Kerf bending?

[Dale Thompson]

Hi Folks,
YUK!!! I just got back from the "kid's" place (daughter and son-in-law) and have another list of "required" projects.

One of them is a table/cabinet which is close to the running path of the grand-rugrats. To avoid the brain damage that I (pa-pa) suffered as a youth, the "kids" want a 4" OD radii on the legs.

OK - I'm not THAT stupid! I know the basic principles of "kerf bending".

My question is: I have a choice between using 3/4" Oak plywood or 3/4" solid Oak. Which one would you use for "legs" that will be about 29" high? Also, if you have any "depth of cut" advice, that would also be appreciated.

I know - your best advice would be to tell them to go to Oak Express or some other "box" store. Forget it! I've already tried that!

Why would they want to rob my "scrap box" when they could get some "respectable" stuff? Go figure!

Dale T.

[Jamie Buxton]

Dale, I don't understand. You say the 4" is Outside Diameter radius. Which is it, the diameter or the radius?

If the 4" is the diameter of a tube, I wouldn't do it with kerf bending. I'd glue up solid stock and round the corners. If it is the radius, I probably still wouldn't do it with kerf bending. Instead, I'd cooper it, using six boards to make a hexagonal cross-section, and then round the corners with a hand plane. That would be much sturdier than a kerf-bent thing, and just as easy.

Or, for less work, I'd buy a premade curved panel from Anderson International. (http://www.aitwood.com/). They sell plywood full cylinder tubes, half cylinders, and quarter cylinders. You can even get them to veneer the tubes for you, for a very reasonable price.

[Dave Richards]

Dale, I think we need a little more info on the table/cabinet.

As far as kerfing is concerned, it's not too difficult to do. You need to make sure your kerfs remove enough material from the back side so that the kerfs don't close up before you reach the desired radius. That means you need to make certain you have enough kerfs.

The kerfs also need to be deep enough to allow the remaining wood to bend without splitting or splintering. That will depend on the wood and its moisture content.

When gluing up the bent piece, you can laminate with a thin piece on the back (kerf side) which will help to hold the piece in its bent position as well as hide the kerf lines.

You might consider steamming the kerfed pieces before bending, clamping them to the form and allowing them to dry before gluing up. This will help to reduce the amount of spring back.

Hope that helps.

[Dale Thompson]

[QUOTE=Jamie Buxton]Dale, I don't understand. You say the 4" is Outside Diameter radius. Which is it, the diameter or the radius?

Jamie,
EGADS!! What is a 4" OD radius? Actually the 4" is the outside radius of the bend. I thought about the "Cooper" thing and may do it. The reason I went with the kerf bend is that they have a store-bought matching table with kerf bent legs. Thank you for the sugestions. I'm also going to check out the website you listed.

Dale T.

[Ted Shrader]

Dale -

I'm with Jamie on the cooper idea. Think that would give you a nice solid leg.

Dain Bramage is snot a baad theeng.

Ted

[Dale Thompson]

Dale, I think we need a little more info on the table/cabinet.

As far as kerfing is concerned, it's not too difficult to do. You need to make sure your kerfs remove enough material from the back side so that the kerfs don't close up before you reach the desired radius. That means you need to make certain you have enough kerfs.

The kerfs also need to be deep enough to allow the remaining wood to bend without splitting or splintering. That will depend on the wood and its moisture content.

When gluing up the bent piece, you can laminate with a thin piece on the back (kerf side) which will help to hold the piece in its bent position as well as hide the kerf lines.

You might consider steamming the kerfed pieces before bending, clamping them to the form and allowing them to dry before gluing up. This will help to reduce the amount of spring back.

Hope that helps.

Dave,

Thanks for the input. I'm sure you know this but some others may not. I have had good luck with the following: Using a piece of similar scrap wood, I cut a kerf depth to within about 1/8" of the uncut surface. I then lay the piece, kerf up, on a flat surface. I then measure a distance from the kerf equal to the desired inside radius of the piece. In my case, this would be 3 1/4". I raise the board until the kerf closes and measure the deflection from the table at the 3 1/4" location. That is the spacing of my kerfs. When finished, all the kerfs should theoretically "close" at my desired radius and there is enough contact for glue in all of them. It's surprising how well this works. Most of the time, I don't need a form board. I just use a regular clamp until the glue dries. Although I've never tried it, I'll bet that this would be a good application for Gorilla Glue because of its expansion tendency.

Thanks again.

Dale T.

[Dave Richards]

Dale,

I've heard of that method of figuring kerf spacing. It works very well. Being an over engineer, however, I generally calculate it on paper by first subtracting the inner circumference of the bend from the outer and dividing it by the kerf width. That tells me how many kerfs I need and from there I can calculate the spacing. I usually add one kerf for good measure, too.

So, if you're going to cooper the legs instead, you could use the birdsmouth method which would make it easier to clamp up when gluing.

Remember, pictures.

[Dale Thompson]

Dale,

I usually add one kerf for good measure, too. Remember, pictures.

Dave,
With that smile on your face, you look like you just got a new sliderule. Is it bamboo or aluminum? I hope that it is bamboo because if you throw aluminum into a fire, it will melt!

Wats dis xtra curf biznus? I gots mi injunearin dagrie frum da U uv Ilanoyes. I dont nead ta ad know "cop-out" xtra curf. Im two teknicaly kurect fer dat "weezil-out" stuf!

You MUST be an engineering/technical type. Your demand for pics tells the whole story. We technical types are "trained" (like Pavlov's dog), not "educated" in the "letters" of communication. OK - I'll try for the pictures. You are certainly not alone on this board. I won't mention any names but I want to communicate with everyone, regardless of their "challenges".

Dale T.

[mike lucas]

When kerf bending is done correctly, it is stronger then wood. But one thing that is very important. The kerfs have to be spaced an extremely consistant spacing, and the depth needs to be exact as well.

The radial arm saw is the perfect tool for this.