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more side bending
hello looths –
i’m building my first guitar. generally following (and enjoying) the gore/gilet books. after 3 tries, i’ve yet to get a crack free bend around the nose of a venetian cutaway with a mahogany side. i made a version of the fox bender – using clamps on hand for the waist and cutaway, and a pid temp controller (verified accuracy), and am using a stew mac blanket and three straps. the recipe for my last attempt was: sapelle (for testing) thicknessed to .083″ narrowed to .068″ around the nose, wetted with supersoft and wrapped in foil for 2 hours, from there into the bender, waist bent first ~ 250 f, followed by the lower bout at ~290 f, then bending and clamping nose slowly ~8 minutes. all bending done at 16 minutes, let it cook at 290 for 10 m, down to 250 for 5 more min, then let it sit clamped 12 hours. alas, still cracking somewhat around the nose (not much springback though).
there is a small (1/32 – 1/16″) gap between the slat and form around part of the nose when clamped up – which i imagine allows cracks to form. i will adjust to make as gap free as possible, but there are even cracks forming where the pressure is consistent on both sides. looking at solutions online i see a few versions of bending jigs that keep the slats tensioned while applying pressure to the cauls – this seems like it would help address the room to crack problem.
from the other recent side bending question here i saw answers to both go more slowly and to go faster. i understand that you don’t want the wood to be dried out in either case.
looking for any advice on how to get better results (including perhaps scrap the mahogany for now and try rosewood, switch to a florentine cutaway, …)
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This discussion was modified 2 months, 1 week ago by
ben.
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This discussion was modified 2 months, 1 week ago by
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6 Replies
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a few more photos: back of cutaway with caul, slats/blanket/side; close up of split at nose; bent side moved to building form.
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This reply was modified 2 months, 1 week ago by ben.
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This reply was modified 2 months, 1 week ago by
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Hey Ben. I can’t *exactly* say where your problems are occurring, but I have a few thoughts. Overall I would say you are too wet, and going too slow.
I bend my sides at .05″ for an outer side and .045″ for an inner side, but I built quite a few guitars with solid sides around .080″ and this method basically worked for me.
I use supersoft like you do. I spray it on the wood about 5 minutes before bending and let it air “dry” while I am prepping the rest of my bending needs. Then right before I bend I spritz / mist the sides with water and mist my craft paper. (I don’t use foil. I had problems with it in the past and went to craft paper).
I have found that making the wood too wet, or letting it soak can cause some delamination in the grain particularly in heavily figured woods or around tight bends. I’ve had this problem more in bindings than sides, but I try to put the moisture on the wood to use the steam it generates vs. putting the moisture IN the wood. Leaving your wood set wet and wrapped in the foil for 2 hours might be a cause for some of the issues.
Also when I bend I move pretty fast. I start my waist bend at 260, slowly but steadily drive it all the way to the center then back off a smidge, bend the lower bout, then start on the upper. For a venetian cutaway I use the normal pressure arm to bend the upper bout as far as I can before starting the cutaway (I use a traditional Fox Bender with a cutaway ram. I go a touch slower here but the whole side is bent in 1:00 – 1:30. (I then tighten the waist back down to snug after the rest of the side is bent.)
I think by going so slowly you are likely driving all the moisture out of the wood and that is causing the problems you are having. It is making the wood brittle.
Also, from your setup, I would take another look at how the cutaway is supported during the entire bend. It seems to me that there isn’t much contact until you are at the final seated position. I suspect that is also causing some of your issues. In a perfect situation you want solid contact on the tight bend of the cutaway through the bend.
Good luck. Feel free to reach out to me if you have more questions or I can help in any way.
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hi brock – thanks so much for your detailed response.
your notion that I’m taking too much time to bend and making the wood too wet – especially for crack prone mahogany – makes sense to me. as does the question of sufficient support around the cutaway. i am able to use the cutaway caul to guide the bend around the nose before driving it into the cutaway, but the recurve is less supported – which coincides with the relatively more cracking there than at the nose. i’m not sure how this differs from the s.m. ram, but i do think more support would help.
i’m going to give it a try incorporating your suggestions. i’ll report back.
again, thanks a lot for the advice.
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hey @patreon_65665670 –
so i substantially followed your recommendations with another test piece of sapele: spray with supersoft about 5 min before going into bender, wet down craft paper, spritz side with water, and get the stack into bender. i was able to get the whole bend including the cutaway done in ~5 minutes.
much better!
attached see pics of the side out of the bender, placed on the bending form for reference and then in the building form. if you look closely at the inside of the recurve on the first picture you can see small cracks forming, these became a bit larger on clamping into the building form. i imagine these could be from runout or perhaps lack of sufficient support.
i took the side out of the bender after ~1.5 hours (and after cycling back up to 290 f for 5 minutes after the initial bend and cool down to ~110 f); they were still a bit damp on unwrapping.
i think that making a shaped caul for the recurve to clamp into the building form could help to limit the cracking.
i’m so pleased with these results after many tries and clearly going in the wrong direction for a while.
thanks a _lot_ for the advice.
-ben
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I agree supporting the piece more will probably help.
You bet.. I am happy to help. I have been at this a long time and still feel like I am constantly banging my head against a wall. It’s part of the fun of it. 😜
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