View Full Version : Help me fix my cracked side.

lefty dan
05-11-2011, 05:02 AM
I learned a lesson the other day. (Take my time)!
I got in a hurry and began bending the side of my koa build. Its a concert and the jig is heated with light bulbs. Im on number 12 bending that way and it has always worked for me till now. I didnt soak the side in water long enough and while bending it cracked.
It cracked on the bottom bend. So I glued it back up with titebond glue. Clamped it between two straight boards. After it dried ya cant even see the crack. So I then wet the side again and put it back in the mold. While bending the bottom it cracked again!!

I have glued it again and again you cant tell. Im going to try to soak and bend it again unless someone can give me a secret on bending it when I get to the weak glued part. Or maybe something better to glue it with?

Just frustrated.



05-11-2011, 05:32 AM
Is it possible to glue it after you finish bending it?

lefty dan
05-11-2011, 05:46 AM
Tattwo, I cant do that because when it cracks it looses the round curve. Its like it snaps in half but not completly in two parts. When I try bending that part is the weakest.

dave g
05-11-2011, 05:51 AM
How about sanding it much thinner, then bending it and a back-up piece of something cheaper & laminating them together? (I'm assuming that you've only got the one piece of nice koa (?).)

lefty dan
05-11-2011, 05:58 AM
Dave I have thought of that. Do you think it will work?
Yes I only have the two halves of koa. Id like to make it work so they stay book matched if possible.
What if I put thin laminate on both bottom and top of the koa side and tried bending it. Would the two pieces prevent the middle piece from breaking? If it would then I could just take the laminate pieces out after.

Moore Bettah Ukuleles
05-11-2011, 07:22 AM
The thing about bending with light bulbs as a heat source is that your wood doesn't get hot until it actually makes contact with the bending for. With blankets, you are heating the wood even before you start the bend. I go as far as using two heat blankets, one on each side of the wood, but that is a little extreme as I am often bending some very difficult wood.
I bend between two slats of spring steel with good success. I use just enough water to create a little steam when the side is heated. From my experience with curly koa, soaking to wood in water for any length of time only makes the wood weaker. I also try to use a lot of pressure when bending. Look at the Taylor videos. They uses tons of pressure and no water.
If I were you I would rebend it as best I could, crack and all, then clamp the sides into the mold, spray it down with water and let it sit until it dries. Then flood the crack with CA. You'll probably still see the crack though and will also probably have a kink there. No matter how many times you've bent sides, this stuff occasionally happens, sometimes for no apparent reason.
I sometimes use Super Soft II for difficult bends. I don't know if it really helps or simply that I pay better attention when I use it.

05-11-2011, 07:30 AM
Hey Dan, as one who has cracked his share of sides, I feel your pain, and I admire your intention to learn from the experience. That said, I think there are a number of things you need to learn. Number one, the properties of Titebond glue, you have been chasing your tail. We use Titebond because it releases with heat and moisture, so we can repair the instrument later. You can not put a repaired glue joint in a side bender and expect the glue to hold, as you have seen. Number two, soaking koa before bending is a bad idea. Chuck Moore, who has some experience with this, says soaking curly koa before bending it is a good way to wind up with a pile of potato chips. He places his sides in a wet canvas sleeve and then uses the side bender. I bend by hand and use a spray bottle to dampen the side right before bending. The only way I know of to effectively glue this back together, is to bend the side first and then glue it. In this case, you probably need to finish the bend by hand. Unfortunately, you attempted the repair with Titebond. Once a Titebond glue joint fails, you need to clean out all of the old glue before regluing, and that is probably impossible in this case. Here is how I would attempt to repair this. First, finish the bend by hand, it does not need to be perfect, just close. Put the side in a mold and use CA glue in the crack. Once this has dried, reinforce the inside of the cracked area with a thin piece of veneer glued over the crack with the veneer grain 90 degrees from the side grain. You can add a piece of veneer on the other side for symetry. Hope this helps.

Moore Bettah Ukuleles
05-11-2011, 07:50 AM
Those are good tips Brad. I'll sometimes bend an area on the hot pipe if I need more control. I found a tip that works great for me when doing so. I'll take a small square of rag (a face towel or a sock works great) and soak it in water and place it on top of the hot pipe and bend on top of the wet rag. It releases at lot of steam and heat. Watch your hands though, it really gets HOT! Keep spraying the rag down with water as it dries out. The usual fire disclaimers apply......

lefty dan
05-11-2011, 01:11 PM
Well, I tried messing with it again and it keeps breaking. Wish I would have taken my time in the first place. Next time I will take the suggestions given to me by you guys the more experienced builders.
I will let ya know what I end up doing.

05-11-2011, 11:26 PM
Just a thought (never tried this but it might work):

Glue a slice of something thin (veneer perhaps?) on either side of the piece, extending 3 or 4 inches beyond the break, using your Titebond (or hide glue for preference if you have any). Grain should run along the side, not across. Then bend over a hot pipe. The Titebond will probably release at the bend but should stay attached at either end of the veneer. This might be enough reinforcement to allow your side to be bent, or at least to get the bend started. If it works, you could complete the bend in your bending jig.

