Fox bender: not enough heat. What wrong?

Show us your DIY shop made Side Bender and the procedure for its use
Will Reyer
Posts: 141
Joined: Fri Mar 15, 2013 5:03 pm
Location: Marshall, MI

Fox bender: not enough heat. What wrong?

Post by Will Reyer » Tue Apr 02, 2013 11:15 pm

Fox bender: No heat! Need advice.

Building first guitar, fabricating necessary jigs/fixtures. Need advice from people with experience.

Built two hot pipes, one is thin-wall (1/16”) 3” O.D. With an electric charcoal lighter element. The other is 1/4” wall, 3-1/2” O.D., that I fire with a propane torch.

Built two part Fox-type bender; base with 3-150 watt light bulbs, and a male guitar side form, both lined with beer-can thickness (.015”) aluminum roofer's flashing, as I read in Allen St. John's book Clapton's Guitar, that Wayne Henderson almost burnt down his shop with a Fox bender left on too long. Left off the tower with acme screw as I figured I could bend the waist on the hot pipe. (photo)

A neighbor drum-sanded my 3 pieces of plain sawn soft maple I had on-hand for sides to .085”. Haven't bent these yet. Jointed one face and band sawed some white ash to practice on with the hot pipe, until I got that drill down (photo).

Wanted to trial the bender, too, before using the intended side pieces. Jointed one face and band sawed three more pieces of soft maple. Bent the waist on one, jointed face out, and clamped it to the male mold with NO heat. The lower bout would bend all the way with no problem. Broke the top bout, as I anticipated, as the bend was sharper.

Wet both sides of the second band sawed piece, bent the waist on hot pipe, clamped in bender with lights on, slowly worked lower bout, then upper bout around to clamp. Got a guitar-making book by an Australian, Jim Williams, that said to leave the lights on for 45 minutes. Set timer and did so. Put oven thermometer on top of the aluminum flashing covering the mold, above the practice side being bent. Thermometer read just under 150 F at the end of 45 minutes. I have the completed practice piece in my female mold now, successfully bent.

Ran bender yesterday with NO side in it, thermometer on top of lower bout again, temp showed 200 F at end of 45 minutes.

Today bent waist of third practice piece on the hot pipe, and clamped it in the bender. This time I used one of my wife's clothes irons to add heat from the top on the aluminum cover to help bend the upper bout. Successful side is now in the female mold.

All I've read says that the Fox bender ought to be 275-300 F for this to work. What is wrong with my construction or methodology that this isn't achieving the proper heat range? I don't believe the inexpensive WalMart oven thermometer can be off that much, but it seems to be working at 100 degrees less than necessary temp. Que paso aqui?
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Kevin in California
Posts: 2807
Joined: Mon Dec 19, 2011 4:19 pm

Re: Fox bender: not enough heat. What wrong?

Post by Kevin in California » Wed Apr 03, 2013 1:41 am

Will, yours doesn't look that different from mine, in basic design, and mine gets up to in excess of 325 degrees in about 15 minutes. 3 @ 150 watt bulbs.
The cavity where the bulbs are is fully lined with aluminum tape. The mold inside is also covered with the aluminum tape and the top of the mold is sheet steel sealed all the way around with aluminum tape. It is sealed up so well that you can see very little light come through when the unit is on.
I wonder too why yours is not getting hot enough. You do need more heat to bend successfully, I would say at least 250 degrees.

Kevin
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ken cierp
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Joined: Tue Dec 13, 2011 11:23 pm

Re: Fox bender: not enough heat. What wrong?

Post by ken cierp » Wed Apr 03, 2013 8:05 am

I don't see a timer on that bender ---- maybe its not in the picture and yes not having one is almost a sure way to burn down a house!!!!

Our benders use a total of 600 watts some have two sockets others have three. The wiring is all heat resistant silcon. The sockets and all the wiring are below a reflector that sits on top of the socket base -- just enough room to screw in the bulbs. A wood buring stove thermometer is handy for checking temps.

I only have the benders heated (on) for a total of 30 minutes -- they get hot enough to make steam in about 10 minutes (that's what I look for before loading). I don't worry at all about exact temperature (I suppose I could dazzle you with some BS about case harding etc.) I am more concerned with not scourching the wood then using too much water or the bend going a little slow. BTW .085 is at the high end .080 is my max thickness.

ken cierp
Posts: 3924
Joined: Tue Dec 13, 2011 11:23 pm

Re: Fox bender: not enough heat. What wrong?

Post by ken cierp » Wed Apr 03, 2013 8:59 am

Will --- looking closer at your form it seems you have some very large tubes? And how are they holding the sides together? My guess is that you have too much mass (longer to heat up) which is dissipating the heat rather than letting it transfer to the aluminum sheet that touches the side material. Plus those bolt things for the cauls are sucking off heat too.

Will Reyer
Posts: 141
Joined: Fri Mar 15, 2013 5:03 pm
Location: Marshall, MI

Re: Fox bender: not enough heat. What wrong?

Post by Will Reyer » Wed Apr 03, 2013 9:18 pm

Duh! I built a radiator!

One look at Kevin's bender photo with the aluminum tape around the edges was all it took. I used 1/2" EMT (electrical conduit) on 2" centers for the "bones" of the male bending form. These sat in 3/4" counterbores down 1/8". Since I wasn't sure how many truss rods (1/4-20 all thread) I would need, I drilled clearance holes for same at all stations (see my photo).

