Replies

1. 
   jackiechan | Feb 09, 2003 08:26pm | #1

   Zoo,

   I do extensive steam bending for the custom furniture that I design and build.  Maple is a very poor candidate for steam bending.  The wood is highly prone to both compression failure (the crushing and collapsing/folding in of the fibers on the inside of the bend), and tension failure (the fibers on the outside of the bend are literally torn apart due to excessive stretching).  This applies to straight grained hard maple - any figure in the wood will only compound the problems.

   The spring back percentage you've chosen of 50% is far too aggressive.  The maximum spring back amount for any wood species should be between 20% - 30%.  Had the maple bend actually succeeded, it would be 20%+ over-bent.

   It is not impossible to steam bend maple if the proper bending form and a compression strap is used.  If you haven't used one, a compression strap is a long strip of galvanized or stainless steel that is cut to the width of the piece being bent, and bolted onto wooden handles/end-stops.  The bending blank should fit snugly between the end stops prior to steaming.  When the steamed wood is pulled from the box, it is immediately placed in the strap with the metal on the outside of the bend.  The strap is then used to pull the end(s) of the bending blank around the form - creating a compressed tension on the outer fibers, preventing a tension failure.

   Even with the bending variable controlled as much as possible, maple only has a small chance for complete success.  Oak, beech, ash, and hickory are the best candidates for steam bending, with oak being the number one choice.

   For maple, lamination bending would offer the best results.

   Definitely send along any questions that you may have.

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

1. 
   ESPEE | Feb 09, 2003 09:48pm | #2

   Dan, thanks for the reply. I have made a small fixture for the small bend of a 16" Radius using plywood as the form. I drilled holes around the Radius so I could use hand clamps every 3". I did use  an  outside support of poplar to protect the maple from the clamps and left the maple in the fixture for 24 hours before being removed.

   I am in Southern Calif. so the weather is not very cold. I have very good steam saturation with my steam box it is 8' 6" long and around 8" sq ID with a  1/2" copper pipe running the length of the box with 1/8" holes drilled every 3" for the steam to escape. When I do the steaming, I leave the doors cracked at each end to let the steam roll through and around the material. The material is supported by wood dowels in the middle of the box.

   I did not over compensate the form as you suggested. this is a very small bend and I did not think it would make a difference.  Boy was I was wrong.

   How would you suggest I make the fixture for the bend, make a smaller diameter like 14" radius or make it more  like a crescent shape?

   I have a another job I am bidding on, for a bull nose on a stage for a school. The material is 1 3/4" x 5" how tight of a bend could be achieved with this thick of material, which will be Northern red oak. The bend is a reverse  bend.

   Think of a swimming pool corner, with the radius of around 20" to 24". Can this be done?  if so how long should this remain in the steam before placing it in the fixture? I would assume I will be using a metal band and a come along.

   Or should I laminate the oak and band saw the shape then use my shaper to cut the Radius, half at a time and flip the material over and shape the other radius.

   Thanks for the reply

   Tony Czuleger

1. 
   jackiechan | Feb 10, 2003 08:20am | #5

   Tony,

   Plywood is the correct material for your bending form.  I use 3/4 ply' for all of my forms, with which I glue up multiple layers to achieve the correct thickness.  Initially I will hold the form together with screws until the glue dries - but, keep track of where all the screws are, you'll be drilling large holes in the form to accommodate the clamps during the bend.

   Regarding your maple bend - When you're laying out your bending form you want it to reflect a 20-30 percent over bend.  For a 90 degree bend, the form should allow the stock to be bent to (approx.) 75 degrees while still maintaining the desired bend radius.  (See figure 1).  For an arc bend (equal bend along the whole length) the easiest thing to do is use a compass.  Draw a line with a ruler the length of the final desired radius.  Place the compass point on one end, the lead on the other and draw the arc with equal travel to both sides of the radius line.  Place the lead back to the radius line (top dead center).  Without moving the lead, shift the compass' point 20-30% closer to the TDC on the radius line.  Plant the point and draw the arc equally to either side.  The form should be built to conform to the second line to achieve the first. (See figure 2)  (See additional post for drawings).

