Bending poplar using Kerf cuts

[Gerry S. Wojtowicz]

Hi all,
I am building q fireplace mantle using poplar. The wall has a slight curve to it. I want to run some kerf cut through the poplar so I can bend it. From what I saw on YouTube, it looked like the cut ended about one quarter inch before going all the way through when people were trying to do this.
I guess my question is, is that a good depth. Will the poplar bend without splitting (most of the videos I saw used plywood). Do I just continue making cuts until I get the bend I want?

Thanks in advance

[Mike Cutler]

Gerry

It seems like it should work, as long as the curve is slight. A 1/4" isn't a lot of material, but it's still got some strength to it.

I don't know if you saw this post, that directs to a calculator for kerf cuts, to bend wood. The website address inside the post is still active.

https://www.sawmillcreek.org/showthread.php?258399-Kerf-Spacing-Calculator-for-bending-wood

[Derek Cohen]

Why not rather scribe it to the wall, and then cut away the excess on a bandsaw. The mantle will fit perfectly flush against the wall, and the face will remain straight ... unlike if you bend it.

Regards from Perth

Derek

[Lee Schierer]

Why not rather scribe it to the wall, and then cut away the excess on a bandsaw. The mantle will fit perfectly flush against the wall, and the face will remain straight ... unlike if you bend it.

Regards from Perth

Derek

I like this idea as well.

[Al Launier]

Why not rather scribe it to the wall, and then cut away the excess on a bandsaw. The mantle will fit perfectly flush against the wall, and the face will remain straight ... unlike if you bend it.

Regards from Perth

Derek

I also think this is a better idea: retains original strength, will fit & certainly look better & it would be a more professional resolution.

[Bill Carey]

Why not rather scribe it to the wall, and then cut away the excess on a bandsaw. The mantle will fit perfectly flush against the wall, and the face will remain straight ... unlike if you bend it.

Regards from Perth

Derek

#3 on scribing to the wall.

[Jim Becker]

I agree with scribing for a slight bend at the wall and if you still want the front to be curved, cut that after you scribe the straight initial workpiece that's intentionally wider than you need it to be when completed.

[Jim Morgan]

Hold on, guys, it depends on how the piece of wood in question is oriented to the wall. If its wider axis is perpendicular to the wall, then scribing is certainly the way to go. However, if it is parallel to the wall (e.g., a frieze), then kerfing is probably preferable to scribing and attempting to resaw in conformance to the curve. Molding scribed to the wall or to the surface of the frieze then conceals the kerfs.

[Bill Carey]

Hold on, guys, it depends on how the piece of wood in question is oriented to the wall. If its wider axis is perpendicular to the wall, then scribing is certainly the way to go. However, if it is parallel to the wall (e.g., a frieze), then kerfing is probably preferable to scribing and attempting to resaw in conformance to the curve. Molding scribed to the wall or to the surface of the frieze then conceals the kerfs.

I have to defer to anyone who quotes Mr. Natural. Parallel to the wall, kerf it.

[Gerry S. Wojtowicz]

Thanks to all for your input. Mike I will check out your link. To those of you who suggested scribing, the piece in question has the flat side of the wood against the wall (the piece is 1 1/4 thick x 8 wide x 58 long; the 8 inch wide face needs the Kerf cut). I guess I should have led with that information.
Thanks again.

[Jerry Miner]

The wall has a slight curve to it.

It would help to know how much "curve"

Kerf-cutting calculator is here (https://www.blocklayer.com/kerf-spacingeng.aspx)

[Tim Janssen]

Steam bending, maybe?

Tim

[Gerry S. Wojtowicz]

Jerry,
Yes, it would be nice to kow how much curve. Not too sure how to calculate that.
Tim, steambending not really an option.

[Jerry Miner]

Jerry,
Yes, it would be nice to kow how much curve. Not too sure how to calculate that..

Put a straight edge (or a straight board) against the curve and measure the gap at both ends. Given the length of the straight edge and the amount of offset, we can calculate the curvature. Nobody can tell you if a 1/2" deep set of kerfs in a 3/4" piece of wood will make the bend until we know how much bend we are talking about.

[John K Jordan]

Jerry,
Yes, it would be nice to kow how much curve. Not too sure how to calculate that.
Tim, steambending not really an option.

After measuring and calculating I would still do a test piece. The radius of a bend can be calculated from the chord length using the straight edge method Jerry mentioned. You can find arc/chord formulas and calculators with Google.

The last time I did a bend like this I left less than 1/4" beneath the saw cut on a 3/4" board. Based on the radius, the thickness, and the kerf width I calculated the optimum distance between saw kerfs to allow the backs of each standing segment to just touch so I could reinforce with glue. It took a lot of saw cuts with a radial arm saw for my 3/4" thick board. (A thicker board will need less space between each cut (or a wider kerf) than a thinner board without extra work on the back.)

