Official Luthiers Forum!

Hello from Sacramento --- I'm trying to bend sides for my sixth guitar, beautifully quartered Honduras mahogany. I'm using an old Charles Fox bending press with internal heat from light bulbs as well as a heating blanket. For the first five guitars, I bent EIR sides easily on this press with no problems whatsoever, without a heating blanket. Now I'm having difficulty bending mahogany. I get "wrinkles" in the waist where the wood crumples, forming funky lines across the width of the side. I also get huge amounts of spring-back, unlike with EIR, which sprung back only very small amounts. After seeing the results with the first side, I re-heated it in the press but managed to take out none of the spring-back and ended up scorching the side. It's a small 12-fret guitar, about like 00-18. The tightest part of the waste has a radius of about 4 and a half inches. I'm not using any water, never needed it with rosewood.

Does anyone have any suggestions to get me on the right track?

Thanks.

Let me first ask a couple questions. You mentioned you are using light bulbs and a heating blanket. I will naturally assume you are using either spring steel or stainless steel slats below and above your sandwich. So my first question is: Are you using a spring tensioned waist support? This is a bar that holds the lower slat and the rest of the sandwich tight against the waist caul during the bend. By holding the whole sandwich pressed against the waist caul form the get go this makes the lower slat give support to the bottom side of the waist bend through out the entire waist bend. If you do not have this or are not using it I highly recommend you rig it up. This will help a lot with wood that want to facet (grain separation) in tight bends

Second question is: Aare you providing any add moisture and how and how much to the wood? Such as wrapping the wood in pre wetted Kraft paper, spritzing the wood with water and then inclosing the wood in aluminum foil?

Third: What temperature are you bending at?

Forth: Are you hearing and seeing steam prior to the bend?

Lastly: What order do you bend in?

Wrinkling sounds like too much water.

Check walmart cooking section for a digital thermometer.

You gotta get some heat in there for the mahogany.

It's gonna bend around 300 degrees or slightly sooner.

It will scorch shortly thereafter.

Blankets are wonderful for sleeping and bending guitar sides.

Blankets are your friend. Bulbs will work, I have both though.

Good luck, just order you some more sides, many sellers will have orphans they'll give or sell you for five bucks each for practice.

_________________
http://www.dickeyguitars.com

I'm sure you'll get responses from more experienced builders than me, but a few questions. Are you using one or two slats? What temp are you going up to? How long did you keep it there?

If I'd venture a guess, you didn't take it up to high enough temp the first round. The second time you scorched it as you used no water and you dried it out. Hog is stubborn and you really need to set the bend in there by getting it up to temp. John Mayes' tut online lists 310F. I've heard higher. Wetting the wood (not soaking it) will help give you some high temp time with out drying and scorching. I've had HOG up way too high a temp for short amounts of time and been alright as I never dried it out too much. Blankets, preferably two (but some will argue only one), will allow you to get a high temp quickly to set in that bend without taking forever getting up to temp with bulbs. Bulbs may take too long, drying out the wood, but I'be never used bulbs.

Also two slats may solve your ripple issue.

What he is refering to as rippels I belive he actually means faceting where the wood microfractures across the grain at tight bends like OM, SJ waists and cutaways due to stress. mahogany is prone to this from time to time

Thanks everyone for replies

My press has a spring-loaded support under the waste, a bar of Al that is about 1/4" by 1" held to the bottom stainless steel slat (of two) by a coil spring on either side.

I have not added any moisture to the wood before inserting it into the press for bending.

I don't have a thermometer that can sense the temperature of the wood, so I don't know the temperature (buying a cooking thermometer at Wal-Mart or somewhere sounds like a good idea). I have a timer/controller that I got from LMI about a 2 years ago. I started off with about fifteen minutes with the temperature controller at ~3/4 of the way from OFF to HIGH.
I have a second heating blanket, but I haven't tried to use both at once.

My order of bending has been:
1. turn on the heat
2. after ~10 minutes, pull the "shoes" down to the ends of the form, starting the bending of the upper and lower-bout curves
3. bend the waist in about about 3 or four steps, pulling the side (hopefully compliant now that it 's got some heat in it)
4. Leave the internal heat on for another 10 minutes or so after the blanket shuts off
5. Let the side sit in the press until cool, overnight if possible

(This procedure has always worked like a charm with EIR, nice tight bend at the waist [classical] with no wrinkling, and clean curves around the bouts with very little spring-back.)

This time, when I started to open things up and saw how much spring-back I got, I kept the side in the press and turned on the heat for a about 15 minutes more at 3/4 heat on the blanket and with the lights on. When the side had cooled a second time I started opening things up again and saw that there was still spring-back, I tried heating it again and scorched the side. Drat!

