Official Luthiers Forum!

Hello,

I'm wondering if anyone has advice on how I can improve my bent sides. What I sometimes get is flat spots in the bend just before or after the curve of the upper or lower bout. I'm putting a masterpiece below to illustrate.



My setup is fairly common I believe. I use a bender from a supplier with a caul like this:



I use the John Hall Universal Waist Caul (tm).

I use a slat -> foil (no wrap) -> butcher paper -> side -> butcher paper -> foil -> slat for the sandwich.

I bend with a silicon blanket and most everything is bent around 150C. For oily woods or woods that tend to cup I just mist the paper. For others I mist the paper and the wood.

I bend most everything at ~0.075.

It just seems like there are kinks at the area indicated in the image. The new LMI bender looks like it kind of rolls the side over rather than a rectangular block like the caul on my bender. I'm not inclined to get their bender though because I have ambitions of making custom body styles.

Is there some technique I'm missing? It's not the end of the world, but I'm sure the sides are getting thinned more in that area once I go to flatten them.

Side note: I got one of those rolling pin sanders from Chris at Elevate. I got a chance to use it last night. Boy, that is something you don't know you need until you have it.

Brad

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

I can tell you what results in perfect, zero-springback, true sides in difficult woods for us:

- A minimum of three plies in the mold so the full width is supported...preferably a solid mold, but much of the additional support needed, especially for shapes with flat runs, is provided by a center laminate

- A dedicated waist caul. We were given a second bender with a universal caul, and after a few uses, modified it to the same dedicated caul as on our original benders. Dedicated cauls ensure the waist is properly shaped, with contact for 1-1/2" - 2" out from the center of the caul...that makes a big difference in accuracy of waist shapes for guitars with more turn in that area than a dreadnaught.

- We don't worry too much about temperature...worry about steam. With a 5 watt/in^2 blanket on FULL, the waist can be brought down to 1/4"-3/8" off the form as soon as visible steam is rising from the bending sandwich. The lower bout and then upper bout follow immediately, with the waist hard set once the upper bout is done. About ten minutes will be required to cook off most of the water in the bending sandwich if the kraft paper is wetted and squeegeed such that it presents a satiny appearance...once the steam stops rising (we use the fogging of a pocket compact mirror - the kind used when noses are powdered - when it is difficult to see any more steam rising to verify that the bending sandwich is empty). We then set the controller to VARIABLE and dry somewhere around 260 deg F - 280 deg F for 40 minutes. Once the timer clicks off, we let things set for an hour for rosewoods and 9-12 hours for mahogany, acacias, etc.

- We use SuperSoft II for anything figured or prone to cross-grain ripples and other bad behavior. Even very difficult woods such as curly ash and highly figured curly anigre behave well when properly treated.

_________________
For the times they are a changin'

- Bob Dylan

Some thoughts.
I was getting similar flats sometimes in some areas until I followed John Hall's suggestions on higher temperatures for bending (more or less) http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=51516&p=678218&hilit=+bending#p678218
I now use 2 slats underneath the stack for support, so slat, slat, alu foil (blue spring steel rusts), wall lining paper, wood, wall lining paper, alu foil, slat, heat blanket, slat. SSII is used for most woods first overnight then dried, spritz water on the wood (much less water on figured woods) water on the paper for everything, but not dripping wet.
The second slat in the stack is tensioned by springs each end, and the outer one is tensioned with turnbuckles each end.
I set my controller to 130C (because the heat rises extremely fast and I don't want to dry the wood out too much during bending) when I see steam I bent the waist to within about 1/4", lower bout, upper bout, then set controller to 165C. As temp rises, then finish the waist (steadily but easy).
When temp reaches the 165C, I cut it back immediately to 125C, and after it cools to that 125C, leave it for 10-15 minutes.
Then cool to room temp, and reheat to 125C for 10 minutes, three times.
Last time after cycling temperature, leave it overnight usually.
My waist cauls are solid wood, and the cauls are rollers which actually turn (below) - I use 3 (1 upper bout and 2 lower) and work them along the hot sides several times during the elevated temperatures.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Woodie, what's your Supersoft II procedure?

Thanks

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

SS II: brush on wet with a chip brush; allow to dry...usually 3-4 hours. Bend before 24 hours have elapsed or reapply and allow to dry.

This is the usual result in curly, rowed SA mahogany; zero spring-back or need for any corrections.

