Green Timber structures- Any experts in the house?
I work for a college on a furniture making course.
We are currently in the process of making a really exciting commission for an outdoor bench for a country park which one of the 2nd year students won a competition to design and make.
The design is essentially like a series of 12 square-edged posts 150mm square mounted together 4 across and 3 high with spacers between.
The square part of the bench runs about 3 metres, then after that, each post runs a further 3 metres approx but tapers down into twisty, spindly, tentacle-like shapes.
Imagine a squid having a bit of a lie down and you get the idea..
Our issue is, although very experienced in cabinetmaking, none of us really have much experience with large-section green timber construction. The bench has to be constructed without screws, glues or expensive fittings etc. Think of it as everything has to come from the woodland.
A small model(does not show full height of layers)
So….what we are needing to do is to join these long, spindly tentacle sections to the main body of the piece. As you can imagine there will be some significant overhang. I’m not even sure this overhang will work due to the weght, but anyway, we’ll see.
Does anyone have any recommendations for joining the ends? I was thinking a halving joint or a scarf-joint with green pegs driven through and perhaps wedged either side…?Posted 5 years ago
Anyone have any thoughts..book recommendations? People to approach for advice?
I know there are some woody types on here so thought I’d ask.
Yep, that’s me!
Having read and re-read your post kayak, I’m still not with you entirely, however I would suggest that due to the weight of these benches, your basic scarf is going to struggle a bit with the overhang. There are of course many other types of scarf and I’ll try to source the best publication for you. Unfortunately, I’ve lent my books and as per, forgotten who to and they have not been returned.
Alternatively, where are you? I’d be happy to pop over to discuss if you’re reasonably local to me.
Sorry not to be of more direct help, but without a drawing of what it is you want to achieve, it’s a tad tricky for me to recommend a specific joint.
Just bear in mind that the strength will come from both the shoulders of the joint and the pegs. These should be dry (i.e. not so green as the beam stock) and the bores will need to be ‘draw-bored’, i.e. offset by circa 1/8th to draw the joint together.
Looks like a fun project and I’m loving the tentacles!!Posted 5 years ago
Hi Slackalice. Thats brilliant.
We are in Leamington Spa, at Warwickshire College.
I didn’t have any decent pictures of the model at the moment but essentially, we have 3-metre sections of square, which we need to join to approx 3-metres tentacles, which start square and then taper down into spindly bits.
I can come up with some better drawings and/or photographs if you think you could help. That would be fantastic.
The square section timber is freshly felled Sycamore and the tentacles are Ash, also green.
I understand what you mean about the seasoned pegs and the offset. Some methods translate across to the cabinet shop.
Thanks for the book recommendations too. I may be able to get our library to get them in.
It would be good if you did happen to be nearby but if you wouldn’t mind me running a few ideas past you then that would also be great by email or whatever.Posted 5 years ago
I,m on the south coast, so a little too far for me to pop in methinks! However, will do all I can to help by email – my addy is in my profile, so please drop me a line.
A line sketch with dims, scanned and mailed to me would be fine and probably enough to get my head around what you want to achieve. In some ways, I’m wondering whether the timber use could be swapped, i.e. use the lighter Sycamore for the tentacles? It might also work better for shaping too, as the Ash may want to split out as it inevitably seasons.
Once I have a sketch, I can do likewise with some suggested joints and mail back to you if that would help.
Can I have your job please??!!! 🙂Posted 5 years ago
Thats great Slackalice, shame you’re on the southcoast.
Basically, the timber choice was dictated to us by the park by what dimensions of trees they had knocking about. Otherwise it makes perfect sense what you say.
I have spoken to the student this morning and he has some better drawings at home which I will get hold of and send them your way if I can and hopefully you can help us out. That would be awesome.
Most of the shaping we have done already. It’s pretty hard work but basically we are holding these huge logs and freehanding them through a band-resaw into curves, and then shaping them further with powerplaners and an Arbotech and sanding. Seems to work ok but as you say, they could potentially split.
This is the sort of thing I think we may have to do initially..
Not only do we have that joint to do, but also we have the perhaps more basic task of joining each square section post to the one below it with a spacer/gap between to allow rain to fall away. Maybe you can help there too? 😀
I’ll get hold of some more stuff and send it your way. Really appreciate this thanks.Posted 5 years agobajsyckelMember
Looks like an interesting project (keep us updated!). Not sure I can add much with the current info and am probably way less of an ‘expert’ than you are after. One question though on the ‘tentacles’ – are they effectively cantilevered from the square section, or do they rest on the ground as they appear to in the model? It would seem to make a big difference to the work that the joint would need to do.
