MIMF

Hello, I’m working on my first guitar, an acoustic flattop, and have some beginner questions for the wizards. I built a mold and I’m practicing bending sides. Here’s my best attempt so far. My guitar’s back and sides will be walnut and this test piece is a pretty good match, it’s just short but fine for practicing.


This is my bending setup. I have an old bending iron I got from LMI a long time ago. I don’t think I‘ll ever have the space or need for a fancier bending system.
The bending seemed to go well, my main issue is bending too far and needing to bend it back. I thicknessed the test piece to around 0.075”, it’s pretty consistent.

First question. How lumpy/bumpy is acceptable for a bent side? The walnut is not homogeneous and different sections bent a little differently. This results is a surface that’s not fully flat along the curves. Should I be striving for smooth and variation free?

Next, the flat ends of the test piece are cupped a little. Is this a problem? With a full length side the ends will be glued to blocks and there’s no cupping where the wood is curved.

Finally, is it correct to assume that I should leave the side in the mold until I can glue in the blocks and maybe the kerfing?

So many questions from this rookie!

Thanks,

-Eric

You have done well. You are good to go. Slight waviness or other irregularities will sand out. And anyway, nobody else will see the imperfections that you imagine are there. Bending by hand on an iron is a great way to work. You don't need anything else, unless you are a factory trying to crank out dozens of sets per day. If you over bend it will often spring back naturally, or you can use the hot iron to correct it.

Bending like you are doing is exactly how I do it and so far I haven't had any issue sanding out any irregularities. Once I started building guitars, I have only built mandolins with very curly maple sides, I found that rosewood is the easiest wood to bend so far for me. I haven't tried walnut. I'm so tempted to buy into the heat blanket and jig thing only because it appears to be so much easier and probably more accurate.

For me, I can leave the sides out of the mold as soon as they are cool, and if you use water, dry. I found that understanding the plastic transformation of the wood solid with heat was critical to getting smoother bends, I try to get around a 4" section up to bending temp, work the bend in slowly across the whole width of the part I'm heating, and try to flow it down the piece. I do go back and adjust the radius of the bend if needed, re-heating seems to be OK. I bend quite dry, I have a little water to generator a little steam if I feel I need it, but I consider that I'm heat bending, not steam bending.

I am a relative rookie, and progressing on my 9th build - a squareneck resonator in curly red maple with walnut accents. I videoed my bend process and posted on YouTube https://youtu.be/NRdh_jVhQUY

Two of my archtop guitar builds had walnut back and sides, and I found that walnut is more forgiving than curly maple or figured cherry. 0.075" thick for the sides is plenty thin enough. I found that steady light pressure while rocking the wood back and forth spreads the heat out, and occasionally turn over to heat the wood on the opposite side. You will feel when the wood relaxes and accepts the bending. About 350 deg F on the pipe is good, and I did not soak the wood before bending, just spray with distilled water to moisten the wood and provide the steam that helps the bending. Clamping in the form with flat cauls can help prevent cupping as the wood dries and sets. I keep the sides clamped to the form for about 2 days to be sure they are good and dry.

As the others stated, a little spring back is expected, and the facets you are seeing would likely sand out on the finished instrument. Getting the wood too wet can actually make the faceting worse.

My video was my first use of SuperSoft 2 veneer treatment and I am convinced that it really helps with faceting or break out with the curly maple. Just finished up the bindings for my build, curly maple with a walnut purfling joined with Titebond 3, and using the SuperSoft these composite bindings bent like butter! That being said, you can bend just fine without the SuperSoft with some patience and a little practice.

One trick I use is to over bend things initially: it's more important that the bend be smooth than that it be right. Allow the side to cool off, preferably over night, and then come back and un-bend as needed. The parts that bent too far to begin with will be easy to straighten out, but it's often hard to get something to take more bend the second time around. Concentrate on getting the spots that don't want to bend to at least 'enough' curve.