_________________
For the times they are a changin'

- Bob Dylan

The paper is the moisture reservoir...we run kraft (the brown stuff) under warm water to saturate, then run it over the edge of the bench to dump excess and get the appearance to a satin sheen when bending most woods. For rosewoods, it really does not matter, so we usually go a bit wetter to better handle the resin bleed. If bending holly or other super white wood, white kraft can be used, but it holds less water than the brown, so needs to be wetter...shiny/glossy works.

Most bending issues we've seen have been related to waiting too long to start the bend and early moisture exhaustion...once steam is seen, the stack will not increase in temp much until most of the water has phase-changed to steam, so we don't use temp as an indicator of when to bend.

_________________
For the times they are a changin'

- Bob Dylan

I do pretty much as Woodie but use two blankets and importantly take the waist only halfway down when the steam is rolling off, then do the lower followed by the upper bout and clamp the cauls tightly in place. Then complete the waist.

The clamped cauls seem to keep tension on the slats for a more even curve. I do use three sections in my bending form so the center is supported.

I also have a custom waist caul for each pattern.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

I have been bending wood for 20 years and here are a few things I learned.
I stopped using super soft and now use windex with ammonia D. I also will say some woods like more heat less water.
I use tempered stainless steel slats and my heat blanket is made in America I found many of the Chinese blankets are lower wattage
and have cold spots
5 watts per in is the blanket requirement.

I also stopped using kraft paper as some suppliers are more acidic that causes staining. Now I use white butchers paper it is untreated and eliminated the staining issue. If your not getting staining with your kraft paper keep using it.

so I spray my slat with winder to hold the paper and pat in down
spray the wood both sides lightly
lay paper on top of wood and spray with paper
slat
blanket
set into bend are bring waist caul down to apply friendly friction
set into bender
turn to full to heat place thermometer between top slat and blanket at the just behind the waist. Apply a weight to the end or spring clamp to hold the blanket and slat in contact with the wood and watch heat
at 240 250 start bending lower bout
turn waist down and leave about 3/4 in high
check upper bout for dampness and proceed.
then set the waist
allow heat to go to 350 375 F
once you hit this temp turn to VAR and hold at about 250 for 10 min
allow to cool

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Colin, this is interesting. The caul in the pic is not attached to the bender by a spring and you run it along the side to sort of “iron” it in place?




Sent from my iPhone using Tapatalk Pro

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

We use both the plans-built Fox bender cauls and a set that came with the bender we were given. We find a single pull under spring tension is sufficient. For tight cutaways, we use the standard Fox-designed ram. Our Weissenborn uses a third caul design for bending the upper bout to neck transition. We usually leave the springs on the cauls, which gives some flexibility on which spring mount/pivot point to use for a given body shape.



A gallon of SuperSoft 2 delivered runs about $50, and will treat sides for about 75 guitars. Mr. Hall's Windex runs about $15 per gallon, and likely treats roughly the same number of sides at about 3/4 ounce per side treated. Either option is inexpensive compared to the labor cost of finding and bending a replacement side should one be spoiled in bending.

_________________
For the times they are a changin'

- Bob Dylan

No Brad, in the caul pic you can see 3 screws into the centre of the rollers (broom handle) at each end, and another sticking out at the bottom of the pencilled cross.
That's my version of a hook for the springs which hold them down to the bending fixture.
I suppose it is a bit like ironing it, but I'm not making any claims for this helping with flats.
I've used the same procedure "mechanically" for quite a long time, the cauls, tension on the slats as mentioned, but the difference for me came when I used temperatures more like John Halls "high heat blast" as I mentioned.
I'd take a pic but everything is disassembled and stowed away and I'm flat out on 2 builds for a show.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

John, to make sure I understand your layers.

Starting at the bottom of the sandwich, you go:
Slat on bottom - sprayed with Windex
White butcher paper - sprayed with Windex and patted down on slat
Side .075" thick- sprayed both sides with Windex
White butcher paper - sprayed with Windex
Slat on top
Heating blanket on OUTSIDE of top slat.
Thermometer is between the top slat and the heating blanket.

Thanks for your help.

Edit. I use the same sandwich sequence, except that I put the heating blanket under the top slat. My reason is that I have always understood that the blanket should not be heated, unless it is against something. One downside of having the blanket next to the wood is that it will burn the element pattern into the wood, if you are not careful.

Colin,
Am I reading this right? You use 4 slats, including the one on the outside of the heating blanket?

What are your layers? Most importantly, where is the heating blanket in the stack?

Thanks for your help.

