Monday, August 6, 2012

A Bit Of A Stretch : Part 2

So if you liked the video in my previous post on how I bent a 1-1/4" thick piece of walnut then you probably want to know what the bend looks like afterward. After all that is the real test, not just getting it to actually bend. I have bent many pieces around a form that have been a total failure. So here are the photos.

Here is the inside of the bend that faced the form. You don't see any compression failures which would look like a wrinkle. These wrinkles can travel into the piece quite a ways. They usually occur where there are pin knots or other defects in the wood. It is very important to select stock that is as free of these as you can. That is one of the challenges with walnut. The wrinkles can sometimes be carved away if your blank is oversized enough.

Here is the outside of the bend. This is the side that is most likely to see failure. It is sometimes a tearing of the wood fibers that appears as a raged vertical break across the grain. If this occurs then the piece is usually a total loss. This failure is generally due to not enough resistance from the end blocks of the  strap to keep the wood from stretching beyond its allowable limit. Wood does not want to stretch as I will discuss in a future post. These will occur at pin knots where the grain swirls around them as well. It also seems to fail this way possibly due to over steaming and weakening the wood fibers and thus causing it to tear instead of stretching as it will allow somewhat.  

The second failure is a lifting of the grain. I show an example of this in the last photo. Walnut is difficult to split so this tends to help minimize those failures compared to oak. However, this benefit seems to be outweighed by the failures from pin knots and other defects that walnut has an abundance of compared to oak.

Here is the lifting of the grain and the resulting split that traveled into the piece. This is not a problem for me since this is located in a place that will be totally carved away. The reason this failed here is primarily due to the grain running out at this point which makes it a weak point during the stretching of the outside wood fibers. Wood does not want to stretch, as I mentioned. The second reason that this failed is that it was allowed to stretch too much at this point because the stainless strap was not covering this spot during the bend and providing proper support.

I hope this example gives you some insight into the process. There are other factors to consider as you dig deeper. I will follow up this discussion on how to prepare a thick bend to minimize failures.

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