Official Luthiers Forum!

Ok, so a search did not yield very good results.

I am bending a cutaway (round) like a Gibson L5. The inside radius is tight and outside radius is round and flowing. I made the cutaway arm for the fox bender and it seats nicely into the cutaway of the frame on the bender. It has a nice angle and appears to work well in theory. Last night I did a practice bend on some old mahogay. The mahogany was about .060, so pretty thin I guess. I assumed this would help with bending the tight radius.

There was not any good examples of how to bend the cutway on youtube or in the forum here. I did read a post by Dave Stewart (I think) saying take it slow and do the waist first. So that is what I did. I bent the waist to about 3/8" from set. I then bent the lower bout. I then use the cutaway arm to slowly bend the raidus over the cutaway until the side was now at a 90* angle to the cutway. Now it seems I just need to slowly bring the caul into the cutaway for it to take shape. Once seated all the way, I lowered the waist the rest of the way. I was bending at about 240 degrees over a lightbulb setup. I then let it go up to about 300 and then I brought it back down. Almost no spring back to speak up. Just a touch.

Two things happend. 1) it went in fine, but I did get two small creases in the sides at the tightest point of the cutaway. No real cracks, but it is not a smooth contour. Did I move to fast with not enough heat? 2) the large bout appeared to lift up a little off the form once the cutaway was bent. Lowering the waist did not remove it completely.

All in all the piece bent fine and fits perfectly in the mold with almost no clamping needed. Yeah!

Issue 3: The sides of the archtop are only 2.5 to 2.75" tall. That means on my fox bender more than half the slat is empty (6" wide slats). I move the side wood all the way to the left or right side. When I do this, the cauls tend to lean toward the empty side until it is seated fully. This also what I think causes the large bout to open up a little. When completed, the wood is straight and not tilted or bend crooked, but while in the process of bending it is a little tough. Can I run the wood up the center of the slats to reduce this an place an even preasure on the sides while bending? Could I keep it straight?

Thanks for any thoughts!

Joe

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Joe

This is the shape I am esentially trying to build...

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Joe

Faceting, not cracking but tearing of the fibers on the outside of a bend and (inside the guitar) and a fold appearance or crease on the inside of a bend (outside the guitar) on non curly or non quilted woods is most often caused by a lack of stable support by the sandwich at high stress point of that bend, if even heating is applied when using a fox style bender.

A sandwich that can remain stretched and tight as the bend helps support the bend and never lets a gap to form as the bend takes shape. If the sandwich is not tight when you start the cutaway bend the inner slat may flex sooner than the outer causing a small gap between the wood and the slat, leading to a facet or faceted bend (fracturing of the fibers leading to a bend made by a series of straights not a fully smooth bend.) A cutaway press caul that fits very well in the cutaway opening will also help avoid this, but a well tensioned sandwich is a big part of the key. But the simplest single thing you can do to help solve this issue is to use a blanket instead of bulbs. With the bulbs you are not getting near as much heat on the cutaway as on the waist and upper bout till you get the cutaway bent closer to the bulbs

Most folks switch to a heat blanket when doing cutaways. When you try your setup with curly maple, you may have problems.

Hello Joe
I would suggest centering the side on the slats. I do this by building the "sandwich" on the bench and using spring clamps at one end to hold the side centered while placing it in the bender. Bring the waist caul down to put a little pressure on the sandwich to hold things in place while removing the spring clamps.
Nelson

Joe, I bent these two sets of curly maple on the weekend with my version of the fox bender-bulbs only (3@150w typ., 200w at cutaway end). Both rams advance at 1/10" per minute (I used 5/8" x 10tpi threaded rod for both so it's easy - one turn of the wheel/min.) This means a total time per side of 40 to 50 min. No idea of temps.
Some thoughts. Keep the sandwich tight as Michael says- I fully bottom the waist before doing anything to the bouts. Set the cutaway ram so it just clears the horn sandwich as it goes 'round. I insert dampened & foil-wrapped side down the centerline of the preheating slats & spring-clamp one end of the sandwich when properly positioned. As I pass beyond the "crown" of each bout, I clamp it down to keep it from rising off the form. As soon as the bending is complete, shut off lights for 2 hrs ... recook 6-8 min....repeat again after 2 hrs....remove next day.

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Dave
Milton, ON

Wow!!!!!!!!!! I had nearly forgotten the pre blanket days when taking more than 5 nim. to bend a side

What, ya got somewhere to go???

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Dave
Milton, ON

Excellent advise from everyone! I will absolutely be upgrading to a blanket, but wanted to get a few test guitars under my belt first. I know that sounds silly, but if I can't justify the money to the honey, its a much harder sell! LOL.

In the mean time, I like the idea of centering the wood in the slats. I will bend some more tonight. Also, I am positive, my slats were gapping in a few places which I was having trouble controlling. I will try to do a better job of that tonight too.

Heat in the cutaway area was my main concern. I may swap the bulbs to apply more heat at that end. I have the 200 watt bulb at the big end not the cutaway. I used Dave Stewarts picture of his bender as a reference when I made the cutaway caul and angles etc.

Thanks you!

Joe

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Joe

It takes less heat in the wood its self to bend the lower bout but by moving the high wattage bulb to the front you run a risk of putting too much heat into the waist and upper bout. Back when I ran my bender on bulbs I used a 600w dimmer on each bulb so that I could control each bulb independently.

That is the real advantage to blankets, the consistent heat for the full length. That and the fact I could bend in 5 min instead of ½-1 hour.

i too use spring clamps to keep my sandwich tight and flush. On my next bender rebuild I think I will incorporate a spring loaded break over center stretcher at each end of the bender to keep the slats pulled tight. i have seen a couple examples on the internet.