Also I wonder about the aesthetics of the thing – jointing at more or less half way along the whole and inevitable differences as the ash and sycamore dry, plus the way in which the join should appear (as well as function). It will be difficult to make a join between the two species that ‘reads’ with the same flow that your model does both short and long term. Might not be a problem but maybe worth thinking around. Obviously the fact that your lengths are already cut (and shaped?) may well limit your options in terms of what is jointed to what and the positions of the joints etc…
One reference book that I think I used to find really useful (and still enjoy reading just for pleasure) is Klaus Zweger’s Wood and Wood Joints. The English version was reprinted recently so you might be able to find a copy online or get your library to borrow it – it’s pretty expensive (but worth it IMO).Posted 5 years ago
Thanks for that bajsyckel, all helpful stuff. I think you are right in terms of joining and different shrink rates etc.
I reckon in the big schem of things it won’t matter too much as it will obviously be outside and weather and generally get nibbled by squirrels etc so a few mis-aligned joints shouldn’t matter too much.
You are right though. The tentacles are effectively cantilevered off the main body of the piece. I hope it’ll work.
Here is a couple of shots of his model, though the model does not show the gaps between each post that are now in the design to allow for rain to pass.
If you imagine that model as about 6m long and more. The tentacle parts need jointing to the square parts…Posted 5 years ago
Ah! thanks for the pics of the model, I can see exactly what you’re trying to achieve and I reckon with some judicious staggering of the scarf joints, they will not need to be quite so complex as your second image.
As far as spacers between the sections, do these need to run both vertically and horizontally? Either way, I would have thought some continuous batten, machined from your Ash stock would do the job. If that was of say 60mm x 60mm, perhaps a rebate could be cut into the sections and the batten fixed with a simple peg, flushed off to hold them in place and also prevent any lateral movement?
Bajsyckel raises some good points, especially regarding the whole leverage/cantilever effect and forces, so the trick will be how to secure the square end of the bench to the ground support (presuming the bench will sit upon something? Dunnage like?). Or will the bench be resting completely on the ground? The latter obviously makes it easier to balance!
Forgive me if I know enter grandmother and eggs territory, but the trick to ensuring the security and aesthetics of the scarf joints will be to lay-up the 2 bits of Ash and Sycamore to be joined so they are perfectly level in the horizontal and vertical. Use a boat level for your vertical rotation and make a small rebate to ensure the level sits flat and also this ensures you check levels in the same place each time you need to set up after trying your joints. Chalk line on the floor will also be invaluable and will help you level up from that to make sure the beam stock is aligned and as straight as it’s ever going to be.
It will then be a case of scribing your shoulders, ideally, you will want to set the pieces up so they are overlapped and you can transfer your scribe lines with your straight edge.
I’ll stop here and tootle off to think about the joints…. I dont think you’re going to need to be too fussy over them except for the one’s on show, but I’ll need to draw some stuff out.
Laters 🙂Posted 5 years ago
tymbian – Member
Where abouts will this be located after it’s finished..forest floor, concrete courtyard etc?
Do the tentacles start immediately after the joint or is there a ‘ run-on ‘ of a meter or so before the tentacles start to narrow?
Hi, the bench is sited in a woodland and should be laid down on some sort of gravel/shale base to allow for drainage I’m told.
I reckon we have on average about 700mm before the poles start to narrow. I was thinking that it wouldn’t be the end of the world if the joint was made, and then cut as part of the taper anyway, providing not too much is taken off.Posted 5 years ago
Just a little update.
So far, we are trying to do the very simplest joint we can and are trying a halving joint on a spare square section timber.
It will of course be harder in the rounder tentacle parts.
We were thinking that we could have the joint running vertically instead of horizontally to retain more ‘meat’ to resist the forces of the cantilevered tentacles…
The halving joint.
Banging in the pegs(broom handles for the practise joint)
Sample joint cut, fitted and pegged.
One issue we have is what pegs to use and where to get them. We need hardwood seasoned pegs. I’ve looked online and you can get them but cost is pretty high for the amount we need. If anyone can suggest an alternative that would be grand…
The practise joint as it is, is strong as hell I’d say. What are your thoughts on the strength of that considering the cantilever/overhang, anyone? Its about a 400-500mm lap.Posted 5 years agobajsyckelMember
Looks fun and I’m a bit jealous of you as I sit behind a computer screen. My thoughts are that making a workable joint isn’t the difficult part given the dimensions of the timber, and that depending on how much meat is being shaped off with your arbtechs then a pinned joint like your sample there could work OK (assuming that it’s oriented the way I imagine it to be, it will be strong with that lap but only in certain applications).
