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| cupping and rippling in mahogany sides http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=9059 |
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| Author: | Pete0530 [ Sat Oct 28, 2006 1:47 pm ] |
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I am relatively new to building acoustic instruments. My one attempt at bending quartered mahogany sides resulted in serious cupping, and ripples that seemed to follow the "ribbons" along the length of the side. I am using a side bender with a heating blanket and stainless steel slats. I suspect it could have something to do with the amount of moisture involved. I sprayed liberally with distilled water, just prior to bending and ran it through several heating and cooling cycles. Springback was also an issue with 1 1/2" at both ends. There seem to be varying approaches to the amount of moisure involved, ranging from bending dry to soaking overnight. Any suggestions would be greatly appreciated. I am anxious to start a couple of parlour guitars, but am reluctant to jump into it without a thorough understanding of what happened the first time! Peter L. |
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| Author: | MSpencer [ Sat Oct 28, 2006 3:52 pm ] |
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Welcome Pete! What did you thickness your sides to? and what temp were you bending at? Do you have any pics to post? Mike |
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| Author: | Michael Dale Payne [ Sat Oct 28, 2006 4:08 pm ] |
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Peter two things in particular come to mind. You said you use the distilled water liberally. This is most likely the main issue. you only want to use enough water to allow the fibers to relax. this is not more than a light spray on each side. I know it use to be common practice to soak the sides but heating blankets more even heat makes this not needed anymore. You want all the water you added to cook off and evaporate out prior to releasing the the sides from the form. The second is related to the first. Since there was excess moisture in the sides left from the bend that moisture is the likely culprit. My guess is you also did not allow the bend to set over night. I don't know for sure but from the issues you have that is my guess. There is always some spring-back. That is just the nature of wood. you have two choices. first leave the bend clamped in the form overnight and till you are ready to assemble the rim or take it out hot and camp it securely to your assembly mold. But the worst of the problems you ran up against I believe had to do with adding too much moisture to the wood. After you have bent a few sets you will find your self using less and less, and even no added moisture. There are ways to remove the ripples tomorrow I will try to explain how I do it. |
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| Author: | Colin S [ Sat Oct 28, 2006 8:40 pm ] |
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Michael, as usual, is quite right, sounds like too much water and taking the sides out too quickly. a very light spritz is all that is needed wrap the sides in kraft paper and, then bend, take the temperature up to 300-320F hold for 3-5mins. allow to cool down. Then, heat back up again to set them really well, hold again a 3mins or so then allow to cool down overnight. Put them in the mould next morning. Or as Michael says next morning run heat up again take them out hot (wear gloves!) and put them in the mould warm and tighten the cauls. Colin |
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| Author: | Marc [ Sat Oct 28, 2006 11:10 pm ] |
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Welcome Pete, the springback on mahogany is typical. What is your clamping like on your bender? Do you have a picture? |
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| Author: | Pete0530 [ Sun Oct 29, 2006 5:24 am ] |
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Thanks for the welcome, Mike! Those sides have long since been trashed, but I believe they were 0.90". At that point I was using 2 or 3 150 watt light bulbs for heat. So I'm guessing around 250 degrees F. |
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| Author: | Pete0530 [ Sun Oct 29, 2006 5:27 am ] |
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Thanks, Michael! I very much appreciate your suggestions. |
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| Author: | Pete0530 [ Sun Oct 29, 2006 5:36 am ] |
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Thanks to you all for your suggestions. I will approach these new projects with renewed vigour. This is a great forum you have here! Very comforting too know that so much experience and wisdom is available. Pete L. |
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| Author: | Michael Dale Payne [ Sun Oct 29, 2006 5:56 am ] |
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you don't want to guess any more, get a thermometer. most mahogany will bend well between 280-320. Knowing you are at temp will help a lot and prevent bending too cool which can cause stress fracturing in the fibers before you even know it is happening. |
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| Author: | peterm [ Sun Oct 29, 2006 6:35 am ] |
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[QUOTE=MichaelP] you don't want to guess any more, get a thermometer. most mahogany will bend well between 280-320. Knowing you are at temp will help a lot and prevent bending too cool which can cause stress fracturing in the fibers before you even know it is happening.[/QUOTE] |
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| Author: | peterm [ Sun Oct 29, 2006 6:37 am ] |
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[QUOTE=MichaelP] you don't want to guess any more, get a thermometer. most mahogany will bend well between 280-320. Knowing you are at temp will help a lot and prevent bending too cool which can cause stress fracturing in the fibers before you even know it is happening.[/QUOTE] Definately! Its better to know where you are at all times. bendinf to hot or not hot enough may lead to undesirable outcomes!! Walmart sells digital food thermometers for $5-$10 that work great! |
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| Author: | Pete0530 [ Sun Oct 29, 2006 8:25 am ] |
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[QUOTE=peterm] [QUOTE=MichaelP] you don't want to guess any more, get a thermometer. most mahogany will bend well between 280-320. Knowing you are at temp will help a lot and prevent bending too cool which can cause stress fracturing in the fibers before you even know it is happening.[/QUOTE] Definately! Its better to know where you are at all times. bendinf to hot or not hot enough may lead to undesirable outcomes!! Walmart sells digital food thermometers for $5-$10 that work great![/QUOTE] |
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| Author: | Pete0530 [ Sun Oct 29, 2006 8:27 am ] |
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[QUOTE=Pete0530] [QUOTE=peterm] [QUOTE=MichaelP] you don't want to guess any more, get a thermometer. most mahogany will bend well between 280-320. Knowing you are at temp will help a lot and prevent bending too cool which can cause stress fracturing in the fibers before you even know it is happening.[/QUOTE] Definately! Its better to know where you are at all times. bendinf to hot or not hot enough may lead to undesirable outcomes!! Walmart sells digital food thermometers for $5-$10 that work great![/QUOTE] [/QUOTE] O.K. I just got back from Walmart with a digital thermometer....any suggestions on how to get an accurate reading of the actual temperature of the wood? |
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| Author: | Michael Dale Payne [ Sun Oct 29, 2006 8:41 am ] |
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put your probe between the wood and the lower slat just outside your lower bout bending caul, in the (have not bent the lower bout yet) position. I use an analog thermometer but the same would be true for any probe. you do not want much tension on the wood to the probe or you end up with an indention of the probes shap inside. I remove the probe just before bending the lower bout. My process is heat to 275-280 bend 3/4 of the waist, bend the upper bout, remove the probe and bend the lower bout then finish the waist. the whole process from first bend to final bend takes about 2-3 min. It is a bit different with a cutaway. The reason I only bend 3/4 of the waist at first is after both bouts are bent, and clamped tight bending the remaining waist pulls from both bouts and makes everything good and tight. |
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