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I'm having issues with bending sides using a Fox Style bender that I built. I tried bending some orphan sides for practice today and am getting some major spring back. See the images below.

The Bender:
Fox Style with 3 150 watt incandescent bulbs

The Wood:
Stripped Sapele

Here was my process:
1: Spritz water on both sides of the wood to be bent.
2: Wipe the excess water off with my hand
3: Spritz down some brown craft paper that is roughly the size of the spring steel slats that I have
4: Create the sandwich: steel slat | craft paper | side to bend | craft paper | steel slat
5: Put this in the bender positioned correctly
6: Turn the Fox bender on
7: Let the wood heat up slowly, whole process took about 33 minutes to get the wood bent, and then cooked it for another 20 minutes before turning the bender off. At this point, the wood was at about 330 degrees for the full 20 minutes.
8: Let the whole thing cool for 4 hours before taking the side out.

What am I doing wrong? Besides not using a heating blanket of course.

Also, The bluing from the spring steel slat was burning off the slat, and going through the craft paper ruining the wood. I tried sanding it out, and no dice. It was almost like an oil stain with the bluing in it. See the pics below. How can I stop that? I can't have this happening to good sides. Thanks for any help. I'm out of orphans to practice on, the next one needs to be right.

This is the amount of spring back that I'm getting. Pretty sure this isn't right.


This is the staining that I'm getting from the slats through the craft paper. One thing that is weird is that the staining is only happening on the outside of the bent side. The inside is fine.. Very weird.



Blueing coming off the slats. This was the first time I used these btw.



This is what the craft paper ended up looking like, the inside where it was near the light bulbs, no staining. Outside, lots of staining.

_________________
Jason McGowan
M&S Guitars
_____________________________
United States of America, looks like another silent night
As we're sung to sleep by philosophies that save the trees and kill the children....
Casting Crowns

I like aluminum flashing for slats to avoid the staining. The reason you are getting it on the outside is most likely because moisture can pool there. Some woods spring back a lot more than others. I like to modify the mold to account for that. FWIW it's not too hard to take what you have done and and tune it up on a hot pipe. It's pretty easy if the wood hasn't dried completely. I had some mahogany that almost went back to it's original shape.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

light bulbs don't allow you to control the heat as well as a blanket. Also spring steel as you can see is a staining issue. Aluminum won't support the wood for the bend as well but you can use foil to help as a barrier for the staining. The amount of spring back you have , I think you can still make it work using a building mold or form.
It appears you didn't get enough heat. Also what was your process? 330 degrees is the low end for mahogany. I like to get that to 350 to set the bend. If you ever do think about a blanket , not all blankets are the same. You should use 5 watts per in as they can handle a better thermal load. 2 1/2 watt per inch can have thermal drops.
One more thing, is the thickness , I like to see .080 to .090 in. You keep playing with it , you will dial in . Just NEVER LEAVE A LIGHT BULB UNIT ALONE. I hope you have a timer on it also.

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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

I'm sure you'll get help from more experienced builders, but here's my 2 cents.
"Blued" spring steel is not stainless. Putting wet paper on it directly and applying heat is going to cause problems like you found.
I use aluminium foil (one piece each side) between my blued slats and the wet paper/wood to protect the slats and the wood.
Aluminium slats are OK but be aware they may stain some woods (especially Koa (green stain) - as will direct contact with steel. e,g, Oak/Walnut (black)). And they need to be staightened each time they are used, but they are cheap and effective..
When I used aluminium slats/bulbs I found 3 x 200s or 2 x 200 w + 150 w (in the middle) bulbs better than 3 x 150s.
Faster heating is my preference, and seems to be most people's.
Sounds like you may be using excessive water too. Moistening sides or paper should be plenty if you heat and bend faster. Start bending as soon as you get signs of steam. It should only take a matter of minutes to complete the bending.
Some spring back is not unusual - did you put the sides in the mold immediately after removing from the bender? Or just compare them to the mold's shape?
Touching up on a pipe is a good solution.
See Todd Stock's Youtube video to get some hints - it's one of the best I've seen, and there are others too.

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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.

Thanks Ken. I designed this mold with about a 7mm spring back correction factor at the head, tail, and waste. I didn't want to put in too much correction, and have sides that ended up being over bent. Maybe I do need a tad more though.. I guess I need to dial in my heat, see how much more correction it needs, and apply it to the next mold. I do have a pipe bender in the process of being made though, that I can try my hand at cleaning these up with. No better time to practice that I guess.



We are definitely going to use a foil wrap for future bends. I remember reading about that now, wish I would have remembered it sooner. At least the sides were just for practice. I want to continue using the spring steel for its support. To me, that seems important. The staining can be avoided. As far as the amount of heat goes, I think you're right on there. We first started with 3 100 watt bulbs, and the spring back was a little worse. We added a 150 watt to the center, and it somewhat improved. Went to all 150's, and it improved a little more. I guess I need to find a source for higher wattage incandescent bulbs, and start experimenting some more. We did have the sides thickness down to 2.2mm as well. I did stay with the bender at all times while it was on, and also have a timer on it. Safety first.



You know, I thought the process was taking a little too long... I was trying to be patient, and allow the wood time to heat up well. I never really saw any steam. I could spritz the spring steel with a little water, and get it to evaporate pretty quick when the bender was at full temp, but that was it. Sounds like the higher wattage bulbs are the ticket. I didn't put the sides straight into the mold because they were just test sides, and I thought they weren't bent enough. I could get them in the mold, but it seemed like a lot of pressure was needed to keep them in the mold. I I know now though, to put them straight into the mold. Thanks for the heads up on Todds tut as well.




All good info Todd. I plan to watch your tut tonight, and get geared up for the real bend this up coming weekend. The bend that I'll be doing this weekend is really old Cyban Mahogany from back in the 1930's. Any idea about what temp I should be aiming for? 350 like John stated? I do want to eventually go to a blanket, but I've invested so much so far, I have to make due for a bit. Trying to keep the wife happy about my hobby is important.

_________________
Jason McGowan
M&S Guitars
_____________________________
United States of America, looks like another silent night
As we're sung to sleep by philosophies that save the trees and kill the children....
Casting Crowns

You've already got a lot of good info.
FWIW, I've only ever bent with a homemade light bulb fox (3 x 150w). (It uses a 5/8" x 10tpi ram). Here are a few things that jump out at me.
-Pre-heat the bender/slats while wetting/wrapping the sides.
-quickly paint/spritz, then wipe with kleenex to remove most......repeat other side, then wrap side in tinfoil
-insert in heated slats & position
-crank some pressure at the waist
-crank 1 to 1 1/2 turns a minute (1 nearing end) 'till waist done, then push out spring hold downs (once/min) 'till bouts done & immediately shut off lights (typically 40min for full .... 50-55 for cutaway)
-cool 3-4 hrs, then REHEAT 6-7 min............repeat (should help springback quite a bit
-remove next day & clamp into mold

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Dave
Milton, ON

I'm sure you are feeling a bit like I did when I built my fox bender and used light bulbs. Seemed like $100 was too much for a little heat blanket. So I struggled with the light bulbs until I could stand it no longer. Called John Hall and ordered one. I now have 2, one for ukuleles. Next, I finally gave in and ordered Super Soft.

Those two things made ALL the difference in the world. Considering the cost of some tonewoods, the blankets are a cheap investment. Also, I went to lowes and bought a stainless steel splash guard and cut it up into strips for use in the bender. Not sure how cost effective that was, but the guard cost $30. And I obtained slats for regular guitars and for ukes.

Mike