   The 1.75" x 5" stock that you have for the stage bull nose can be bent to a radius of 20" - 24".  To give an example, I recently completed a Norweigian Kick Sled (think of a scaled down, light weight dog sled, without the dog, that you propel yourself along like you were riding a scooter or skateboard.).  One of the parts was a 1"x1"x46" long that I bent into a 180 degree arc with a 6" radius.  The key is proper tension and compression, plus an incredibly stout bending form.  There is no such thing as building a bending form and a compression strap too strongly.  There are literally thousands of pounds of pressure being exerted on the stock during a bend, which grows exponentially as the stock gets thicker.

   The trade off between steam bending and milling is this: Steam bending will give you consistent grain across the whole bend, and will remain almost as strong as the stock was prior to the bend.  The downside is that it is more difficult to control the variables to achieve a consistent bend over many pieces.  Milling will give you the shape you want with no concerns about spring back, but is much more wasteful of the wood stock.

   Give me a detailed description of the stage's bull nose design.  There are a few options that can be used, and will give you the same end shape result.  (And may take less time too.)  Will the grain of the wood run with, or perpendicular to the stage?

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

   Edited 2/10/2003 12:23:16 AM ET by Jackie Chan

1. 
   ESPEE | Feb 11, 2003 03:14am | #8

   Dan, thanks for the quick reply.  I have done exactly the proceture in the diagram and have steamed the Maple.  I also had forgotten that I had spoken to the Folks at the Nevada State RR museum 1 1/2 years ago while in Nevada.  They do Restorations on old 1800's railroad passenger cars and later. They spoke of using salution of water & downey fabric softener,  soaking the material over night then steam bend the material. Any thoughts on this? 

   I did try this on the maple and it did seem much more flexible when I placed it in the fixture, compared to the other two with out using the Downey.

   On the other note, the oak project is in the bidding stage and I need more info from the sub contractor. I do have a floor plan and a section of the detail and could copy and fax to you or scan & E mail it to you.

   Oh yea , I forgot to stick my burrito in the steamer so no hot lunch today.

   Hehehe.

   Thanks Tony Czuleger

1. 
   jackiechan | Feb 11, 2003 06:11am | #9

   Tony,

   Soaking the wood creates a whole new set of variable, that in the end will thwart a perfect bend.  The wood fibers will swell as they absorb water, which will give it an increased flexibility (marginally).  This may allow the wood to stretch on the outside of the bend, but it allows the inside of the bend to crush and fold in on itself.  You'll also find that soaking will most likely discolor the wood, especially on a light species like maple.  As the wood dries, checking along the grain is inevitable.  Because the wood is now soaked through, as opposed to just heated through, controlling spring back will become much more problematic as the wood dries from the outside in.

   When fabric softener (or any chemical) is introduced into the soaking process you end up with Forrest Gump's box of chocolates - "you never know what you're going to get".  Will the glue joints hold now that the wood is full of a chemical additive?  Will the desired finish be achievable for the same reason?

   The reality is that the above method can work only marginally well.  If you're willing to give it a go - test it on a piece of scrap from the wood you'll actually be using.  Run the test from first cut to final finish (and any glue-ups there might be).  I'd load the piece into a PVC tube, capped at both ends, using distilled water.

   Changing gears - If you want to scan and e-mail me what you've got for the stage plan, I'll give it a look.

   ... a burrito in the steam box, you've got me on that one.  But have you ever tried fresh corn on the cob with a little ceramic dish to melt butter in?!  Outstanding.

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

2. 
   ESPEE | Feb 12, 2003 09:54pm | #10

   Dan the bend went well on the maple I  had about what you had said I would in the spring back of 20%. 

   It fit the 1:1 pattern from the customer almost right on the money,  Thanks.

   Now I just need to get the next job signed on the Stage.

   Regards

   Tony Czuleger

   Edited 2/12/2003 1:55:33 PM ET by zoo

3. 
   DonStephan | Feb 16, 2003 01:17am | #11

   This thread has been a very interesting read. If I may "hijhack" it somewhat, I have been experimenting with bending 1/4" cherry strips into U shapes, 4" radius and 8" radius. Using a compression strap, I overbent only slightly, keeping the intended radius but angling the two sides of the U only a bit beyond parallel. The springback made both the radius larger than intended and the two sides of the U splayed out. Using your 20-30% overbend guideline (wish that had been included in the steam bending books I found at the Woodcraft store) would I shrink the radius AND angle the sides towards each other? Would be glad to experiment some to better learn steam bending.

