# Steam bending form



## bugman1954 (Apr 1, 2008)

I'm bending a board for an architects house who has an arched group of three windows. The rest of the encasment is with straight boards. The form is to the exact radius. I thought if I steam bend the board which I cut in 1" wide strips I knew it would spring back some and when I clued and pressed again they should be close to staying to form. I took them out of the press after 24 hour drying. They unsprung more than I thought. I am considering resteaming them and bending them in the opposite direction to break more of the fibers in the board so it will retain its shape better. Any ideas would be appreciated.


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## woodnthings (Jan 24, 2009)

*Some comments*

Those 1" strips look a little heavy or thick... 3/4" or 1/2" maybe? Thinner strips would be more flexible of course. The "steaming" form and the gluing form are the same, I assume. Once the strips are glued up there will be very little spring back in my experience. I think you'll be fine, but you could use thinner strips next time. :thumbsup: bill
BTW I would not rebend in the opposite direction.


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## bnew17 (Nov 10, 2009)

very interested to see what input others have on this


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## jlhaslip (Jan 16, 2010)

Thinner strips would be better. Less tendency to spring back.

Also, are the forms cut at different radii?
You can't make a jig saw cut and use it as a form. 
The top and bottom of the built-up molding are 2 different radii. Might be a factor.


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## woodnthings (Jan 24, 2009)

*A jig saw will work in my opinion*

A fine cut blade will not affect the curvature. My friend who makes bending forms for arched top doors uses a Bosch barrel type jig saw to make forms up to 8'' thick of particle board. Another method is to use a router and trammel like this:
http://www.handymanclub.com/ExtraContent/ExtraContentDetail.aspx?id=39040
:thumbsup: bill


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## bugman1954 (Apr 1, 2008)

The forms are two different radius. This problem is not about the forms its about steam bending wood. I found another board and I am going to cut thinner strips. I am still going to glue the origional boards to see how it works out also. Thanks for the suggestion.


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## ~WoodChuck~ (Jan 17, 2009)

if you want less spring back you not only can use thinner strips but also do it in 2 separate glue ups.there is much less pressure this way and thus less spring.also once out of the form attach a spreader to keep radius true.


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## ~WoodChuck~ (Jan 17, 2009)

also by making your workpiece longer than you need will reduce spring...say 2' on each end if you have the material


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## Julian the woodnut (Nov 5, 2008)

There is a reason most bent laminated casing is layed up from 1/8" to 3/16" stock. The more layers(thin ones that is) the less it can spring back. Your 1" strips are WAY to thick for bent lamination. Also if you use a plastic resin glue you can easily lay up all the layers in one glue up without worrying about springback


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## Dave_G (Dec 21, 2009)

Hi - I've been doing quite a bit of curved woodwork recently and have concluded that laminating is the easiest way. My curved parts are only one inch thick and I have used either 3 or 4 layers.

I did some calculations on the amount of spring back which have been borne out by experiment. Approximating the bending using beam theory the spring back will be one divided by the square of the number of layers. This assumes that all the layers are the same thickness and the same material.

My pieces are one inch thick. A typical item might deviate from straight by, say, three-quarters (three-fourths for those in the US) of an inch at the middle of the bend. This is in a piece about 10 inches long. If I use three layers, three squared is nine, so the spring-back is one-ninth. For a "curvature" of 3/4 inch this gives a spring back of a bit over a sixteenth of an inch (0.0833 inches). This is borne out in practice. Using four layers will reduce this considerably (four squared is 16, giving a spring-back of one-sixteenth of the curvature).

I use a jig with a radius of curvature reduced by the expected spring back so that the final shape is close to what I want.

I find that I can get very repeatable results in this way. I have made a number of drawer fronts which are curved, and of course any difference in curvature between the different drawer fronts would be very noticeable when all the drawers are inserted. However, the differences between my drawers I reckon to be less than 1/32 inch and quite unnoticeable.

I find that the two inches at each end of the bent piece don't take the curvature very well, so I make my pieces 4 inches over length and trim the ends afterwards.

As others have noted, you can build it up bit by bit, rather than try to do all the laminations at once. This reduces the forces considerably. Even my small pieces needed a (calculated, not measured) force of around 3 tons to bend them all up in one go. The bigger items I have done I have built up a layer at a time.


