Apparently, I Need a Tutorial on Side Bending

[Greg Carter]

I have bent cherry and purpleheart, but mahogany is kicking back at me. I use a blanket on a Fox-style bender; the sandwich is blue steel strip, paper soaked in water, mahogany, soaked paper, heater, and blue steel strip. The mahogany is quartersawn, grains running perfectly up and down, just like a soundboard, and about .080". all four sides are clipped to keep the sandwich tight. With an adjustable rheostat, I bring the blanket to 290, then crank the voltage down to keep the temp there. This takes 3-5 minutes. I slowly crank the waist in, then slowly bring the sides down to the form, one at a time, pressing the sandwich to the form with a full-width block along the way. When fully engaged, there is a press at the waist, and full width blocks and the end of both the upper and lower bouts keeping the sandwich tight to the form.

I have seven cracks in the board, and the mahogany is straight between the cracks, as if there was no bend.

What am I doing wrong? Ideas include not using quartersawn, but flatsawn, wood. Perhaps soaking the rib in water for 5-20 minutes before putting it in the sandwich, or leaving it at 290 for 5-10 minutes before applying pressure.

Any and all help appreciated.

Greg

[Bob Gramann]

Sometimes, the mahogany has some runout that you can't see until you try to bend it. If you've got runout, it can be very hard to bend without breaking. I don't have experience with a bender but use a pipe. Sometimes when I bend mahogany, it stops wanting to bend when the water I have spritzed on it is gone. Another spritz in that spot lets me get more bend there. Soaking it won't help with this but spritzing it again might--it's the steam that carries the heat into the wood. Many woods I bend dry--mahogany seems to like the steam. I can't speak to your temperature. I don't measure. When the pipe is hot enough that a spritz flashes off instantly, I consider it hot enough for most woods I use.

[Michael Lewis]

It sounds like the wood is not getting hot enough. I think you need the wood to be around 350F or even more, and only long enough to make the bends, then let it cool.

Bending with metal slats and a heat blanket remove a lot of the feel from the operation, so it is more difficult to tell when the wood is ready to bend. You probably need to feel when the wood becomes relaxed with the heat, then it will droop a bit and go pretty much where you put it. But when using metal slats you have to use some force to bend them as well as the wood, this is where the feel is mostly lost, overcoming the metal and breaking the wood before it is ready to bend.

How do you know how hot your wood is? A dial on a control may tell you how hot your blanket is but not reflect accurately how hot the wood is.

[G. M. Seigmund]

I had the same thing happen to me. I had successfully bent Sycamore, White Oak, Curly Maple, and messed up a really nice set of Honduras Mahogany sides. John Hall at Blues Creek had the answer. I use a Fox style bender, heating blanket, and stainless slats, & craft paper. Wood and paper slightly damp. I use a shop rag on the rear bout and about 110/120 degrees you can feel the wood relax, using hand pressure bend the side and lock down with the caul. Immediately do the same to the front bout and then crank down the waist. Should be finished and watching your temp in three minutes or less. Good Luck

[Chuck Tweedy]

G.M., I assume you are saying "110C/120C" - which is still not very hot. You really need more like 175C to get it going
Mahogany is a tough wood to bend. And it springs back a lot even after.

[G. M. Seigmund]

Start bending at "110/120C" or it will break, buckle etc. I just followed John Halls recommendations.

[Greg Carter]

Interesting ideas, all. This opens up a new question. Do you crank down the waist first, or the bouts then the waist?

[G. M. Seigmund]

Rear bout, front bout, waist. By the time my thermometer hits "150C" I've bent the rear bout, the front bout is bent by "190C" and the waist by "220C" I let it get to "300C" and back off on the heat it gets down to "250C by the time the 15 minute timer shuts the blanket off. Lay your hand on the rear bout and you can feel it relax around "120C"/"130C" I bend it with hand pressure and use the spring caul to hold, then the front and waist as quick and smooth as you can. Its worked perfectly for my last three Honduras Mahogany side sets.

[Chuck Tweedy]

I'm confused.
GM - you are showing buckled sides.
I assume you are giving instructions in that last post for a SUCCESSFUL bend - not the bend pictured.

