Spring back with fox bender

[dwizum]

Hi all,

So far I've been bending sides by hand over a bending iron. I recently built a fox-style bending jig and I've been practicing in an attempt to learn to use it.

I've got an eBay heating blanket with a temp controller. I'm bending black walnut "rejects" as practice - these pieces were either too short, had wormholes, or were cut too thin. Most of them are between 1.7 and 1.8mm.

I'm wiping the sides down with water, wrapping in aluminum foil, then stacking them between two spring steel strips with the blanket on top (although I did one with the blanket underneath). Then I put the stack on the form and turn the blanket on. I've tried a few temps between 300 and 350 F. Once it hits temp (which only takes a minute or two), I clamp the waist down, then the lower bout and the upper bout. I've been leaving the heat on for a minute or two, then unplugging it and letting it cool to room temp (a few hours).

My problem is this - every single time I've gotten the side out of the jig, it springs back really badly. The top and bottom end up maybe an inch or two out of alignment - much too far to feel comfortable moving forwards.

I suppose I could fix them with the iron but I'm certain I could get better results, I just don't know what to try. Anyone have ideas? Higher temp? Let the sides soak with the blanket on for a while after bending? More water? Less water? Anything? I'd rather get some guidance so I don't burn through all my rejects trying to figure it out on my own...

[charles Tauber]

Once the sides are bent and held in place on the bender, I leave the heat on for about 15 minutes before turning it off. I then let them cool in the bender.

The inserts that define the bent shape can be sized to over-bend the waist and upper and lower bout curves. Doing so reduces the amount of the spring-back relative to the desired shape. There will always be some spring-back. If spring-back is excessive, as you said, the shape can be touched-up on a hot pipe.

[dwizum]

Thanks for the suggestion. I'll try one with that method today. What temp are you bending/heat soaking at?

I'm sure this is subjective, but how much spring back is too much?

This will also be my first time building with an outside mold - my past instruments were totally freehand, so I'd get things as close as I could on the iron then just proceed with that shape - this was "easy" in the sense that I wasn't concerned about ultimate precision, but it did result in a lot of slop in dimensions - my instruments are sometimes 1/4"+ off the plan. With the bender and outside form I'm trying to get things "right" in terms of matching the plan and getting a consistent shape. I can force these sides into the mold but I don't want to introduce all that stress. On the other hand, I don't want to kill myself trying to get the bends perfect (especially if that would mean a lot of time on the iron, since the whole point of the bender was to reduce time on the iron!).

[Dyson Guitars]

I leave my sides on the heat for about 10 minutes then let it totally cool in the mold/bender. I also have my side molds shaped with a bit of overbend in them to account for some of the springback after bending. I don't get much springback, but I don't know how much is too much either. Just getting more heat on the side once it is to shape is really going to help you out. I also throw the bent side into a mold as soon as it exits the bender and it stays there unless I'm working on it and have to take it out.

Just my thoughts.

[redir]

I have found that I need to always touch them up on a pipe after bending and when they are ready to mount to the top. Makes me wonder why I bought a bender in the first place

[charles Tauber]

Originally Posted by dwizum

What temp are you bending/heat soaking at?

A good question. I place the thermocouple/probe near the waste between the bottom slat and the wood: it seems a convenient and secure location. At temperature readings on the control unit of just under 200 F, the wood begins to burn. That reading could be due to where the probe is placed or that the control unit isn't calibrated correctly. Regardless, the answer is at a temperature just below burning the wood: some minimal scorch marks are fine.

I should probably boil some water and calibrate the controller, but, just like bending on a hot pipe, I don't really know or care the actual temperature: it needs to be hot enough to do the job, but not so hot as to produce too much scorching.

I'm sure this is subjective, but how much spring back is too much?

It is subjective. Different people build to different theories. Some believe there should be no tension anywhere in the build, including the sides. Others purposely tension some components.

For me, it isn't about the amount of spring back, as a distance, it is about how much force is involved to have the sides conform to the building method. That is up to you. If the sides are "easily" made to conform to the templates, that's close enough for me. In some more "resilient" woods, ziricote, ebony, etc., I'll touch up the bends on a hot pipe.

In my current build, I'm using laminated sides for the first time. Two 2.2 mm thicknesses are epoxied together in a vacuum bag over the form, no heat during epoxy curing. When the sides come out, they are STIFF and have near zero spring-back and are exactly to template. I also use pre-bent, laminated solid linings, the result of which, when combined with the laminated sides, is a very stiff assembly that can't be forced into shape: they are the shape of the gluing fixtures/cauls used in laminating them. In that case, it doesn't matter all that much how closely the original bent sides are to the desired final shape.

