What went wrong?

[patrick corry]

My first attempt at bending a stem blank for an Atkinson Traveler. I steamed my pre-soaked Ash blank (7/8"x1 7/8") for 55 minutes. 38F outside temperature, so maybe it cooled too much on the way to the barn. The compression strap stop at the top comes loose at 1:25, and the failure at mid-bend at 1:35. Did I bend too fast? Bad grain structure? Compression strap not tight? Steamed too long - too little? I had intended to cut the blank into two stems once bent; maybe I should bend two individual stems at the same time. Or singly?

When I first started driving nails as a kid, and I mis-hit the nail, my Dad said I wasn't holding my mouth right! Everybody's a comedian! Maybe that was it....

Close up of failed grain; it clearly stretched.

 

[Michael Grace]

Hi Patrick - I think you've hit on a few possibilities but I think the main issue is probably that the compression strap doesn't appear to be doing the compression it should. There is some play in the strap relative to the wood at the start, and then the tail end rises up and relieves tension early on. Also, how is the compression strap attached at the end that you first put on the form? It's unclear whether the strap could have moved there. Loss of compression coupled with cooling might had led to failure. Finally, was the wood air-dried or kiln-dried, and how long did you pre-soak? If kiln-dried, you would do well to let it soak for several days, preferably at a warm temperature.

 

[patrick corry]

The Ash was air dried, probably 18 months old. I sawed out the blank from a 2" thick slab. I don't think the failure was because of grain runout. The grain appears to be pretty parallel to the surface. I did soak for 4 days fully immersed in water. The compression strap has an angle iron bolted to it at the lower end:

I may need to apply the C clamp to the compression strap where the stem blank ends. The problem there is that the clamp in that location may interfere with the tail end of the form surface. It appears modifications are in order.

I found a video today of the crew at Headwaters Canoe bending a stem. There were 3 people involved with one pouring (presumably) boiling water on the stem blank as the bend progressed! I need helpers!

 

[dtdcanoes]

Oh Boy. I wish I lived closer. I think it would have been good to have had the bending done in the time you took to get to the shop door. I have my box within 5 feet of the boat and a boiling kettle at hand to pour some on the piece just before I start the bend. I do it because I usually work alone and that buys me a lot of time. I have a 1/2" bend in the strap ( half the thickness of yours ) at the beginning end and it fits into a notched receiver at the bottom of the form. The far end is loosely held with a little clamp and the piece goes in the box to steam. When steamed I take it out, tighten the little clamp at the far end and pour some boiling water onto both sides , put the piece into the notch and bend away slowly. I usually can clamp after the piece after all bent or even as you go around the tightest curve almost done with the bend and add clamps at the hole points.
Time for some mods, but we have all been there. Good luck.
Dave

 

[Dave Wermuth]

To add: I think I counted a minute or more from the time it came out of the box. I prefer a few seconds. Your technique of bending looked spot on. I like to think of the wood as taffy. Go slow and it stretches. Go fast and it breaks. You seemed to go slow and it looked like you had a good feel for the pressure and when it wants to give. I stick my backer strap on pretty loosely and just hold it by hand as tight as I can by hand. Sometimes they just break. I don't put the wedges in until it's all the way. Then I can hold it by hand and put the wedges in with the other. My best guess is that your taffy cooled off to quickly.

 

[MGC]

What the Dave's said.....too much time before you had it on your form and started to bend it. I have my form right under my steam box and waste no time getting it into the form. My compression strap does not get attached to the piece. I simply lay it over the top and jam it into the pocket in the form and start bending. I've not needed to pour water on the piece as I bend but I bet it helps. I'm also a one man band so I concentrate on getting the piece into the form and bent. I put my bolts through and only place a wedge if I can't get the wood down to the form on a really curvy stem. After I have the piece rough formed I hammer in my wedges and add clamps between. In my forms the compression strap is held in place only by the lower pocket and the wedges.
And....sometimes wood breaks for absolutely no good reason. It's good to have a few extra pieces ready to go just in case.

 

[patrick corry]

I agree that there was too much time out of the steam box prior to bending. The cold day was no help either. We learn from our mistakes, and I’m thinking next time i’ll put the steam box just inside the barn with the burner outside for safety. Also, I will have a helper to quickly place the first clamp and maybe an intermediate clamp as I continue with the bend. I don’t think the failure was a grain issue. I have a second blank ready to go. Maybe the compression strap could be attached to a ‘pocket’ on the form into which the blank is quickly jammed, and once the upper stop is engaged the bend can begin with little delay.

Thanks everyone for your thoughts.