   Would you consider writing a steam bending basics article for the magazine? Found good explanation of compression straps in a couple books but nothing about overbending guidelines, and the books addressed only European woods.

   Thanks for your generous sharing of experience. Don

4. 
   jackiechan | Feb 16, 2003 09:22am | #12

   Hello Don,

   Cherry is a notoriously poor steam bending wood.  You should feel good that any sort of bend was achieved!  The wood's fibers will almost always crush and collapse in on themselves on the inside radius of the bend.  Easing off the pressure on the compression strap would be the appropriate action to take to avoid compression failure - but, without that end pressure, the cherry's outside (radius) fibers will literally tear apart due to to over tension (over stretching) failure!

   Spring back is caused numerous factors, some of which are controllable (to a point).  The top variable for the amount of spring back that occurs are: was the wood steamed for the right amount of time?  (If the wood isn't flexible enough, it will want to spring back to its initial straight geometry.)  Was the piece under enough compression from the compression strap?  Too much compression is possible, but it is better to have too much pressure than not enough.  How fast did you bend the wood over the form?  As soon as the wood leave the steam box it begins to cool very rapidly.  The outside fibers will naturally cool first, so their elasticity is the first to go.  If they aren't able to stretch, spring back will occur as the outer fibers try to return to their initial straight position.  Was the wood allowed to cool completely before being unclamped from the bending form?  To achieve the greatest stabilization, the bent piece should remain clamped onto the form for at least 24 hours*  Other spring back factors can be anything from the wood's species, to the dimension of the wood trying to be bent.

   When multiple bends are being made on the same form, a series of drying forms are necessary.  The drying form should have an over bend amount of (approx.) 12 - 15% of the desired final radius.  Leave the bending blank under compression on the first form for about 10 minutes to allow the wood to give in to the bend, then transfer it to the cooling form.  This frees up the strap and the form to continue bending.  To accommodate the ten minute intervals, load the wood into the box ten (plus) minutes apart - just keep track of the order in which they were loaded.

   The greater the compression, speed of the bend, and the length of time and temp. in the steam box will reduce the amount of spring back.

   Regarding your bending form geometry question, increase the angle of the sides only (the legs should be coming closer to each other).  The greatest amount of compression has occurred at the apex of the bend, which makes it the most stable.

   Buy some white oak (as straight grained as possible!) and reproduce the bend you tried with the cherry - after adjusting the degree of over bend on the form!  After bending oak a few times (which will produce night and day results), then try other species. 

   Please feel free to send me any additional questions that you may have. 

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

2. 
   jackiechan | Feb 10, 2003 08:25am | #6

   Tony - The pictures that were supposed to be inclosed in the previous post.

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

   
2. 
   RANGERP75R | Feb 10, 2003 02:29am | #3

   Dan

      I will over-work you on this thread. I was getting ready to e-mail you concerning the type of wood to use on the band on the semi-round counter we spoke of several weeks ago. I think you answered the question with oak, as it will be 4 coated with gloss porch and floor paint anyway. The wood will not have to match anything.

      If this is a go, no answer will confirm. If not I would appreciate your recomendation of what would be the best bet. Thanks for the past help. Everything turned out very well.

   You are a gentleman and a scholar of curves, sir...

   sarge..jt

1. 
   jackiechan | Feb 10, 2003 05:56am | #4

   Sarge,

   I thank you - and you are correct, oak is the way to go.  My personal preference is white oak.  It must be as straight grained as possible, with little to no grain run out to achieve the best possible bend.

   Bend well good sir...

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

   ... of course we'll all want to see photos of the finished product!

1. 
   RANGERP75R | Feb 10, 2003 08:42am | #7

      Thanks Dan. I will pick up some quarter-sawn straight grain as it looks like I've got a trip to Atlanta Hardwoods for another item anyway.

       I will get a picture in several weeks as this has been an adventure so far. Never done a carcass this large and never worked with laminates that were curved. Pain working those big panels alone, but with a few little clever things I built it got a lot easier. Actually turning out to be a fun project. I will say that everyone has bent over backwards to give and assist. Looks like I might have to throw a big barbecue. ha..ha

   Evening...

   sarge..jt