First attempt at heat bending: questions

[Marty Jacobson]

SUCESS!!!

Thanks for the guidance everyone!... before investing in a bunch of new stuff, (pipe, electric heaters, etc.) I opted to give it another go with what I have. I cut the flame waaaaay back on the torch and applied the heat indirectly by having the flame licking the side of the pipe rather than strait down the middle. I could tell the heat was in the right range because, as Graham said the water just bounced right off the pipe. I successfully bent the entire side without scorching it at all! I did try the wet rag, but that didn't seem to be working, so I abandoned it. You can tell the difference between last night's attempt and tonight's:

That looks perfect! There's no smell like it, is there?

[HoGo]

The thin stuff, seems to be plated? Thanks, I've been trying to figure out what might be a good budget option for making a strip, seeing as metal stockholders only want to sell sheets by the acre, or that's what it feels like...

Most folks use 0.25-0.3mm (0.010-0.012") thick stainless spring steel. Search for precision shim material, here I can buy strips of various widths and thicknesses also in small quantity for fraction of what luthier supply shops ask.

[soliver]

Yippee! I hope you will post pix of your challenges and successes on your journey.

Didn't want to get annoying and post pics all over the place, so I just posted a pic of the Rim all glued up on the "Post a Pic of whats on your workbench" thread.... but why not:

[putnamm]

Didn't want to get annoying and post pics all over the place, so I just posted a pic of the Rim all glued up on the "Post a Pic of whats on your workbench" thread.... but why not:

That looks great, Spencer. I'm a newish builder, myself and appreciate seeing your progress. As for worrying about the number of pics/posts, I've been quite prolific myself and so far no one has complained (to me, anyway)!

My two cents: If you're going to build more of these you might look in to a bending form/mold. I fully admit that bending sides by hand is a valuable skill, it is transferable to many different projects and probably should be at the top of the list for things that serious luthiers know how to do... But I personally honed in on F5s specifically as all I want to build (for now). After getting frustrated with trying to bend by hand (That scroll!!!) and trying many different techniques, tools and jigs I went with an inner form/molds, heat blanket and a metal strip. It's worked great so far.

I realize this may be regarded as the "lazy" way to do it. On the plus side, it helped me produce sets of ribs the way I wanted so that I could move on to learning and working on other parts of the process.

[amowry]

The thin stuff, seems to be plated? Thanks, I've been trying to figure out what might be a good budget option for making a strip, seeing as metal stockholders only want to sell sheets by the acre, or that's what it feels like...

I use aluminum flashing, which works great and doesn't oxidize or stain the wood. I buy it in wider rolls and cut the whole roll to width (carefully) on my bandsaw after taping it so it won't spring open as I cut it. I've had the same strips for 20+ years.

[soliver]

One more question that's relative but not necessarily about heat bending:

Do I need to keep the Relative Humidity near 40% during the entire build, or just during glue ups?... I'm imagining my power bill going up.

[graham darden]

The thin stuff, seems to be plated? Thanks, I've been trying to figure out what might be a good budget option for making a strip, seeing as metal stockholders only want to sell sheets by the acre, or that's what it feels like...

Jim,
I bought a roll of 2 inch wide stainless shim stock. .010 thick.

[sunburst]

...Do I need to keep the Relative Humidity near 40% during the entire build, or just during glue ups?... I'm imagining my power bill going up.

Frankly, you can get away with some fluctuation in RH during a build (just as we can get away with RH variation with completed instruments), but ideally we start with wood that has been kept in conditions of RH of 40% to 50% until moisture content is stable, and continue to maintain that throughout the build and the life of the instrument.

