Side Bending Practice

[scamp]

Hope this is the correct place for my question. I am trying to bend mahogany sides on a pipe. I have tried to practice on old kiln dried mahogany used for home details. Resawn to 090 and planed down o 075. Cannot bend it on the pipe. I have tried temperatures from 250 to 450. I set that aside thinking it was the kiln drying that prevented me from bending. I'm now trying to bend cut offs from my guitar sides and am having the same problem. I have tried spritzing, wet parchment paper and wet cloths to no avail. What am I doing wrong?

I’m a rookie at this myself but I will give my two cents. The temperature of the bending iron that I used to hand bend was 375 degrees ( this is about what LMI recommends ). I found it helpful to watch a lot of on line videos of experts hand bending to get a feel for how to do it before I gave it a try. When I did it I used a wet rag between the iron and the wood to protect the wood on the outside of the guitar but nothing when the iron was on the wood on inside of the guitar After heating the wood a little while you could feel it soften and under light pressure bend so I’m not sure what you’re doing wrong. I did, however, discover that it helps to hold the wood in its bent shape, remove it from the iron and wait until it cools down a bit otherwise it tends to spring back. Maybe that’s something you can try. Hopefully others with more experience can chime in.

[warren47]

I prepared another sample. Planed it down to 0.075 tried to bend it on a 375 degree pipe. Nothing. Just held it against the pipe until I could smell smoke. Still no luck.
I cut the end of a 2 x 4 to about a 6 inch radius (cut along the wide surface). Clamped this and my wood sample to a work bench and heated the assembly to 275 degrees. I bent it very slowly but it still snapped.
Next experiment will be a metal/paper/wood/paper/metal sandwich on the same 2 x 4. Again, heating with a heat gun.

[scamp]

I have no idea what you’re doing wrong. I could understand having a problem with kiln dried wood but it sounds like you are using air dried wood now. Maybe someone else has an idea

[MaineGeezer]

How/where are you applying the bending force? You should be applying, as much as possible, force over a wide area, not just at the ends.

[warren47]

I have been applying the force at the edges. I guess this might be a problem.

Earlier I mentioned my next effort. Still using the kiln dried mahogany I was able to bend it using a metal, wet paper, wood, wet paper, metal sandwich over a curved edge on a 2x4. Pressure was applied to the free end when my thermometer read 235 degrees while heating it with a heat gun. At this point I am looking at my hot pipe as the problem.

[scamp]

How are you measuring the temperature of your bending iron? If there is poor thermal conductivity between the heating element and the pipe and you are measuring the temperature close to the heating element then the temperature of the pipe may be lower than you think. Just an idea.

[Stray Feathers]

For what it's worth, I have had difficulty using a hot pipe (mine is a piece of aluminum boat mast built by a friend). My best success in my first builds was adding a heat gun clamped in position to augment the hot pipe. I later replaced the 300w bulb in the pipe with a heating element from an old dish type heater, which had a screw base on it. It seems to get hotter. But I never really was sure about temperature etc. and this was confirmed when I compared the laser-type thermometer I used initially with a meat-thermometer type sold by John Hall. See photo to see the discrepancy. In the end I made a Fox-type bender - much better. I still use the pipe for incidental work but am not crazy about it. Bruce W.

[scamp]

When I built my bender using a solid aluminum bar I screwed a thermal sensor into it as a feedback to the temperature controller. I then checked the temperature on the controller against an external temperature sensor using a probe on the surface of the pipe. They were relatively close. It’s what I expected because the aluminum is such a good thermal conductor.

IMG_0170.jpeg

What I did find interesting as I monitored the temperature while bending was that the temperature of the iron went down significantly when I first started bending. The controller tried to correct this and after awhile the iron came back up to 375 degrees. It did show that an electric heat iron with less thermal mass ( like a hollow pipe ) and without a controller with temperature sensor feedback will probably get significantly cooler when in contact with cool wet wood. Something else to consider.

[warren47]

scamp and stray feathers. You both anticipated what I found this morning. I am measuring temperature with a "dot" instant read therometer strapped to the lower extent of the pipe. As scamp found, I too was getting quite a drop in temp when the wood was in contact with the pipe. I suspected a problem with how I packed the pipe. Upon opening the assembly I was presented with a few melted pellets. They obviously are not aluminum. I moved the heating element into contact with my working surface on the pipe and put the pellets back in.
I no longer saw a reduction in temperature but I was still un able to bend air dried nor kiln dried mahogany. I tried water spritz, Windex and finally water augmented with household ammonia. I was able to bend 1/2 wide 075 thick strips but they were still breaking.
Realizing I was working on the hottest part of the modified pipe I began to get gradual bends without breaking. Using a wet paper towel I was able to get these 1/2 inch wide strips to match the waist of my template. I will practice a bit more with the kiln dried wood before attempting the sides.
I am also considering another disassembly and wrapping the heating element with enough aluminum foil to get a snug fit in the pipe.

[Stray Feathers]

I am sure not an expert in this area - maybe others will weigh in - but I wonder if you are worrying too much about filling the pipe with metal? I have seen far more descriptions of pipe benders that just have heaters inside than those with metal packing in them. They might be propane torches, barbecue heaters, light bulbs or other heating devices. The original Fox benders I think just had three large light bulbs in them, plus a little foil as a reflector. I find it seems to take forever to get the wood to relax on a pipe bender, but then I recall that it takes two or three minutes of direct contact with the silicon blanket to get a side hot enough to bend in a Fox bender. The fellow that made mine used a 300w bulb successfully, but I modified it with the cone heater in the photo - which itself was scavenged from another luthier's homemade round pipe bender, which is to the left in one photo. Bruce W.