Sounds to me like I need to use water, a digital thermometer and get the wood to 300F or so right away, then let it start cooling so it doesn't scorch.

Thanks for the assistance. I'll check back here again tomorrow.

WATER
James W B

_________________
James W Bolan
Nashville Tennessee

This is hilarious, you couldn't get any more diverse answers if this was about French Polishing!
Mahogany is a bit like that.
I bend on a hot pipe so I'll stay out of the fray and watch.

_________________
https://www.facebook.com/profile.php?id=100008907949110

10 minutes is wayyyyyy tooooo long. You need to finish bending within 4 to 6 minutes and at that point use the heat and time to set and dry the wood. Also if you are using the slatted waist cawl put something under that to avoid faceting.
At 225 start bending the lower bout then check to see the wood is still damp at the upper bout and bend that , then do the waist. Waiting too long and using too much water is not a good thing. I bend at .090 thick. Take the heat up to 375 hold for about 1-2 minutes and hold 10 min at 220-250 to dry and set. You should have a good result

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

What is the order of your sandwich? Is the blanket on top? do you have a solid form? Rosewood bends like butter and not mohogany.

I bend on both a hot pipe and with a fox bender and in both cases I bend the waist first followed by the bouts.

Had the same problem earlier this year. John Hall helped me out. I too was cooking the wood too much. I was initially measuring the temperature of the wood with the thermometer under the wood. Should be between the blanket and the top slat. And those blankets heat up really fast.

With my mahogany troubles, what worked for me was to lightly mist the wood and wipe off excess with a paper towel. Sandwich slat-wood-slat-blanket. (spring steel - wrap with tin foil or it will stain) Tighten up the bending cauls a little to make sure there's good heat transfer. Fire it up on high power and begin bending shortly after I hear a little water sizzling which is maybe around 240-250 F. Ease into it at first. Faster as the temperature comes up. I think I start my waist bend about 2:30 after the heat and am completely done with everything in about 5 minutes maybe 6. Let it cool 30-60 minutes and then reheat at around 280-300 F for maybe 5 minutes. Cool before removing from the bender. On one set I used too much water and needed to cook it some more to get it dry.

No springback. Everything goes together like legos. Those machines are awesome.

Thanks again John!

_________________
Mike Lindstrom

Well I guess if you are using bulbs and blanket that the form isn't solid and the blanket is on top.
You could also try and preheat the envolope to get both slats up to heat and then slide in the side and then bend. Of coarse wear a good set of cotton gloves.

I prefer to sprits mahogany with SupersoftII , wrap in Kraft paper and sticker for 2 days. I then remove the mahogany from craft paper. Lightly sprits with water and wrap in aluminum foil. My sandwich then is spring steel slat, heating blanket, aluminum wrapped mahogany and lastly spring steel slat on the bottom I put all this in the bender and attach the spring loaded waist caul and the upper and lower bout caul and heat to 275f. Once I see steam I bend the waist 1/2" to 3/4. by this time the heat is at 290f to 300f I cut the power back to maintain 280F and bend the upper and lower bout full. I then finish the waist bend. This takes 4-5 min so far. Once all is bent I cut the heat to 220 and cook for 8-10 min and cut the power If at any time I start to smell the wood getting too hot I cut power then because this will indicate that the moisture is gone. Keep in mind I only lightly spritzed the wood not soaked it. I too place my thermometer probe between the blanket and the slat. Once the wood has cooled over night I remove the wood and go straight to the mold. I do see a small amount of spring back but nothing of consequence.

I have found that mahogany scorches quickly so attention needs to be paid at all times to the smell. In fact If I do not hear or see steam still coming out of the wrap I get a bit concerned with mahogany.

What your getting is compression.
The wood is indeed fractured.
The biggest reasons for this is bending to fast-or the side to thick-and not enough heat.

or all three.

mike

_________________
Mike Collins

I've found some mahogany to be much harder to bend than other pieces, and it's impossible to tell which ones will be problematic until you try. The crush lines on theinside of the bend are pretty much the normal failure mode for mahogany. I suspect that a lot of the little Gibsons and such were painted black to hide the crush lines in the waist; you see them a lot in those old boxes when you clean up the paint.

It's sort of unfair comparing almost any wood with Indian rosewood for workability. It's about the easiest wood to bend, and the most stable, of all.

Thank you all for sharing your experience and knowledge. I now own a digital thermometer and I will try bending with H20 on the surfaces and wait until I get to about 300F before bending the waist. I'll let you know the results.

All the best now.

Cheerio! (That's British for "ciao!")

I've been waiting til when I lick my finger and tap the slat and get that sizzle sound. Sort of like spitting on an iron, that's about right.