 

[Rollin Thurlow]

An excellent looking stem jig.
Too long before starting the bending is one concern.
In the steam box you had the stem located in the lower half the the box. The temperature at the top of the box is going to be much hotter than lower down. The stems should always enjoy the prime steam box location at the top.
Steaming a bit longer , about 1-1/4 hrs would be a bit better. If the lumber is kiln dried it should soak in water for a few days before trying to bend.
The bending strap was not working to its best advantage. Instead of having a positive stop at the end of the strap, the stem was pushing its way past the stop which is why the strap was bending so much at the very end. In the video at 0.41 you clamped on a handle on the bottom of the stem. this seems like the right thing to do, but instead you need the handle on the top of the steel band. It wants to be clamped so about 8" is covering the last 8"of the stem and another 12" is extending beyond the stem. I know this seems backwards but will prevent that serious bending of the band and it will provide an excellent handle to bend the stem with. With the improved strap you should be able to bend your stem in a tight circle.
A helper would be most helpful. One person to apply the clamps and another person holding the handle and bending the stem. The inexperienced person can do the bending with the clamp person telling them how fast or slow to go.

 

[chris pearson]

Expansion failure = band not containing the wood enough. Did you clamp it tight? Yes, wood probably cooled off too much combined with not enough moisture content to begin with. Moisture meters are worth their weight in gold.

 

[chris pearson]

Rollin is such a show off!

 

[Rollin Thurlow]

Says mister diorama man who shows paint brush brisles down to 1/16 scale!

 

[JClearwater]

Patrick,
I agree with the others who said that too much time was lost between when you removed the wood from the steamer and when you began the bend. Likely a fatal error. Also you did not mention and no one else chimed in asking how hot did your steam box get? My cooker is directly below my box so the steam goes directly in. Water boils at 212 degrees Fahrenheit but the steam cools rapidly as it rises from the water surface. You can demonstrate that yourself next time you boil water on the kitchen stove. You can hold your hand directly over a pot of boiling water at say 8" above without getting burned because it cools so quick. Use a meat thermometer thru the side of your steamer. Your steamer should get up to 200 degrees at least before you start your clock. Just my 2 cents worth and you may be over paying at that.

 

[chris pearson]

Says mister diorama man who shows paint brush brisles down to 1/16 scale!

Lmao

 

[patrick corry]

Rollin, thank you for the observation about the position of the stem within the box! That makes perfect sense. Clearly, the strap needs work, which I will address before the next attempt. I do want flat grain against the form, correct? Viewing your corkscrew bend closely… am I seeing vertical grain oriented against the bend?

My stem stock is air-dried, and I pre-soaked 4 days. My steaming time may have been too little if I started the timer before reaching full temperature. The cheap meat thermometer I used got cooked beyond function the first time I fired the rig up!

 

[Todd F]

Patrick, thanks for posting the video. While probably frustrating to you, I learned alot from it and the comments from others. Look forward to a successful bend in the near future!

 

[samb]

A technique that I use to make outer stems on an all wood boat could probably be adapted. It means there is no cooling between heating and bending.

In the photo, you can see the over length and still straight stem is inside the plastic sleeve and under the towel for a bit of heat retention) already screwed to the keel. The screws also hold the compression strap in place. The compression strap has a handle at the end measured to act as a stop. The steam is applied by the wallpaper stripper on the left.

After the right time, the strap is hooked round the end of the stem (the steam tube has to run on top of the stem to allow this). While the steam is still on, the stem is bent slowly round and just needs the one handed application of a previously prepared G clamp to keep things in place. At that time, switch the steam off and split the tube away to allow things to start drying. Don’t forget the thick gloves! The boat needs some weight in it otherwise you end up lifting it up. If you lift the boat after the right amount of time, you can let it steam longer and lift more slowly

All finished and fastened, waiting to be trimmed flush with the hull.

Sam

 

[Rollin Thurlow]

Patrick, a couple more suggestions; If you put your steamer or the steam box outside, any (and I mean ANY) little wind will greatly cool off the steamer, the hose and the box. Put up a wind shield around the steamer and wrap the steam box with some blankets.
Make a couple of practice stems to steam at the same time. The practice stems just need to be the same size as the real stem but don't have to be of the same quality of wood , in fact they can be real reject stock. Practice with these sticks, especially if you have a helper. Don't worry about the practice stick breaking, just go through the whole bending operation and it will tell you a lot about your setup and procedure. It will help tell you if you have the right clamp positions, if your steel strap is adjusted correctly, if your helper is really helping, etc. You'll bend with a lot more confidence when you do the real stem.
For all practical purposes, and all things being equal, the flat or vertical pattern does not really matter for bending purposes. The wood should bend just as well either way. Most of the time I'm placing the wood in a position that I think will give the appearance that I want.

 

[Dave Osborn]

Compression strap needs to be clamped tightly. If the clamp interferes with the mold, your stock needs to be longer to accommodate it.
Your mold looks good.
On bends with larger stock like this I sometimes use a pumping action, pull down, release, pull down, release, repeat.
I think it helps to get the wood fibers to slide.

 

[John Sand]

I've only done a little bending, frames without a strap. I think your cold strap and form are contributing to heat loss. I might put the strap on top of the steam box and a portable heater near the form. Good luck, keep us posted.

 

[Fitz]

I agree with samb. You could try the plastic bag method. Steam it in the bag and bend it and clamp it bag and all on the form. No cool down whatsoever. Wear good gloves and watch out for hot water and steam.

You could also laminate stems using thinner bent material. I saw a canoe with laminated stems hit a rock dead on in rapids one time and it didn't have a problem.