Help with Steam Bending

[handtoolenthusiast]

I'm new to this forum and am excited to post my first question. I'm trying to steam Mahogany that is 18" long by 4.5" wide. It is 3/4" thick. I'm putting a 17" radius on it and am having trouble steam bending. I know this is a hard wood to steam, but am trying it anyway to learn and attempt. Is this a lost cause? I can't get it to bend much. I've left it steaming for 2.5 hours at the most. Has anyone successfully bent Mahogany? Any tips? Thanks!

Robert

[bywc]

I have never done bending before but from all the things I have read it seems like 1/4 is about the thickness people generally use I am not sure how well 3/4 would work but I know of all the articles I have read they have never used wood that thick but as I say I have never done it so I could be wrong but that may be your issue.

[ddg]

Robert,

Do you know if the wood is kiln or air dried. From everything I've read, kiln dried wood is out for steam bending because the wood changes in the drying process. Air dried wood will still allow steam penetration. Your problem might be with kiln dried wood.

It's been a long time since I've read about steam bending, but bending 3/4" thick material to a 17" radius should not be a problem.

I don't know where you can get air dried mahogany (they might have to kiln dry as part of the import process). You might be able to get these folks to work up some mahogany:

http://www.flutedbeams.com/home.html

Good luck,

Doug

[handtoolenthusiast]

Thanks for your fast replies. It is kiln dried, I'll try find an air dried peice. I also read I may be over steaming it... I'll have to try it a couple more times. I may end up steaming a more suitable species and then glue a thinner mahogany piece over it, but I hate to give up. Thanks again.

[Onboard]

Robert, I’m guessing that you’re not going to be able to bend your mahogany successfully, but that comment may be a challenge or maybe an encouragement to proceed with your experiments. Please understand the following is from my researching the subject on the web and not personal experience.

Mahogany has a high lignin (and hemicellulose) content relative to other hardwoods. Lignin works like a glue to bond the cellulose fibers together which in essence is the structure of the wood itself, and forms the wood fiber cells. You have to heat and soften the lignin (and hemicellulose) to allow the cellulose fibers to slip past each other. The problem is getting hot water (heat) to the lignin sites, which will facilitate heating the lignin molecules. As Robert has already mentioned, steaming air dried wood is better (higher moisture content), and green wood better yet (the water is still in the wood fiber cells for heating the lignin). However, green wood will take a long time to dry out, such as freshly cut wood drying to an air dried state, although you could also kiln dry as well to shorten the time.

Mahogany is at the low end of the bendable scale via steaming. By steaming, you are trying to get hot water, or moisture to the lignin sites to soften them and allow you to bend the wood. Some woodworkers will flex the steamed wood after steaming, in both directions to help “break” down the lignin binding. In the case of Mahogany, you are trying over the course of “uninterrupted” steaming (as close to 212 deg. as you can get it) to heat the lignin molecules up to a point they will allow the cellulose fibers to move. It's recommended that you mount a thermometer at the opposite end of the steam box or pipe, to monitor temperature. Also, make sure your steam generator (whatever you've chosen) does not run out of water during the steaming period.

One post I read is from a boat builder who presoaked a 1/2” thick strip of mahogany for 2 to 3 days, and only then did he steam the mahogany. A rough rule of thumb is 1 hour of steaming for every inch in wood thickness, although depending on the species this could take longer. For 3/4” thick wood, of any species, that would be 45 minutes, so 1 1/2 hours would probably be the maximum.

Some “steamers” will put the wood in a trough that they can heat the water up in to aid in getting the moisture inside of the wood prior to steaming. You are right in when you said you may be “…over steaming it”, so after the rule of thumb time, it may be worth pulling the wood out of the steamer after the standard time and attempt a bend.

You haven’t mentioned what kind of jig you’re using to bend the wood into the desired radius. The bent, wet wood does need to dry out after steam bending, before releasing from the jig. Some benders use a metal strap to allow the wood to release moisture and dry quicker. The reason for drying, is that the wood will swell when water has been forced into the interior of the wood, and when it dries it will “shrink” to a smaller dimension. So you need to allow a drying time after bending to allow the wood to shrink and stabilize, and measuring for final stock in whatever your project is. Even then, depending on the radius of bend, it will have a tendency to “spring back” and lessen the radius of the desired bend. Remember, the wood is under compression on one side and tension (stretched) on the other side.

Here’s a webpage that hopefully will give you some direction and encouragement. However, he was bending 1/2” mahogany and not 3/4” mahogany as you’re attempting, however I believe the information should still be helpful. Not the same person as I mentioned above.

Have fun with your experiments. Oh, and if all else fails, there's always bent lamination.

[Vic]

How hot is the steam chamber staying? 200 or above is what you should be shooting for (from what I've read). I plan on doing a lot of steam bending in the future along with lam bending a curved veneer pressing. Let us know how it goes.

[handtoolenthusiast]

Very helpful information... and I will read that webpage after this reply. I may end up laminating it after a few more tries. The temperature is monitored with a thermometer and it is consistently at 212 deg. I do think, however, that I need to rebuild my box to have a little more pressure build up in it according to some other sources I've read. Mine loses a lot of steam throughout the chamber. Currently have a sprang ankle, once I'm walking better in a couple of weeks I'll build a new box and try again. Thanks for the help!

[handtoolenthusiast]

It took a while, but I do have an update. I was able to finally bend the Mahogany, but it had too much spring back, even though I made a few jigs to increase the radius. It also didn't compress well, which I knew it wouldn't, but it created a "lumpy" effect on the face of the board. In the end, I have switched to white oak and will veneer Mahogany over it. The oaks bends much better. Maybe I cold get the Mahogany to work, but I've learned most furniture makers wouldn't have done so anyway. Thanks for all the tips. It was a good learning experience.