i have to create a curved section 1 1/2 by 1 1/2 to a 9 1/2 inch radius, 180 degree bend in approx 4 ft of material
my stock is freshly dressed, air dried quarter sawn white oak, 3/4 by 1 1/2 and i have the Lee valley bending apparatus, a bending template and a drying rack per their instructions
any suggestions, hints etc etc
when should the bent pieces be glued up? i assume the hot freshly steamed pieces wont glue up successfully
Replies
steve,
Green would be better than air-dried. The rule of thumb for steaming is 1 hr per inch of thickness. Once out of the steamer, the wood is pliable for a limited amount of time, (a minute, two tops) so you must "make haste, slowly". Haste, because you must get it bent before it cools and loses its plasticity. Slowly, because sudden jerky moves will cause a split or break- the wood fibers must be allowed to slide past one another, and smooth moves will allow this. So have your form, clamps, straps, whatever you are using, to hand and adjusted beforehand. Go through a dry run while you are waiting. If you can, orient the bend so that the growth rings follow the line of the curve, as if the layers of the rings were laminations, in other words.
Once the bendings are dry, in a couple weeks, glue them together in a separate operation.
Ray
thx, my thoughts exactly, the LV manual is pretty explicit, but doesnt cover lamination of bent pieces at all
"make haste, slowly"
LOL .. I LOVE THAT!
Somebody posted in here for a Woodworkers dictionary. THAT phrase is a MUST include! Tells it ALL in three words!
Steve,
The fact that you’re starting out with air dried quartered white oak is a big plus; it steam bends very well. That said, I would be hesitant to try to bend a ¾” thick piece at a 9 ½” radius unless the grain in the piece ran almost completely true the entire length. Also, with only two ply’s, there will be a bit of spring-back.
I would use between 3 and 4 ply’s for the total lamination, and expect to have very little to no spring-back after steaming and gluing.
For some great info on steam bending, check this out.
If you use polyurethane glue, (like gorilla glue, but I like titebond’s poly glue better. It spreads easier than gorilla, and is easier to get out of the bottle) you can glue-up the day after you do the steam bend, (although the more time you give the wood to relax after steam bending, the more it will want to hold its shape). Polyurethane glue kicks with moisture and works very good in this application. Clamp well!!! The foam has no strength. I use both cellophane tape on the form and clamps, and wax paper between the wood and the form and backing strip to keep clean-up to a minimum; poly glue can get very messy!
If you wish to use a regular PVA glue (titebond II and the like), you’ll need to wait at least a couple of weeks for the moisture content of the oak to lower enough to get an excellent glue bond.
There are a few other variables to consider… could you give us some more detail on the overall design and how this bent component fits into that design? It would be very beneficial. Thanks.
-Kevin
edit: p.s. CAUTION!!! Any steel that comes in contact with the oak after steaming will leave a black mark on the wood as the iron in the steel reacts with the moisture and tannins in the oak.
Edited 9/3/2008 10:18 pm ET by grainwise
Edited 9/3/2008 10:21 pm ET by grainwise
When I took a steam bending course - at Lee Valley in fact - the instructor stated that the mc in steamed wood was actually substantially *lower* once steamed. He agreed that was counter intuitive, but what he told us was that the steam is there to heat the chemicals in the wood and allow the fibers to slide, but that the fibers "swell" in the process and expel moisture and so wood might be mc 12% going in, and mc of 6% coming out of the steamer. He seemed pretty certain of this, so we all wrote it down. But if that is the case, would that prevent effective gluing ? or is the issue perhaps the chemicals in the wood other than water that prevent a good bond. All I know is that this guy spent a good chunk of every day bending wood, and taught us a variety of tricks as well as the basics (if I could just find my notes !!)Gavin Pitchford
"Sail fast - live slow" (build even slower)
Hum, Did he put a meter on it to show you this? I have metered the wood before and after steaming, and found that mine came out ~ 18º MC.Grainwise, if you have a meter, why don't you check it before and after, and get back to us with your results. 9" R is not too tight to just bend 8/4 material, and a heck of lot easier to do than getting a good joint between two separate bends, especially when you consider that there will be some distortion in each of them causing them to no longer to be flat.