Thought that effect would be marginal though, figured the bulbs would heat the steel. This afternoon I had time before we went out to celebrate me turning 70 to do a little rework. I taped the edges, added 1x2's on the ends to be a floor under the overhanging male bending form, and took Ken's excellent suggestion and made a reflector out of roofer's flashing for below the light bulbs on the porcelain bases.

Had time to run it only for half an hour, without loading in a side to bend, just putting oven timer on the top of the lower bout. In five minutes it showed 150 F. At 10 min. it was 175 F. At 15 & 20 min. it sat at 200 F so it may peak there, just getting hot sooner.

Expect it will do to bend two soft maple sides for the square neck resonator I've designed. At this point, I've successfully bent two practice sides that have the band saw marks on the interior face. Used one of my wife's clothes irons to assist the upper bout on the second one of these.

My neighbor on the next mile road is talking about laminating sides. I've done laminations for curved baseboards and truck campers and they're easy to do. I've also got enough practice in on both my bending irons now I think I could just do sides with one of them.

Greatly appreciate the feedback, Kevin & Ken. When I was doing mechanical design I was usually the only one to lay out my proposals at group meetings. I always wanted to get it right the first time, and if somebody else found an obvious fault before we whittled chips, that was way more useful than boring on blindly, hah!

Ken Hundley
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Location: Chicago Area
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Re: Fox bender: not enough heat. What wrong?

Post by Ken Hundley » Thu Apr 04, 2013 1:06 pm

I use two 300 watt bulbs in the bouts, and a single 150 at the waist. You definitely need to get hotter than 200 F, will mean the difference between partial success and easy bending. I also would go thinner than .085", I tend to stay between .070" and .080", unless it is an easy to bend wood.

I have a sauna timer in mine that won't stay on for more than 15 minutes. I am up to temperature in 7-10 minutes, Cook at full heat for 5 minutes, then use my dimmer to back off some of the heat to prevent scorching, resetting the timer as I go. Then cool to room temp before pulling from the mold. I have only broken one set that way because it was a tight cutaway, and I forced it instead of letting it heat up and bend normally.
So, my big brother was playing guitar and I figured I'd try it too.
- Stevie Ray Vaughan

http://www.nocturnalguitars.com

Robert Hosmer
Posts: 167
Joined: Tue May 08, 2012 7:30 pm
Location: Southern IN

Re: Fox bender: not enough heat. What wrong?

Post by Robert Hosmer » Thu Apr 04, 2013 5:01 pm

Will Reyer wrote:Duh! I built a radiator!

One look at Kevin's bender photo with the aluminum tape around the edges was all it took. I used 1/2" EMT (electrical conduit) on 2" centers for the "bones" of the male bending form. These sat in 3/4" counterbores down 1/8". Since I wasn't sure how many truss rods (1/4-20 all thread) I would need, I drilled clearance holes for same at all stations (see my photo).

Thought that effect would be marginal though, figured the bulbs would heat the steel.
The bulbs do indeed heat the steel. Problem is you only have so much heat output available, and the more heat you use heating the steel, the less you have going to the wood.
This afternoon I had time before we went out to celebrate me turning 70 to do a little rework. I taped the edges, added 1x2's on the ends to be a floor under the overhanging male bending form, and took Ken's excellent suggestion and made a reflector out of roofer's flashing for below the light bulbs on the porcelain bases.

Had time to run it only for half an hour, without loading in a side to bend, just putting oven timer on the top of the lower bout. In five minutes it showed 150 F. At 10 min. it was 175 F. At 15 & 20 min. it sat at 200 F so it may peak there, just getting hot sooner.
Even if 200F is not your peak temperature, that is the time frame you should be targeting. This is another indicator that the heat output in relation to thermal mass is not tailored to your needs.

Expect it will do to bend two soft maple sides for the square neck resonator I've designed. At this point, I've successfully bent two practice sides that have the band saw marks on the interior face. Used one of my wife's clothes irons to assist the upper bout on the second one of these.
Yes, the iron works in a pinch. I've also confiscated one of my daughter's curling irons to touch up bindings. But both are the same concept as a bending pipe- they are a single-point-of-contact heat source. The purpose of a bending mold is to make the bending process quicker and safer for the wood, especially for the "trickier" woods.
My neighbor on the next mile road is talking about laminating sides. I've done laminations for curved baseboards and truck campers and they're easy to do. I've also got enough practice in on both my bending irons now I think I could just do sides with one of them.
Nothing wrong with laminating sides. More involved from a prep standpoint. I think Dave Bagwill (deadedith on this forum) can give you plenty of insight on this process. If you're looking for detailed literature, John Bogdanovich uses laminated side construction in his book classical guitar making.
What was that show where the actor always said "More power!"?
Seriously, though, I believe the simplest fix for your application is to follow the advice already mentioned by Ken C. and Ken H., which is increasing the output (more total wattage).
Allow me to clarify by stating that I have no experience building or even using using the bulb-heated benders. But I've built more than my share of forges and kilns, and the principles of thermodynamics remain the same.
Use what you've already built; just increase the wattage.
Always have plenty of sandpaper; it's rough out there!

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