WoodNet Forums: Does a chipbreaker really break chips?

I didn't run out to the shop to do some new experimenting, but am just stating what I think. I have done experimenting now and then over the last few decades, but could be mistaken in my recollections. Who knows.

- I do not think chipbreakers break chips when smoothing or otherwise making very thin shavings. Very thin shavings are just too weak for that to be needed, or to work.

- I do think chipbreakers help by breaking chips when doing coarse work with a plane provided the chipbreaker is close enough to the cutting edge. I believe a well set chipbreaker can be a big help when wanting to take thick shavings. I've had cases where I was convinced that a plane could successfully cut thicker shavings with the chipbreaker down close to the edge than when it is pulled back farther than 1/64" or 1/32".

- I don't think the breaking function of chipbreakers is of any importance planing end grain or across the grain, again because the shavings are too weak to need to be broken to prevent them from pulling up fibers, causing tearout.

- Chipbreakers have to be fitted to the blade well. If thee is any hint of a gap or the transition from the blade to the top of the chipbreaker isn't smooth, the mouth will clog and all planing will stop. I don't consider that a problem with chipbreakers per se, just a need for careful tuning. However if all that tuning didn't improve the performance, why bother. I'm with Chris on that.

- I think when the chipbreaker is properly fitted to the blade, if the tight mouth or small setback of the chipbreaker causes clogging it's because the mouth is too tight. On many planes the solution is simply moving the frog back. But on many others the tightest part of the throat, the part where the clogging occurs isn't the mouth itself, but is a little bit higher where the top of the chipbreaker may close the throat tighter even than the mouth. There are two solutions to that that come to mind. One is filing the front of the throat of the plane so it isn't vertical but rather leans forward. I've seen 15 degrees suggested several places. But it depends on the curvature of the tip of the chipbreaker. The angle filed on the mouth has to be at least great enough so the throat doesn't close tighter than the mouth where the chipbreaker comes closest to the front of the throat. 15 degrees usually accomplishes that. One can also flatten out the curvature at the front of the chipbreaker, too, but I wouldn't do that unless it is obviously unusually steep.

- I think in all cases the chipbreaker helps to support the blade, and is especially important to thin blades. In this function it reduces chatter, not tearout. It works not only by adding mass to the blade but also by prestressing it. Both make it resist vibration better.

- Chris's comments about old wooden planes not having chipbreakers is true. But there also came a time when nearly all but the very cheapest one had them. The same kind of case could be made from that that they do work.

- For planes tuned to do fine work I agree with him that the added support for the blade is the only thing they accomplish.

It sounded to me like Chris was mostly complaining about poorly fitted chipbreakers, but maybe that's just because he didn't want to waste the space making that clearer.

Here's an example of how bad a poorly fit chipbreaker can be.

I was showing a group of people how to sharpen plane blades and how to adjust and use wooden planes. Everyone had brought in one of their own to work with.

One of the first fellows to finish sharpening his blade asked me to help him set the plane up. He already had the blade, chipbreaker, and wedge in, so it was just a matter of showing him how to make the blade project the right amount.

I showed him how to make the initial setting by laying the plane on a flat piece of wood, drop the blade in and set it flush against the wood under it, and tap the wedge in. As usual, when I took a couple of strokes down the edge of a piece of wood it cut fine. Everyone was happy and impressed that the wooden plane could be set so quickly.

But after several strokes it hit me that there weren't any shavings coming out the throat. Nor were there any down inside when I took a look. Not knowing what was going on, I pointed that out, and demonstrated by taking a few more strokes. There were some wide open eyes at that point.

About then I suspected what had to be happening. So I pulled the blade out and showed everyone that all those shavings were packed in between the blade and chipbeaker. Soon that plane would have jammed up and stopped working.

He'd forgotten to fit the chipbreaker tight to the blade, and actually had a pretty decent gap there.

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Cheers,
Bob