mahogany is more important than how it's joined at the corners, you can make it work by....(you guessed it) using breadboard ends. They'd work pretty damn close to how you've drawn them, although I'd be inclined to use just 5 M&T's across the width of the top (centre+each end of the pine section & 1 in each of the side pieces). As Frank said, lock each M&T with a dowel through each tenon; you can glue the central dowel in place to centralize any expansion, but you must slot all of the outer tenons to allow them to "float" across the dowels. Remember to keep your slots tight in the long grain axis (barely wider than the dowel) but loose in the cross grain axis (to let them float freely). You can lock the outer dowels in place by driving them most of the way through the joint, adding a dab of glue to the holes just prior to driving the dowels home.
As your sketch (nice sketch btw) suggests, you can reinforce the whole with additional tongues, but don't glue them.

For the record, I'd still edge the lot with razor wire to deter errant backsides.. :wink:
 

tim

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Now i understand - I was thinking that Razor wire inlay was a fancy term for really thin detailing :lol: :lol:
 

Philly

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Thom
Only other way (if you want to do the wrap thing) is to make the top like a framed panel. This has the pine section "floating" within the mahogany frame, allowing the pine to move happily without causing problems.
I reckon if you used mitred bridle joints for the corners you'd have a nice strong joint.
Hope this helps,
Philly :ho2
 

frank

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or what about veneered MDF with mahog wrapped round and mitred joints ,ive got me coat on and i'm off .
 
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