However, I’m not sure I am with you on the location of the joint relative to the supports, and making a joint that is still strong in a cantilever is tricky. And the fact that it’s green means that more complex joints of the sort you might have adopted in dry timber to combat this doesn’t help either. Then the reality is that the tentacles will likely sag over time given their length unsupported (though that might not be as bad as it might seem). That said – and looking at an earlier post – do you mean that you have 700mm to the ‘tentacle side’ of the joint before they are shaped? If so can you support or pin the tentacles (discreetly obviously) on the tentacle side of the joint before the cantilever. That could help reduce some of the issues before anything else.
Still looks good and I’m sure you will get something to work…
RE the pegs – what timber and dimensions are you looking for? square or circular section? And quantity? If you can’t find any anywhere I could see if I know somewhere that could help out…Posted 5 years ago
RE the pegs – what timber and dimensions are you looking for? square or circular section? And quantity? If you can’t find any anywhere I could see if I know somewhere that could help out…
We are looking for a hardwood that’s fairly resistant to weather. Oak I suppose is ideal.
It needs to be seasoned, about 25~30mm in diameter and I’d guess at about 10 metres or so considering all the joining we have to do.
I’ve seen some timber framing pegs for sale on eBay which are actually hexagonal, but they cost a bomb.
I think the tentacles will inevitably sag over time but that’s fine I think. Personally, I’m a little worried about kids climbing on them, falling in and hurting a leg or something. Hopefully it’ll be obvious that, that’s not a good idea.
Matt_outandabout, that office project is fantastic. What a great project to do.Posted 5 years ago
Apologies for my absence, its been really busy here, however Ive sketched out the scarf joint I believe will work for you. Have you got an email addy I can send the scanned image to?
As for pegs, go to Coyles Timber near Bath, they will do you 3/4″ drawn oak pegs, they work out at less than a £1 each IIRC.
If you dont have an email I can send the sketches to, Ill try and post them onto flickr or the suchlike and get them onto here…Posted 5 years ago
Im still gobsmacked as to how easy that was to scan and post!! hey ho. Im sure that would have taken me much longer on my old PC with much associated swearing….
Ok, to try and explain my sketch…. I havent put any dims for the length of the tenon but I reckon a man of your calibre will be able to work that out and you’ll have a ‘feel’ for what is going to be the right length, possibly circa 250mm/10″? Tenon and shoulders I would suggest are 1/3 – 1/3 – 1/3, i.e. 50mm/2″ each.
As you can see, the shoulders are cut at 45degree’s, this will increase surface area to be loaded and increase cantilever effect. These joints are commonly used (albeit slightly different) in wall plates and some sill plate applications.
I suggest using no more than 3/4″ dia drawn pegs (as mentioned above) and obviously, dont forget to draw-bore them (approx 1/8″ offset to bring the tenon stock (malke) into the mortice stock (female).
Any comments, questions or suggestions, please let me know. I have an easier day tomorrow!
CheersPosted 5 years agotimberMember
Nice looking project, I won’t offer anything for the jointing method, my craftwork is rather more agricultural and my approach would have just been to find a decent 6m length to mill rather than join two 3m pieces. That sycamore has some nice colouring in it.
We make pegs for our gate making from seasoned oak off-cuts, skimming them down to size before hammering them through a 1/2″ hole in a bit of angle clamped in a vice, but guess you’ll want a bigger dia. and that will need a big mallet and a strong arm / bench. Small woodland crafts set-up could churn them out easy enough, but the closest one I know is a social enterprise in West/Mid Wales. Coyles look convenient.Posted 5 years ago
Glad to see everything is ‘ coming along nicely ‘. There is some good advice above. Slackalices’s joint looks spot on as it would stop the joint ‘ opening up ‘ ie. the two pieces of stock moving away from each other.
