Replies

1. 
   Lataxe | Mar 19, 2009 05:03pm | #1

   rian,

   When I make a ladderback chair with four curved back slats those slats are only 1/4 inch thick.  They bend when lent back into, especially if one tips the chair back on two legs. 

   The 1/4" is more than strong enough for the job because the slats are rived with the grain then steam bent into shape.  The fact that the grain is continuous throughout the length of the slat means they are very strong, despite being so thin.  Also, the force on them of a sitter is absorbed by the tension of them bending, rather than manifesting as a sudden failure of the weakest fibres.

   So, although your chair might need 1/2" slats for reasons of traditional proportion, it probably doesn't need them to be 1/2"for strength - if they are rived and steam-bent. Or, put another way, 1/2" will be plenty strong enough with such slats.

   If your slats are sawn and there is grain running out along their length, they will be much less strong  In a case where the grain is at a significant angle to the length of the slat (and so runs out a lot) there is definite danger of a failure in 1/2" slats, should a big heavy bloke flop down into the chair, with a grunt and a bang.

   Lataxe

1. 
   bmyyou | Mar 19, 2009 07:59pm | #2

   I'm making the slats by resawing two 1/4" slats on rift-sawn white oak and using a small, manual vacuum bag to shaped them over an MDF form.   I am also planning to mortise the seat-back posts at an angle to accommodate the curvature of the seat-back rails (don't want to change the grain direction at the end of the rails).

   Will that be sufficient?

1. 
   Lataxe | Mar 19, 2009 08:42pm | #3

   Brian,

   A lamination is usually a stronger piece than a similar one made of solid timber, as there are two (or more) sets of grain so one laminate layer usually offers reinforcement where another might be weak. 

   Also, you will be able to select 1/4" pieces that are relatively straight grained, as laminating is providing the bend rather than cutting a bend out of a single piece of straight grained wood, which is what usually causes grain to run out.

   And, as you say, keeping the tenons in line with the rest of the rails' grain will keep  them strong along their whole length.

   Lataxe

2. 
   KeithNewton | Mar 21, 2009 03:04am | #5

   If you are going to re-saw the vacuum bag. You should use at least 4 1/8" plys if you want it to hold the shape. You will not like what you get from two plys. Find and read the Spring-back thread which has been running for about a week for more information.

1. 
   bmyyou | Mar 21, 2009 03:46am | #6

   I have glued up one set of the two 1/4" thick pieces already and it has held shape very well; will check out the thread you've mentioned

2. 
   stanley2 | Mar 20, 2009 05:52pm | #4

   Brian, my steam-bent slats are 1/2 inch for the Morris chair.  Keep in mind the design affect of using anything thinner because of the large scale of the chair.  As for strength, I don't see the issue since you can't really tip a Morris chair back on two legs and the depth of the seat means the person's back is simply resting against the back cushion.