Correct.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

The blanket is on top of the sandwich, with an additional slat above it for abrasion protection. From bottom to top as the sandwich is built:
- Spring steel slat
- Separate sheet of aluminum foil
- Separate sheet of brown kraft paper
- Wood
- Separate sheet of brown kraft paper
- Separate sheet of aluminum foil
- Spring steel slat
- Blanket
- Spring steel slat

_________________
For the times they are a changin'

- Bob Dylan

Woodie,

The paper and alum are not wrapped around the wood in a package? You are lying down separate layers cut to size like they were the metal slats?


Sent from my iPhone using Tapatalk Pro

Sorry for the delay in answering...yes - separate sheets. This avoids the wrinkling and pulling of envelope-type foil or paper wrap, and related cleanup. We also think it aids in allowing the bending package to dry more evenly and the side to set.

_________________
For the times they are a changin'

- Bob Dylan

I use pretty much the same deal as Woodie except for the foil. I thought there must be a better way
than using that much foil, so I got some aluminum flashing at a big box store and made strips. It works
just as well and adds a little more support.
Brent

Woodie G et al, this past weekend I used the procedure(s) outlined in this thread to bend a set of koa sides. They came out great; nice and tight to my form, no cracks, cupping, or other distortion--all good. However, both sides came out heavily water stained. The staining is mostly gone after a light sanding (see comparison pic below) so that's more of a nuisance than a problem. However, the sides are displaying a very noticeable color shift. After going in with rich, vibrant oranges and chocolate browns, they came out rather pale and with a greenish cast. I'm curious about what happened and concerned about a mismatch with the color of the back. (This is a rather unique set of koa and was not inexpensive.) I've bent many sides before (not koa, though), using the same basic process, and never had this occur.

Here's my bending sandwich (as a sideview of the stacked layers):

{top of stack}
top slat made from rolled aluminum flashing
silicone blanket
stainless slat
aluminum foil sheet
wetted brown butcher paper sheet
lightly spritzed koa side
wetted brown butcher paper sheet
aluminum foil sheet
stainless slat
{bottom of stack}

Here's what I did:

Misted Supersoft II onto sides and let them stand to dry (about 9 hours)
Quickly raised temp on FULL until visible steam appeared (about 2 minutes, thermometer ~300 F)
Switched to VARIABLE and reduced temp to ~265 F, working as temp slowly dropped
Tightened waist to around 1/2" of final shape
Tightened lower bout
Tightened upper bout
Finished tightening waist
(At this point ~10 minutes had elapsed)
Increased temp to ~330 F
Reduced temp to 265 F and let it sit
After 30-40 minutes there was no more steam visible (used the mirror trick)
Cut power and let everything cool for 9+ hours

Thoughts? Thanks!

George :-)

A few ideas:

- No extra water should be needed on the sides - the kraft paper has what is appropriate. Mahoganies and acacias (e.g., koa, bay laurel, Tasmanian Blackwood) do not benefit from extra water, and that likely contributed to what is a lot more water staining than we typically see.

- There is no issue with letting the blanket run full open for up to 10 minutes or so...even without a careful watch on the steam. The idea is to get a burst of steam for long enough to get through the bending process, and even on Venetian cutaway, we see a few minutes of waiting for steam to burn off after everything is nice and put to bed. Stainless slats and kraft paper without the foil would eliminate the issue unless your water is very high in aqueous Al compounds.

- Some acacias will show some greenish staining when bent with aluminum slats or aluminum-rich water (as in water heated in aluminum pan, etc....I assume the aluminum foil and - more importantly - the extra water were not your friends in making the aluminum ions more mobile.

_________________
For the times they are a changin'

- Bob Dylan

Thanks, yeah, that makes sense. The two elements that I hadn't done before were the koa and aluminum. Hmm ...

_________________
George :-)

yes you have my stack correct
slat
wet paper
wood
wet paper
slat
blanket

bend lower bout first
then upper
waist last

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

There are only a few species which see issues with aluminum staining, with koa being the most common. For these woods, you might consider wetting out the paper with distilled water and either using spring stainless slats (thus, avoiding the need for the foil barrier) or substituting Quilon-treated parchment paper for the foil (not the silicone-treated parchment that is marketed as an upgrade) if the additional support offered by spring steel slats is desired in tight cutaways or hard waist turns.

_________________
For the times they are a changin'

- Bob Dylan

If you have a source for scrap laminate you can use that in place of metal slats. It's non staining.