I asked previously as to where the tentacles start as I’m think you need to minimise the gravitational/ cantilever effects on the joint. I’m gonna think out loud for a minute…Longevity…I take it you’ld like this to last as long as possible. Standing water will kill this quicker than you would want so locking 12 pieces tightly together won’t be ideal and a small gap should be left longitudinally vertically and horizontally between each piece. Sanding a slight cuvature into the top face on each will aid water run-off and machining a drip-edge on the underneath of each piece would stop water creeping along underneath and staying damp. The principal being rain would run off the top piece, drip onto the underneath piece, then run off etc etc. The complete piece should be raised off the ground….so dig a rectangular hole into the forest floor ( I know…you might get dirty ) 3.5m long x 0.7 m wide 0.6 m deep and stand/ support 4 rows of 3 posts each, 160mm apart in the width, 100 mm x 100 mm x 650 mm long so that 100 mm protrudes ground level. The posts need to be at 0.5 m, 1,75 m and 3.5 m from the beginning of the piece. You’re gonna have to be pretty accurate with the placing of the posts and direction of the tenons ( explained later ). Fill the void with a anguler type gravel/ stone ( Type 1 works fine and locks together and compacts really well ) This is the drainage you require. A top each post that you have buried is a tenon 20mm wide 50mm long 70mm high. in the center of the tenon is a hole to take a peg. The bottom row of your 150mm posts need to morticed 20mm wide, 50mm long running along the center of the piece at 0.5 m, 1.75 m and 3.5 m ( the 3.5 m mortice is after the join in the tentacle piece thus supporting it ). The mortice needs to run all the way through the piece. Two holes are drilled into the piece one being the same height as the peg hole in the tennon ( the second hole to take a second peg later see *** )The bottom 2 center pieces 6 meters long can now be fixed into place. Place the mortices of the center two pieces onto the tenons of the middle two rows of buried posts and peg.
Second Row….Again cut 12 pieces of stock 100 mm x 100mm x 150 mm long and cut a tenon as above ( 70 mm high 50 wide ) from either end ( this leaves a 10mm collar ) drill the center of each tenon to take a peg and place this connection piece into the mortices in the bottom middle two rows of your bench ( *** this is where the second hole comes in ) the 70 mm tenon sits above the other 70 mm high tenon ( with a 1 cm gap in the middle of the stock ) Peg. Now fit the two bottom ‘ outside edge ‘ pieces onto the tenons and peg.
…and repeat, connection pieces as above, 3rd middle row then second outside row then the third and final outside row. Please note all four pieces of the top row only need to be morticed 70mm deep so that the tenons ( tenii ? ) can’t be seen . the only visible fixing methods to be seen are the ends of three pegs on the outside three rows.
I hope you can follow this…it’s only a suggestion and i do tend to over think things but it has to work or there’s no point.. I just need to work out how to stop the top 4 pieces moving apart. this will then tie-in the whole bench…
I’ve a headache..
sleep…..Posted 5 years ago
Blinkin flip! Thanks everyone. What awesome help! 🙂
Just got in and seen this, had a quick scan but it’s late so will have a proper look at work tomorrow. Really, really grateful for all this help.
I phoned round a few places about pegs and was put on to Coyles timber as it happens.Posted 5 years ago
They seem quite reasonable on price but if we can help it, we are trying to make it free as ultimately the student has to pay for extra non- woodland materials from his prize fund.
We have loads of Iroko worktop from the old science labs. I’ve seen octagonal pegs made. Does the panel think Iroko pegs would work if we machine them up? Its got fairly good weather resistant properties so I thought…
I’ve not any real world experience in using seasoned pegs to hold green stock. My logic says if you put the finished piece out in the ‘ Summer ‘ the green timber will want to dry/ shrink around a peg thats not shrinking/ drying. I’d hate this to split the stock. Why can’t you use a green peg out of the same material as the stock at least this way any thermal or moisture affect will cause the peg to shrink/ expand at the same rate as the stock..
As before, just thinking out loud.Posted 5 years ago
Yeah I think the whole idea is that the seasoned peg will expand and the timber around it shrink, to lock the joint.
It’s fairly inevitable that cutting away such heavy sections will release some tension, particularly as we are largely using branches as the tentacles which usually will have more tension built in.
We have had a couple split already but hopefully have enough that we can allow a few to do this….
Slackalice, thanks so much for the drawings. Makes perfect sense what you say about the cantilevering forces.
I reckon the joint is pretty feasible, I’ll take it in the workshop today and maybe we’ll have a go at one.
The main problem we have is trying to cut the vast majority of the joint on a bandsaw when you have 3-4 metres of heavy and bendy tentacle to support and limited room! 🙂Posted 5 years ago
Just got in from another favourite shopping trip…. Axminster Tools!! I’ve had fun!!
