Side bending machine problems...

[Eddie Nias]

Hi all, I need help!

I’ve bend a couple of sets of Indian Rosewood by hand before with no trouble at all (well, ok, some effort, and it’s never quite as smooth as I’d like), but when I came to do a set of mahogany last week it cracked all over the place; nothing I did seemed to make this wood want to bend.

I’d been thinking about making a ‘machine’ for a while and though maybe now is the time to do it. So I made a mould, got a heating blanket, some steel slats, and made a temperature controller. I’ve enclosed a picture.

I have tried it on two pieces of mahogany now, the pair from the one I ruined by hand bending and part of a new set I just got...neither went well! Again they both just didn’t go bendy and cracked really badly.

I’ve read a lot of other peoples’ suggestions on how to do it; ie temperatures, thicknesses, method etc.. and nothing seems to make any difference. I’ve also seen a few videos on youtube, and when other people do it their sides go almost like plastic, mine never do.

Help! I have tried just about everything that I’ve seen/read others do and as
I say, nothing seems to work.

The slats are from Lmi, and I've tried with them, without them, with tin foil, without tin foil, completely wrapped up in tin foil (so it could stew in its own juice!). Nothing, just not bendy.

Am I letting it dry out too much maybe?
Thanks, Eddie.

[gmm55]

Hmmm... 28 hours, 45 views, and no reply from a builder? Oh well. That means you get a reply from me, a non-guitar builder in the strict sense, but much experience bending wood in other ways for other purposes.

Any runout of the wood will significantly increase the likelihood of breaking, but regardless, you have either inadequate wood moisture content, inadequate heat, or most likely both. I'd be cranking up both significantly. Cut up and use the already ruined pieces for experimenting in the waist. If you get the waist to bend without issue, the bouts should be a breeze. Using foil sounds like a mistake to me, but perhaps a pro will chime in.

In addition, your mechanism looks very slow and cumbersome to operate. the blocks must be advanced incrementally, since you have to stop to grab the wing nuts each half turn. Of course both sides should advance at the same time to maintain parallelity with the mold.

I'd work on a new mechanism that advances the block faster, and continuously. A quick and dirty way might be to replace the wing nuts with threaded rod couplers, and use a pair of 200 rpm cordless power-drivers tipped with deep sockets. I automate some of my lathe carriage motions by doing exactly this with a cordless driver.

Edit: Then again, maybe I am reading your mechanism wrong. Perhaps you actually use your hand(s) to push the block down and only tighten the wing nuts at the end. Can you clarify?

[thebolo]

I still bend my sides by hand. It takes some practice, but I get a good feel for the wood and generally can avoid cracks and stuff like that. I know that doesnt help with your problem, but it is another option if you get too fed up with the machine.

[kennyk]

I think you should have a look at Chris Wynne's 10 DVD set Guitar building masterclass. (available direct from him at Thomas Lloyd Guitars in Oz, or via Stew-mac, or as you're in UK, maybe madinter might be the best method.)

He uses a side-bending jig which uses 3 200w filament bulbs to heat the wood.

also the dampened sides are covered top and bottom with 0.7mm aluminium sheets, NOT tinfoil, and put into the jig that way. he does the larger, lower bout first, gives it a few minutes then the upper bout, and finally screws down the waist, then leaves it with the heat on low for around 45 minutes.

he does each one slowly and he says you can feel the wood 'give in' as you ease the sprung bars round.

[Brackett Instruments]

Every piece of wood bends differently. As a general rule I bend hot, and quick. With the right temperature, and timing it shouldn't take much effort to bend. You get busted sides from using too much effort. If you try to bend before the wood it hot, and ready it will be too hard to bend. If you cook it too long before bending it will take a set, and be hard to bend. Practice is the key, but the real key is paying attention to how it feels when you're bending. Here's a video I made a while back.
http://youtu.be/CHxTUiErChA

[snowwis]

On my bending "machine", I use 3 stainless steel "slats" , one on each side of the heat blanket, and the third on top of the wood.(slat, blanket, slat, wood, slat) I preheat the slats for about ten minutes BEFORE putting the wood into the sandwich. I dampen both sides of the wood using a spray bottle. If you preheat enough, the dampened wood will "sizzle" just a bit when it first touches the slats. The resulting steam is what helps the wood conform to the mold. As stated by another poster, some pieces of wood bend better than others. If the two sets you tried are "sisters", you may want to try wood from another source. Also I usually bend sides at about .090 - .100 thick. From your pictures, it looks like you are in that ballpark. Bend the waist slowly and evenly. Slow the bend if you feel strong resistance. Finally, no matter how carefully you bend (or who you are, or how much experience you have), you will occasionally end up with some slight cracking. Hope this helps.
George

[charles Tauber]

Basically, what others have already said:

1. runout
2. hot enough
3. water/steam
4. with sufficient heat and steam, the wood will plasticize allowing you to feel when it wants to bend
5. thickness, around 2 mm, give or take.
6. preheat before adding the wood

In my experience, mahogany is easier to bend than rosewood and at a lower temperature.

I use a Fox bender with heated light bulbs and spring steel slats, one on each side of the wood. I preheat for 5 to 10 minutes at which point I spray a little water on the assembly in the region of the waist: the water beads and jumps when hot enough. Then I spray the wood with water and add to the assembly, leaving the form dimmed to retain some heat while it is taken apart, the wood inserted and then reassembled. Once the wood is between the slats, and the slats in the mold and heat turned back up, I wait until I start to hear the wood sizzling in the area of the waist. Then start to crank down the waist, which I do first. Then the upper bout, then lower bout. I let the feel of the wood determine the pace at which it is cranked down. The cranking down takes several minutes.

[Kitchen Guitars]

Replace your bolt downs in the front and back with springs. This is a Blues Creek Bender
John will sell pieces to make your bender right, and be your best help