| Official Luthiers Forum! http://www.luthiersforum.com/forum/ |
|
| Tiger _Myrtle - Need some bending help http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=29585 |
Page 1 of 1 |
| Author: | Tom Armstrong [ Thu Oct 21, 2010 1:42 pm ] |
| Post subject: | Tiger _Myrtle - Need some bending help |
First off, I'm not new to bending with a heating blanket and Fox style bender. I've bent many species of wood and have mostly had success. I don't mind learning by mistakes but this is getting expensive! I searched the forum and found posts that led me to believe tiger myrtle would bend relatively easy. I have ruined two sets and am now down to my last book-matched pair. The first set was scraped down to .078, misted, wrapped in foil, and pre-heated to 315 degrees for four minutes before cranking her down. My set-up is slat, side, heating blanket, slat. This set cracked at the upper bout. Not salvageable as a side. Second set I took down to .070, but this time I used Supersoft 2 and let dry overnight. I once again, misted lightly, wrapped in foil and used the same slat arrangement and temperature. I waited and additional 5 minutes before slowly tightening down the waist caul. C-R-A-C-K. Also un-salvageable as a side. BTW I took a moisture reading from a second set cutoff (prior to Supersoft) and it reflected 11% which is common for my shop. Before bending I looked very carefully at the second set to make sure that was no tight curl grain in the area of the waist. For those of you who have actually bent this stuff, any constructive advice would be appreciated. As I said, I'm down to my last set and I am paranoid about putting it back in the bender and thinking about bending this set by hand as opposed to a Fox bender. Thanks, Tom Armstrong |
|
| Author: | meddlingfool [ Thu Oct 21, 2010 2:36 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
My guess would be that during your preheating stage your wood is drying out entirely and you're trying to bend completely dry wood. I've cracked sets where the upper bout dried right up while I was bending the waist and lower bout. I leave the edges of the sandwich exposed so I can remist the upper bout immediately prior to applying the bend. I know you don't want to drench the sides and have them cup, but I think there is such a thing as not enough. And I never 'preheat' the sides at all, they're thin enough to be heated all the way through almost immediately. I follow John Hall's instructions, which is to gently start bending the waist as soon as the thermometer hits 240, then do the lower bout by which time the thermo usually reads around 300 or 320, then the upper bout. I add a step of remisting the upper bout area lightly. That being said, I've never bent myrtle specifically so... Let's hear from other folks. |
|
| Author: | Michael Smith [ Thu Oct 21, 2010 3:17 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I have been using the same setup as you. The only thing I have been doing differently is placing a piece of brown paper bag soaked on top of my side. I was using a thermometer but found it works better to start my bend when the bottom metal slat will sizzle with a spit coated finger. Not very scientific I know but it has worked without fail for me. |
|
| Author: | Mark Groza [ Thu Oct 21, 2010 3:49 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
From what you said, slat, wood ,heating blanket, then slat. That might be the problem.I don't put the blanket against the wood i'm bending,(not enough support there). I go slat ,wood,slat, then blanket, slat when i bend. Yes, i use 3 slats. That also protects my blanket when pulling down the cauls on the lower and upper bouts. |
|
| Author: | Alan Carruth [ Thu Oct 21, 2010 3:55 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
With figured wood I often to a rough bend by hand to about 50% of the final shape. I allow that to sit overnight, and then it goes into the bender. The last set of mytle I bent was offering to crack in the upper bout during the initial bend. I superglued a sheet of plain white printer paper on the outside, and it bent fine. It's all about doing whatever you have to to keep those cracks from starting. Scraping the paper off at the end was not too much of a chore. |
|
| Author: | Tom West [ Thu Oct 21, 2010 4:12 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
Alan: Nice Idea....!!!!! |
|
| Author: | mhammond [ Thu Oct 21, 2010 5:59 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I just wonder if that tree is just an ornery one. The set I bent was done on a hot pipe and it behaved beautifully. The set was expensive so I supersofted first just for insurance, at .070 I bent a cutaway by hand. Sorry I can't be more help...... Mikey |
|
| Author: | TonyKarol [ Thu Oct 21, 2010 6:24 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I think you are waiting too long and its case hardening .. especially the second time around ... I am totally done in 4-5 minutes, and thats from turning the blanket on full .... note .. never bent tiger myrtle though .. but lots of other stuff ... |
|
| Author: | peter.coombe [ Thu Oct 21, 2010 7:02 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I have used Myrtle in my mandolins for around 14 years so have a fair bit of experience bending it, but not in guitars. Most Myrtle will bend easily with no breaks, but there is some that is more difficult, and a few pieces are close to impossible to bend without breaking. Sometimes the difficult pieces will suddenly snap clean in two with no crack to warn you. So there is a fair amount of variation between trees. Fortunately the near to impossible pieces are rare, but of course Murphy rules and they are just the pieces that you really need to bend because the matching back you have looks stunning. Tiger Myrtle is a very rare form of Myrtle, limited in quantity (and maybe not available for much longer), and hence is expensive and should not be wasted, so if it were me I would go super careful and bend slow and carefully by hand. My guess is that you have Myrtle wood from one of the more difficult to bend trees. The suggestion to pre-bend by hand before putting it in the bender I think is a good suggestion. I would do that with all Tiger Myrtle. |
|
| Author: | Tom Armstrong [ Thu Oct 21, 2010 8:06 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