[charles Tauber]

Originally Posted by redir

I have found that I need to always touch them up on a pipe after bending and when they are ready to mount to the top. Makes me wonder why I bought a bender in the first place

Well, view the side bender in context.

The side bender was created by Charles Fox in the early 1970's. He used it in his school of guitar making to facilitate students - such as me - who had limited woodworking - and no wood bending experience - to make acceptable guitars in a 6-week course. It removed from the process a potentially difficult step and ensured a consistent result.

Later, the side bender was used in factory settings to deal with custom work. Eventually, it caught-on with many one-off luthiers, some of whom were already skilled at bending over a hot pipe. It also caught-on with many first-time makers for the same reasons as the bender was used in Charles' school.

What is interesting to see is the many variations and permutations of the original bender, including replacing the light bulb heat source with industrial blankets and controllers.

In short, if you are skilled at bending over a hot pipe, and enjoy doing so, there really isn't much need for a side bender. One thing it does do is reduce the "workmanship of risk": I've never broken a side in a side bender and never produced sides with kinks. Once one is skilled at bending over a pipe, one rarely does either. That takes time and practice. The bender largely eliminates that: it replaces personal skill with technology.

In industrial factory settings, a foot-operated, heated, steam press/bender saves time, effort and improves consistency. But, that's a different animal, different magnitude, than the Fox-style bender.

[redir]

Originally Posted by charles Tauber

Well, view the side bender in context.

The side bender was created by Charles Fox in the early 1970's. He used it in his school of guitar making to facilitate students - such as me - who had limited woodworking - and no wood bending experience - to make acceptable guitars in a 6-week course. It removed from the process a potentially difficult step and ensured a consistent result.

Later, the side bender was used in factory settings to deal with custom work. Eventually, it caught-on with many one-off luthiers, some of whom were already skilled at bending over a hot pipe. It also caught-on with many first-time makers for the same reasons as the bender was used in Charles' school.

What is interesting to see is the many variations and permutations of the original bender, including replacing the light bulb heat source with industrial blankets and controllers.

In short, if you are skilled at bending over a hot pipe, and enjoy doing so, there really isn't much need for a side bender. One thing it does do is reduce the "workmanship of risk": I've never broken a side in a side bender and never produced sides with kinks. Once one is skilled at bending over a pipe, one rarely does either. That takes time and practice. The bender largely eliminates that: it replaces personal skill with technology.

In industrial factory settings, a foot-operated, heated, steam press/bender saves time, effort and improves consistency. But, that's a different animal, different magnitude, than the Fox-style bender.

That is precisely why I purchased a side bender. I had recently begun working with a client who has a sizeable stash of very old BRW. Long story short I have 4 guitars commissioned and possibly more but anyway on the 2nd guitar I cracked a set of BRW. As soon as applied pressure on the pipe the wood just snapped across the grain. I'd never seen anything like it. After consulting around online Alan Carruth had introduced me to 'brash wood.' So I easily wrote that one off.

I simply grabbed another set and then it was no problem.

On the 3rd guitar I cracked yet another side. These sides had the same characteristics as the first leading me to believe they were all cut off the same board. But this one did not crack so easily. They were terribly difficult to bend even though BRW has a reputation for bending like butter.

So I lost it! After building 65 guitars in the last 25 years and never once cracking a side I totally just lost it in one weekend. In baseball they call it the 'yipps.' Like when a pitcher completely looses control over pitching the ball accurately and pretty much has to give up playing the game till they get their head right, some never do.

As a result of that experience I got a bender. When bending $1k plus wood it makes sense.... I practiced on about ten sets of wood before going to the BRW and... CRACK! Right inside the bender it cracked again.

So I don't know what to think except this BRW is so old (cut in about 1950) that it's just gone through some changes that make it difficult to deal with.

But the bender was nice for those sets that I bent successfully. I might just make or buy a few molds and bend 20 sets each 000, Dread, and classical guitars and then just pass it on to someone else.

[dwizum]

Well I'd like to report back that I've gotten the hang of it. Cooking longer helped, as did getting the side directly into a form. But the biggest obstacle I had was the wood, I think - I had been practicing mostly on a stack all cut from the same board and I think that board was just really moody - I tried bending one of the practice sides on my pipe iron and it was almost hilariously difficult in terms of just not wanting to take a set no matter what I did.

I switched to sides from a different stock and they bent much easier and had so little spring back that I'm comfortable using them just fine straight out of the machine, which was my ultimate goal.

Thanks everyone for the input.