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Re-inforcing a piece of elm burr

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OldWood

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I had this idea sometime back about making a Vienna Regulator clock case and bought a suitable movement. My woodworking skill and commitment lead me into looking around for a simpler design than the traditional case with many examples of the wood worker's capabilities, so the project did not progress.

Recently my mind came back to this and the movement being a wasted investment, and realised that there was no actual reason for there to be a case and that a simple back board would be adequate, with perhaps some protection round the movement.

Chance had me in the local hardwood saw mill today looking for some oak for another project when my eye 'unfortunately' fell on a metre long wainy edged burr elm plank - it was a bingo moment but then it may not be, as it may well not work, but I bought it ! I know, I know, but we all do it, don't we !

The question is this - there is a narrow neck at about 1/3rd length that is some 60mm wide and <20mm thick, and is clearly a weak point - it's good feature but not if the plank fails there. My feeling is to rout a groove in the reverse side and insert a steel 'backbone' in that area - does the collective think that I should glue this in (PU glue?) or use short screws into the 22mm thickness of the body of the plank ?

Many thanks
Rob
 

Dodge

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Personally I wouldn't add steel as a reinforcing, over time the tanin in the elm will react with the steel and unsightly stains could appear in the wood - its not as common for the reaction in elm but with oak it is very common. I would route a chanel/groove as you suggest and glue in a good strong straight grained piece of oak or even elm if you have enough.

I am not a fan on PU glue and have had experiences of it failing - Just use good old Titebond 1 and it will hole forever.

getting to you comment regarding the clock movement not needing a case, well that depends upon the type of movement - If it is a quartz/battery powered movement then as you say there is no need for a case but if it is mechanical you must case it - it only takes a little dust to ruin a good movement and stop it running.

Hope this helps.
 

Midnight

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side-stepping the mechaniklanky mechanism (cos I know nuffin about em Guv), if I'm reading this right, you want to create a channel in an already "physically challenged" area of the board (that's as close to PC as I get btw) making it weaker in the hope of securing a piece of something stronger in the void as reinforcement. By all means skelp me up side the lug if I misinterpereted... see... elm is kinda queer stuff in that rather than possessing grain that resembles nice orderly n regimented straws, it's more akin to a grenade going off in a pan of spaghetti... absolutely random direction and density... Now while it's a challenge to work with the stuff (sometimes), that random nature of the grain give it some pretty kewl physical properties, strength and elasticity being 2 of the main ones. My point is it might be a "wee bit nippit", but it punches well above its weight in the strength/cross-sectional area dept.

That said... you've seen the board, obviously, I'm working on speculation, and if you're heart-set on adding a spline there's a few options open to you. Dodge has already outlined potential probs associated with using any iron based material (316 grade stainless steel being a probable exception) so I won't rehash. I'd advise trying to find a particularly dense piece of elm that can be shaped to fit tightly into the channel you're thinking about, primarly because it'll have very similar behavoiral charactoristics re expansion/contraction/mechanical loading etc and should "move" at a similar rate as the rest of the board, whereas fitting a spline of a different species just may cause a mechanical failure as the different materials try to move at their own rates. Alternativly you could perhaps use a piece of aluminium bar stock which can be "machined" to allow the board to move around it while lending the support you're looking for.

As for bonding one piece to the other, if using a wood spline, you needn't use anything more exotic than PVA... if opting for a metalic spline, mechanical fasteners are probably best, fixing through slotted / oversized holes to allow the board to expand and contract with the changes in humidity...
 

OldWood

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Mike
That's a pretty 'kewl' piece of comment. If it wasn't for the fact that the plank (if that is what you could call a bit cut off the very edge of a burry trunk) is out in the workshop and it being a bit stormy here this evening, I would post a picture - I'll do so tomorrow.

Your logic is first rate, with the caveat that I do wonder if burr wood follows the general rule of main trunk timber. On my understanding that burr is an almost cancerous growth of twigs that are forced into one another, does that have the same strength as wood that has grown from sap wood into main timber. Or is this very interlocking in the burr area what you are referring to. From the way that I have found elm burr turns, there doesn't appear to be any fibrous element - there's no grain in any direction and you get no tear out; it turns and sands very easily. It seems amorphous which implies brittleness and lack of strength.

I've worked with elm in my brother's cabinet making shop - chair seats are a traditional use because of the cross bonded fibres and scraping those to shape is hard work.

Rob
 

Midnight

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Rob, hand on heart, I've no experience with working with anything other than small (golf ball-tennis ball) sized burrs. With them, the heart/sap wood tends to contour (sorta kinda) the burr with the grain direction going nutz in the locality, but grain density can be totally random, some so soft you can push a fingernail into them, some hard as glass, there's really no way of forecasting what it might be until ya get up close and personal with it n kick its tyres... I thought you turner dudes worked stock while still green? (the stock, not the turner)... Talking to my supplier today, remembering first experiences with elm and gaining an insight of how the stuff mills from his perspective; even after logs have been drying for over a year, green elm slices like cheese when being milled into boards...

Forgive the short reply, I spent the day loading/unloading and storing almost 3 ton of new stock... I'm technically knackered... gettin too auld for this caper I reckon... ;)
 

Phil Pascoe

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:idea: if your elm is weak enough to need reinforcement 1/ it won't take screws very well, 2/ you shouldn't have to worry about it moving at a different rate than any insert, whether wood or metal.......... Why not put a strip of something like a piece brown oak or teak or anything straight grained and approx. the same colour in? At least then if it shows through anywhere it's the same colour.
 

OldWood

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Thanks guys - I've now moved the topic onto a new thread about finishing the piece. The wood throughout the plank is in good condition, but at this neck it clearly flexes slightly. My solution is to screw a bit of steel to the back while I'm hand preparing it and remove it once finished.

Mike - wet/green wood has the advantage of being easy to turn, and is invariably used by demonstrators as the shavings are impressive to the audience, plus there is no dust to cause bothers. The downside is that the wood will move and crack horribly. There is a case for turning the shape approximately and then controlling the drying over a month or so

Rob
 
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