How strong are glued and headless pinned, are obeche butt joints likely to be?

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Not your average framer
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How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by Not your average framer »

I have for several years had it in mind to produce a gothic style spandrel frame in obeche to frame a medium sized mirror. I have quite a significant stock of 34mm x 12mm obeche moulding and I am thinking of marking out pieces of the moulding using backing board templates and a pencil, so that I can out the individual pieces of the spandrell and assemble and join them using glued and pinned butt joints. I plan to have a flat marked out assembly template to enable accurate placement for gluing and pinning using my pneumatic head less pinner. I need to keep the size and therefore the final sales price within sensible limits. This is not something which is necessarily simple, quick and easy to build as I try to plan other projects to be, but this is something which has always interested me and I will be doing it partly for my own satisfaction, recognising that getting a good profit margin out of this, while also being able to sell it at an affordable price will have it's limitations. The outer frame will be produced from a deep rebate pine moulding to provide something robust and solid.

This outer frame needs to provide something really strong and rigid to stabilise every thing else which will be fixed inside. The headless pins will hopefully not be visible after a couple of stipled coat of a stone coloured paint. I have several recycled sheets of 4mm mirror glass, which cost me nothing and still need using up, but I still need to be careful about my expediture on everything else in order to maximise my profit. You just don't see items like this at sensible prices anywhere, so there should not be too much pressure to sell something like this too cheap. Most of the gluded and pined joints, will be T shape butt joints featuring long grain and end grain facing each other. My headless pinner will not significantly disrupt the internal wood grain structure, but since one face is likely to be end grain the strength of the glued join with be of limited strength, so the pins will be required to increase the strength of the joints. Since my longest headless pins are only 20mm long the width of my obeche sections will only be about 10mm wide to ensure there is still 10mm of the headless pins to penetrate into the end grain at the other side of the joint.

It should be an interesting project and I plan to take some photos for the furom. The various shaped pieces will be a straight forward opporation to cut them by cutting along the pencil lines using my band saw. I was hoping to be producing a trefoil design, but if the open circular bit where to be cut out od obeche somewhich around the circular pieces, part of this circular part would be formed across the end grain of the obeche and therefore lack any durable strength, so this needed to be avoided. I have no experience of what sort of strength, I am likely to be able to obtain from such butt joints in obeche and although I think that they will be adequate, I would be interested to hear from anyone else who may have better knowledge about this. I an not expecting any of the joints to be under any tension whatsoever.
Thanks,
Mark.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
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Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by JFeig »

Obeche wood by itself in an average strength wood. A miter joint is the weakest style of joinery. A lap joint would be much stronger. As for a glued and pinned miter joint, the joint is stronger that the end grain of the wood.
Jerome Feig CPF®
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Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by Not your average framer »

Hi Jerome,

Yes, that is already my understanding as well. A butt joint is both long grain and end grain facing each other, but the end grain sucks some glue away from the area of the joint into the area away from the joint, resulting potentially a lack of an adequate amount of glue remaining at the site of the joint. The way that these gothic spandrels are constructed, I am expecting the pieces which present the end grain faces to these joints will be facing long grain joint faces at both ends. The headless pins will be driven in through the long grain pieces at either end which will not be piece which are acting to pull away from the joint, so no tensile stain acting on the joint.

However the area of glued contact at each joint is likely to be 12mm by 10mm square. It won't help me to to be allowing a coat of glue on the end grain side of the joint and allowing that to set before finally gluing and assembling the joint, as this would made the job a much slower job than I would expect to be financially viable. I really need to glue each joint and complete the assembly in one go! Fitting the shorter end gain pieces into a dado cut out might be a possibility, but perhaps not ideal, however it would add some extra glue contact area. Adding a dado cut out would enable me to widen the pieces of obeche by bringing the short part into the longer piece and reducing the distance before the headless pins penetrate the other piece.

I think that maybe this is starting to make more sense now and maybe a single bandsaw cut dovetail might be something which could be practical!
Thank for making me think straight,
Mark.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
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Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by JFeig »

Have you tried a "seal coat" of glue for the end grain and reapply glue after for the final joint assembly?
Jerome Feig CPF®
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Not your average framer
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Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by Not your average framer »

Not so far, but I do have a dovetail router bit and I am strating to think about using a dovetail joint to join these piece together and this is really starting to make better sense to me. Thanks your your help.
Mark.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
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Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by prospero »

Have you thought about using a biscuit joiner?

For narrower mouldings you can cut the biscuits in half. You do need to contrive some sort of strap clamp or use
spring clamps, depending on the nature of the joint. With biscuits the vertical alignment is set, but the joints can be
moved laterally. You can't do that with dovetail joints. :D
Watch Out. There's A Humphrey About
Not your average framer
Posts: 11013
Joined: Sat 25 Mar, 2006 8:40 pm
Location: Devon, U.K.
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Interests: Lost causes, saving and restoring old things, learning something every day
Location: Glorious Devon

Re: How strong are glued and headless pinned, are obeche butt joints likely to be?

Post by Not your average framer »

Hi Peter,

I'm not that keen on buying a biscuit joiner, as it is makes it a more time consuming job. It is a fairly simple matter to attach a sacrificial fence to the mitre gauge on my mitre table and line up the position for cutting a dovetail. I even cheat a bit sometimes and cut two emply dovetail cutouts and align them with a piece of masking tape at the rear and then fill the bow tie shaped hole with Aradite. It's like casting your own hoffman bow tie inserts and if you over fill the holes, you sand off smooth afterwards.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
― Geoffrey Chaucer
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