Thread: Resawing - Kerf - how does it work?

It effectively turns the kerfs into a mitre box attached to the sides of the board.
Do you wonder why the saw does not cut into the mitre box?

Hi Evan,

Don't you need the kerfing plane to have same blade thickness as the saw you will be using? Shouldn't the saw come first?

I believe the kerf creates a path of least resistance which the saw will fall into as long as it's steered reasonably close to the line. I maybe wrong on that though.

Regards

Saws are lazy; they’ll take the path of least resistance unless forced into a different direction. By running a kerf along at least 3 edges you effectively reduce the length of wood in touch with the teeth; they would rather stay in that narrower bit than drift off to one side where the path is longer.

The least resistance idea sounds highly plausible.

I like it.

Doug3030, I see what you're saying, but my query is about the underside, at the saw teeth. I could not fathom how the teeth followed what would essentially be a tiny influence. The two halves above those teeth, absolutely I see it acting like a mitre box.... But the saw face bamboozled me

Is this really the answer then for cutting boards? The saw is "lazy" and cuts toward the "thinner" path preferentially?

Fascinating!

Originally Posted by woodPixel

Doug3030, I see what you're saying, but my query is about the underside, at the saw teeth. I could not fathom how the teeth followed what would essentially be a tiny influence. The two halves above those teeth, absolutely I see it acting like a mitre box.... But the saw face bamboozled me

Evan, I sense that you and I share a similar trait of having to analyse and understand all the fine detail. This has advantages and disadvantages at times. An advantage is that sometimes we can see some things that others miss and can come up with a better procedure. A disadvantage is that sometimes we get bogged down in analyzing something to death instead of just accepting it and the consequence is that nothing gets done. It is a blessing as much as it is a curse.

I don’t think I’ve ever noticed the bandsaw blade being lazy.

As Doug said, the kerfs act as a guide for the saw in a similar way to the slots of a mitre box. I don't think we need to invoke any human attributes, the only thing that's lazy in my shed is me. Have you ever tried to saw a saw-width off the end of a piece you cut over-long with a handsaw or backsaw? It's virtually impossible, the saw tends to slide off the face due to the pressure on one side not being balanced by any pressure on the other. And that's exactly what is happening when sawing with guide kerfs, if the saw starts to wander to one side, pressure is reduced against the "loose" side of the saw & it will be pushed back into the kerf until the pressure is equal on both sides.

And taz01, re your question "do the guide kerf & re-saw kerf need to match?", the answer is no, it can be a bit thinner or a bit wider & still work ok. I frequently pre-kerf pieces for the bandsaw using the tablesaw, and even a thin-kerf blade makes a wider kerf than the bandsaw blades I typically use. However the wider kerf usually works fine - when it lets me down it's usually my own fault for forcing a dull blade. Such problems don't arise if re-sawing by hand because a) you can more quickly feel the increased resistance if the blade wanders into the wood beside the guide kerf, and b) you normally switch the board around every so many strokes, so you can be certain the cut is going where you want it to.

Obviously, you try to keep the guide kerf to something approximating the main saw's kerf width because that will reduce the amount of planing to clean up your re-sawn boards..

Cheers,

have to admit, this kerfing thing has me quite fixated.

I did a sketchup with a 10mm kerf and a "saw" at 15° and 30° angle simulated.... the kerf that guides the saw under the very tips of teeth, at the furthest point is barely 2mm.

Fascinating that this tiny tiny amount of guide actually works.



Im building my mini-bench right now (glueup today! yay!) and once that is finalised, its onto a kerfing plane, then the mighty frame saw.

Obviously, once I actually do it once I'll see that it works.... though Id admit Im still baffled at how it works



On a slightly different tangent, Ive seen a few dudes using circular saw to kerf with..... vaguely interesting....

Originally Posted by woodPixel

On a slightly different tangent, Ive seen a few dudes using circular saw to kerf with..... vaguely interesting....

Someone needs to tell old mate in the video that if he turns the board over end to end between cuts instead of side to side he won't get that step in the cut.

Here is a low tech kerfing plane

kerfing saw.jpg

Probably be alright if it wasn't for the plastic handle on the saw.

62woollybugger was generous enough to email me and share his own plans for an EXCELLENT kerfing plane.

I hope he doesn't mind me sharing them

kerfing saw (4).JPG kerfing saw (1).jpg kerfing saw (2).jpg kerfing saw (3).JPG

That's fine, I'm happy to share. I'll see if I can upload the PDF template for the body to this thread, so anybody who wants it can use it.

Here's the PDF for the body template, hope it works. The numbers are the diameter, in mm, of the spade bits I used.
Kerfing Plane.pdf