Then remove the veneers (perhaps putting the side in your bender to heat the full length?). Repair the crack (probably reinforcing on the inside) and clean up glue residue.

Would this be too thick to bend? 0.6mm veneer either side would increase the thickness by 1.2mm so it would be hard work. But I can't see this doing any harm.

lefty dan
05-12-2011, 05:04 AM
Chris, I dont have a hot pipe so I would have to make one. I like the idea.
Im going out of town for a week and I know I will be thinking of ways to make this work.

I tried bending it again this time no heat put a weight on the far end of the side hoping that it would slowly bend over time. Looked like it bent past the crack area so I started messing with it and again it broke. I then re-glued it and again it looks as if it never had a crack in it. Did I mention that it the crack is almost all the way through and two parts.

Im thinking about trying this. Make a gig the shape of the bend I need Take the dowel off and but the wood with the shape of the bend on the threaded rods. Then clamping it down. Maybe the wood will have no other option but to bend close to what I need.

I do have other wood I can use but I like the book matched look and for that I will need to make this work or throw both sides aside and start on new sides.

Someone said the mark of a good luthier is a guy that can fix his mistakes. Its become a challenge at this point.


Moore Bettah Ukuleles
05-12-2011, 06:28 AM
I think the correct quote refers to how well he can cover his mistakes. Meaning that the end product should show no evidence that a mistake was ever made.
With such a deep crack it may be impossible to make make an invisible repair. If this is the case, and you are insistent on continuing, this is the time to think out of the box and start considering tobacco stain, sunburst finishes etc. If your repaired bout is smooth and kink free there are probably some things you can do to disguise the damage if you are creative.

The cracking may have been out of your control. I run into this problem only when a set of sides has excessive runout in it or if it has a particularly wide band of curl or other inclusion. Some builders advise to never glue up tops and backs until you've bent your sides; too many things can go wrong in the building process.

Liam Ryan
05-12-2011, 11:11 AM
time for a bit of advice that might sound a bit harsh.

Dan, I think you're putting the cart before the horse. The problem here is not the cracked side but the under-performing light bulb bender. Here's a list of things to keep in mind
1. Not many builder's use a light bulb bender these days. They've gone the way of the do-do.
2. If you continue building you'll find plenty of timber harder to bend than curly koa.
3. +/- a few bucks, one ruined set of curly koa in the bin could have been spent on a blanket; which has a much shallower learning curve and much less chance of sending expensive timber to the bin.
4. Learning to bend on a pipe is highly recommended. Most of the time you can feel when the wood wants to bend without having to force it.

So..... I'd suggest a solid form, a blanket and some practise. And most importantly you sides should be 1.8mm thick at most.

And remember, instrument building is a wicked mistress. No body stops learning in this game.

lefty dan
05-12-2011, 11:48 AM
All very good advice.
I keep saying that Im not going to build another so why invest in more tools ex.... But then Im buying wood and at it again. Its kind of addicting.
So this side is done. Gotta find a place I can buy just the sides in koa.

05-12-2011, 11:22 PM
My pipe bender is very low tech and as cheap as could be.

1. Go to your local auto shop (garage in England) and scrounge a short length of discarded exhaust pipe - 1.5 inch diameter or so.

2. Take a piece of thick MDF or ply and make a hole in it for the pipe to go through.

3. Cut 4 slots around 1 inch deep in end of pipe, splay to 90 degrees with a hammer (no need for exactness here).

4. Drill holes and bolt to MDF (pipe goes through, bolt on "inside").

5. Clamp MDF in bench vice, sit blow torch on bench pointing into pipe. (I should have said the MDF needs to be the right size to make this work - clamping area + height of blowtorch = length from bottom of MDF to mid-hole). No, the MDF or ply doesn't scorch, atl east if you only use it for 10 mins at a time.


Moore Bettah Ukuleles
05-13-2011, 07:44 AM
Or, as I do, simply clamp a galvanized pipe into a bench vise. Stick your torch inside and light. Couldn't get any easier.
All usual fire disclaimers apply.

Vic D
05-13-2011, 06:31 PM
Those are good tips Brad. I'll sometimes bend an area on the hot pipe if I need more control. I found a tip that works great for me when doing so. I'll take a small square of rag (a face towel or a sock works great) and soak it in water and place it on top of the hot pipe and bend on top of the wet rag. It releases at lot of steam and heat. Watch your hands though, it really gets HOT! Keep spraying the rag down with water as it dries out. The usual fire disclaimers apply......

Another gem of a tip. Thanks Chuck.

Vic D
05-13-2011, 06:38 PM
I hate to say it but, once a side splits it's not a side anymore, it's binding or inlay. I bend my soprano stuff on a stew mac style egg shaped aluminum "pipe" bender I bought off a violin maker on ebay, you can really feel the wood give way when it wants to and if you take your time and feel it out you can bend crazy tight curves in woods. I'd take all the advice given from everyone who posted here... it's all gold.

lefty dan
05-17-2011, 09:48 AM
"I'd take all the advice given from everyone who posted here... it's all gold. "
Vic I agree, Thanks to everyone for the great advice over the years. I love being able to ask questions and get advice from builders who are much better than I. It makes me want to keep trying to get better. Its also why I love this forum.