If you end up with a tight bend in one spot and relatively flat areas next to it due to grain changes, don't start by straightening out the over bend. The wood tends to stay pliable for some time, even after it cools off. If you straighten that area and then come back a while later to bend the flats, you'll find the straightened out section will be right back where you started before you get much bend into the flats. The trick is to start by over bending the flats. Allow everything to cool off and stabilize, and then go back and straighten it out.

The moisture content of the wood seems to help by lowering the temperature at which it becomes plastic. The longer you heat it the dryer the wood gets, and eventually you end up scorching the wood without it bending. Ideally the iron should be almost hot enough to scorch the wood, so that you can heat it through fast and bend it before the moisture is driven off. Add water to the inside of the bend; the bonds between cells get weaker when the wood is wet, and you get splits on the outside.

Thanks, everyone. I am trying to apply all the hints and make some progress. I have completed four test runs and the last one seems really nice. It was also a scrap that is much closer to the actual side pieces. I’m going to start working on thicknessing and cutting the sides to shape next. Bending wood like this seems a bit magical, very fun.

-Eric

One other thing: I usually start with the hardest (tightest) bend first. That way, if it breaks, you have not spent a lot of time on it. Usually that's the waist, or a cutaway; some Dreads have a sharper shoulder than the waist.

Alan Carruth wrote: ↑Mon Feb 22, 2021 10:45 am One other thing: I usually start with the hardest (tightest) bend first. That way, if it breaks, you have not spent a lot of time on it.

Me too! Although I have cracked bends at the lower bout after successfully bending the tight ones.

For some time I had problems using the bending blanket and form wit sides the broke in the upper bout. I'd bend the waist and the lower bout, and by that time the water was all gone off the upper bout, so I'd give it another spritz. The logic of doing it in that order was to give the usually tighter upper bout bend a little more time to warm up, but even a light spritz on the inside surface was enough to cool it off and it would break. Now I spritz the upper bout inside before[u/] I bend the lower bout, and I've had 'way less trouble. Sometimes it's those little things...

When I use a bender. I put the side in wet (not dripping wet) craft paper and seal that in aluminum foil. The craft paper steams when it gets hot but the steam can’t just go away wherever it wants. It has to find its way out of the folds of the foil. That seems to keep everything fairly uniformly wet and hot. I go waist, upper bout, lower bout. Then I cook off the moisture let it cool and cook again a time or two to make sure it is set.

I don’t have a set up to do the cutaway in the bender. I wish I did! I have to do waist, part of the upper bout then lower bout. Afterwards I have to do the cutaway by hand. It sure would be nice to be able to do that part first as has been suggested!

Mark McLean wrote: ↑Fri Feb 19, 2021 6:56 am... unless you are a factory trying to crank out dozens of sets per day. ...

I would say 100's of sets rather than dozens.

Some one with a bit of experience, assuming the wood was at an appropriate thickness, can bend side quickly and accurately. Certainly half a dozens sets an hour would be routine. A bending iron is certainly faster than something like a fox bender but needs a good bit of experience to get the factory like repeatability.

I saw a video at one point where someone was doing sides in a small industrial machine. Two sets of sides at a time. The machine was using steam and hydraulics. If memory serves it took about 4 mins. While the machines was cooking the sides, the guy doing it was trimming and fitting the previous two sets in to the moulds. The timing was to perfection. Sorry I don't have a link. It was a great little video. I suspect it was south american.

I have to admit to having "exaggerated" about the machine I mentioned above.. Put it down to old age. I am not 100% sure this is the video I saw originally but it is very close. He is only doing two sides at a time. But is sure looks pretty fast. There is also a video on the same channel for cutting fret slots and another for the head slots.

They have two videos showing the machine. If you look at this one it will throw up a few more similar machines from other makers.
https://www.youtube.com/watch?v=kVgNhqhuOrw

Something like 30 seconds with the fretboard already glued to the neck. Most of the video (4 mins or so) is assembling the machine.
https://www.youtube.com/watch?v=TgCREU3VOeM

How to cut the head slots on a classical guitar in about 20 seconds.
https://www.youtube.com/watch?v=CKZvttu4KZ0