Also, 300C = 572F !! That is scorch temperature.

[Barry Daniels]

I always bend the waist first and I think the majority of other builders do the same. What is the purpose of doing it your way, GM?

[David Ackley]

I haven't bent mahogany yet (have some in the wings), but included the layers you listed, I always wrap the wood and damp craft paper in aluminum foil and fold it to seal. This keeps the steam in and has always worked fine for me. I also always do the waist first at around 240 F, and finish the bouts as it heats to 310 and keep it there for 10 min. So far, so good - no burns nor cracks.

[Jeff Highland]

The last mahogany I did earlier this year, I just sprayed veneer softener on both sides wrapped it in plastic for an hour then put it in the bender without the plastic. SS slat, heating blanket, kitchen paper, rib, SS slat.
Turn the blanket on full count to 60 seconds, crank down the waist fully bend the lower bout, bend the upper bout, turn off the blanket
All done in 2 to 2 1/2 minutes maximum.
Let it cool fully then turn back on at low temp for an hour to set.
Works for me and gives minimal springback, this is with an LMI Blanket others may vary
I suspect by allowing 3-5 minutes before bending, you are allowing the wood to dry too much.
YMMV etc

[John Hall]

Side bending is all about temp control. There are a number of ways to do this. Some woods like hot and dry , others wetter. Maple and Mahogany tend to be hotter and dryer.

I have a tutorial on You tube that will show the process but here is the basic process I use.
KEEP A LOG OF TEMPS AND WOODS
the more you do the less you may have to reference the list but having one helps.

First off , the lower the grade of wood the higher the risk, same as figure. If I have high figure woods I will use super soft ll . This helps a lot but you don't really need this for most woods. I don't use a light bulb bender and I will change the stacking method , depending on what I am bending.

Simple process is this
slat wet paper ( any unreated paper will work ) wood , wet paper blanket slat

I set the stack into the bender and apply a little friendly friction with the waist cawl to allow for adjustment .
I have a little notched stick that allows me to locate the neck end to the waist. I apply the hold downs and turn on the heat. I usually start bending the lower bout first at 220F . I walk the hold down so it take about 30 seconds or so. then I move to the upper bout. I do check to see that the wood is still damp. This should be about 250F by now and then again a 30 second walk down with the hold down then I set the waist. I find this gives a good repeatable process for most woods.
I allow the temp to hit 275 to 300 F then use the VAR cycle set at 220 to 250 F for 10 to 15 min to get the water out. Allow to cool then place in mold .

On Mahogany I will use less water and often will do slat wood slat blanket and allow the set heat to rise to 350F

I don't get in here as often as I would like but please feel free to contact me and I will be happy to help where I can.

[John Hall]

Bending the waist first will not give you the best replication of the pattern. Do a dry run and you will see gaps form on the bouts . Also by doing the bouts first then the waist , you are allowing the heat to get to the cellular level. As you drive the waist cawl you can see the slats pull down on the pattern to get a better set. I always had repeatability issues before I went to this method.

[Barry Daniels]

Good information John. I will have to give it a try.

[Tom West]

Barry: I do the waist first but not all the way. Leave it up about 3/8" or so. Do the bouts and then set the waist. I think as John says it gets you tighter to the form. I also have the outer slat set up with bungee cords attached to ends rather then coil springs on the hold down slats. They tend to set the bouts real tight.
Tom

[Barry Daniels]

Tom, I do the same thing. The thing I don't understand on the "waist last" approach is how you do you align the side. I do mine by placing a masking tape flag at the waist and line it up through the slot in the bender. And bending the waist first locks the side into this position and keeps the wood from shifting toward the tail or the neck.

[John Hall]

When bending , how you locate things is important. I don't register off the waist I use the neck block. I have bent over 4000 sets of wood , so I have developed a pretty repeatable process. I use a special ruler that fits into the waist slot and align the end to a given point. I only need to get this close as there is trim on both ends. Then I set the sides into the mold and do the truing of the rim and set the body geometry.
Like Tom I allow a certain amount of tension on the waist then lower bout and upper , then the waist. If you do the waist first , you will see more variation in the sides. Unless you use a tension spring or tie down on the slat.