Hot Pipe Bending

[John Abbett]

All,

So I set up my hot pipe, and resawed some nice curly maple into strips for my 335 I'm making. I sanded them smooth, and was all set to go. I soaked them for about 5 minutes, and tried to bend it on the hot pipe - NOTHING. I Cooked and cooked these thin strips, they would not bend. Sure, I could get a slight radius out of it, but not the curvy hornes I needed. Every time I tried to bend the strips, they broke. They broke across the grain. I spend 2 hours playing with different moisure and temperature settings. I ended up with a pile of half bent strips on the floor. Very disappointing. So I mixed up a stiff drink and wondered what I did wrong.

The next evening I grabbed a hunk of air dried maple from the woodshed and cut and thicknessed it, soaked it about 5 minutes in the tub, put it on the hot pipe, and wham, bent like a champ. I got both the left and right bent perfectly, and broke one where I got a little carried away. It only took a few minutes, they are drying in the mold for a few days then they should be ready to go.

The only thing I can think of is that the first set of Maple boards were kiln dried, and the second set of wood strips air dried. They were hard maple, but they should still bend. I don't know what type of maple the second set was, just maple.

Anybody run across this, the wood that wouldn't?

-John

[Prostheta]

Curly figure shows on the quarter, so perhaps being quartered made it more difficult to achieve bends or at least more prone to cracking? What were the cuts of the second set (of sets)?

Edited May 19, 2008 by Prostheta

[John Abbett]

It was curly, the grain was all over the place. The grain at the end was off at an angle, not quartered, not flat.

I think it was either the type of wood was resistant to bending or it was kiln drid to death.

It wouldn't budge, then it cracked across it's width. It was like the hot pipe wasn't getting hot enough, but it would sizzle water instantly. It was plenty hot. (Same heat I used to successfully bend the second set of strips).

-John

[fryovanni]

What was the thickness of the wood you were bending? That can make a huge difference. Temperature also may have varied(maybe you were getting the timing down on the second set)?

Curly wood is a huge variable(quarter or flatsawn, although curly does show best on the quarter). Your dealing with a lot of short grain and runnout(you have it around every single curl, at side thickness). Depending on how strong the angle of the grain, it can be extreamly weak(think of slicing an end off a 4x4(a bit less than 1/8" thick) then glue a couple flatsawn plates spaced about 3/16" apart, it will be very prone to breaking. That is basically what your up against with extra strong curl. The grain is rolling up and down.

One Really good idea is to be sure you support the sides with a flexable slat from above as you bend. This helps heat the wood evenly, and spread the pressure so it doesn't focus on the weak points. David Myka had a really nifty slat he showed at the last instrument show here in Portland that had made up for bending binding. Very simple, but a great idea.

Rich

Edit;

Curly figure shows on the quarter, so perhaps being quartered made it more difficult to achieve bends or at least more prone to cracking? What were the cuts of the second set (of sets)?

Quartersawn wood is prefered for bending sides, because you avoid possible seperation with flatsawn wood. I have bent flatsawn wood(quilted figure), and never really had any issues with seperation, but it is a consideration.

Edited May 19, 2008 by fryovanni

[Prostheta]

Thanks Rich. I'm still getting ready to get my feet wet in side bending, and this is all really useful information to me. I hope that the flame stock I have will bend well, but I do have the quilted as a backup. One step at a time, at all times for me when I attempt this :-\

I'd be interested to hear how thick your pieces were, John.

[fryovanni]

FWIW, When I am bending sides or bindings. The range of thickness is usually between .065-.090"(thin to thick). There is a WORLD of difference in how hard it is to bend wood that is on the low vs the high end of that range.

[MiKro]

FWIW, When I am bending sides or bindings. The range of thickness is usually between .065-.090"(thin to thick). There is a WORLD of difference in how hard it is to bend wood that is on the low vs the high end of that range.

+1 on that that

[John Abbett]

I went back and checked, my second batch is slightly thinner then the first batch. Not much, but I'm sure that contributed. Also, I don't think I soaked the first batch long enough, I think they dried out. I was learning how to do the bending, and set two on a bench while I worked on the the first, the two on the bench dried out. The second batch, I pulled out of hot water and stuck in a plastic bag to hold the moisture.

So it's a combination of things..

1. Thinner is easier

2. Retain Moisture

3. Wait for the wood to give a little before bending.

4. Heat all around the bend, not just the middle of the bend.

5. Pay attention to grain direction, quarter sawn may break, and flatsawn may tear.

6. Use some sort of backing to help with smooth bending.

7. Practice on cheap wood before trying to bend that nice quilted maple.

Any others?

-John

[Prostheta]

"8. All of the above"

[fryovanni]

I went back and checked, my second batch is slightly thinner then the first batch. Not much, but I'm sure that contributed. Also, I don't think I soaked the first batch long enough, I think they dried out. I was learning how to do the bending, and set two on a bench while I worked on the the first, the two on the bench dried out. The second batch, I pulled out of hot water and stuck in a plastic bag to hold the moisture.

So it's a combination of things..

1. Thinner is easier

2. Retain Moisture

3. Wait for the wood to give a little before bending.

4. Heat all around the bend, not just the middle of the bend.

5. Pay attention to grain direction, quarter sawn may break, and flatsawn may tear.

6. Use some sort of backing to help with smooth bending.

7. Practice on cheap wood before trying to bend that nice quilted maple.

Any others?

-John

#2- You don't want to use too much water, this can cause problems. You do want to use the moisture efficiently(steam helps heat the wood evenly). You may want to consider super soft, or a veneer softener if you want to soften the fibers before bending.

#5- Quartersawn is not prone to breaking per sey, and flatsawn may expose the possiblility of seperation(but certainly does not mean it will happen, just a possibility). Figured wood thats structure has a lot of short grain and end grain(flat or quartersawn) is where the risk increases drastically because the orientation is much weaker. Exposed endgrain also drinks and releases moisture very fast.

Using a side bending jig and heat blankets allows you to use slats which can be sealed(control moisure *get more out of less). Provides even support top and bottom. Delivers heat evenly and efficiently if a controllable manner. Allows you to heat and leave to wood in place to cool(no cooling as you transfer it to a mold). Provides extreamly repeatable results. Is fast because it is so efficient. Just FWIW, this is why many builders have gone to this method of bending. It doesn't take many broken sets to pay for a heat blanket

Luck,

Rich

[Hector]

I don´t use much water when bending maple, and get good results.

I do have a question, the fist pieces you bent and broke, were curly maple. the other pieces that bent well were flamed too?

When reading your post, the first thing that came to my mind was " the second set is plain maple, not figured." but you didn´t specified.

plain or figured?

Edited May 31, 2008 by Hector

[John Abbett]

Second set plain maple.

Generic maple board out in the woodshed.

It's going to be burst-dark on the edges anyway.

-John