[Schneidly]

Hello all, reviving this thread because I too am trying my hand with bending and was wondering if you might be willing to pass along your insights. I'm trying to bend big leaf maple that is relatively flat sawn. The bending pipe is 2" NPS (2.38" actual OD) heated with a halogen bulb. My practice pieces came from the same pieces of wood as the real ones and went fairly smoothly, so I jumped in to the real pieces. Unfortunately my luck ran out and I'm struggling with it splitting on the outside of the bends. I've tried clamping the wood to my bending strap similar to Hogo's setup to keep the wood in compression, but doesn't seem to be helping much. Also bends are sort of scalloped with sharper and shallower bend radii depending on the figure at that area. I've tried going a bit hotter with the pipe, only lightly damper ing the pipe side surface of the maple and stopping once the strap over the pipe is too hot to touch in hopes of keeping the lignen there from relaxing. Sides are handplaned down to about .080-.085" thick, nice and smooth surface prior to bending.
I welcome and appreciate any knowledge, tips or tricks you kindly pass along!

[ProfChris]

I think you need to go thinner. For ukulele sides (similar bend radiuses) in tricky wood I thickness to nearer .070 inches (1.8 mm). Unfortunately that's a bit late for these sides.

My other thought is that you might be forcing the wood too much. When I get it just right (which is not that often!), the wood feels flexible in my hands and I just ease it into shape rather than feeling as if I'm making it bend.

That said, some boards won't cooperate, so you might just be unlucky. I hope someone has more helpful ideas.

[HoGo]

What is temperature of your iron? You need 160-190 degrees Celsius so water turns into steam immediately upon contact and will hold that temperature even with longer/repeated exposure. Halogen bulb doesn't seem to have the power. It may be enough to warm up straight grained wood which will bend easily with little moisture and heat but curly or otherwise figured wood requires very even heat distribution and even bending force.
I'm not sure if I posted in one of my bending experiment threads about bending with ordinary iron. I soaked and wrapped the portion of the wood in aluminium foil (yu can add wet paper towel inside) then heated with ordinary clothes iron till the steam escaped for few seconds and then bent around the bending iron using steel strap, this makes the wood much softer as it heats and steams the whole volume evenly and the bending iron helps keep heat in the bent area.

[sunburst]

Those portions of lifted grain look like the wood was too wet, and/or bending was progressing too slowly and the wood at the outside of the bend got too hot.
It is a delicate dance, but if we can get the inside of the bend hotter than the outside we improve our chances of compressing the wood inside the curve while not fracturing the wood at the outside of the bend. In other words, as soon as the wood starts to bend easily, bend what you can and if it is not enough, stop before the outside wood is too hot, let it cool and try again. In difficult wood I try to keep the outside of the bend cooler than the inside. It usually requires stopping and starting several times, and using a bending strap is essential.

Quilted maple will most likely leave you with some "facets". I've bent quilted bigleaf maple sides that I simply had to settle for facets in the bends. Rather than going thinner, I ended up using slightly thicker sides material and sanding the surfaces smooth later.

[Schneidly]

Thank you all for your feedback! I see a few different angles to try which lead me to a few follow up questions if I may...

Adrian - Does the effectivness of using the clothes iron pre-steam method depend on grain orientation? It seems that steaming the outermost fibers would cause them to more easily let go (crack). Does the clothes iron method only work if you're able to clamp both ends of the wood to the bending strap to keep the neutral axis in the bending strap (all wood would be in some amount of compression)?

John - To try going a bit thicker (I have one more piece of stock to handplane down from the rough sawn .190"), what thickness would you recommend trying? .095"? Intriguingly my practice pieces were off cuts from the "real" sides, but I had left them slightly thicker. I also appreciate the insight that the quilted maple will likely end up faceted, even in your skilled hands.

Anyone - What diameter pipe do you use? The 2.38" outside diameter I'm currently using is making the scroll area ok, but I can't seem to get the tighter bends leading into the points or around the neck block. I'm guessing I should use a smaller diameter, but curious what size I should grab at the hardware store. Lastly, I've heard that minor cracks could be repaired using some CA, but my attempt at it made a very dark streak in the area, which I'm guessing will not absorb dye like the surrounding wood. Is it possible to repair minor bending cracks? If so, what method have you had success with?

Thank you all again for all of your help to fledgling builders!

[sunburst]

John - To try going a bit thicker (I have one more piece of stock to handplane down from the rough sawn .190"), what thickness would you recommend trying?

