Bending some Mahogany questions

[Eric Knapp]

Hello, I’m still very much a rookie and I have this nice board of ribbon mahogany that I’d like to use for a guitar. I’ve had it for over 30 years and it is perfectly quarter-sawn. Here’s a scrap with no finish, water, or mineral spirits.

I’m trying to figure out how to bend this. I have an old electric hot-pipe bender. The first scrap I tried would just plain not bend at all. It would burn and break but not bend. I tried it dry and with some water sprayed on the surface. Next I tried soaking a scrap over night. These scraps were thinned to 2.2mm/0.087”. The soaked scrap bent easily but the inside of the bend got these crumpled or folded sections. (I’m not sure what the best term for those are.)

Can these be avoided? If so, how? The wood is lovely and should make a nice looking guitar, if I can bend it.

Thanks,

-Eric

[Alan Carruth]

Yup, that's mahogany alright; unless it's sapele, which gets sold as mahogany sometimes (or used to). That's how mahogany fails in bending, when it does, though. Usually its the softer stuff that does that, but I've had problems with mahogany more or less at random forever.

A friend of mine noticed once that that an old Gibson L-0 had nice stripe mahogany, and scraped off the black lacquer to re-do it and show off the wood. It had crush marks on the waist on both sides.

[Barry Daniels]

Eric, you are discovering why most builders quickly upgrade to a Fox bender and an electric heat blanket.

[Eric Knapp]

Yup, that's mahogany alright

I was afraid you were going to say that. Oof.

Eric, you are discovering why most builders quickly upgrade to a Fox bender and an electric heat blanket.

Does a bender fix this issue consistently? Are there benders that aren’t the price I’d have to pay at LMI? That’s what’s been holding me back as I’m retired and this is a hobby.

Thanks,
-Eric

[Barry Daniels]

Yes, benders can easily be made in-house, and there are simpler designs. Mario Proulx had a very simple design that may be in the library. I will go look for it. The only real cost is the blanket and controller, and metal slats.

[Barry Daniels]

Here is a bender by Waddy Thompson that is based on Marios design. You can see it is basically half of an inside form, metal slats and some quick clamps. The electric blanket is sandwiched between the two metal slats, along with the side.

[Alan Carruth]

A bender doesn't totally solve that problem, but it helps. That Gibby was done on a bender, after all...

The real problem IMO is that mahogany is not consistent; some of it bends well and some simply doesn't. Usually the harder stuff bends better, but not always. Sadly, the only way I've seen to find out which you have is to try bending it. Thinning the waist and using a less sharp bend helps, of course.

[Mike Conner]

Does SuperSoft 2 help with mahogany? Been really great with curly red maple.

[Barry Daniels]

I use it on all side wood. Sure can't hurt.

[Eric Knapp]

Thanks for the input, guys. I tried an experiment today. I thinned a test piece to 1.5mm/0.06” and tried bending it with just a splash of water. It bent just fine with no damage to the inside of the bend. I also have some SuperSoft and I’ll probably use that too. Making a Fox bender is not in the cards for this summer so I’m going to make do.

-Eric

[Mark McLean]

I think that SuperSoft or similar products do help - and sometimes you want to employ all the help you can get. There is no disadvantage to using it that I have experienced or heard about. If you find that you need to go to less than 2mm to get that wood to bend one option could be to do laminated sides. You could bend it at something like 1.5mm and glue it up with a nice looking veneer on the inside. It would have the advantage of being stiffer and stronger, and well as being bendable. There is no shame in using laminated sides. A number of high-end luthiers are specifically choosing to do laminated rather than solid sides for the advantages that it offers - and there is really no concern of it causing a compromise in acoustic performance.

[Barry Daniels]

Some folks actually think that laminated sides can improve the sound cause it makes them stiffer so they don't rob vibrational energy from the top or back.

[Alan Carruth]

Side mass probably makes more difference in 'keeping the sound in the top' than side stiffness. Gore adds weights to the sides just below the waist for that purpose.

[Barry Daniels]

Not trying to be argumentative, but my in reading of Gore's books, the weights main purpose is for tuning the fundamental peak frequencies. In other words, if the peaks are already at the optimum positions, then he would not add the weights.

All of the laminated sides that I have made are heavier than my single ply sides, so they apparently help both parameters.

[Eric Knapp]

I’m about to attempt to bend these sides so all the shared information is very valuable.

Thanks!

-Eric

[Alan Carruth]

Barry Daniels wrote:
"Not trying to be argumentative, but my in reading of Gore's books, the weights main purpose is for tuning the fundamental peak frequencies. In other words, if the peaks are already at the optimum positions, then he would not add the weights."

It's been a while since I reviewed Gores' books. My understanding is that the main purpose of adding side mass is to move the node line of the 'main top' resonance outward, reducing 'phase cancelation' around the edge where different areas of the top move in opposite directions. This changes the pitch. At the same time there is an optimum pitch for the top resonance that couples most strongly with the 'main back' resonance and pumps the most air through the hole at the 'main air' pitch. If the couple is already as strong as it can get reducing the cancelation at the cost of further dropping the pitch would not make the guitar any more powerful.

Back before his books came out Gore and I exchanged e-mails about this. Fred Dickens had tried adding side mass by putting weights in the neck and tail blocks, and had not gotten useful results, so I was skeptical. Gore described his experiments, and I was able to more or less repeat them, using spool clamps to add mass around the edge and a small accelerometer to read the side motion. This substantially confirmed his model. Fred's weight placement was probably working against him in other ways (such as the 'first corpus' resonance) and muddying the results.

[Marshall Dixon]

The picture you posted of the crack in the sides was on the inside of the bend. I usually experience cracks on the outside, but have only used mahogany twice (without any problem).

I fold a piece of aluminum foil or parchment paper around the side piece after spritzing both sides with water. It helps trap the steam.

I use a Fox bender but I've done it free hand too. I think steam helps. I'm aware that some peoples' experience contradicts this and have caution against wetting the (maybe just a particular figured) wood, but it works for me.

[Alan Carruth]

Woods that are low in density often crush on the inside surface, especially if they have an open pore structure. Softwoods do generally. I've seen this also with willow, butternut, poplar, cedro, and even some soft maples.

Years ago I repaired two pre WW1 Martins with mahogany tops that had that sort of crush failure along the front edge of the bridge on the outer surface of the top. They made the mistake of having the front edge of the bridge plate align exactly with the edge of the bridge, producing a really large stress riser. A 'fun' repair...