Thanks to all that responded. The two sets were from different suppliers so it's doubtful they came from the same ornery tree. I don't usually use aluminum foil but kraft paper instead and then begin the bending process when I see steam. Using foil I thought would give better penetration of steam to the wood. At least that was my thought process and that was recommended in archived posts. Guess we were wrong. I haven't bent over a hot pipe in years but I think it may be time to dust it off. Live and learn. Thanks again. Tom |
|
| Author: | peterm [ Thu Oct 21, 2010 10:42 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
Tom Armstrong wrote: ....pre-heated to 315 degrees for four minutes before cranking her down. My set-up is slat, side, heating blanket, slat. This set cracked at the upper bout. Thanks, Tom Armstrong That's your problem right there. I bend as soon as the wood gets to the desired temp... You are over drying the wood and making it brittle. |
|
| Author: | Kim [ Thu Oct 21, 2010 11:29 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
peterm wrote: Tom Armstrong wrote: ....pre-heated to 315 degrees for four minutes before cranking her down. My set-up is slat, side, heating blanket, slat. This set cracked at the upper bout. Thanks, Tom Armstrong That's your problem right there. I bend as soon as the wood gets to the desired temp... You are over drying the wood and making it brittle. You would think the issue must be something along those lines Peter. I have not yet built with Tiger Myrtle, (Nothofagus Cunninghamii which has absolutely no relationship whatsoever to Umbellularia Californica which is the tree bearing the wood most commonly referred to as 'myrtle' in the USA) but I have enough of it in my shed to have allowed a little bending experimentation and if there is anything remarkable about that experience it is that this wood normally bends beautifully. Cheers Kim |
|
| Author: | Allen McFarlen [ Sat Oct 23, 2010 3:28 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I use Tiger Myrtle quite often, and never had an issue. In fact I look forward to using it as it's the easiest wood I've ever come across for bending. I believe you are letting it preheat way too long. My method is slat / wood just dampened and excess water wiped off and then wrapped in al-foil/ blanket / slat. Turn blanket on full, bend lower bout starting in 1 1/2 minutes, proceed to pulling waist 1/2 way down, then do upper bout. Finally pull waist all the way down. Fully finished in 2 minutes. No worries. |
|
| Author: | Beth Mayer [ Sat Oct 23, 2010 4:02 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
Everyone seems to have a bit different way to bend (different order of layers of slat/wood, etc); different temps and timing. Yet most of you seem to agree that his problem with the tiger is that he is letting the wood dry out. Something I read in another thread or saw on a dvd said that it is the heat and not the moisture that causes the wood fibers to relax in order to bend them. If that is true, then why is the drying a problem? I don't question that it IS a problem, I would just like to understand what's happening at the level of the fibers and how these variables are affecting them. Beth |
|
| Author: | Darryl Young [ Sat Oct 23, 2010 5:31 pm ] |
| Post subject: | Tiger _Myrtle - Need some bending help |
Maybe he has waited so long the wood case hardens becoming brittle and cracking. |
|
| Author: | Alan Carruth [ Sat Oct 23, 2010 7:52 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
I can't give you 'scientific' data on this, but here's the way it seems to work to me. There is always a certain amount of moisture in the wood, of course. If you heat the wood up quickly to the bending temp, and bend it before all the moisture is driven out, it usually goes well. However, I have noticed that the longer I heat wood, the hotter it has to get in order to bend, and at some point the wood gets so dry that you end up scorching it before it will bend. This suggests to me that the presence of moisture effects the softening temperature of the lignin: more moisture, lower temp. Too much moisture on the wood weakens the bonds between cells, and you get 'peeling' failures, particularly on curly wood. These are on the tension side, so always put the moisture on the compression side. One big exception to this is mahogany, which will develop crush failures on the compression side if it's too wet. Other similar woods, like cedro and willow, will too. Bending is thus one of those 'Zen' things, like French polish, that requires you to be really aware of what's happening, and to be ready to stop at any time to allow things to equilibrate. Before we got the bending blanket jig this was always one of the hardest things to teach the students. There would be a kink here and a flat spot there, and it might seem like a few more seconds of bending each one in a different direction would result in a smooth curve. Generally these were due to people not moving smoothly enough from one part to the next; a little too fast leaves a flat here, and slowing down to get more bend results in a kink, and a dry area in the wood. You put the other side against the iron, and instead of bendiing it just scorches where the kink is, while the flat bends the wrong way. With stuff like this it's just best to turn off the iron, put the wood down, and find something else to do until tomorrow. The wood will have absorbed enough moisture from the air to relieve a lot of the stresses, and to make it bendable again, and you won't be in nearly as much of a hurry. Remember the 'koan of Lutherie': "The slower you work, the faster you will get done." |
|
| Author: | Tom Armstrong [ Fri Oct 29, 2010 7:04 pm ] |
| Post subject: | Re: Tiger _Myrtle - Need some bending help |
As a follow up, I had two orphan sides from the two previous "screw-ups" and bent each successfully by commencing the bend at around 230 degrees F. Since I changed nothing else, including the slat/wood/blanket slat layup I am deducing that drying out from too hot a temp. was probably the cause. Once again thanks to everyone that responded. |
|
| Page 1 of 1 | All times are UTC - 5 hours |
| Powered by phpBB® Forum Software © phpBB Group http://www.phpbb.com/ |
|