Regards cutting the joints, personally I’d go for skil saw and big mortice chisels for the tenon and if you’ve got a chain morticer to get the flesh out of the mortice, the joints wont take long at all. Doing them on the band saw will be border line dangerous and it’s only a few cuts with a 9″ circular saw.
Remember that you’re dealing with big heavy section stuff, so forget about your cabinet making tolerances for these joints. So long as you scribe the shoulders correctly and use the chisels to those scribe lines, the rest of the joint (i.e. the bits that don’t show) wants to be loose, as in 2 -3 mm loose. Otherwise youll spend most of your time and effort trying to separate the joints after trying them for the first time! Although you may have already worked that one out!!
Your thoughts re dry pegs in green timber are good re shrinkage/expansion, the strength of the draw-bore mortice and tenon joint is is the peg. Ive personally never used Iroko for pegs (probably because it’s quite expensive! It’s also quite short grain and may be prone to shattering while you’re thumping them in with your club hammer. If you have no oak, I would be tempted to use the Ash and if you’re all about doing it yourself, then here’s some info on shaving horse’s and plans for making one. All you need then is a drawknife and a willing student…!!
Just got in, so in need of a cuppa. Cheers
I’m deeply envious of your machinery!! Mr Wadkin??!!Posted 5 years ago
NIce one Kayak!! Looks like you’re getting there! I’m liking your jigs too.
Thinking about it, you’ll be able to use the same approach for the female/mortice. I can imagine it will take some time cutting the joints as you are, however as you know, the first one or two are as much finding out how-to missions, so they will get quicker. In oak framing, it’s all mortice and tenons, scarf’s et al and quite frankly, we pay attention to the shoulders and the seen bits, so tend to go at them with big skill saw and bigger butt mortice chisels with a 4lb hide/hide Thor mallet 😀 . Great anger management stuff 😀
Just remember to allow some tolerance in the joint! Either hollow the cheeks of the tenon or the inside of the mortice, whilst keeping to your scribe lines. Apologies if I’m teaching you to suck eggs.
It’s going to be a great looking piece of functional art.Posted 5 years ago
My understanding of using dry pegs in green timber is good. As the green stock dries out and shrinks, it contracts around a seasoned peg (i.e. the peg has already done it’s shrinking and is hammered in tightly). It follows that as the green stock seasons, the joint strengthens.
As I mentioned in one of my earlier posts, the stock will split as it seasons anyway, especially as the tentacles are being made from ash, which has a tendency to split. I don’t believe the integrity of the draw bore joints will be compromised by the splitting, in fact as you probably know, one of the best ways to arrest a split is to drill a hole at it’s narrowest point.
I have seen split tenons in really old frames that I’ve dismantled for repair/restoration and from what I’ve learned so far, these have occurred to either a knot in the tenon, or excessive movement in the frame.
Does that help answer your question?
Tim tooPosted 5 years ago
Bigger trees would work for sure, but we are working with what is available for us, plus any bigger and I don’t think we could physically lift and work them. They’d have to stay put and we’d go to them, and we can’t do that, especially as we don’t have chainsaw training etc…Posted 5 years ago
We are essentially trying to do big scale work with furniture making sized tools..
We have altered the joints we are trying to achieve a bit as they were proving overly complicated for what we are trying to do.
We now have a halving joint, with a mitred shoulder at one end, and a square shoulder at the other. Hopefully this will resist the forces ok and hold together.
As someone suggested above, the short grain of Iroko is not ideal for the pegs. It’s actually quite brittle and we have had one or two split and crack.
Banging the pegs through the joint.
One of the joints with some blending in done with a power planer.
Blending in the jointed components.Posted 5 years ago
I’m not so keen on the half lap joint. I’m sure it will support the weight, but it wont prevent the parts separating as well as a mortise & tenon. Needs must I suppose.Posted 5 years ago
In my ignorance I have used pieces of brush handle to make dowels before. They are seasoned, the right size, have a long straight grain and are readily available. I didn’t have any oak either.
I share your thoughts on the half lap really beaker2135. I think the problem is we are not really set up to cut these joints as well as we’d like, we are finding our cabinet making tools a bit limiting.
I borrowed a small electric chainsaw which has made some aspects easier.
We took the elements of what slackalice was saying about resisting the cantilever forces but simplified it.
To address the potential for the joint coming apart, we have added extra pegs at all different angles, thinking this would create some sort of mechanical lock of sorts. Hopefully we’re right.
Detail of angled pegs
View of side of one joint
A few tentacles starting to come together
You can just about see here the mitred shoulder we have kept on the halving to resist the cantilever.Posted 5 years ago
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