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Bending machine help http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=54613	Page 1 of 1

Author:	jfmckenna [ Mon Dec 13, 2021 10:23 am ]
Post subject:	Bending machine help
Long story short I only purchased a bending machine because I had started building with BRW and after building some 65 guitars I cracked a set of BRW sides on the hot pipe. Turned out it was brash wood and nothing I could do about it but none the less sit gave me a case of the yipps and I didn't want to make another expensive mistake like that. The form I have is the OM form and it's either different from my Antes plans or I'm doing something wrong. The waist is coming out wrong and there is a terrible amount of spring back. I even sanded down the end block and head block curves to allow for spring back. I don't mind touching them up on a hot pipe but it kind of makes me wonder why bother having one of these machines if you have to bend by hand anyway? My process The sandwich is: Steel sheet - wood (spritzed with water and wrapped in foil) - steel sheet - blanket - steel sheet Place everything in the machine then start to heat up. At about 240deg I start bending at the waist Then I pull out the lower bout Then the upper bout. Now at about 300deg I shut the heat off then bend the rest of the waist and really tighten it down. Then I turn the heat back on to about 220deg for ten minutes. Let cool completely, seems to be at least an hour. Then this is what I see:

Author:	Terence Kennedy [ Mon Dec 13, 2021 10:40 am ]
Post subject:	Re: Bending machine help
That looks pretty normal to me. Technique is close to what I do. I use two blankets, moist brown wrapping paper with the slats covered with foil, bend the same way with similar heat points, and keep the heat at around 300 degrees for 15 minutes and allow to cool fully followed by a re-heat to 300 for 15” and cool. After the waist is partially bent and the bouts are bent I clamp the bouts snugly with long bar clamps before finishing the waist. Seems to ensure good support from the lower slat. I’d have to say I may get slightly less spring back than you got but not much and it conforms easily to the mold with spreaders and clamps. I’d say you are pretty good and that side should do the same.

Author:	Clay S. [ Mon Dec 13, 2021 10:42 am ]
Post subject:	Re: Bending machine help
Looks pretty good to me. If you are using an outside mold can you get it to fit with moderate pressure? I always assume I will have some spring back unless I am molding (laminating) the sides from veneer.

Author:	SteveSmith [ Mon Dec 13, 2021 10:55 am ]
Post subject:	Re: Bending machine help
Your bends look ok. Went through much the same with the bending machine till I learned to use it. Bending machines work fine but I went back to the pipe, I just prefer doing it that way, I guess.

Author:	jfmckenna [ Mon Dec 13, 2021 11:22 am ]
Post subject:	Re: Bending machine help
Ok so that is considered normal. I do not use a mold so I have to bend very precisely. It does look as though if I did use a mold the spreaders would probably force fit it. But since I build top down it has to be better. Still I can see an advantage to using the machine as it gets everything pretty close. They are kind of fun to use too.

Author:	SteveSmith [ Mon Dec 13, 2021 11:44 am ]
Post subject:	Re: Bending machine help
Yeah, I always needed a mold with the machine.

Author:	Robbie_McD [ Mon Dec 13, 2021 12:24 pm ]
Post subject:	Re: Bending machine help
Your process looks good - same as mine except I don't use the 3rd slat over the blanket. Your results look similar to mine at the start. I had a run of 5 EIR sides to do, so I modified the form incrementally over the run of 5 until it came out almost perfect. That meant modifying the form to overbend the bouts to account for the spring-back. The waist seems to hold it's shape. I have used the same form for about 35 builds - EIR, Bubinga, Ziracote, Cypress, Peruvian Walnut, and the resulting bends are all very close after modding the form....

Author:	mountain whimsy [ Mon Dec 13, 2021 12:39 pm ]
Post subject:	Re: Bending machine help
I have a similar approach. No third slat over the blanket. For me, the second cooking after things have cooled down is key and has removed most of my springback. Also having a properly shaped waist caul helps with that bend.

Author:	jfmckenna [ Mon Dec 13, 2021 12:43 pm ]
Post subject:	Re: Bending machine help
So how long do you allow to cool down? Till it's room temperature? Over night? I'll have to try the second heating a bit more intently on the next set.

Author:	johnparchem [ Mon Dec 13, 2021 1:06 pm ]
Post subject:	Re: Bending machine help
I cool down until it is room temp. I do a second fifteen minute cycle at close to 300°. Still I have a bit of spring back, close to what you have. I think the issue is you can not over bend on a form. When I bend on a pipe I naturally overbend and it springs back to the desired shape.

Author:	Toonces [ Mon Dec 13, 2021 1:56 pm ]
Post subject:	Re: Bending machine help
Design some "overbend" into your bending form as that helps a lot. Also, I find I just have a bit of springback and I spend a few minutes on a hot pipe correcting for this. IMO, you never want to force anything into a mold - very light pressure is different but certainly make sure your sides are close enough that easily conform to your mold shape before continuing on.

Author:	Woodie G [ Mon Dec 13, 2021 4:06 pm ]
Post subject:	Re: Bending machine help
I'm assuming your mold and bending form are commercially sourced, but not from the same vendor? If you are building in an outside mold, the minor variation I see does not appear to be something to be concerned with, and you might use some spreaders to assess whether you actually have any fit issues. For solara-type construction, the neck block and tail block may be appropriately contoured to help keep the design body shape something close to the template, but no one should expect solara-built instruments to be a 100% match to what is at best a shape that reflects either a specific prototype instrument or an average of measured guitars. Again, no issue I can see that would be a real problem. In summary: If the body shape varies a bit from 'idea', it's a custom guitar, and the variations seen are fine as long as it fits in the case you intend on providing with the guitar.

Author:	jfmckenna [ Mon Dec 13, 2021 4:14 pm ]
Post subject:	Re: Bending machine help
Ah yes I had not thought of that. If I got the mold from the same vendor it probably would have been a better fit. So that brings up a question. Assuming Martin is the designer of the 000/OM did they change the design over the years? Like the waist used to be tighter and they widened the curve out or visa versa?

Author:	Clay S. [ Mon Dec 13, 2021 6:11 pm ]
Post subject:	Re: Bending machine help
From what I have read there is some acceptable dimensional variation from instrument to instrument ,especially among the older instruments. If you want to build top down (which I often do) you can use a work board and screw L brackets around the perimeter where necessary to control the shape.

Author:	bluescreek [ Mon Dec 13, 2021 8:46 pm ]
Post subject:	Re: Bending machine help
antes isn't close to a martin if you want martin plan the Greillier plan is closer antes is overbraced the amount of spring back you have is normal and will most likely fit the mold

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