I wouldn't try over .090" most likely. Bending gets more difficult as thickness goes up. Also, concentrate you efforts in the flat places and avoid the kinked places. If the piece starts to kink, let that place cool and concentrate your efforts in the flats next to it.
I have a 2" ID pipe and I have a 1" ID pipe fastened inside the larger pipe so that a couple of inches stick out towards me. The tight point and scroll bends require the small pipe.

And just so you know and take it into account when considering my advice, my "skilled hands" are really not very good at bending sides. It's one of the things I wish I could get better and faster at.

[HoGo]

Adrian - Does the effectivness of using the clothes iron pre-steam method depend on grain orientation? It seems that steaming the outermost fibers would cause them to more easily let go (crack). Does the clothes iron method only work if you're able to clamp both ends of the wood to the bending strap to keep the neutral axis in the bending strap (all wood would be in some amount of compression)?

Most important is understanding grain direction in the piece and forces that you apply. Wood (nearly) always breaks on the layer on the outside of the bend when it is stretched above its limit (which is different for each piece) The inside layer of the bent wood gets compressed and rarely fails there. Any grain runout from the ouer surface creates inherent weak spots and water/steam always penetrates MUCH faster in the direction of grain than across.
Heat and moisture softens wood but the two main strategies of bending - dry or wet differ a bit.
Dry bending (or with minimal use of water sprayed on the inside surfaces only) relies on heating/ softening only the inside of the bent wood so the outside will stay strong to withstand the bending but inside will give. You need to heat the inside quickly (very hot iron close to burning temperatures) and evenly (It is best to have oval iron so you can start on the flatter side that allows heating of longer part of wood at once - notice the burnt spots in th epics of OP in this thread) and use bending strap to apply the bending force evenly. Oval iron also helps eliminating too severe bending at once. As Sunburst noted you need to bend immediately when you feel the wood reaches flexibility (requires experience) and stop before the heat and steam gets to the outside and makes it too vulnerable and let the piece cool down for a while. You must count with fact thta the steam follows wood cells and figured wood means the cells go across the piece and lead the steam and heat right towards the weak spots of grain running out of surface.
Wet/steam bending counts with FULL softening of the piece using steam and/or soaking the piece and longer exposition to iron (but need to watch if the piece is not drying). The metal strap has two functions - it holds the water in a bit longer and allows more even distribution of bending - ideally the strap is stretched with REALLY high force and the iron shape matches desired curvature so you don't apply too large stress to a single point, you are sort of wrapping the rib around the form in one continuous move. Ideally (with thick wood) the metal strap is anchored to both ends of bent wood so it becomes a part of it and takes all the stretching instead of the wood. Fixing the wood to smooth metal strap is not easy as it can slip even with quite brute force of clamps. Sandblasted (or otherwise roughened surface) strap helps, some folks even use cloth based sanding belt as strap for bending which grabs the thinner pieces even without clamps just with hands.
Now, all this is theory and you need to learn which wood reacts best to which method and at what thicknesses.
Folks like Ellis can bend quilted maple very smoothly at quite a thickness (looks like 1/8") with his bending fixture - notice how he has all the straps pre-tensioned to apply even pressure right at the bends to the steamed ribs.

[Richard500]

Thinking about Hamlett’s advice about restricting heat to the compression side, I suppose the bending strap, instead of being a poor conductor (stainless, fabric) could be made as a heat sink. Aluminum would be a start, or copper if handy. That way a large gradient could be held across the wood. Comments?
And I think I posted this before, but here’s a clothes iron with a bolted-on aluminum rod with a flat side intended to provide very uniform heat over the sharp bend of 8” or so on a bowl back cant, plus a (non S-M) dial type surface thermometer. Lacking skill or experience, I opted for measurement.

[Schneidly]

Thank you all for your advice and knowledge! I also appreciate John’s encouragement that even experienced hands crack sides from time to time. There’s a lot of great information here that you’ve shared and I’m going to enact one of Mr. Condino’s most wise pieces of advice and shut off my computer and spend some time with my hands on the tools and wood and get my feel for it. I’m also going to change the pipe setup I’m using to have a smaller diameter to aid in forming the tighter bends. Thank you again!