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A little help on wood expansion/contraction?

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Forum topic by Seeharlez posted 02-12-2011 03:57 AM 1209 views 0 times favorited 3 replies Add to Favorites Watch
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Seeharlez

83 posts in 2461 days


02-12-2011 03:57 AM

Topic tags/keywords: question oak joining

Hi guys,
I have a quick one… I am just finishing up a step stool project and did the glue up last night. Now after looking at it and standing on it I would like to add a little reinforcement where the treads meet the side. They are through mortise joints but they were my first try and not as tight is they should have been. Now I’m a little worried about racking and also about the integrity of the side pieces under load. My concern is, I would like to add a small glue block underneath each tread at the joint but in order for it to be strong I need to have the grain of the block running perpendicular tp the tread and side piece. Wood is Oak. Treads and side pieces are 3/4” thick. Tenons are ~1/2”. See the photo.

The length of the glue block will be ~5”. Any concern with differential contraction if the block is fully glued?

A second question – I’m using a stain and wipe on poly. Any thing to look out for applying the poly? Is it fairly easy to wipe in to the corners without getting pooling or build up? Does it have any affect on the glue if it penatrates the joints?

Thanks in advance for the help! Will post the project soon – after finishing.

Greg


-- Greg - Vancouver, BC


3 replies so far

View Bill Davis's profile

Bill Davis

226 posts in 3392 days


#1 posted 02-12-2011 06:49 AM

I’d say you may be looking at some problems because of the grain directions. Both the tread and side look like they are plain sawn. You didn’t say what kind of oak but oak’s tangential shrinkage varies from under 9 to over 12% depending on species. The shrinkage along the grain of the glue block you have shown is likely less than 1%. If all pieces are at the same MC and you keep them in a RH stabilized environment it should be OK but likely that won’t happen. A finish that minimizes moisture to pass through will somewhat help but will not eliminate the potential problem because RH changes will cause the hygroscopic wood MC to change and the wood pieces to shrink and swell and because of your intended layout the tread and side will try to change dimension approx ten times more than the glue block putting shear force on your glue joint. A design asking for failure.

View SergioC's profile

SergioC

82 posts in 2135 days


#2 posted 02-12-2011 06:56 AM

Two guesses, not sure if either would work. You could cut the glue blocks as cross-cuts to keep the grain the same and have similar expansion. Or, a better one I think, you could skip the glue and put screws in slightly slotted holes to allow for some movement. You probably could glue one side and allow the other to float with screws in oval holes.

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Don

514 posts in 2541 days


#3 posted 02-12-2011 08:23 AM

With a 5 inch span you shouldn’t have a problem. That’s a pretty normal width for the styles and rails on external panel doors of houses and those are glued with opposing grain and are under extreme conditions.

-- Don - I wood work if I could. Redmond WA.

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