grain direction????

[engineerone]

so try as i might i cannot get the flat top of my cherry table to stay flat it is a couple of planks biscuited together, but for whatever reason has not stayed as i wanted.

being a cheap skate i still want to use it, so am thinking
about putting a couple of cross pieces underneath which will
be screwed on with slotted holes.

question then is which way should the grain go??
by that i mean, should you screw through with the grain
vertical, or horizontal???

should i guess be easy to understand, but is it??

paul

[mrgrimsdale]

They would be stiffer with the grain vertical but not so much as to make much difference to a table top - but important on some things such as a string instrument sound board.

cheers
Jacob

[Sgian Dubh]

Studs and joists are cut so that the wide faces of, for example, 4" X 2" pieces show tangential figuring and the edges show quarter sawn or near quarter sawn figuring.

Put another way, when you look at the end grain of a stud or joist the growth rings are orientated concave or convex to the wide faces. Wood resists stress or loading much better perpendicular to the growth ring orientation than it does when applied parallel to their orientation. Loading the edge of a tangentially cut 4 X 2 causes less deformation than loading the edge of a quartersawn 4 X 2.

This characteristic of wood applies in all structures, therefore arrange the end grain of your bearers such that the circumferential direction of the growth rings is as near perpendicular to the underside of your table top as you can.

I'm guessing I answered your question, but it is possible I misunderstood what you were asking. Slainte.

[engineerone]

thanks both,

richard i am glad you even put forward a consideration.
you know what they say no such thing as a bad teacher,
only students who don't understand ;D

actually in this case the table width is quite narrow, about
400mm, then less the framework for the drawer case on which the top will rest, held by its expansion blocks. internally the
width is about 280mm, so i guess the other question would be
will it do anything, to have cross braces?

paul

[Sgian Dubh]

I say going on your further description that additional bearers aren't necessary. Presumably there will be some part or parts of the drawer casing you are going to include in the construction that go across the grain of the cupped table top you are bothered by.

You could fix the top to this casing directly, or through the use of an additional piece added to the drawer casing part; this extra piece just makes room for you to get in there and add a screw in the centre and a couple of slot screws at the extremities, or something along those lines anyway. Slainte.

[dunbarhamlin]

Oct 26, 2008 9:45:29 GMT Sgian Dubh said:

This characteristic of wood applies in all structures, therefore arrange the end grain of your bearers such that the circumferential direction of the growth rings is as near perpendicular to the underside of your table top as you can.

As an aside (in the context of guitar and mandolin necks) I have read reports (by folks with science backgrounds, whose findings I'd trust) which suggest it isn't clear cut, but varies from species to species (and indeed between samples)
So a beam can be stiffer radially than tangentially.
Cheers
Steve

[Sgian Dubh]

Oct 27, 2008 16:49:42 GMT dunbarhamlin said:

So a beam can be stiffer radially than tangentially.
Cheers, Steve

Steve, your post prompted me to do a little extra research and it seems your instrument making associates have a point. In the Wood Handbook: Wood as an engineering material (1999) Forest Products Laboratory, USA, the authors Green DW, Jerrold EW and Kretschmann DE briefly discuss this topic in the section Annual Ring Orientation (p 4-30 to 4-31).

Their conclusions after looking at numerous test results is that some timbers are stronger when stressed radially, others when stressed tangentially; and yet others where there is no difference in strength between stressing the wood either radially or tangentially. They go on to note that intermediate stress loads, ie, somewhere between tangential and radial loading also show mixed deformation results.

What this means to me is that through needing to do a little extra research to respond to you I have found a snippet of information that will make the book on timber technology I am writing more accurate and better researched. I can juxtapose this research against the research that informed me that wood is stronger when stressed tangentially, that is following the direction of the growth rings around the trunk or branch.

Thanks and Slainte.