Rib re-flexing: How much is to be expected?

[putnamm]

I am using outer molds with inside spacers to hold the form of my ribs once they're bent. They're usually locked in the mold for a day or two while I prepare the block sets.

When I unclamp the inner spacers and begin to place my blocks I find I still need to apply a fair amount of clamping pressure to get the ribs back in to the right form, as they are trying to flex back in to their original, straight orientation. This is particularly true of the bottom (point-to-point) rib.

I'm using plain hard maple because my skills don't justify spending a lot of money on fancy wood. My ribs are 0.070" thickness. I use molds with a heat blanket to bend them initially before locking in to the form.

Somehow I got the idea in my head that the pros bend their rib wood in such a way that it never flexes back, and so the blocks just naturally fit without any pressure. Not sure this is accurate.

Thanks,
Mark

[amowry]

Do you cycle the heat a few times with the heating blanket? I usually heat once for bending, let cool, then heat again for about 15 minutes at a lower temperature (maybe 180 degrees F). That should help set the bend.

However, non-figured maple will spring back more because the fibers can't slip past each other during the bending process. Highly figured quartersawn maple will have almost no springback.

It may not be too much of an issue because the plates and blocks will hold the sides to shape and the stresses will eventually dissipate, but if you still can I would try cycling the heat once or twice more.

[putnamm]

Do you cycle the heat a few times with the heating blanket? I usually heat once for bending, let cool, then heat again for about 15 minutes at a lower temperature (maybe 180 degrees F). That should help set the bend.

However, non-figured maple will spring back more because the fibers can't slip past each other during the bending process. Highly figured quartersawn maple will have almost no springback.

It may not be too much of an issue because the plates and blocks will hold the sides to shape and the stresses will eventually dissipate, but if you still can I would try cycling the heat once or twice more.

I have not tried cycling the temperature, no. I will give it a shot. I also may be pulling the wood out of the bending mold early, as I only leave it in there for maybe five minutes.

I figured that it would eventually settle once all the parts are assembled and glued. For now I'm most worried about small indentations or marks from clamping so hard!

Thanks for the suggestion, Andrew.

-Mark

[j. condino]

"Somehow I got the idea in my head that the pros bend their rib wood in such a way that it never flexes back, and so the blocks just naturally fit without any pressure."

That's the goal.