Steambending

      How much time to allow based on thickness and proposed radius of wood. September 23, 2003

Question
Approximately how long would I need to steam a 45"x7"x3/4" piece of maple or ash if I wanted to bend it into a 14" circle?

Forum Respones
From contributor B:
The general rule of thumb is 1 hour per inch of thickness. You can put the wood in as the box is coming up to temperature. If you do this, then it should be ready to bend when the box reaches 195 to 200 degrees. This is typically what I do, but I rarely bend anything thicker than 1/2".

Or, you can put it in after the box is up to temp and leave it in for 1 hour.



You could not get a piece wet enough or hot enough to make a bend this severe. If you would steam for a severe n=bend, the initial MC must be no less than 22% and no wetter than 28% MC, with a flat MC gradient. Steaming has to be done at about 212 F with 100% RH. Never cooler and not much hotter, unless it is done under pressure.

Ash is often too brittle unless it is quartersawn. You would have to supply a large amount of end pressure for severe bends. You will find a discussion of this in THE WOOD DOCTOR'S RX, a book available through the FDM bookstore.

For the bend you want, I would try ammonia steam bending, as it is the only way I know that you can get the wood pliable enough.

Gene Wengert, forum technical advisor



From contributor B:
Gene is right... I misread your post as a 14" radius. I've tried bending 3/8" x 4" cherry on a 6" to 7" radius and it was not very successful. It bent but not cleanly, although a better bending form than I put together might have helped.


From the original questioner:
Thanks. I was looking into building a solid drum shell.


What kind of drum? I have used stave building techniques for ahsikos and djembes, as well as turning djembes out of solid blocks of wood, and have hand carved djembes from tree trunks. By far the easiest is to stave build the shell. I have worked with people who have built cylindrical drums from staves, and who have turned them out of solid log sections. I've included a link to a variety of drums I have made, in a variety of woods.


What about laying up steamed 45+" x 7" x 1/8" pieces on a form to the thickness desired? That is, steam them on the form, holding the bend with straps until dry (off the form) and then glue up (rotating the seams) on the form. This would be more stable over time and you may not have to go all the way to 3/4" thickness for the strength.

Is the 3/4" wall thickness necessary for sound resonance characteristics?



From the original questioner:
What I had in mind is a 14"x7" snare drum shell, of which most (also stave shells) are built 3/4" thick.

I've thought of building a stave shell, but most people build them the hard way. Most are built using vertical staves, then the builder has to figure out how to round the shell. This is not too hard on the outside, but rounding the inside becomes troublesome.

If I were going to build a stave shell, I would cut 1/6 circumference "slices" from a planed plank of wood, then glue up the shell horizontally (like Legos).

Building the shell out of 1/8" plys is how ordinary shells are made, which I can get at decent prices. What I wanted to see was if I could make a solid/one-piece shell like several custom drum makers offer (except at a lower price).



A lathe might be the way to go - I have seen dununs up to about 14" in diameter by 24" deep turned out of a solid piece - given the proper tools it is a fairly straightforward process.


Here in Maine the boat builders bend some amazing stuff but they have a dozen or more guys with come-alongs and serious jigs. Chairmakers will split (not saw) their stock out of a log with froes and wedges, thereby having the best grain to stretch and compress.

As far as ammonia steam goes, I think we need a reality check. Drum Workshops has a video called American Dream III. In it they show some of their manufacturing process with lamination. Highly $pecialized equipment. The drums you are talking about are constructed in segments like a barrel. They often had additional support rings laminated to the inside for extra strength due to the staves coming apart. You not only have to turn inside and out but you have to flatten and bead or bevel the drumhead contact surfaces. There are a number of drum part suppliers that sell shells, hoops, lugs, strainers, etc. This may be the way to experiment with making your own.



I have over the last 20 years bent 1/2 a million pieces in cherry, oak, walnut, hard and soft maple. The average moisture content should be between 15 to 22 percent. The smallest radius I have bent successfully is 6" with a failure rate of 20 percent. Each piece must be straight grained and free of defects in the bend area. The piece to be bent must be steamed for an average of 1 hour per inch of thickness. The bent piece must be supported in a tray with end pressure while bending and left in the tray to set for 1/2 hour before removing. During this time support strips are attached across the bend from each end and left in place until the piece is dried to 5 or 6 percent moisture in a hot room (100 degrees F.). Removed from the hot room and left to adjust to room temperature for 24 hours, then the holding strips removed. The piece will spring in .954 percent of the length of the bend between end point and end point. This average was arrived at by measuring from end point to end point when the parts were wet (just bent) and after they were dried to six percent moisture content and the holding strips removed. I tested four different species (oak, walnut, cherry and hard maple) with five different bends for 5000 pieces to arrive at the spring back formula.

The formula is (length of bend on left of centre) x (length of bend on right of center) x .954. This will give you the amount of spring back of the bent part. With this formula you can calculate what the form should be to produce the bent part without trial and error. Bending a small radius or large, you must be prepared and do dry runs until you are comfortable with the process.



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
I am a professional drum maker specializing in exactly what you are seeking information on. It is most assuredly not impossible to bend a 14 snare shell. You might want to mill the board down thinner before you start, as 3/4 is a little thick to start with. 5/8th would be better. Keep in mind you are going to have to round in the shell on a lathe after the glue up of the scarf joints and the reinforcement rings. So you will need some material to lose on the inside and outside as the rounding process takes affect.

I have built both steam bent and stave style shells since 1996 and I will tell you one thing is for sure, stave are way better in every way.



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