I tested on a piece of scrap to make sure the bend would work correctly and give a smooth surface. It's been over 30 years but I THINK I moistened the face of the board before bending. It probably was not poplar, possibly oak.

JKJ

[Ernie Miller]

I do a lot of kerfing in my work. In half inch thick stock (poplar), I kerf 7/16" deep, and never have a problem with strength. However, the bends I'm making are fairly severe. I think making slight bends is more difficult because conventional wisdom says the slighter the bend, the further apart the kerfs can be. If you draw a circle in SketchUp and zoom in on it, you'll find you don't have a circle at all - just a series of straight lines. The more segments you put into the circle, the smoother the curve. Same thing with wood. If you spread your kerfs wide apart, you'll get several straight segments that "curve" only at the kerf locations. To get a smooth curve you need the kerfs close together - something you may not want to do for a slight bend.

[Gerry S. Wojtowicz]

Hi all,
I have to admit I don't understand the kerf cutting calculator.
I wanted to attach a sketch of what I am trying to kerf cut but I can't figure out how to attach a file.
Can anyone step me through it?
Thanks

[Jerry Miner]

Hi all,
I have to admit I don't understand the kerf cutting calculator.
I wanted to attach a sketch of what I am trying to kerf cut but I can't figure out how to attach a file.
Can anyone step me through it?
Thanks

The calculator starts with the radius of the bend. What's your radius?

To attach a pic, click on the "picture" icon (the middle one) in the bar above your new post to "insert an image"---- then choose the file (in jpg. jpeg. png. or gif. format) to upload.

[Gerry S. Wojtowicz]

Jerry,
I don't know the radius.
What picture icon

[Jerry Miner]

The picture icon is here when you start a post:
376788

We can figure your radius if you give us some info (as requested above)

[johnny means]

Does this board sit on the wall alone? Or does it get trimmed out somehow?

[Gerry S. Wojtowicz]

376865

I have a sneaking suspicion my attachment did not work. It is supposed to be a scan of a sketch I made of what I am trying to do.
The board in question is 55 3/4 inches long, 5/4 thick and 8 inches wide.
The 8 inch wide face will be against the wall (which would appear as a half circle on the sketch if it worked) and will receive the kerf cuts.
The bow in the wall is such that there is a 1/2 inch gap between the board and the wall at either end.

That is all the information I have. I don't know how to calculate any additional information that might be needed in order to use the kerf calculator.
If any help can be provided it would be appreciated. If not, I certainly understand.

Thank you all again in advance for your efforts to help out this hapless soul.

[Gerry S. Wojtowicz]

376868

OK this should work

[John K Jordan]

376865

I have a sneaking suspicion my attachment did not work.

I see a blank PDF. If it's a scan, maybe just attach it as a JPG file.

I didn't look at the kerf calculator but I assume you need the radius of the curvature of the wall.

You could calculate the radius of the curved wall from this (see the diagram and equation here):
https://math.stackexchange.com/questions/564058/calculate-the-radius-of-a-circle-given-the-chord-length-and-height-of-a-segment
55.75" would be the chord length (l)
1/2" would be chord height (h)

Or a little calculator here: https://www.mathopenref.com/arcradius.html
where Height would be .5 and Width would be 55.75

I'd offer to take a closer look at the math but my day started early at the doctor's office and is proceeding this evening with my brain modified by pain pills. Calculating two ways the wall radius looks to be about 777 inches but you better check it yourself!

JKJ

[Jerry Miner]

Calculating the Radius: [(height of arc)^2 + (1/2 chord length)^2]/ 2(height of arc)

John's got it: at 55 3/4 the radius would be 777. At 56 the radius is 784. No matter, really.

I don't think the kerf calculator will help you here, because--according to the kerf calculator-- two kerf cuts will get you there--- but you wouldn't have a smooth curve.

I would kerf every inch or so to get a smooth curve. That bend is pretty slight. Kerfing every inch will give you plenty of flex.

Another alternative, of course, is to build up the thickness from several thin layers, either in a bending form, or right on the wall. (Or a series of spacer blocks and a final "skin" of thin solid stock).

[Gerry S. Wojtowicz]

John K Jordan,

Thank you. I got the same figure

[Gerry S. Wojtowicz]

Jerry,

I think you're right. Besides, I couldn't get the kerf calculator to work. My numbers seemed to fall outsides its range of input parameters.
I'll try a kerf every inch or so. I agree its not much of a curve.

Again, thank you all for your help.

[Gerry S. Wojtowicz]

Hi all,

One final comment. Photos of kerf cut.
Sorry the second pic is off 90Deg
Thank you all for all your help and support376996376997