We're going to be doing a fair bit of steam bending at the high school over the next month or two, I'll take some readings and report back then. I'll agree that using a single piece bend is the most elegant way to work, if at all possible. Once you get into laminating, the more laminations you have, the less distortion and spring back you get.-If you didn't learn something new today, you're not looking hard enough!
If I want to do a laminated bend, I just make the laminates thin enough to bend without steaming, and save having to do that step. I don't see the point in doing both. There isn't any point in making life harder than it already is, and doing both steaming and laminations on this seems unnecessary for this bend to me. I don't know why the OP needs to worry about the spring-back with that 9.5" r bend if he has a sufficient supply of steam for long enough, and starts with enough compression on his straps, then works fast enough. However, I don't know enough about his or your set-up to know what parts you might not have right or wrong. There are plenty of pitfalls. If he got the LV book and has read and understands it, and follows every step, then he should do fine. I have done steam bends on a full 2" thickness down to about 4" r before without any spring-back, so this doesn't sound like it should be a problem.
Gavin, I would believe that the MC content of green wood would go down after the bending process, but all the way to 6%!?! That I would have to confirm with the moisture meter.One of the chemicals in the wood that the steaming process activates/loosens is lignin. Basically, this is the "glue" that holds the wood cells together. I don't believe that the lignin being steamed will interfere with the gluing process at all.-If you didn't learn something new today, you're not looking hard enough!
..the instructor stated that the mc in steamed wood was actually substantially *lower* once steamed..
Interesting. Somebody had better get a memo to the dry-kiln operators about this phenomenon.
I make Archery bows when I am in the mood for it. I have steam bent and even bent wood wrapped in paper kept wet for several days and then left to dry. Start over, a never ending process but worked if you have the patience for it!
The above is Preface to you question..
But if that is the case, would that prevent effective gluing?
I would say nothing if a Poly glue. It loves moisture to make it react! But not my original thought..
I always wait as long as possible. Just let the wood sit there (as is) and see what happens.. A few days will tell you alot and how it acclimates to it NEW surrounding and shape!
Threw MANY 'sticks' in the trash for making a bow. BUT I would think most would be MORE that useable for a cabinet or other item if glued down to a strong base of some type to keep it tame for the rest of it's days!!
Thanks for the link..
Never ran across it before.. Good read and then some!
Steam bending is the best part of any project. To make parts consistantly is key. Your steam bent parts must also remain stable. We experimented for many years. What we learned was interesting. You must eliminate the internal stress created by the bending process. Hot, wet, steam works quite well, amonia probably better. If your requirements are tite bends with no laminations, amonia works best. Commercial benders would most likely use this process. A word of caution, it is both expensive to set up and dangerous.
We found that the length of the inside of the radius is much shorter than the outside of the radius on thicker pieces. Cutting thin laminations [we use a resaw] sanding them smooth, and reassembling them as they were, will look like one piece when you are done. The steam is still key to reduce bending stress. The thinner the laminations, the less time in the steamer. We would experiment with each specific situation.
When Gorrilla glue was first introduced, we began testing. This greatly sped up our process. We now use a very short bending process, before gluing. Maybe only a few minutes. We spread G.G. on one side of everything and wrap the laminations with green stretch wrap. We put the part back in the bending form and let dry 4 hours. We would then remove the stretch wrap and let dry until needed. We could easily complete sereral parts in each form each day. Go to steambentrockingchairs.com OOPS, I tried this and the site is temporarily down, go to utahcustomcabinets.com and click on the rocking chair link.
Edited 9/6/2008 1:47 pm ET by Lixer
Gorrilla glue .. Why do folks hate the foam? Easy mess to get rid of. In fact easier than most other glues!
I have used Gorrilla glue on many laminations. Curved or otherwise. It is a wonderful glue for many projects. NO, I do not use it for general woodworking.
I have made more than a few archery bows using it and NONE have failed BECAUSE of the glue.. Wood broke but NOT at the glue line!
Yes it is a bit dark on a glue line but never bothered me at all. Looked like it belonged there!
This forum post is now archived. Commenting has been disabled