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## woodnthings (Jan 24, 2009)

*Thickness is 1"?*



Julian the woodnut said:


> There is a reason most bent laminated casing is layed up from 1/8" to 3/16" stock. The more layers(thin ones that is) the less it can spring back. Your 1" strips are WAY to thick for bent lamination. Also if you use a plastic resin glue you can easily lay up all the layers in one glue up without worrying about springback


 We were never told the *thickness* of the strips in the photo, I just guestimated them to be about 3/4" to 1/2"...which is too thick. The height of the stack or *thickness* of the stack, I believe is the 1" dimension referred to. A clarification would be helpful.
Based on Dave_G's information which was excellent BTW, thinner strips would probably not have to be steamed to be bent, thinner being 1/4" or less. Once the glue set up it acts like a tension member to keep the fibres from sliding or returning to their original shape. 
Quote Dave_G" 
*My pieces are one inch thick.* A typical item might deviate from straight by, say, three-quarters (three-fourths for those in the US) of an inch at the middle of the bend. This is in a piece about 10 inches long. If I use three layers, three squared is nine, so the spring-back is one-ninth. For a "curvature" of 3/4 inch this gives a spring back of a bit over a sixteenth of an inch (0.0833 inches). This is borne out in practice. Using four layers will reduce this considerably (four squared is 16, giving a spring-back of one-sixteenth of the curvature)."

Do you mean after laminating them as a *total* *thickness* of the stack of laminations?
The word *thickness* is used in 2 different ways here and it's confusing.

Dave_G, For your curved drawer fronts which have to be rather tall to be useful, say 4"-6" maybe more, do you steam them? And can you post a photo?
:thumbsup: bill


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## Dave_G (Dec 21, 2009)

Woodnthings - thanks for the interest. I've done two types of curved work, all part of my bedroom, which is still "work in progress". The curved things are (1) drawer fronts and (2) the frame members for curved panelled doors.

(1) My drawer fronts are 3/4 inch TOTAL thickness. I've done some with four laminations and some with three laminations. The height is 5 1/2 inches. For this height rather than use 5.5 inchwide 1/4 inch thick laminations, each layer is made up of more than one strip of 1/4 inch thick wood. I've then used a layer of veneer over front and back (which effectively makes two additional thin laminations.

(2) My door frame members are one inch total thickness, three inches wide. I've made these in three laminations and they are different thicknesses. The front lamination carries an ovolo moulding and is 3/8 inch thick. The middle lamination is 1/4 inch thick and is slightly narrower than the others to make the groove for the panel. The back lamination is 3/8 inch thick.

The radius of curvature is about 18 inches so the "bend" in each piece is not excessive. Nevertheless, the force to bend the laminations round the former I made is large - I calculated 3 tons and I use several strong G-clamps (5 clamps for the door frame and 8 cramps for the drawer fronts since I need to cramp it from both edges, it being 5 1/2 inches wide.

I don't bother steaming them. I did try making a contraption to steam them but it wasn't very succesfull. I couldn't get repeatable results and they bent out of the plane of curvature (if that makes sense) and came out rather distorted.

I think the force required to bend one-inch thick wood round anything but the gentlest of curves would be excessive and would also lead to fracturing of the wood. I even had this problem to some extent with my 3/8 inch laminations - I would think that this is about the limit for bending wood round an 18 inch radius. Some woods might be better in this regard; I'm using beech.

It is slow work. I only made one former for bending the wood around so I can only do one piece at a time. Allowing one day for glue drying, and doing one lamination at a time, with four laminations and 10 drawers to do, this is 40 days minimum. And I only have weekends to do it!

I'll borrow a camera and take some photos when I've finsihed. It all looks a bit of a half-done shambles at the moment! Since I'm still doing these things I'll take some photos showing how I do it.

Dave_G


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## Dave_G (Dec 21, 2009)

*A sketch of my curved piece*

I sketched the door frame member in sketchup so you can see what I mean. The radius of curvature is actually 20 inches. It took some force on the G-clamps to bend this; I think it would be difficult to bend a greater thickness at this radius (or a smaller radius at the same thickness)


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## J R in MO (Feb 2, 2010)

Woodmaster Owner Gary Striegler Has a very indepth of how to tape/disc showing how make both curve casing and molding. More information and video on Woodmaster Tools web site. 
Almost perfect results every time. Hope this helps. J R


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