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material "movement"

distortion after machining

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Thomas Phillips22/05/2010 23:26:04
21 forum posts
1 photos

Sorry to be cadging info yet again but I am presently making a run of shelf brackets out of quite good quality red pine they are screwed and doweled and glued and I go to a lot of trouble to make them accurately. They are as square as I can get them when made. They are not painted. After a few weeks in  storage they distinctly loose their accuracy and are no longer dead square . Maybe not much but back to back on a plate say 1 degree some maybe a touch more. I guess, but don’t know that it is something to do with the water content of the wood. They are wanted unpainted so the customers can paint them to suit themselves. Any ideas how to stop this apparent distortion in storage. Thank you.


John Phillips

Edited By sparky on 23/05/2010 19:18:28

Olly Parry-Jones23/05/2010 09:26:15
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2776 forum posts
636 photos
Hi Thomas,
 
Do you have any photos of these brackets? It may help to explain.
 
Otherwise, it sounds as though the timber wasn't dry enough when you started working on them. Did you start machining the same day you bought the wood? Often, it's best to leave the timber indoors for a few days to 'acclimatise' before machining. With pine, even a couple of days can have a positive effect. A week should be plenty.
 
It could also be because the conditions in your client's home are again much different to your workshop... Not a lot you can do about that. But, I think that's where the photos may come in handy - perhaps there is an alteration you could make to the design that would allow for some expansion and contraction?
Thomas Phillips23/05/2010 12:25:50
21 forum posts
1 photos
Dear OPJ,
 
Thanks for your note. Here are a few pics as requested.
Will try to keep wood for a week  before machining on the next batch. Wonder if there is anything you can spray on to limit these changes without affecting the ability for the clients to paint them to suit?
 
Sorry can't get my head round posting pics on this note.
Will you please send me your email address and I will send them that way.
Julian23/05/2010 15:07:10
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547 forum posts
28 photos
Hi Thomas
 
       This is something that my old woodwork teacher told me years ago and has worked for me.
Just imagine the stresses and strains that the tree undergoes during its growing period, and the we come along after 50 - 100 years and jop it down, dry it out and start slabing it up. When you get it home and reduce the moisture content even more those stresses build up again, don't forget that timber is a living material. When we rip the wood to size on our table saw we again release tensions in the wood and it will spring about a bit so try to cut it a bit oversize and then when you need to use it trim to the exact dimensions required. You won't eliminate the movement totaly but you will reduce it to a usable tolerance.
As OPJ says always good to acclimatise for a few days prior to working on it.
 
                          Julian
 
Olly Parry-Jones25/05/2010 17:07:38
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2776 forum posts
636 photos
Hi Thomas,
 
I've just sent you an e-mail.

Olly.
Sparky25/05/2010 18:25:24
7631 forum posts
22 photos
Thomas
 
If you follow these links, you should be able to upload images for an album and place one in a thread as well as a link.
 
 
 
Just ask if you need help mate.
 
Marc
Olly Parry-Jones30/05/2010 08:15:38
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2776 forum posts
636 photos
Hi guys,
 
Thomas last night e-mailed me three photos (he's also been having PC trouble, by the sounds of it!) and I've just uploaded them to a new album:
 



From his description, it sounds as though the timber is distorting to the extent that the back edge and end-grain edge directly below the shelves are no longer square (possibly as much as 1mm out).
 
If these were to be painted, I've have suggested edge-jointing three narrower boards to make up the width - however, that's not the case, as these must be left unfinished.
 
First thought that came to my mind was to cut new ones with the grain running diagonally; that may prove to be better and would certainly overcome any short-grain issues on the front end... That would also mean that getting the same edges square in the first place becomes more difficult (though, a template should work).
 
Any other thoughts from anyone?
Toothy30/05/2010 16:51:08
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458 forum posts
67 photos
Hi
 
A simple correction to the problem would be:
1   Glue a thin veneer to the under shelf surface and square this with the wall surface,
2    If necessary do the same with the wall surface  if deemed essential bearing in mind that few walls are absolutely flat.
 
The holes already made for fitting can be recreated in the veneer with a sharp knife.
The veneers must be the same wood as the brackets. Bear in mind that oak MOVES and any future brackets should have an allowance made for final squaring after manufacture.
 
Toothy

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